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Glossary - Page 1 of 15
Space Science Glossary
Glossary Topics
Binary & Multiple Stars
“Lifecycle” of a Star
Star Clusters
 Globular Clusters
 Open Clusters
Extra-Solar Planetary Systems
Zodiac, Ecliptic, Celestial Equator
Solstice and Equinox
Mapping the Sky
Magnitude System and Star Brightness
Naming Objects
Active Galactic Nuclei, Seyfert Galaxies, and Quasars
The Milky Way
Gaseous Nebulae
Interstellar Space
Scales, Sizes, and Units of Measurement
Meteor Showers and Meteor Storms
Our solar System
Glossary - Page 2 of 15
Binary & Multiple Stars
A double star is two stars that appear close to one another in the sky. Of these:
Some are TRUE BINARIES, meaning that the two stars actually orbit around one
Some are OPTICAL DOUBLES, meaning that they just appear together when
viewed from the Earth because they lie along the same line of sight.
Related Terms:
 Wide Double – two stars that can easily be resolved (seen as separate stars)
 Visual Double/Binary – two stars that can be resolved with the naked eye or a
 Spectroscopic Double/Binary – two stars that can only be recognized as separate
stars when looking at their spectra for Doppler shifting from their orbital motions.
 Eclipsing Binary – a pair of stars whose plane of orbit is such that, when viewed
from Earth, the stars pass in front of and behind one another at certain times. This
means that the total light from the pair fluctuates.
 Multiple Star System – this is a system containing more that two stars. It may
consist of two pairs orbiting one another, a pair orbiting a single star, or some other
 Astrometric Binary – a pair of objects – the invisible companion is detected by the
“wobble” of the visible companion.
 J. Pasacjpff and D. Menzel. 1992. Peterson Field Guides – Stars and Planets. New
York, NY. Houghton Mifflin Co.. (p 190)
“Lifecycle” of a Star
The lifecycle of a star depends mostly on the initial mass of the star, though chemical
makeup and interactions with companion stars can also have a profound effect. (Most
stars are born in double and multiple star systems.) The following is a basic outline:
1. Stars are formed within molecular clouds, mostly made of hydrogen (~70% by
mass), helium (~28%), and other elements and dust particles.
2. In denser parts of molecular clouds, gravity causes gas and dust to collapse into
small, dense objects called cloud cores. These cores collapse until the center starts to
heat up, forming a protostar. Magnetic fields erupt from the forming protostar and
interact with the disk. Some of the disk's spin energy converts into a pair of oppositelydirected jets of gas and plasma. The luminous shockwaves of these jets, and of the
surrounding gases, act as signposts of starbirth. Dust and ice in the disk collide to form
larger and larger particles, eventually leading to the formation of protoplanets. Violent
collisions mark the very last stages of planet formation, which does not end until only
Glossary - Page 3 of 15
about a dozen Moon- to Jupiter-sized objects form in well separated orbits to avoid
further merging.
3. At first, the protostar shines from the heat generated by contraction. As gases at the
center of the protostar heat up, thermonuclear fusion starts as hydrogen is “burned” to
make helium. When this fusion stabilizes, it is said to have reached the main sequence
(see Hertzprung-Russell diagram). Massive stars reach this stage more quickly; low
mass stars take much longer.
4. The star spends most of its life in the main sequence stage. The gravitational forces
that try to collapse the star are almost balanced by the outward pressure of radiation
and hot gasses that are heated by the thermonuclear fusion. Massive stars consume
their fuel of hydrogen much faster, living luminous but short lives.
Any star more massive than about 100 Suns will blaze with such a furious light that the
outward pressure of radiation will exceed the inward pull of gravity and the star will
break up. Very small stars (about one-tenth the mass of the Sun) can live hundreds of
billions of years since they burn their fuel very slowly. Note that this is far longer than
the current 14 billion-year age of the Universe! Stars less massive than 0.08 times the
mass of the Sun fail to ignite the fusion of hydrogen, and will just fade and cool to
become brown dwarfs.
5. Stars start to die when the hydrogen in their cores is completely consumed, at which
point the core shrinks and heats up. This heating allows helium to "burn" to form carbon,
generating energy up to ten thousand times faster. The extra energy causes the star's
outer layers expand outward, where they then cool; the star becomes a red giant.
6. The ultimate fate of a star depends on its initial mass.
The most massive stars (60 times the mass of the Sun or more) shed much of their
mass during the main sequence phase as powerful stellar winds, including all of their
hydrogen. In the cores of such massive stars most of the matter is converted into
heavier elements. Near the end of a massive star's life, it swells, first forming a blue
supergiant, then a red supergiant, even oscillating between the two as processes cause
the star to heat and cool, expand and contract. The very most massive stars are so
luminous they blow off their outer layers. The remaining star, called a Wolf-Rayet star,
is recognizable by its strange spectrum.
