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M Abbreviation for the ¤ Messier catalogue of galaxies, nebulae and star clusters.
Mab A small inner satellite of Uranus, discovered in 2003. It is about 25 km
(16 miles) across. Mab orbits Uranus at the same distance as one of Uranus™s
rings and may be the source of the material for that ring.
Magellan A US spacecraft placed in orbit around ¤ Venus to map the surface by
means of ¤ synthetic aperture radar. It was launched from the Space Shuttle
Atlantis on May 4, 1989. The use of radar was essential because Venus is
perpetually covered by opaque cloud. Magellan arrived at Venus on August 10,
1990, and completed its ¬rst phase of operations in May 1991, having mapped
84 percent of the surface. The next phase of observation involved ¬lling in gaps
and making more detailed observations. Magellan burned up in the venusian
atmosphere in 1994.
Magellanic Clouds Two small, irregular galaxies, which are satellites of our own
¤ Galaxy. They are visible as hazy patches in the southern sky. The Large
Magellanic Cloud (LMC) is in the constellation Dorado and is about 170 000
light years away. The Small Magellanic Cloud (SMC), in Tucana, is about
210 000 light years distant.
Magellanic Stream A long streamer of neutral hydrogen gas apparently spanning
the 200 000 light years between the ¤ Magellanic Clouds and our own ¤ Galaxy.
It forms an arc 150 long in the southern sky.
Magellan Telescopes Two 6.5-m telescopes at the ¤ Las Campanas Observatory in
Chile built by a consortium led by the Carnegie Institution of Washington. The
¬rst instrument, called the Walter Baade Telescope, was completed in 2000
and the second, the Landon Clay Telescope, was ¬nished in 2002. They are
designed to observe large areas of sky simultaneously.
magnetar A ¤ neutron star that has a much stronger magnetic ¬eld than normal.
Magnetars possess magnetic ¬elds far more intense than any other known type
of object. The powerful ¬eld results in X-ray emission and periodic
˜˜starquakes™™ that crack the surface crust of the star and release bursts of
energy in the form of gamma-ray ¬‚ashes. Occasionally, extremely intense
gamma-ray ¬‚ares are observed, thought to be when the entire crust of the star
is shattered. ¤ Soft gamma repeaters are a type of magnetar.
magnetic star A star with an exceptionally strong magnetic ¬eld. Magnetic ¬elds
more than a thousand times stronger than the Sun™s general ¬eld have been
measured for a group of ¤ A stars, which also have peculiar spectra.

magnetic substorm

An infrared mosaic of the Large Magellanic Cloud from the Spitzer Space Telescope.

magnetic storm A major world-wide disturbance in the ¤ geomagnetic ¬eld,
caused by an enhancement of the ¤ solar wind, such as a ¤ coronal mass ejection.
The onset of a storm is marked by a sudden increase in Earth™s magnetic ¬eld
when it is compressed by the oncoming solar wind. The magnetic ¬eld
varies rapidly during a storm, which normally lasts for about a day. Major
magnetic storms can induce electric currents in power lines on Earth,
sometimes disrupting the supply when safety cut-outs are triggered.
¤ magnetic substorm.
magnetic substorm A disturbance lasting about half an hour in the ¤ geomagnetic
¬eld in the region of Earth™s geomagnetic poles. Substorms may take place
during a ¤ magnetic storm, or when there is no other magnetic disturbance. An
effect of a magnetic substorm is to propel charged particles in the ¤ magnetotail


Earth™s magnetosphere.

both away from and towards Earth. The particles directed towards Earth
produce ¤ auroras.
magnetograph An instrument used in solar astronomy for mapping the strength,
direction and distribution of magnetic ¬eld across the surface of the Sun.
magnetopause A region between 100 and 200 km (60 and 120 miles) thick at the
boundary between the ¤ magnetosphere and the ¤ solar wind.
magnetosphere The region around the Earth, or any other planet, within which
its natural magnetic ¬eld is constrained by the ¤ solar wind.
magnetotail The part of the ¤ magnetosphere of Earth, or of any other planet,
extending like a tail in the direction opposite the Sun on the planet™s nightside.
Earth™s magnetotail begins about 8“10 Earth radii away on the nightside and
stretches for between 80 and 1000 Earth radii according to the state of the solar
magnitude A measure of the brightness of a star or other celestial object. On the
magnitude scale, the lowest numbers mean greatest brightness. The brightness
of stars as observed from the Earth “ their ¤ apparent magnitude “ depends on
their luminosity and their distance. ¤ Absolute magnitude is a measure of
luminosity on the magnitude scale.


The magnitude system grew from early attempts to classify the apparent
brightness of stars. The Greek astronomer ¤ Hipparchus (c. 120 bc) ranked stars
on a scale of magnitude from ¬rst, for the brightest stars, to sixth, for those
just detectable in a dark sky by naked eye. This qualitative approach was
standardized in the mid-nineteenth century so that a difference in magnitude
of 5 corresponds to a brightness ratio of 100:1. If two stars differ by one
magnitude, their brightnesses differ by a factor equal to the ¬fth root of 100,
which is 2.512. The zero point of the scale was set by assigning standard
magnitudes to a small group of stars near the north celestial pole.
The magnitude of an object varies with the wavelength of radiation
observed. Visual magnitude corresponds to the normal sensitivity of the
human eye. Bolometric magnitudes take account of all radiation, both visible
and outside the visible range. Magnitudes measured over a de¬ned wavelength
range are often described as ˜˜colors.™™ The accurate determination of
magnitudes is called ¤ photometry.
main sequence ¤ Hertzsprung“Russell diagram.
major planet Any of the eight planets Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus or Neptune. Before 2006, Pluto was included with the major
planets but since then has been considered a ¤ dwarf planet according to
criteria adopted by the ¤ International Astronomical Union.
mantle The layer inside a planet or satellite lying below the crust but above the
core. Earth™s mantle contains 65 percent of the mass of our planet.
mare (pl. maria) The Latin for ˜˜sea,™™ which is applied to the extensive dark areas
on the Moon. Its use for features on the Moon dates from a time when it was
believed that the darker areas were liquid water.
The lunar maria are actually ˜˜seas™™ of solidi¬ed lava. They were created
more than 4000 million years ago when the Moon was volcanically active. The
molten lava ¬‚owed into huge basins that had been excavated by the impacts
of large meteorites. By this stage in the Moon™s history, the frequency of
meteoritic impacts had fallen. There are fewer craters on the lunar maria than
on the brighter ˜˜highland™™ areas.
Margaret A small moon of Uranus discovered in 2003. It is about 11 km (7 miles)
Maria Mitchell Observatory An observatory in Nantucket, Massachusetts, that
was founded in 1908 as a memorial to Maria ¤ Mitchell (1818“88), a pioneering
scientist and teacher in an era when very few women took on such academic
work. She won international fame after discovering a comet in 1847. The
observatory houses 18-cm (7-inch) and 20-cm (8-inch) telescopes used primarily
for education.
Mariner A series of spacecraft launched by the USA during the 1960s and 1970s in
a program to explore the planets Mercury, Venus and Mars.


Part of a lunar mare region, the Mare Tranquilitatis (Sea of Tranquility), photographed
from orbit by the crew of Apollo 15.

Mariner 2, the ¬rst successful interplanetary spacecraft.


In 1962, Mariner 2 achieved the ¬rst successful ¬‚yby of Venus, to be
followed by Mariner 5 in 1967. Mariner 4, launched in 1964, was the ¬rst
successful mission to Mars and revealed the existence of martian craters.
Mariners 6 and 7 followed in 1969. Mariner 9 was put into orbit around Mars in
1971 and returned over 7000 images. Mariner 10, launched in 1974, was the
¬rst two-planet mission. It made three separate encounters with Mercury,
providing 10 000 images, and a ¬‚yby of Venus. There were a total of seven
successful missions in the Mariner series. Numbers 11 and 12 were renamed
¤ Voyagers 1 and 2.
Mars The fourth major planet from the Sun, often known as the Red Planet
because of its distinctive color, noticeable even to the naked eye. It takes 687
days to complete one orbit at an average distance of 1.52 AU from the Sun.

The Hubble Space Telescope captured this view of Mars on Jun 26, 2001 when Mars was 68
million km (43 million miles) away. Several dust storms are active, one large one over the
north polar cap and another over the Hellas impact basin in the southern hemisphere (to the
lower right).


