Astrophysics and Cosmic Engine Glossary

This glossary is designed to support the needs of senior Physics students in Years 11 and 12 and covers the terminology used in the NSW course and more. (*Updated)

A spectral class
Hot white stars (7,500 -10,000 K effective temperature). They show strong H lines and ionised metal lines in their spectra. Examples include Sirius A (A1 V), the brightest star in the night sky and Deneb (A2 Ia), a supergiant.
absolute magnitude (M)
The magnitude a star would appear to have if at a distance of 10 parsecs from Earth. Absolute magnitude is a measure of a star’s intrinsic luminosity and allows us to directly compare stars.
absorption spectra
Also called dark line spectra, these arise when photons of specific frequency are absorbed by cooler gas, leaving a darker line on a background continuum region. Most stellar spectra, including that from our Sun, are absorption spectra.
active optics
Corrects distortions in the shape of large, thin primary mirrors due to gravitational and structural flexing to provide a smoother surface. Typically operates at a few hertz and uses actuators to align the mirror.
adaptive optics
Corrects for the distorting effects of the atmosphere on incoming light. Some systems use a beam splitter, a reference or artificial star and powerful computer processing to deform small tertiary mirror at a frequency of several hundred hertz to compensate for the distortions. Current systems work better at IR wavebands than in optical light.
Anglo-Australian Observatory (AAO)
This joint British-Australian organisation operates the largest optical telescope in Australia, the 3.9m Anglo-Australian Telescope (AAT) which is located at Siding Spring in the Warrumbungles near Coonabarabran, NSW.
A radio telescope: a collecting ‘dish’ and associated equipment, used for radio astronomy.
apparent magnitude
A measure of a star’s (or other celestial object’s) brightness as measured from Earth. Originally developed as a six-point scale by Hipparchus, now extended and open ended. Sirius, the brightest star in the night star has an apparent magnitude of – 1.47 whilst the faintest detectable by the naked eye is magnitude 6.
astrometric binary
A binary system in which the fainter member of the system is detected due to its gravitational effect on the proper motion of its brighter companion. The first one detected was Sirius B, now known to be the white dwarf companion to Sirius A.
The branch of astronomy concerned with the accurate measurement of the position and proper motion of celestial objects.
The scientific study of the Universe and what it contains (planets, stars, galaxies, etc), and how these things formed and evolved over time. Not to be confused with astrology, which is a set of beliefs about the influence of heavenly bodies on human affairs. Astrology is not generally accepted to be a science.
Australia Telescope Compact Array (ATCA)
Comprises six 22m radio dishes on a 6km baseline. Located at Narrabri in northern NSW it operates as a radio interferometer.
Australia Telescope National Facility (ATNF)
Australia’s largest astronomical organisation, part of CSIRO, the ATNF operates the Parkes radio telescope, the Australia Telescope Compact Array at Narrabri and a single 22m dish at Mopra (near Siding Spring). Its headquarters at Marsfield in Sydney has extensive workshop facilities where new receivers and amplifiers for radio telescopes are designed and constructed. It is also involved in developing the next generation of radio telescopes such as SKA, the Square Kilometre Array.
The apparent magnitude of an object as measured through a B or ‘blue’ filter. Also called a photographic magnitude as it approximates the blue spectral response of photographic film.
B spectral class
Very hot blue-white stars (10,000 - 28,000 K effective temperature). Characterised by neutral He lines.
Balmer lines
Big Bang theory
The theory that the Universe ‘started’ with an event that created time and space, about 13 billion years ago.
binary system
Two stars that revolve around a common centre of mass. Most star systems are binary. Binary systems are important because they allow astronomers to determine the masses of stars.
black hole
A highly condensed region of matter, with a gravitational attraction so strong that anything that comes too close to it – even light – cannot escape. Black holes can’t be seen, but can be detected through the gravitational effects they exert on other objects. Some black holes are the end product for massive stars as they evolve. Many galaxies have supermassive black holes at their centre.
A measure of the amount of electromagnetic radiation from a celestial object. it is expressed as a magnitude, which can be either apparent, whcih depends on an object's intrinsic luminosity and its distance from the detector, or absolute.
Cepheid variable
An important class of periodic pulsating variable star that obeys a period-luminosity relationship discovered by Henrietta Leavitt. Type I or classical Cepheids are giant luminous yellow stars with periods of 5 -10 days. Type II Cepheids or W Virginis stars are about teo magnitudes dimmer than Type Is for the same period and have periods of about 12 - 30 days. The longer the period of a Cepheid, the more intrinsically luminous it is. This allows them to be used as "standard candles" and are thus important distance indicators for the extragalactic distance scale.
Chandrasekhar limit
The theoretical upper limit at which a stellar remnant can form a white dwarf. Beyond this value (~1.4 solar masses) the electron degeneracy pressure is unable to withstand gravitational collapse and the electrons are forced into the nucleus where they combine with protons to form a dense sea of neutrons. The remnant star is then a neutron star.
The continuous spectrum that any object would produce if no absorption or emission lines were present. Any body above absolute zero emits a spectrum, the shape of which is dependant on its temperature. thepeak of a continuous spectrum can be used to determine the effective temperature of a blackbody-like object such as a star.