Once the material at the core is burned to iron, the star faces the ultimate energy crisis
since iron cannot be fused to gain energy. In a flash, the iron core collapses and the
star releases more energy than the star produced during its entire lifetime! This is a
supernova explosion that expels material at 10% of the speed of light and leaves
behind a black hole.
Glossary - Page 4 of 15
Stars with initial masses between eight and 50 times that of the Sun do NOT
evolve to the Wolf-Rayet stage: they never completely lose the hydrogen in their outer
layers. Such stars also become blue and red supergiants. As they build up an iron core,
they too explode as supernovae. The remaining core then begins to collapse. If the core
is larger than five solar masses, collapse continues until it becomes a black hole. If the
core is less than five solar masses, the collapse is stopped when electrons and protons
are squeezed together by the extreme pressure to form an ocean of neutrons. These
Neutron stars have giant magnetic fields that produce powerful beams of electrons. If,
like a lighthouse light, the neutron star's spin sweeps the beams past Earth, we see
pulses of radio energy—a pulsar.
Stars approximately the size of the Sun stop fusing elements after they form a core
of helium or carbon. Their cores collapse until they are about the size of Earth and
electrons can’t be squeezed any closer, resulting in a white dwarf star. As the white
dwarf forms, it gently expels the outer layers of the progenitor red giant star out into
space, which it then lights up with UV radiation from the hot white dwarf core. This
whole object is called a planetary nebula.
Sometimes a supernova can occur when a white dwarf accretes matter from a
companion star that is evolving to its red giant phase. When the mass of the white dwarf
reaches 1.4 Solar masses, the electrons can't support the star against the inward pull of
gravity, and it collapses. But since most white dwarfs contain lots of helium and carbon,
the collapse triggers rapid nuclear burning, and within seconds blows up the star. This
type of supernova (Type Ia) does not leave behind a collapsed remnant.
Hertzprung-Russell (HR) Diagram
This is a plot of increasing luminosity against decreasing surface temperature. A
random survey of stars plotted on an HR diagram gives a consistent pattern. The long
thin diagonal of stars is called the MAIN SEQUENCE, and this is where stars spend
most of their lives (see 4. above). Above and to the right of that are red giants and
supergiants; below and to the left are white dwarfs. Other types are spread around the
 (click on the relevant file)
Star Clusters
Star clusters are groups of stars that are gravitationally bound together. There are two
kinds of star clusters:
Open Clusters (or Galactic Clusters)
These are groups of tens to thousands of stars that were formed from the same
molecular cloud, and so are similar in age, chemical makeup, and location. They
Glossary - Page 5 of 15
are thought to have formed from clouds of gas and dust in the Milky Way, and are
distributed in the plane of the galaxy. Some open clusters are still surrounded by the
gases from which they were formed, and are areas of new star formation.
 Globular Clusters
These are groups of around ten thousand to one million stars. They are very old,
around 12 to 20 billion years, and are thought to have formed in an earlier
generation of stars, called Population II stars. (Our Sun and the stars we see in
open clusters are Population I stars, born of the material that was left over from the
galaxy’s formation after Population II stars had already been born). These globular
clusters are distributed in a spherical “halo” of the Milky Way.
 (click on the photo near Globular or Open)
 Ridpath, Ian. 1998. Norton’s Star Atlas and Reference Handbook. Harlow, Essex,
England. Addison Wesley Longman Ltd. (pp 154-155)
Extra-Solar Planetary Systems
Exoplanets are planets that go around stars other than our own. Over 100 exoplanets
have been found to date, mostly in singles, though some are in pairs or even triples
around a given star.
The planets are not observed directly – they are too small and too dim. Rather,
scientists observe the star wobbling to and fro from gravity. Much the same way
binary stars pull each other in orbit around each other, a planet pulls on its parent
star just a little bit. We use this information to determine the size of the orbit and the
mass of the planet. In a few rare cases we can determine the planet’s size by
measuring the reduction in the amount of light received from the star when the
planet passes in front of the star.
It is entirely possible that there are smaller planets orbiting stars, but present
techniques do not allow us to detect anything smaller than 0.1 the mass of Jupiter.
The study of extra-solar planets is a very active field of research and the numbers
change every month.