Mars is one of the terrestrial planets with a diameter of 6 786 km
(4217 miles), which is just over half the size of Earth. It has long been regarded
as the planet (other than Earth) most likely to have life, a view encouraged by
the presence of polar ice caps and observations of seasonal changes.
Nineteenth century observers, notably Percival ¤ Lowell, convinced themselves
that they could make out systems of straight channels and speculated that they
might be arti¬cial constructions. Exploration of the planet by spacecraft has
so far failed to produce any evidence that life exists currently on Mars.
However, studies of meteorites believed to be of martian origin have fuelled
speculation that microscopic life at least may have existed on Mars in the
remote past when the climate was wetter and warmer, and the possibility of
microscopic life in subsurface rocks cannot be excluded.
Successful missions to Mars include ¤ Mariner 4 in 1965, Mariners 6 and 7
in 1969, Mariner 9 in 1971, ¤ Vikings 1 and 2 in 1976, ¤ Mars Path¬nder, which
landed in July 1997, ¤ Mars Global Surveyor, ¤ Mars Odyssey, ¤ Mars Express and
¤ Mars Exploration Rovers.
The relatively low density of Mars (3.95 times that of water) suggests that
25 percent of its mass is contained in an iron core. There is a weak magnetic
¬eld, about 2 percent the strength of the Earth™s. The crust is rich in olivine
and ferrous oxide, which gives the rusty color.
The tenuous martian atmosphere is composed of 95.3 percent carbon
dioxide, 2.7 percent molecular nitrogen and 1.6 percent argon, with oxygen as a
major trace constituent. The atmospheric pressure at the surface is only 0.7
percent that at the surface of the Earth. However, strong winds in the atmosphere
cause extensive dust storms, which occasionally engulf the entire planet.
Clouds and mist sometimes appear. Early morning fog forms in valleys and
clouds occur over the high mountains of the Tharsis region. In winter, the
north polar cap is swathed in a veil of icy mist and dust, known as the polar
hood. A similar phenomenon is seen to a lesser extent in the south.
The polar regions are covered with a thin layer of ice, which is a mixture of
water ice and solid carbon dioxide. High-resolution images show a spiral
formation and strata of wind-borne material. The north polar region is
surrounded by stretches of dunes. The polar ice caps grow and recede with the
seasons, which Mars experiences because its rotation axis is tilted by 25 to its
orbital plane.
The martian year is about twice as long as Earth™s year, so the seasons are
also longer. However, the relatively high eccentricity of Mars™s orbit makes
them of unequal duration: southern summers, which occur when Mars is near
perihelion, are shorter and hotter than those in the north. Seasonal changes in
the appearance of features as observed from Earth are explained as physical
and chemical changes.

Mars Exploration Rovers

There is a marked difference in the nature of the terrain in the two halves
of Mars roughly either side of a great circle tilted at 35 to the equator. The
more southerly part consists largely of ancient, heavily cratered terrain. Major
impact basins “ the Hellas, Argyre and Isidis planitiae “ are located in this
hemisphere. The north is dominated by younger, more sparsely cratered
terrain, lying 2“3 km lower. The highest areas are the large volcanic domes of
the Tharsis and Elysium planitiae. Both areas are dominated by several huge
extinct volcanoes, the largest of which is ¤ Olympus Mons.
These volcanic areas are located at the east and west ends of an immense
system of canyons, the Valles Marineris, which stretches for more than
5000 km (3000 miles) around the equatorial region and has an average depth of
6 km. It is believed to have been caused by faulting associated with the
upthrust of the Tharsis dome.
There is evidence, in the form of ¬‚ow channels, that liquid water once
existed on the surface of Mars. Channels from the Valles Marineris appear to
have been created in some kind of sudden ¬‚ood. There are also sinuous, dried-up
river beds with many tributaries, found only in the heavily cratered terrain.
Mars has two small moons, ¤ Phobos and ¤ Deimos, which are in
near-circular orbits in the equatorial plane, close to the planet. They are very
dif¬cult to see from Earth. They are so different from Mars that it seems likely
they are captured asteroids.
Mars Exploration Rovers Two robotic rovers that were landed on Mars by NASA
in 2004. Each weighs 185 kg (408 lb) and carries instruments to investigate the

Mars Exploration Rovers. An artist™s concept of one of the rovers on the surface of Mars

Mars Express

geology of Mars and return images. The rovers travel slowly on six wheels,
controlled by radio signals from Earth. The rover called ˜˜Spirit™™ was launched
on June 10, 2003 and landed in Gusev Crater on January 3, 2004. Its twin,
named ˜˜Opportunity,™™ was launched on July 7, 2003 and landed on January 25,
2004 in Meridiani Planum, on the side of Mars opposite Gusev Crater. They
bounced down protected by airbags. The performance of the rovers far
exceeded initial expectations that they would operate for about three months.
Both were still working in 2006.
Mars Express An ESA mission to Mars, launched on June 2, 2003. The spacecraft
carried seven instruments to study Mars over at least one martian year. It was
successfully put in orbit on December 25, 2003 and later maneuvered into its
operational near-polar orbit. It carried a lander, Beagle II, equipped to search
for signs of past or present life, but the attempted landing on Mars failed.
Mars Global Surveyor A NASA mission to Mars, launched on November 7, 1996
for arrival in September 1997. It was successfully put into a high elliptical orbit
around Mars on September 11, 1997. In the following months, ¤ aerobraking
was used to maneuver it gradually into an almost circular near-polar orbit from
which to carry out systematic mapping beginning in March 1999. Many
thousands of high-resolution images of the martian surface were returned.
2001 Mars Odyssey A Mars orbiter launched by NASA on April 7, 2001. It arrived
at Mars on October 24, 2001, and its main program of scienti¬c work was
carried out between February 2002 and August in 2004 though its mission was
extended beyond that date. The spacecraft mapped the distribution of minerals
and chemical elements over the martian surface and discovered that there are
large amounts of water just below the surface in the polar regions. Another of
its tasks was to study the radiation environment near Mars to see what risks
there may be for any future astronauts. 2001 Mars Odyssey also acts as a
communications relay station for lander missions, including the Mars
Exploration Rovers.
Mars Pathfinder A NASA Mars mission, which was launched on December 4, 1996
and arrived at Chryse Planitia on July 4, 1997. The main objective was to test a
low-cost means of sending a spacecraft and a surface roving vehicle to land on
the martian surface. It carried a 10-kg (22-lb) miniature rover, named
˜˜Sojourner,™™ equipped to measure the chemical composition of the surface
rocks and soil and to take images around the landing site in Ares Vallis.
The lander™s impact was cushioned by airbags, which bounced several
times before coming to rest. The performance of both the lander and the rover
exceeded expectations and they were able to continue operating for nearly
three months, until September 27. Panoramic views of the landscape were
returned and Sojourner successfully traveled on expeditions covering about 80
m. Instruments on the lander monitored atmospheric conditions at the

253 Mathilde

An artist™s concept of the Mars Reconnaissance Orbiter.

surface. Measurements of the martian atmosphere also were made during the
parachute descent.
Mars Reconnaissance Orbiter A NASA mission to Mars, launched on August 12,
2005. It arrived in orbit in March 2006. Science operations began in November
2006, after the spacecraft™s orbit had been gradually adjusted by ¤ aerobraking.
The orbiter is equipped with more advanced instruments than any previous
mission and is expected to return several times more data than all previous
Mars missions combined. A particular objective of the mission is to investigate
the history and distribution of water on Mars.
Mars Science Laboratory A robotic rover NASA plans to send to Mars in December
2009 to arrive in October 2010. It will be twice as long and three times as heavy
as the ¤ Mars Exploration Rovers and will carry more ambitious equipment. It will
be able to collect samples of martian soil and rock and analyze them.
mascon An area of anomalously strong gravity on the Moon. The word is a
contraction of ˜˜mass concentration.™™ Mascons are presumed to indicate the
presence of rocks denser than average, though there is no consensus about
exactly how they formed. The areas are roughly circular and are associated
with ¤ mare areas.
mass transfer The ¬‚ow of material from one star to another in a close binary
system. The material transferred may stream directly onto the star™s surface or
form an ¤ accretion disk.
253 Mathilde An asteroid that was imaged by the ¤ Near-Earth Asteroid Rendezvous
(NEAR) mission in a close ¬‚yby on June 27, 1997. It is a uniformly dark ¤ C-type

Mauna Kea Observatories

Asteroid Mathilde imaged by the NEAR spacecraft.

asteroid. NEAR found its mean diameter to be 52 km (33 miles). Five craters
with diameters in excess of 20 km (12 miles) were identi¬ed on the side
of the asteroid in sunlight at the time of NEAR™s encounter. Mathilde™s
rotation period was also measured and found to be unexpectedly long
at 17.4 days.
Mauna Kea Observatories Observatories on a mountain-top on the island of
Hawaii at a height of 4200 m (13 800 feet). The site is one of the best in the
world for optical, infrared and submillimeter-wave astronomy and there have
been observatories there since 1970. The ¬rst large telescope to be installed
was a 2.24-m (88-inch) re¬‚ector belonging to the University of Hawaii. In 1979,
three more major telescopes began operation: the ¤ United Kingdom Infrared
Telescope (UKIRT), NASA™s ¤ Infrared Telescope Facility (IRTF) and the
¤ Canada“France“Hawaii Telescope. The ¤ James Clerk Maxwell Telescope working in
the millimeter wave region, and the Caltech 10.4-m (34-foot) Submillimeter
Telescope were opened in 1987. The ¬rst of the two telescopes of the ¤ Keck
Observatory was completed in 1992, and the second in 1996. Two 8-m
instruments, the Japanese ¤ Subaru Telescope and one of the international
¤ Gemini Telescopes, were completed in 1999. The ¤Submillimeter Array became
fully operational in 2003.
Maunder diagram ¤ butter¬‚y diagram.

McMath“Pierce Solar Telescope Facility

An aerial view at sunrise of Mauna Kea Observatories with Gemini North (the large silver
dome) on the foreground ridge.

Maunder minimum A period of about 70 years, starting around 1645, during
which ¤ solar activity was consistently at a low level and sunspots were rare. For
37 years no aurora was recorded.
Maxwell Montes The highest mountain peaks on Venus, located on Ishtar Terra.
They are up to 11.5 km (38 000 feet) high.
Mayall Telescope A 4-m (158-inch) optical re¬‚ecting telescope at ¤ Kitt Peak
National Observatory, belonging to the US ¤ National Optical Astronomy
Observatories. It has been in operation since 1973.
McDonald Observatory An observatory belonging to the University of Texas
located on Mount Locke in the Davis Mountains near Fort Davis. It was
established in 1932, ¬nanced by a bequest from a wealthy Texan banker and
amateur astronomer, William J. McDonald. The original instrument,
completed in 1938 and still in use, is a 2.08-m (82-inch) re¬‚ector, known as the
Otto Struve Telescope after the observatory™s ¬rst director. In 1969, a 2.72-m
(107-inch) re¬‚ector came into use. The latest addition is the ¤ Hobby“Eberly
Telescope, completed in 1996. There are also several smaller telescopes and a
5-m (16-foot) millimeter-wave dish.
McMath“Pierce Solar Telescope Facility A large solar observatory located at
¤ Kitt Peak and belonging to the US ¤ National Solar Observatory. The main
telescope was completed in 1962 and consists of a 1.6-m (60-inch) mirror,
mounted on a tower, which directs the sunlight down a long shaft inclined at
32 to the horizontal. Much of it is below ground. The telescope produces a


The McMath“Pierce Solar Telescope Facility.

high-resolution image of the Sun 75 cm (30 inches) in diameter. The entire
building is encased in copper, and coolants are piped through the outer skin to
maintain a uniform temperature inside.
Megaclite A small outer moon of Jupiter discovered in 2000. It is about 5 km
(4 miles) across.
megaparsec (symbol Mpc) A unit of distance equal to one million ¤ parsecs.
Mensa (The Table or Table Mountain) A faint southern constellation introduced in
the mid-eighteenth century by Nicolas L. de Lacaille with the longer name
Mons Mensae, the Table Mountain. It contains no stars brighter than ¬fth
magnitude but part of the Large ¤ Magellanic Cloud lies within its boundaries.
Merak (Beta Ursae Majoris) One of the two stars of the ¤ Big Dipper in Ursa Major,
called the ¤ Pointers. The stars in the Plough were designated by position rather
than in brightness order. It is therefore actually the ¬fth-brightest star in the
constellation, with a magnitude of 2.4. Merak is an ¤ A star 79 light years away
and its name, derived from Arabic, means ˜˜the loin.™™
Mercury The nearest major planet to the Sun and the smallest of the terrestrial
Observation of Mercury from Earth is very dif¬cult, partly because it is a
small planet and partly because it is never more than 28 from the Sun on the
sky. Because it lies closer to the Sun than Earth, Mercury (like Venus) goes
through a cycle of phases, similar to those of the Moon. Hardly any surface
detail can be discerned and very little was known about the planet until the
¬‚ybys of ¤ Mariner 10 in 1974 and 1975. Mariner 10 followed an orbit around
the Sun that made it pass close to Mercury three times before it ran out of
attitude-control gas. The images it returned cover about 35 percent of the
surface of Mercury.


A mosaic of Mariner 10 images of Mercury.


Ancient, heavily cratered terrain accounts for 70 percent of the area
surveyed by Mariner 10. The most signi¬cant single feature is the Caloris Basin,
a huge impact crater 1300 km (800 miles) across “ a quarter the diameter of the
planet. The basin has been ¬lled in and is now a relatively smooth plain. The
impact took place 3800 million years ago and temporarily revived volcanic
activity, which had mostly ceased 100 million years earlier. This volcanism
created the smoother areas inside and around the basin. At the point on Mercury
diametrically opposite the impact site, there is curious chaotic terrain that must
have been created by the shock wave. Cliffs between a few hundred and 3000 m
high are characteristic feature on Mercury. It is believed that they formed when
the planet™s crust shrank as it cooled. In places they cut across craters.
The length of Mercury™s orbital and rotation periods means that a ˜˜day™™ on
Mercury lasts two mercurian ˜˜years.™™ This leads to immense temperature
contrasts: at perihelion, it reaches 430  C where the Sun is overhead; the
nighttime temperature plunges to À170  C. The high daytime temperatures
and Mercury™s small mass make it impossible for an atmosphere to be retained.
Small amounts of helium that are detected may be the product of radioactive
decay of surface rocks or captured from the ¤ solar wind.
The average density of Mercury is only slightly less than that of Earth. Taking
account of its smaller size and lower interior pressure leads to the conclusion
that Mercury has a substantial iron core accounting for 70 percent of its mass and
75 percent of its total diameter. There is also a magnetic ¬eld of about 1 percent
the strength of the Earth™s ¬eld, providing further evidence for a metallic core.
A NASA spacecraft to study Mercury, called ¤ Messenger, was launched in
August 2004 and will enter orbit around Mercury in 2011.
meridian (1) The great circle on the celestial sphere passing through the celestial
poles and the zenith.
meridian (2) A line of longitude on the Earth, or on another astronomical body.
On Earth, the meridian through Greenwich marks the zero of longitude and is
sometimes called the prime meridian.
MERLIN Acronym for Multi-Element Radio Linked Interferometer Network, a
network of radio telescopes at various locations in the UK, operated by the
University of Manchester from ¤ Jodrell Bank Observatory. MERLIN is used for
¤ long-baseline interferometry and can be linked to other telescopes and networks
worldwide to carry out ¤ very-long-baseline interferometry.
Merope One of the brighter stars in the ¤ Pleiades.
mesosphere A region of Earth™s atmosphere above the stratosphere, between
heights of about 50 and 85 km (30 and 50 miles), in which the temperature
decreases with height to À90  C at its upper boundary, the mesopause.
Messenger A NASA mission to Mercury launched on August 3, 2004. The ¬‚ight
plan involves ¬‚ybys of Venus in October 2006 and June 2007, and three ¬‚ybys


of Mercury in January and October 2008 and September 2009. It will go into
orbit around Mercury in March 2011. Messenger carries eight instruments to
investigate Mercury™s structure, composition, ¤ magnetosphere and ¤ exosphere
and is expected to operate for a year.
Messier Catalogue A catalog of about a hundred of the brightest galaxies,
star clusters and nebulae, compiled by the French astronomer Charles
Messier (1730“1817). His initial list, published in 1774, contained 45
objects but it was supplemented later with additional discoveries and
contributions from Messier™s colleague, Pierre Mechain. Objects in the catalog,
which is still widely used, are identi¬ed by the pre¬x ˜˜M™™ and their catalog
The list was not compiled systematically. Messier™s main interest was
searching for comets, and he noted hazy objects he spotted during comet
searches. Some were ¬rst recorded by Messier, but others were already known.
There are some errors and discrepancies in the list as published. M40 is a
double star and M73 a group of four stars, but not a true cluster. The
identi¬cation of M91 is uncertain from the original source, and M102 was a
duplication of M101. Messier™s own list stopped at number 103 but a further
seven objects were added in the twentieth century.
meteor The brief luminous trail observed as a particle of dust or a piece of rock
from space enters Earth™s upper atmosphere. The popular name for a meteor is
˜˜shooting star™™ or ˜˜falling star.™™
Earth is constantly bombarded with material from space. The individual
objects range in size from rocks of several kilograms down to microscopic
particles weighing less than one millionth of a gram. It is estimated that more
than 200 million kilograms (200 000 tons) of meteoric material is swept up by
the Earth in the course of a year. One-tenth of it reaches the ground, in the
form of ¤ meteorites and ¤ micrometeorites. The remainder burns up in the
atmosphere, becoming visible as meteor trails.