Zodiac, Ecliptic, Celestial Equator
The ECLIPTIC is the path across the sky along which the Sun, Moon, and planets
appear to travel; it is the plane of the solar system. The CELESTIAL EQUATOR is the
circle that would be made if you took the Earth’s equatorial plane and extended it out
into space. The Ecliptic and the Celestial Equator are at 23.5 degrees to one another.
The ZODIAC constellations are those that lie along the Ecliptic. The NORTH
23.5 deg
Celestial Equator
Glossary - Page 6 of 15
CELESTIAL POLE is the place in the sky where the North Pole would point if it
extended out into space.
Glossary - Page 7 of 15
Solstice and Equinox
The hemisphere facing the Sun experiences summer; the hemisphere facing away
from the Sun experiences winter. In the Northern Hemisphere, the SUMMER
SOLSTICE is when the North Celestial Pole of the Earth is tilted directly toward the
Sun, and the WINTER SOLSTICE is when the North Celestial Pole is tilted directly
away from the sun. At the solstices, the Sun appears farthest from the Celestial
Equator. VERNAL EQUINOX and AUTUMNAL EQUINOX happen when the Sun is
at the points where the Celestial Equator meets the Ecliptic. We say that the
solstices and equinoxes are the first days of each season.
Earth undergoes many motions. It spins around its axis causing day and night. It orbits
the Sun each year. It also PRECESSES, meaning that it “wobbles” like a top. The
Earth’s axis is tilted at 23.5 degrees to the vertical as it orbits around the Sun, and so
the circle described on the sky every 26,000 years by the North Celestial pole is (2 x
23.5) 47 degrees wide in the sky. At the moment, the North Celestial pole points to
Why the Sun is no longer in a person’s sign/constellation on his or her
Forms of astrology have been around in many cultures for thousands of years.
However, the classical Greek signs and the accompanying zodiac (from the Greek
work “animal”) were published around the time of Hipparchus (circa 150 ACE) to that
of Ptolemy (circa 150 ACE). At that time, the Earth was at a different point in its
precession. The Sun was in Aries on the Spring Equinox (Northern Hemisphere),
and this point was called the FIRST POINT OF ARIES. Now, because of
precession, that point happens when the Sun is actually in Pisces, one sign to the
East. This means that all of the signs are shifted by one. This First Point of Aries
continues to shift by 1 degree every 72 years.
Mapping the Sky
Several methods are used to measure the positions of stars and other celestial objects.
1. Right Ascension and Declination
Glossary - Page 8 of 15
This coordinate system is used to uniquely identify the positions of astronomical
objects when viewed from anywhere on Earth.
 Right Ascension - measures the east-west location of the object, starting at the
Celestial Meridian (an imaginary line connecting the Celestial Poles and passing
through the First Point of Aries) and measured in hours, minutes, and seconds
west of that line. It is somewhat analogous to the Prime Meridian line that
passes from North to South Pole through Greenwich, England, and defines the
zero-line of time.
 Declination – measures the North-South location of the object, starting at the
equator and measured in degrees North (+ve) or South (-ve). It is analogous to
the lines of latitude on a globe.
2. Altitude/Azimuth
This coordinate system is used to identify the position of an astronomical object as
measured from the viewer’s location.
 Altitude – measures the vertical position of the object in degrees above (+ve) or
below (-ve) the observer’s horizon.
 Azimuth – measures the horizontal position of the object in degrees, starting at
north and moving eastward.
3. Constellations and star names
The 88 traditional Greco-Roman constellations are used to divide the celestial
sphere into sections. Within each constellation, stars have been assigned a Greek
letter beginning with the brightest star. Many of the brightest stars have also been
given a common name. For example, the brightest star in Cygnus (The Swan) is
Deneb, which means “tail” in Arabic; it is also called Alpha Cygni, or  Cyg.
 – fun article about how you CAN’T buy a
Mapping Sources:
Magnitude System and Star Brightness
The brightness of a celestial object is measured using the magnitude system. The
brighter the star, the lower its magnitude. It is also a logarithmic scale, such that a star
Glossary - Page 9 of 15
of magnitude 3 is about 2.5 times brighter than a star of magnitude 4, and a star of
magnitude –1 is about 2.5 times brighter than a star of magnitude 0.
This system may seem to be awkward and lack sense, but in fact it is based on the
natural capabilities of the human eye. Hipparchus was the first to catalogue star
brightness this way in 120 BCE, assigning a number from 1 to 6 where 1 represented
the brightest stars. Astronomers are now able to precisely measure a star’s brightness,
and so are not limited to whole numbers.
Related terms:
 Apparent Magnitude – this is the actual magnitude of the star as we measure it
from Earth.
 Absolute Magnitude – this is the magnitude that the star would be if it were at a
distance of 10 parsecs (about 33 light years).