A meteor trail recorded during the
Leonid meteor shower in 1998.

Meteor Crater

Meteor crater imaged by a remote sensing instrument aboard the Terra
Earth observation satellite.

Meteoric material typically enters the atmosphere at speeds of around
15 km/s. Heating due to friction causes medium-sized particles to vaporize.
They give off light, leaving a temporary trail of ionized gas. This trail is capable
of re¬‚ecting radar signals. Radar has been used to detect meteors during the
daytime when they are too faint to be seen by eye.
Much of the meteoric material in the solar system orbits the Sun in distinct
streams. Many meteor streams have the same orbits as known comets. The
particles may be strung out all along the orbit or concentrated in a swarm. When
Earth happens to cut through a stream, a ¤ meteor shower is observed. In addition
to the dozens of regular meteor showers, a background of sporadic meteors is
observed throughout the year. They may come from any direction. ¤ ¬reball.
Meteor Crater (also known as Barringer Crater; Canyon Diablo Crater; Coon Butte)
The best-preserved and most famous ¤ meteorite crater on the Earth, formed
about 50 000 years ago. Located between Flagstaff and Winslow in Arizona,
USA, it was discovered in 1891. It is a bowl-shaped depression in the ground,
1200 m (4000 feet) across and 183 m (600 feet) deep, surrounded by a wall
30“45 m (100“150 feet) high. The original meteorite is thought to have been of
the iron type and to have weighed more than 10 000 tonnes but most of it was
destroyed on impact. The many scattered fragments of what is called the
Canyon Diablo meteorite total only about 18 tonnes.
meteorite A piece of a ¤ meteoroid that has survived its passage through Earth™s
atmosphere and has landed on the ground. Individual meteorites are normally
named after the place where they fell. Studies of the paths of a small number
of meteorites observed as ¤ ¬reballs, and subsequently recovered, show they
were in orbits originating in the ¤ asteroid belt. The chemical composition of

meteor shower

A piece of the stony-iron Huckitta
meteorite from the Northern Territory
of Australia. This sliced and polished
piece is about 2.5 cm (1 inch) across.

meteorites and the minerals they contain provide clues to the origin and
evolution of the solar system.
There are three main classes of meteorite: irons, stony-irons and stones.
Stony meteorites are further divided into two important categories: achondrites
and chondrites. Chondrites contain chondrules, small spherical inclusions,
made of metal or of silicate or sul¬de materials. Chondrules are not present in
The chemical composition of chondrites is very similar to that of the Sun
and they are thought to represent primitive solar system material that has not
been altered signi¬cantly by heating. Carbonaceous chondrites have the
highest proportion of volatile elements and their composition is closest to the
Sun™s. Among the achondrites, there are many subtypes with differing
chemical and mineralogical composition.
Stony-iron meteorites contain free metal and stony material in roughly
equal proportions. Iron meteorites consist almost entirely of iron and nickel.
Over 40 different minerals have been identi¬ed in them, though the basic
constituents are two forms of iron“nickel alloy, kamacite and taenite.
meteoroid A small piece of rock or dust in space, especially one with the potential
to become a ¤ meteor or ¤ meteorite. The word ˜˜meteoroid™™ is normally used to
mean an object about 100 m (300 feet) across, or smaller.
meteor shower ¤ Meteors appearing to come from one point in the sky over a
period of several hours or days. Meteor showers result when Earth cuts
through a stream of meteoric material in space. Many showers recur around
the same time each year when Earth reaches the same place in its orbit. Dozens
of such annual showers are known, though only a handful give good regular

meteor stream

displays. Very occasionally, a particularly dense swarm of particles produces
tens or hundreds of meteors every minute. A more typical, average rate for a
good regular shower would be between 20 and 50 meteors per hour.
The trails of shower members, if traced back on a sky map, appear to
intersect at a point, called the radiant. However, this is only an effect of
perspective. The meteors are actually traveling along parallel tracks when they
enter the atmosphere. Meteor showers are normally named after the
constellation in which their radiant lies. For example, the radiant of the
Perseids is in Perseus.
meteor stream An extended swarm of meteoric material in orbit around the Sun.
Many meteor streams are known to be spread out around the orbits of
particular comets. The particles tend to become distributed more or less evenly
along the orbit over time. A meteor stream that has formed relatively recently
may still be concentrated near its parent comet. ¤ meteor, meteor shower.
Methone A tiny moon of Saturn, about 3 km (2 miles) across, orbiting at a distance
of 194 000 km (120 546 miles) between Mimas and Enceladus. It was discovered
by the ¤ Cassini spacecraft in 2004, but may have been imaged once before,
in 1981, by ¤ Voyager.
9 Metis An asteroid discovered in 1848 by Andrew Graham from Ireland. Its
diameter is 190 km (118 miles).
Metis The moon of Jupiter™s innermost known moon, discovered by Stephen P.
Synott in 1979. It is about 40 km (25 miles) in diameter, irregularly shaped
and reddish in color.
Metonic cycle A period of 19 tropical ¤ years, after which the phases of the Moon
recur on the same days of the year. This happens because 19 tropical years
equals 6 939.60 days, which is almost exactly 235 synodic ¤ months (6939.69
days). The discovery of the cycle is attributed to the Greek astronomer Meton,
who worked in the ¬fth century bc.
Mice A popular name for the pair of interacting galaxies, NGC 4676 A and B. Long,
tail-like streamers of material extend from the galaxies, giving them shapes
reminiscent of a pair of mice.
microlensing The result of an invisible stellar-sized object acting as a ¤ gravitational
lens and amplifying the light of a more distant star directly behind it. The star
brightens temporarily in a characteristic way. Surveys for microlensing events
have been used to search for invisible dim stars or ¤ brown dwarfs, which may
account for as much as 90 percent of the total mass of the Galaxy.
micrometeorite A particle of meteoritic material so small that it does not burn up
in the atmosphere. Micrometeorites fall to Earth as a rain of minute dust
particles. It is estimated that four million kilograms of them reach the ground
each year. The size of the particles is typically less than 120 mm. ¤ meteor,

Milk Dipper

The interacting galaxies nicknamed ˜˜the Mice.™™

micrometer (1) In general, any instrument for measuring small distances
accurately. In astronomy micrometers attached to telescopes are used by visual
observers to measure the angular separations of pairs of objects, such as
¤ binary stars.
micrometer (2) (micron, symbol mm) A unit of measurement equal to one
millionth of a meter.
Microscopium (The Microscope) A small, insigni¬cant, southern constellation
introduced in the mid-eighteenth century by Nicolas L. de Lacaille. Its brightest
star is magnitude 4.7.
microquasar A stellar-mass ¤ black hole, which is physically similar to a ¤ quasar,
but on a much smaller scale. Microquasars are characterized by powerful
jets traveling almost at the speed of light.
microwave astronomy The study of radio waves from astronomical sources
across a wide band of the electromagnetic spectrum from the far infrared at 1
mm wavelength to short-wave radio at about 6 cm. At the shorter wavelength
end, these waves are absorbed by Earth™s atmosphere. ¤ Cosmic Background
Explorer, millimeter-wave astronomy, radio astronomy, submillimeter-wave astronomy.
microwave background radiation ¤ cosmic background radiation.
Milankovic cycles Small variations in the tilt of Earth™s rotation axis and the
eccentricity of its orbit around the Sun. They have been linked with long-term
climate variations and the incidence of ice ages.
Milk Dipper An ¤ asterism formed by the stars Zeta, Tau, Sigma, Psi and Lambda in
the constellation Sagittarius. It is presumably named because of its ladle shape
and the fact that the most intense part of the Milky Way lies in Sagittarius.

Milky Way

Milky Way A band of hazy light circling the sky. It is the combined light of vast
numbers of stars in our own ¤ Galaxy. The term Milky Way is also used to
mean the Galaxy itself.
The band of light we see around the sky is the disk of the Galaxy viewed
from within. The Sun is situated two-thirds of the way out towards the edge of
the galactic disk, and the Milky Way appears brightest in the direction of
the bulge at the galactic center, which lies in the constellation Sagittarius.
Clouds of obscuring dust, such as the ¤ Coalsack near the Southern Cross, gives
the Milky Way a patchy appearance in places.
The main constellations through which the Milky Way passes are Perseus,
Cassiopeia, Cygnus, Aquila, Sagittarius, Scorpius, Centaurus, Vela, Puppis,
Monoceros, Orion, Taurus and Auriga.
millimeter-wave astronomy Astronomical observations of radio waves with
wavelengths between about 1 and 10 mm. This part of the radio spectrum
contains many features caused by complex molecules and it is specially
important in the study of ¤ molecular clouds, regions of star formation,
¤ circumstellar disks and ¤ comets. ¤ Atacama Large Millimeter Array,
submillimeter-wave astronomy.
Mimas A moon of Saturn, discovered by William Herschel in 1789. It orbits
185 520 km (115 277 miles) from Saturn and measures 418 · 392 · 383 km
(260 · 244 · 238 miles). Its surface is heavily cratered. The largest crater,
Herschel, is 130 km in diameter, one-third the size of Mimas, and has a central
peak. The impact that created Herschel must very nearly have shattered Mimas.
Mimosa (Beta Crucis ) The second-brightest star in the constellation Crux. It is a
giant ¤ B star of magnitude 1.3 and is 350 light years away. Mimosa is slightly
variable, changing by 0.1 magnitude in about 6 hours.
Minkowski™s Footprint A double-lobed nebula surrounding a star in Cygnus,
regarded as a ¤ planetary nebula in the process of formation. It was discovered
by Rudolph Minkowski in 1946.

Mimas imaged by the Cassini


The Mir space station photographed by the crew of the Space Shuttle Endeavor in 1998.

minor planet An alternative name for an ¤ asteroid.
Mintaka (Delta Orionis) One of the three stars forming the belt of Orion. At
magnitude 2.2, it is the seventh-brightest star in the constellation. It is actually
an ¤ eclipsing binary and varies in brightness by 0.1 magnitude in a period of
5.7 days. In addition, it has a seventh-magnitude visual companion. The
primary star is a ¤ supergiant ¤ O star lying 915 light years away. Mintaka is
a name of Arabic origin meaning ˜˜the belt.™™
minute of arc ¤ arc minute.
Mir A Russian space station, launched into Earth orbit in 1986. It was occupied by
astronauts regularly, sometimes for many months at a time, before ¬nally
being abandoned. In a controlled maneuver, it was allowed to re-enter the
atmosphere and crash into the Paci¬c Ocean in March 2001.
Mira (Mira Ceti; Omicron Ceti ) The prototype of a class of long-period variable stars.
The name is Latin for ˜˜wonderful.™™ Mira was the ¬rst variable star to be
discovered. The Dutch astronomer David Fabricius noted it at third magnitude in
1596, but found it to be invisible to the naked eye a few months later. He
noted it again at third magnitude in 1609. Mira is a giant ¤ M star that varies
between about second and tenth magnitudes in a period of roughly 332 days.
It is shedding large amounts of gas and dust, which form a strong ¤ stellar wind.
Mira also has a ¤ white dwarf companion and material is being transferred
onto it.
Mirach (Beta Andromedae) The second-brightest star in the constellation
Andromeda. It is a giant ¤ M star of magnitude 2.1 and is 200 light years away.
Derived from Arabic, Mirach means ˜˜girdle.™™


A Chandra X-ray image and an artist™s impression of Mira. The Chandra image shows Mira A
(right), a highly evolved red giant star, and Mira B (left), a white dwarf. To the right of the
image is an artist™s concept of the Mira star system.

A Voyager 2 spacecraft image of
Miranda taken in 1986

Miranda A small moon of Uranus. It was discovered by Gerard Kuiper in 1948.
The ¤ Voyager 2 spacecraft passed Miranda at a distance of only 3000 km
(1800 miles) in 1986, returning very detailed images of its surface.
Though only 472 km (293 miles) in diameter, Miranda has several
contrasting types of terrain. Alongside cratered areas, typical of planets and
satellites, there are large tracts of grooves and ridges. It seems unlikely that
such variety could have been caused by geological activity in such a small
satellite. One theory suggests that the satellite was once shattered by a massive
impact into several parts that subsequently coalesced again.

MMT Observatory

Maria Mitchell.

Mirfak (Alpha Persei) The brightest star in the constellation Perseus. It is a yellow
supergiant ¤ F star of magnitude 1.8 lying 200 light years away. The name
comes from Arabic and means ˜˜the elbow.™™
Mirzam (Beta Canis Majoris) The second-brightest star in the constellation Canis
Major. It is a giant ¤ B star of magnitude 2.0 and the prototype of a class of
slightly variable stars. It changes in brightness by just a few hundredths of a
magnitude every six hours. Mirzam is 500 light years away.
Mitchell, Maria (1818“1889) Mitchell was the ¬rst woman in the USA to be
appointed to a professional position in astronomy. She was born on the island
of Nantucket, then the world center for the whaling industry, into a Quaker
society that expected capable women to work independently. Her father
taught her astronomy. With his telescope she discovered a comet in 1847, for
which the King of Denmark awarded her a gold medal, and this led to
worldwide fame. In 1849 she became the ¬rst woman to work full time for the
US Nautical Almanac Of¬ce, where she computed the position of Venus. She
was the very ¬rst person to be appointed to the academic staff of Vassar
College, Poughkeepsie, New York.
Mizar (Zeta Ursae Majoris) The fourth-brightest star in the constellation Ursa
Major. It is an ¤ A star of magnitude 2.3 and is about 78 light years away. The
Arabic name means ˜˜girdle.™™ In the sky, it happens to lie very close to the
fourth magnitude star ¤ Alcor. However, it is not known whether the two stars
are linked. Mizar does have a known fourth-magnitude companion. Both it and
Mizar are ¤ spectroscopic binaries.
MMT Observatory ¤ Multiple Mirror Telescope.


Mneme A small outer moon of Jupiter discovered in 2003. Its diameter is about
2 km (1 mile).
mock Sun (parhelion; sundog) A circular patch of light in the sky, 22 away from
the real Sun. Mock Suns usually appear in pairs, one either side the real Sun,
on a circular halo of light, though one may be brighter than the other. The
effect is caused by ice crystals in Earth™s atmosphere.
molecular cloud A cloud of interstellar matter consisting mainly of molecules
of gas. There are two distinct types, both found close to the plane of the Galaxy,
within the Milky Way. These are small molecular clouds and giant molecular
clouds (GMCs).
The small clouds are typically a few light years across, with 1000 to 10 000
molecules per cubic centimeter and temperatures of around 10“20 K. They
may contain even colder condensed ˜˜cores,™™ with densities ten or a hundred
times greater. These small clouds contain mostly molecular hydrogen (H2).
They are very cold because there is no radiation from stars within to
heat them.
Giant molecular clouds are made up primarily of molecular hydrogen and
carbon monoxide (CO), but they also contain many other ¤ interstellar molecules.
They are the most massive entities within our Galaxy, containing up to ten
million solar masses, and are typically 150“250 light years across. Their density
can be as high as ten million molecules per cubic centimeter. Infrared
emission from these clouds is evidence that they are regions of star formation.
GMCs are nearly always associated with clusters of hot, massive, young
stars. Luminous clouds of ¤ ionized hydrogen (H II regions) are created by young
stars near the edges of a GMC. The ¤ Orion Nebula is one example. A GMC lies
behind the glowing nebula. Up to 4 000 GMCs are thought to exist in the
Molonglo Observatory An Australian radio astronomy observatory, located near
Canberra, belonging to the University of Sydney.
Monoceros (The Unicorn) A faint constellation, but one rich in stars and nebulae
by virtue of its location in the ¤ Milky Way, straddling the celestial equator next
to Orion. It is not one of the ancient constellations but seems to have gained

Type of month How measured Length in days

Anomalistic Perigee to perigee 27.554 55
Draconic Node to node 27.212 22
Sidereal Relative to ¬xed stars 27.321 66
Synodic Cycle of phases 29.530 59
Tropical Equinox to equinox 27.321 58


The Moon photographed from space by the Apollo 11 astronauts.

general acceptance in the mid-seventeenth century. Its brightest two stars
are third magnitude; it contains the ¤ Rosette Nebula, the ¤ Cone Nebula and
¤ Hubble™s Variable Nebula.
month The time the Moon takes to complete one orbit of the Earth, or to complete
one cycle of phases. There are various kinds of month, their length depending
on what is used as the start and end point for the measurement.
moon A natural ¤ satellite of a planet.
Moon The Earth™s only natural satellite. It is a barren, heavily cratered world,
without liquid water or an atmosphere. It was explored by American
astronauts who traveled there during the ¤ Apollo program landings of 1969“72
and has been extensively mapped both from orbiting craft and from Earth.
Tidal forces are responsible for the fact that the same side of the Moon now
always faces Earth, apart from the minor effects of ¤ libration. As the Moon
travels around Earth over the course of a month, it goes through its familiar
cycle of ¤ phases. The Moon shines only by re¬‚ected sunlight. The Moon™s phase
changes according to the proportion of the sunlit side visible from Earth, and it
depends on the relative positions of the Sun, Earth and Moon