 Visual Magnitude – this is the magnitude of light that is received from the object in
the visual spectrum, about 550 nanometers.
Naming Objects
While many cultures throughout time have given names to the stars, planets, and
celestial objects, the scientific community has given them “official” names so that they
can be uniquely identified. The organization charged with this task is the International
Astronomical Union (IAU). There are several naming methods. Some of the main
methods are explained below:
Stars Names
See “Mapping the Sky.” There are many catalogs that also give their own names
(e.g. HD93521 is the 92521st star in Henry Draper’s catalog). There are
commercial companies that sell star naming rights for a fee. These are
recognized only by the companies themselves, and are not official according to
the IAU.
Messier Objects (M)
Charles Messier (1730-1817) was a French astronomer with a strong interest in
finding comets. During his searches, he kept coming across other objects in the
sky that could easily be mistaken for comets. He compiled these objects into a
catalogue, and we still use this catalogue today. There are 110 Messier objects,
such as M31, the Andromeda Galaxy.
 Messier objects with thumbnail
Glossary - Page 10 of 15
New General Catalogue (NGC) and Index Catalogue (IC)
In 1888, a Danish astronomer named J.L.E. Dreyer published the New General
Catalogue of deep sky objects that combined the many lists present at that time.
Objects found subsequent to this were collected in the Index Catalogue. These
objects still bear their catalogue names. Examples are NGC 772 (the Seyfert
galaxy in Aries) and NGC 6530 (a star cluster in M8, the Lagoon Nebula in
Sagittarius). This catalogue is presently being updated to eliminate
inconsistencies and overlaps.
 and click “Historical Perspective”
Comets and asteroids
Comets and asteroids are typically named after their discoverer. They are also
given a prefix and number that tells when it was discovered, the type of orbit, etc.
Planets and their moons
The names of the planets go back to the time of the Romans, when planets were
named after the gods and goddesses they represented in that culture. As the
moons of the planets were discovered, most of them were given names of
characters who were associated with the name of the planet. For example, the
Galilean system (the moons of Jupiter discovered by Galileo) were named for the
loves of the god Jupiter. The moons of Uranus are named after characters in the
works of William Shakespeare and Alexander Pope.
Naming sources
Galaxies are groups of millions of stars. There are different kinds of galaxies, among
Glossary - Page 11 of 15
Spiral Galaxy –disk-shaped with a
bulge in the center and “arms” of
bright, young stars that appear in a
spiral pattern. Some spiral galaxies
have a bar of stars across the center –
these are called barred spirals.
Elliptical Galaxy – usually shaped
like a round or elongated ball.
Elliptical galaxies contain older stars
and very little gas and dust.
Different Types of
Irregular Galaxy – undefined shape,
containing many young stars, dust,
and gas.
Classification of Galaxies
Active Galactic Nuclei, Seyfert Galaxies, and Quasars
(Active Galactic
Seyfert Galaxy
Quasar or QSO
Some galaxies appear to release enormous amounts of energy in
their centers, probably resulting from the presence of a
supermassive black hole. Many AGN have powerful jets emanating
from their centers.
Seyfert galaxies are one type of AGN that have very bright centers
appearing almost star-like (though in fact the centers are made up of
many stars and may contain a black hole).
QSOs look just like stars, though some have faint “fuzz” around
them. They emit as much energy as galaxies from a space the size
of our solar system and are found primarily at very large distance “at
high redshifts.” They are usually strong radio sources, and are
probably powered by matter falling into a central supermassive black
hole. They are extreme forms of Seyfert galaxies.
We are seeing the light from the quasars now, but the quasars are
probably all dead now– that is why we don’t see any nearby (they
died long ago and so their light reached our position and kept on
going long ago).
Glossary - Page 12 of 15
 - active
galaxies, all types
 - seyfert galaxies
 Hubble looking at quasars
The Milky Way
The Milky Way, our own galaxy, is a spiral galaxy. It is about 100,000 light years across
and contains some 100 billion to 200 billion stars. The Sun is located about 2/3 of the
way out from the center, in one of the spiral arms. From our position we look back
toward the center of the galaxy when we look in the direction of the constellations
Scorpius and Sagittarius (the very center of the galaxy sits at the border between them).
When we look at the Milky Way, the band of stars visible on a dark night, we are looking
out along the plane of the galaxy where there is the greatest concentration of stars. All
of the stars that we can see with the naked eye are inside the Milky Way.