Morehouse, Comet

The terrain on the nearside consists of two basic types: the heavily
cratered, light-colored highlands, and the darker, more sparsely cratered
˜˜maria™™ (seas). The maria have roughly circular outlines, a relic of their
formation early in the history of the Moon by the impact of large meteorites.
A further type of surface terrain is formed by ¤ ejecta. Signi¬cant areas are
marked by material ejected from the large Imbrium and Orientale basins.
The way the Moon formed is uncertain, but it has existed as a separate
body for around 4500 million years. The most favored explanation is that it was
created in a giant impact event when an asteroid collided with the recently
formed Earth. Evidence from the ¤ Lunar Prospector mission suggests that the
Moon has a very small core about 700 km (450 miles) in diameter, consistent
with the impact theory of its origin.
Early in its life the Moon became hot and molten. As it cooled, the crust
formed but it was heavily cratered by the impact of large numbers of
meteorites, the largest of which created the mare basins. These subsequently
¬lled with dark basaltic lavas. Signi¬cant volcanic activity then ceased, at least
2000 million years ago. The farside of the Moon differs from the nearside
in that it lacks any large lava-¬‚ooded mare areas.
Morehouse, Comet A comet discovered from the USA in 1908. It was the ¬rst
comet to be studied extensively by photography. Remarkable changes were
observed in the structure of its tail. Throughout September 30, 1908 the tail
changed continuously. On October 1, it broke off and a tail could not be seen
visually, though a photograph of October 2 showed three tails. The breaking
off and subsequent growth of tails occurred repeatedly. ¤ disconnection event.
morning star Venus or Mercury when it appears in the eastern sky in the early
morning before sunrise.
Mount Graham International Observatory An observatory site located on
Mount Graham near Safford in south-eastern Arizona. There are three
telescopes there: the 1.8-m Vatican Advanced Technology Telescope (VATT),
the ¤ Heinrich Hertz Submillimeter Telescope and the ¤ Large Binocular Telescope.
Mount Stromlo and Siding Spring Observatories The astronomy research
facilities of the Australian National University (ANU), operated by the
university™s Institute of Advanced Studies. All the telescopes at the Mount
Stromlo site, near Canberra, were destroyed in a bush ¬re in January 2003. At
the Siding Spring site, which was established in 1962, the Observatories has
four telescopes, including a 2.3-m (90-inch) re¬‚ector. Siding Spring Mountain is
at an altitude of 1000 m (3200 feet) in the Warrumbungle range, most of which
forms a national park. This observatory site now also houses a number of
telescopes owned by organizations other than the ANU, including the
¤ United Kingdom Schmidt Telescope, and the ¤ Anglo-Australian Telescope.

Multiple Mirror Telescope

Mount Wilson Observatory An observatory near Pasadena, California, located on
Mount Wilson at an altitude of 1750 m (5700 feet). The ¬rst instrument located
there was a horizontal solar telescope built in 1904. Two solar tower telescopes
were added in the next few years. A 1.5-m (60-inch) re¬‚ecting telescope was
begun in 1904 and brought into service in 1908. It was the largest telescope in
the world until the opening of the 100-inch (2.5-m) ¤ Hooker Telescope at the site
in 1917. The observatory is owned by the Carnegie Institution of Washington.
The Mount Wilson site is now also home to optical and infrared
¤ interferometers. The largest, completed in 2000, is Georgia State University™s
CHARA array. (CHARA stands for Center for High Angular Resolution
Astronomy.) It consists of six 1-m telescopes arranged in a Y-shape on a
400-m-diameter circle. The University of California™s Infrared Spatial
Interferometer (ISI) consists of three 1.65-m (65-inch) telescopes mounted on
trailers which can be positioned up to 85 m (280 feet) apart.
M star A star of ¤ spectral type M. M stars have surface temperatures in the range
2400“3480 K and are red in color. Molecular bands are prominent in their
spectra, particularly due to titanium oxide (TiO). Examples of M-type stars
include the nearest star, ¤ Proxima Centauri, which is a dwarf, and the
supergiant ¤ Antares.
The coolest M dwarfs of spectral types M7 to M9.5 and temperatures of
2400 K and below, include both stars and substellar objects with masses below
the lower limit for a true stars (0.08 solar masses). ¤ brown dwarf.
Mullard Radio Astronomy Observatory (MRAO) The radio astronomy
observatory of the University of Cambridge. It was founded in 1957 and Sir
Martin ¤ Ryle, who had begun research in radio astronomy at Cambridge in
1946, was its director until 1982. The major telescopes to have been built at the
site, just outside Cambridge, include the Ryle Telescope, consisting of eight
13-m (43-foot) dishes on an east“west baseline 5 km (3 miles) long.
Historically, the MRAO specialized in cataloging radio sources, producing
the Third, Fourth, Fifth, Sixth and Seventh Cambridge Catalogues (abbreviated
to 3C, 4C, etc.) at different frequencies. These led to the discovery of many
¤ quasars and ¤ radio galaxies. The ¬rst ¤ pulsars were detected at the MRAO
in 1967.
Since the 1990s, the focus of research has shifted, with the development of
an optical ¤interferometer (COAST) and instruments for studying the ¤ cosmic
background radiation.
Multiple Mirror Telescope (MMT) A telescope of unique design that operated at
the Fred Lawrence Whipple Observatory on Mount Hopkins in Arizona
between 1977 and 1997 as a joint venture of the Smithsonian Institution
Astrophysical Observatory and the University of Arizona. It combined six
individual 1.8-m (72-inch) mirrors in a circular arrangement on an altazimuth


mount. Together they had the light-gathering power of a single mirror 4.5 m
(176 inches) in diameter. In 1989, in view of developments in mirror
technology, the decision was taken to substitute a single light-weight mirror of
6.5 m diameter in order to double the light-gathering power and substantially
increase the ¬eld of view. The conversion was completed in 1999 and the
facility renamed the MMT Observatory.
Mundilfari A small outer moon of Saturn in a very elliptical orbit. It was
discovered in 2000 and is about 6 km (4 miles) across.
Murzim Alternative spelling of the star name ¤ Mirzam.
Musca (The Fly) A small southern constellation containing one second-magnitude
star and three of third magnitude. Its origin is obscure, but it has been
attributed to Johann Bayer.

nadir The point on the celestial sphere diametrically opposite the ¤ zenith.
Naiad A small moon of Neptune discovered during the ¬‚yby of ¤ Voyager 2 in
August 1989. Its diameter is about 340 km (211 miles).
nanometer (symbol nm) One thousand-millionth (10À9) of a meter.
Narvi A small outer moon of Saturn, discovered in 2004. It is about 7 km (4 miles)
NASA Abbreviation for ¤ National Aeronautics and Space Administration.
Nasmyth focus A point where the image made by an altazimuth-mounted
reflecting telescope can be located on one side of the telescope tube. Light is
brought to a focus there by an extra mirror in the optical system, which directs
the beam along the altitude axis, and through a hole in the supporting
trunnions. It was first used by the inventor James Nasmyth in the nineteenth
century. It has the advantage of remaining at a fixed position relative to the
telescope wherever it is pointed. This means that bulky or heavy instruments
can be mounted there on a permanent platform, which rotates only in
¤ azimuth. In practice, there are two possible Nasmyth foci, one either side of
the telescope tube. ¤ altazimuth mount.
National Aeronautics and Space Administration (NASA) The US government
agency responsible for civilian manned and robotic activities in space,
including launch vehicle development and operations, scientific satellites
(such as orbiting observatories) and spacecraft, and advanced satellite
technology development.
NASA was created on July 29, 1958, when US President Dwight D.
Eisenhower signed the National Aeronautics and Space Act of 1958. This
legislation was widely acknowledged to have been passed by the US Congress
in response to the successful and unexpected launch of the ¬rst arti¬cial Earth
satellite (Sputnik 1) by the Soviet Union. The headquarters of NASA are in
Washington, DC, and it operates ¬eld centers and other facilities at various
locations in the USA as well as several tracking stations around the world.
National Optical Astronomy Observatories (NOAA) An organization formed in
the USA in 1984 to bring under one administration the national facilities for
optical astronomy at ¤ Kitt Peak National Observatory, ¤ Cerro Tololo Inter-American
Observatory and the ¤ National Solar Observatory. NOAA is also responsible for
the operation of the ¤WIYN Telescope at Kitt Peak, and the US contribution to
the international ¤ Gemini 8-meter Telescopes project.

National Radio Astronomy Observatory

National Radio Astronomy Observatory (NRAO) Combined radio astronomy
facilities operated in the USA by a private consortium of universities. It obtains
its funding under a cooperative agreement with the National Science
Foundation. The NRAO runs the ¤ Very Large Array (VLA) in New Mexico,
and the telescopes at ¤ Green Bank, West Virginia.
National Solar Observatory (NSO) The solar observation facilities of the US
¤ National Optical Astronomy Observatories. They consist of the ¤ McMath“Pierce
Solar Telescope Facility at ¤ Kitt Peak, Arizona, and the Sacramento Peak
Observatory in New Mexico.
NEAR Abbreviation for the ¤ Near-Earth Asteroid Rendezvous mission. ¤ NEAR
Near-Earth Asteroid Rendezvous mission ¤ NEAR Shoemaker.
near-Earth asteroid ¤ near-Earth object.
Near-Earth Asteroid Tracking system (NEAT) A NASA survey for ¤ asteroids
approaching relatively close to Earth. It started in 1995, using a CCD camera
installed on a 1-m (39-inch) telescope operated on Mount Haleakala in Maui,
Hawaii, by the US Air Force. From 2001 it has also used the Schmidt telescope
at ¤ Palomar Observatory.
near-Earth object An ¤ asteroid or ¤ comet in an orbit that can bring it
exceptionally close to Earth. They are believed to be asteroids from the main
¤ asteroid belt that have been perturbed by the gravitational attraction of the
major planets, together with inactive nuclei of comets.
NEAR Shoemaker A NASA mission, launched on February 17, 1996, to
rendezvous with the asteroid ¤ Eros. The original plan was for the craft to enter
orbit around Eros in January 1999. The misfiring of a rocket made this
impossible. NEAR flew past Eros on December 23, 1998 and, after completing a
further orbit of the Sun, entered orbit around Eros in February 2000, enabling
it to study the asteroid for a period of about one year, from distances as
close as 24 km (15 miles). At the end of the mission, it was allowed to descend
to the surface of Eros. Though it was never designed for a landing, the
touch down was soft enough for some instruments to continue to operate.
On the way to Eros, the spacecraft flew past asteroid 253 ¤ Mathilde in
June 1997.
The initials NEAR stand for ˜˜Near Earth Asteroid Rendezvous.™™ The craft
was named in honor of the planetary scientist Eugene M. Shoemaker
(1928“1997) in March 2000, after it had successfully entered orbit around Eros.
nebula (pl. nebulae or nebulas) A cloud of interstellar gas and dust. Nebula was at
one time also used for objects we now know to be galaxies, such as the
Andromeda Galaxy, which used to be called ˜˜the great nebula in Andromeda.™™
An ¤ emission nebula glows because of the effects of ultraviolet radiation on the gas;
a ¤ re¬‚ection nebula shines by re¬‚ecting starlight. An ¤ absorption nebula is dark


and is usually visible only in silhouette against the background of a luminous
nebula or star¬eld.
Other objects consisting of luminous gas are also known as nebulae, in
particular ¤ planetary nebulae and ¤ supernova remnants.
Neptune A major planet of the solar system, it is the eighth in order from the Sun
except for the periods when Pluto™s eccentric orbit brings it closer to the Sun
than Neptune. It is one of the four giant planets, having a diameter of
49 528 km (30 777 miles), almost four times Earth™s. It has a small rocky core
surrounded by an icy mantle of frozen water, methane and ammonia. The
outer atmosphere is mainly molecular hydrogen with 15“20 percent helium
(by mass) and some methane.
Neptune was discovered by Johann G. Galle of the Berlin Observatory on
September 23, 1846 following predictions made independently by John Couch
Adams in England and Urbain J. J. Leverrier in France. Their calculations
were based on discrepancies between the observed and predicted orbits of
Uranus since its discovery in 1781, which were attributed to gravitational
perturbations by an unknown planet.
Viewed from Earth, Neptune is seventh or eighth magnitude and so not
visible to the naked eye. With high magni¬cation and larger telescopes, it is
seen as a faintly bluish disk, the color being due to the presence of methane in
the upper atmosphere.
Close-up images were obtained by ¤ Voyager 2 during its ¬‚yby of Neptune
in August 1989. Observations with the Hubble Space Telescope (HST), capable
of resolving atmospheric detail, began in 1994. Neptune has distinctive and
varying cloud features in a highly dynamic atmosphere. The most prominent
feature found by Voyager 2 was termed the Great Dark Spot. Located about 20
south of the equator, it rotated anticlockwise in a period of about 16 days.
Bright cirrus-like clouds had formed over this and other small dark spots.
However, it had completely disappeared when observations were made with
the HST in 1994. Meanwhile, another dark spot, not seen by Voyager, had
formed in the northern hemisphere. It too was accompanied by bright clouds.
Subsequent observations with the HST have revealed that the pattern of clouds
is constantly changing, though the underlying banded structure of the
atmosphere remains stable. The atmospheric features rotate at different rates,
and also drift in latitude. Wind speeds up to 2200 km/hr (1400 mph) have been
measured. Neptune™s average temperature is 59 K. It is not understood why
Neptune radiates 2.7 times more energy than it receives from the Sun.
There are two main cloud layers in Neptune™s upper atmosphere. The highest
consists of crystals of methane ice, and this lies over a lower opaque blanket of
cloud that may contain frozen ammonia or hydrogen sul¬de. There is also a high-
altitude haze of hydrocarbons produced by the action of sunlight on methane.

An increasing amount of cloud on Neptune between 1996 and 2002 revealed by visible light and infrared data from the Hubble Space Telescope,
which were combined to make these false color images.
neutrino astronomy

Regular radio bursts detected by Voyager 2 revealed that Neptune has a
magnetic ¬eld and is surrounded by a magnetosphere. The bursts occurred at
intervals of 16.11 hours, apparently the rotation period of the planetary core.
The magnetic axis is tilted at 47 to the rotation axis and it is thought that the
asymmetric magnetic ¬eld may originate in the planet™s mantle rather than
in its core.
Observations made from the ground during occultations by Neptune had
suggested the presence of incomplete ring ˜˜arcs.™™ Voyager 2 detected four
tenuous rings, one of which is ˜˜clumpy™™ in a way that can account for the
occultation observations. The mission also discovered six new moons around
Neptune, bringing the total number known, with ¤ Triton and ¤ Nereid, to
eight. Five more were found subsequently, in 2002 and 2003.
Nereid The small outermost moon of Neptune, discovered by Gerard Kuiper in
1949. The best ¤ Voyager 2 image obtained was from a distance of 4.7
million km (2.9 million miles), not sufficiently close to reveal surface detail but
good enough to determine its diameter as 170 km (105 miles).
neutrino astronomy The collection and analysis of neutrinos from cosmic
sources, especially the Sun. Neutrinos are elementary particles with no electric
charge and almost no mass, which interact only very weakly with other matter.
They travel essentially at the velocity of light and are produced in vast
quantities by the nuclear reactions that take place in the centers of stars and
in ¤ supernova explosions.
Because they hardly interact with matter at all, neutrinos are very dif¬cult
to detect. The ¬rst long-term experiment to search for solar neutrinos, at
Homestake Mine, South Dakota, took advantage of the fact that neutrinos
sometimes interact with a chlorine atom, converting it to a radioactive isotope
of the gas argon. The detector consisted of a tank containing 400 000 liters
of the cleaning ¬‚uid, carbon tetrachloride. It was located in a mine because
neutrino detectors need to be deep underground to protect them from
¤ cosmic rays.
In another form of neutrino detector, sensors in a large tank of water pick
up radiation generated by the interaction of electrons with solar neutrinos.
Detectors of this type, the Kamiokande experiment in Japan and a similar
detector in Ohio, made the ¬rst observation of neutrinos from a supernova “
those from ¤ SN1987A. In 1996 Kamiokande was superseded by a larger version,
¤ Super-Kamiokande. In 1999, the ¤ Sudbury Neutrino Observatory (SNO) opened in
Canada, and work began on ¤ AMANDA, a cosmic neutrino ˜˜telescope™™ in the
Antarctic ice at the South Pole. Following the success of AMANDA, a much larger
neutrino experiment, called ¤ IceCube, was constructed in the Antarctic ice
around AMANDA. A European collaboration (GALLEX) and a Russian experiment
have made use of the interaction of neutrinos with gallium.