Gaseous Nebulae
Gaseous nebulae are vast, glowing clouds of gas (mostly hydrogen) and dust in
interstellar space, and they play a major part in the lifecycle of stars. Stars being born
light up the gas out of which they were formed. Old stars and supernovae throw off
materials that then form nebulae. Nebulae are classified according to their different
 Reflection Nebulae - The dust in these nebulae simply reflects the light from nearby
stars and usually appears blue.
 Emission Nebulae - These nebulae are made of very hot gas that is energized by
UV from a nearby star, causing the gas to glow at select colors; the most dominant
to our eyes is red.
 Planetary Nebulae - These are the shells of gas that are thrown outward at the end
of some stars’ lives. They have nothing to do with planets; early astronomers
thought they might have had to do with planets since they appear to encircle stars.
We now know they do not.
 Supernova Remnants – Even though they do not have the word “nebulae” in their
name, supernova remnants are also gaseous nebulae. In this case, the gas is
heated by the explosion of the supernova. Messier had several in his catalog.
Glossary - Page 13 of 15
Interstellar Space
Even though there are some 10 21 stars in the universe, there is still a lot of “empty”
space left over. So, what’s between the stars? In galaxies, that space is partly filled
with gas and dust. When interstellar gas and dust are near bright, hot, stars, they glow
as either reflection nebulae or emission nebulae. Between the galaxies, that space is
even emptier.
Scales, Sizes, and Units of Measurement
Because of the vast distances involved, astronomers use special units.
meter (m) - the fundamental unit of length in the metric system. Equal to about 3.3
feet or 1.1 yard.
kilometer (km) - equal to 1000 meters.
Astronomical Unit (AU) - the commonly used unit of distance in the solar system; it
is equal to the average Earth-Sun distance, or 149,000,000 km.
light year (ly) - a commonly used unit of distance outside the solar system, equal to
the length traveled by light in one year. It is equal to 9,460,000,000,000 km.
parsec (pc) - the preferred unit of distance outside the solar system. Developed
because it is connected to the apparent change in position (parallax) of a star as
measured from two opposite points on Earth’s orbit, such as the winter and summer
solstices. Defined as the distance at which 1 Astronomical Unit subtends an angle
of one second of arc (1/3600 of a degree), or the distance an object has to be for its
parallax to equal one second of arc. Equal to 3.26 light years or 30,800,000,000,000
kiloparsec (kpc) - 1000 parsecs.
Megaparsec (Mpc) - one million parsecs.
 – the scale of things
Meteor Showers or Meteor Storms
These are caused when the Earth pass through the debris left over from a comet (or,
rarely, some other object). The comets that go orbit the Sun leave a trail of debris along
their orbit, and when the Earth is at the place where Earth’s orbit and the comet’s orbit
cross, the debris collides with Earth’s atmosphere and burns up, leaving a meteor, or
streak of light, across the sky. This debris is typically smaller than a grain of rice.
Meteor showers have a particular place in the sky from which they appear to come,
called the RADIANT. This is the place in the sky that is directly above the part of the
Earth that is headed into the debris field.
Glossary - Page 14 of 15
Meteors from tiny bits of space debris throughout the solar system can be seen on most
nights, but occasionally larger chunks of debris collide with out atmosphere. If they do
not burn up completely while passing through the atmosphere, they land on the ground
and are called meteorites. They originate from broken asteroids, planetary debris that
has been ejected during a meteorite crash, etc. These are occasionally seen as
fireballs passing overhead.
 - Meteor Showers
 - Types of
Our Solar System
Our solar system is a fascinating place. The planets, moons, asteroids, and other
bodies of our solar system all have unique features for us to study.
 - This is an overview of the history, mythology, and
current scientific knowledge of each of the planets and moons in our solar system.
Each page has text and images, some have sounds and movies, and most provide
references to additional related information.
 - This is a collection of many of the best
images of the planets, moons, and small bodies of the solar system from NASA's
planetary exploration program, and includes extensive information for each image.
 - This site takes a close look at each of the
planets and other bodies of the solar system.
 - This site gives details of NASA’s past, present,
and future exploration of our solar system.
 – NASA KIDS looks at the planets.
 - a GREAT
astronomy glossary/dictionary with great pictures, etc.
NASA Planetary Photojournal – high resolution images of all bodies in the Solar
System. The “cream of the crop,” but many images are quite large to download, so
be patient!
The JPL home for web pages covering all missions related to Mars exploration. You
can see the latest images and data released from the currently operating Mars
Global Surveyor and Mars Odyssey missions at:
Glossary - Page 15 of 15
The web site for NASA’s Galileo mission to Jupiter. The latest mission news, plus a
gallery of returned images, is available.
Home page for the Cassini-Huygens mission to Saturn.