neutron star

The early neutrino experiments detected fewer neutrinos from the Sun
than they expected if standard particle physics theory was correct. The
discrepancy was called the ˜˜neutrino problem.™™ It is now known that
neutrinos are capable of changing from one form to two others, and the early
experiments picked up only one kind. The occurrence of this phenomenon
implies that neutrinos do have some mass, albeit very tiny indeed.
neutron star A star with a mass between about 1.5 and 3 solar masses that has
collapsed under gravity to the point where it consists almost entirely of
neutrons. Neutron stars are formed in ¤ supernova explosions. They are
typically about 10 km across and have a density of 1017 kg/m3. The greater a
neutron star™s mass, the smaller its diameter. Neutron stars are detected by the
radiation they emit, as ¤ pulsars or ¤ magnetars.
Once a star has exhausted the nuclear fuel available in its core, the core
starts to cool and the internal pressure falls. This leads to the core contracting.
It is a sudden and catastrophic event for stellar cores of more than 1.4 solar
masses, which implode until the pressure between neutrons balances the
inward pull of gravity. In the resulting supernova, much of the original star is
blown off into space leaving behind a dense core that forms a neutron star or “
if it contains more than three solar masses “ a ¤ black hole. The process leaves
the neutron star spinning rapidly and with a strong magnetic ¬eld.
A neutron star is believed to consist of a solid crust of heavy atomic nuclei
and electrons, about a kilometer thick, overlying a superconducting liquid of
neutrons, with a few protons and electrons. There may be a small solid core.
Newcomb, Simon (1835“1909) Newcomb became a mathematician in the US
navy and in 1877 was made the superintendent of the American Nautical
Almanac. He is best remembered for his work on new tables for calculating the
positions of the planets and the Moon and for introducing a unified, more
accurate system of standard astronomical data. His tables were the most
accurate ever made without the aid of a digital computer and remained in use
by professional astronomers for a century.
New General Catalogue of Nebulae and Star Clusters (NGC) A catalog of non-stellar
objects compiled by John L. E. Dreyer of Armagh Observatory and published in
1888. It listed 7840 objects. A further 1529 were listed in a supplement that
appeared seven years later, called the Index Catalogue (IC). The Second Index
Catalogue of 1908 extended the supplementary list to 5386 objects. The original
General Catalogue of Nebulae was compiled by ¤ John Herschel and published
in 1864.
New Horizons A NASA flyby mission to ¤ Pluto, launched on January 19, 2006.
After a close flyby of Jupiter in February 2007 designed to increase its speed,
the spacecraft is expected to fly past Pluto in 2015 and may then go on to one
or more other bodies in the ¤ Kuiper Belt.

Newton, Sir Isaac (1642“1727)

Artist™s impression of the New Horizons spacecraft at its encounter with Pluto and

new Moon The Moon™s phase when it is at the same celestial ¤ longitude as the
Sun. At new Moon, all of the illuminated half of the Moon faces away from
Earth and the Moon is not visible from Earth.
New Technology Telescope (NTT) A 3.5-m (138-inch) reflecting telescope of the
¤ European Southern Observatory, located at the ¤ La Silla Observatory in Chile.
Regular observations with the telescope started in 1990. The name reflects a
number of features incorporated in the design that were innovative at the
time. The relatively thin mirror is kept in the correct shape by means of an
¤ active optics system, which analyzes a stellar image and controls the
mirror supports about once per second. The altazimuth mount and
special enclosure are designed for maximum stability and pointing accuracy,
and minimum disturbance of images from air turbulence. The telescope
can be operated remotely from ESO™s headquarters in Germany via
satellite link.
Newton, Sir Isaac (1642“1727) Newton was one of the greatest scientists of all
time, remembered for his work on gravitation and mechanics. Born
prematurely after the death of his father, he had an unsettled childhood. In
1661 he went to Trinity College Cambridge, where he studied mathematics,
graduating in 1665. His Cambridge career was interrupted by the plague. He
spent 18 months at his mother™s house, developing the ideas to do with
gravity, optics, and calculus for which he is most famous.
On his return to Cambridge in 1667, he was elected a Fellow of Trinity and
two years later was elected Lucasian Professor of mathematics. He invented the
methods of calculus but published very little until 1693. In optics Newton

Newtonian telescope

Sir Isaac Newton.

investigated the refraction of light, breaking white light into its constituent
colours using a prism. He concluded that an astronomical ¤ telescope made with
mirrors rather than lenses would give superior images. As a demonstration, he
presented a 6-inch re¬‚ector to the Royal Society in 1672.
His ¬rst research on gravity, in 1665, concerned the motion of the Moon,
which he described as being in free fall around the Earth. He returned to this in
1684, when he began to write his greatest work, now known as the Principia
Mathematica, which was published in 1687. He set out three laws of motion as
well as the law of universal gravitation. Importantly, Newton showed that the
laws of physics governing the universe are the same as those applying on Earth.
Newtonian telescope A simple type of reflecting telescope, designed by Isaac
¤ Newton (1642“1727), who demonstrated it to the Royal Society in London in
1671. The main mirror is a paraboloid (or spherical if very small). The
secondary mirror is flat and is positioned in the reflected beam at an angle of
45 to form an image just outside the main tube. The design is suitable for
small amateur instruments but not for large telescopes.
NGC Abbreviation for ¤ New General Catalogue.
Nix A small moon of Pluto discovered in 2005. Its diameter is estimated at 45 km
(30 miles).
nm Abbreviation for ¤ nanometer.


noctilucent clouds (NLC) Luminous bluish clouds high in the atmosphere that are
sometimes seen in the summer twilight sky. They form at a height of about
80 km (50 miles). Noctilucent clouds are very thin and cannot be seen from the
ground during daytime or in bright twilight. They occur in summer so are
difficult to observe from the highest latitudes, where the sky never gets dark
enough at that time of year. However, they are also a high-latitude
phenomenon, so the range of latitudes from which they can be seen “ between
50 and 65 “ is very limited. The number of sightings decreases when ¤ solar
activity increases.
nocturnal A simple instrument for telling the time at night by observing the
position of the two stars in the constellation Ursa Major known as the
¤ Pointers. The continuation of the line joining these stars passes very close to
the north celestial pole, and it acts like a giant clock-hand in the sky that
sweeps around daily as Earth rotates.
A nocturnal consists of two concentric disks and a sighting arm, ¬xed at
the center by an eyelet through which the Pole Star can be sighted. The lower
disk is graduated with the days of the year and the upper one with the 24 hours
of the day. The sighting arm is aligned with the Pointers and then the time can
be read off.
node A point on the celestial sphere where the plane of an orbit intersects a
reference plane.
Norma (The Rule) A small and insignificant southern constellation introduced by
Nicolas L. de Lacaille in the mid-eighteenth century. It contains no star brighter
than fourth magnitude.
normal galaxy Any spiral or elliptical ¤ galaxy that does not have unusual
structure, a disturbed or active nucleus or non-thermal radio emission.
North America Nebula (NGC 7000) A complex region of nebulosity in the
constellation Cygnus. It includes ¤ emission nebulae, ¤ reflection nebulae and
¤ absorption nebulae. It was discovered by William ¤ Herschel in 1786 and was
first photographed in 1890 by Max Wolf. The photographs showed its shape to
be suggestive of the North American continent. It is about 1 across and is just
detectable with the naked eye under ideal conditions. Its estimated distance is
2300 light years.
Northern Cross A name sometimes given to the constellation ¤ Cygnus and, in
particular, the cross formed by the stars Alpha, Beta, Gamma, Delta, Epsilon
and Eta.
northern lights A popular name for an ¤ aurora when observed from northern
North Star A popular name for the star ¤ Polaris.
nova (pl. novae) A star that suddenly increases in brightness by about 10
magnitudes, then declines gradually over a period of months. The word nova is


An artist™s impression of how a nova occurs as a result of material streaming onto a white
dwarf in a double star system

a shortening of the Latin ˜˜nova stella™™ “ a new star. In any one galaxy, typically
a few tens of novae occur in a year.
Novae are close binary systems in which one star is a ¤ white dwarf. When
the companion star evolves and expands to ¬ll its ¤ Roche lobe, material
streams towards the white dwarf, forming an ¤ accretion disk around it.
Material accumulates in a layer on the surface of the white dwarf until the
temperature and pressure at the base of the layer become high enough for
nuclear reactions to be sparked. The energy produced is unable to escape as
more material is deposited in the overlying layers. The temperature may rise to
100 million degrees and, at some point, explosive nuclear reactions are
triggered, producing the nova outburst.
Some novae are surrounded by an expanding envelope of gas. At speeds of
up to 1500 km/s, the envelope soon disperses into space. It is estimated that the
mass of material lost is about one ten-thousandth (10À4) the mass of the Sun,
and the energy released is only a millionth of that released in a ¤ supernova.
˜˜Classical™™ novae are observed to erupt only once, though outbursts may
recur every 10 000 to 100 000 years. Recurrent novae, such as ¤ P Cygni, repeat their
outbursts on timescales of 10“100 years. Recurrent novae differ from classical
novae in that their brightness increase of six to eight magnitudes is smaller, and
their spectra during outbursts are different. At other times their spectra seem to
indicate the presence of a ¤ red giant in the binary system. ¤ dwarf nova.
nucleosynthesis The making of chemical elements in naturally occurring nuclear
reactions. Nucleosynthesis takes place in the interiors of stars, in ¤ supernova
explosions and other astrophysical situations where high-energy collisions can


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