CNO cycle
Abbreviation for "Carbon-Nitrogen-Oxygen cycle (or carbon-cycle)". This is a form of stellar nucleosynthesis in which hydrogen is fused into helium in main sequence stars. This cycle uses carbon-12 nuclei as a nuclear catalyst. It requires core temperatures of at least 14million K to initiate and dominates at 20 million K or greater as is found in more massive main sequence stars, those of O, B and A spectral class.
colour index CI = B – V
It provides a numerical value for the ‘colour’ hence effective (surface) temperature of a star that can be measured directly by measuring the apparent magnitude of the star through B and V filters.
dark nebula
A cool cloud of dust and gas. Due to its low temperature it does not emit light in visible region of spectrum so appears dark, often blocking out light from stars that are behind it. Best example is the Coal sack nebula in Crux.
distance modulus
Defined as m – M. A negative distance modulus means that the object is closer than 10 parsecs to us whilst a positive value means that it is further away than 10 parsecs.
Doppler shift
Refers to the apparent shift in spectral lines If a galaxy is moving towards our Galaxy, or away from it, the light we see coming from that galaxy appears different from what it would be if the galaxies were ‘standing still’. If the two galaxies are approaching each other, the light becomes higher in frequency (shifted towards the ‘blue’ end of the spectrum). If the galaxies are moving further apart, the light becomes lower in frequency (shifted towards the ‘red’ end of the spectrum). As most of the galaxies we see are moving away from ours, this redshift effect is much more common than a blueshift. The Doppler shift applies to radio waves and other forms of radiation as well as to light. (It also applies to sound.)
dwarf star
A potentially misleading term that refers to stars of luminosity class V, that is main sequence stars. Not to be confused with ‘white dwarfs’.
eclipsing binary
A binary system that is detected by the periodic eclipsing of each component star by the other. The resultant light curve shows primary eclipses when the dimmer star passes in front of the brighter star and a scondary eclipse or minimum when the brighter star eclipses the dimmer star. The component stars in an eclipsing binary are generally close to each other.
effective temperature Teff
The temperature of a blackbody that has same luminosity and size as a star. In effect it represents the temeprature of the photosphere ('surface') of a star. Expressed in Kelvin, K.
A fundamental particle of matter. A lepton, it has a rest mass, me of 9.109 x 10-31 kg and a charge, e of -1.602 x 10-19 C. Its antiparticle is the positron.
emission spectra
Also called "bright-line" spectra, these are produced by electrons de-exciting and jumping back down to lower-energy orbits, emitting photons of specific frequencies. These so-called emission lines appear as bright lines on a dark background in photographic recordings or narrow peaks on an intensity plot. Emission lines are produced by many astrophysical processes such as those found in emission nebulae (including planetary nebulae and HII regions), quasars and the some stars such as Wolf-Rayets.
exosolar planets (exoplanet)
Planets orbiting stars other than our Sun. To date only one has most likely been directly imaged; most have been detected by their gravitational effect on their star causing a periodic shift in the spectral lines of the star. A few planets have also been detected by their transiting actross the face of a star.
F spectral class
Stars with a 6,00 - 7,500 K effective temperature and a white-yellow colour. Spectral characteristics are weak Ca+ lines and medium H Balmer features. Examples include Procyon (F5 IV), the supergiant Canopus (F0 Ib) and the main sequence star π3 Orionis.
galactic cluster
Another term for open cluster as they are typically located in the arms of spiral galaxies where there is abundant dust and gas essential for star formation.
An Italian astronomer and scientist (1564 - 1642) who first systematically used a telescope to study celestial objects and publish his findings. His observations helped consolidate the Copernican concept of a heliocentric model. He also made valuable contributions to the phsyics of motion.
A huge collection of stars – often many thousands of millions – and associated dust and gas. Galaxies are separated from each other by enormous stretches of empty space. They may be spiral in shape (like a Catherine-wheel), elliptical (like a football), or irregular. The galaxy we live in is referred to as the Galaxy (or sometimes, the Milky Way Galaxy): it is a spiral galaxy, about a hundred thousand light-years in diameter.
Gemini Observatories
Two 8.1m optical/near-infrared telescopes, one on Mauna Kea in Hawaii in the northern hemisphere, the other in Cerro Panchon in Chile, in the southern hemisphere. Australia is a partner in the seven nation international consortium that owns and operates the telescopes.
globular cluster
Gravitationally-bound spherical clusters of densely-packed stars, normal found in the halo of galaxies. Globular clusters contain thousands of stars, maybe up to a million. Globular clusters contain old, highly-evolved Population II stars that have low metallicities. A prominent southern hemisphere example is 47 Tucane.
helium flash
The sudden onset of helium core fusion (or "burning") in post-main sequence stars is called the helium flash. It occurs in stars with mass less than about 2-3 solar masses once post-main sequence hydrogen shell burning has dumped sufficient helium nuclei into the core, heating it up to about 100 million K. Once the temperature is hot enough for helium fusion in one part of the core, the reaction quickly spreads throughout it due to the behaviour of the electron degenerate gas, the whole process taking minutes or hours.
Hertzsprung – Russell (HR) diagram
An extremely useful diagram that plots luminosity (or Absolute magnitude) on the vertical axis and spectral class, colour, colour index or effective temperature along the horizontal axis. It shows that stars fall into several main groups including the main sequence, giants, supergiants and white dwarfs. Comparing stars on the diagram allows u to infer their properties and better understand their evolutionary path.
Hubble Space Telescope (HST)
An Earth-orbiting optical, ultraviolet and near infrared space telescope with a 2.4 m primary mirror. Launched in 1990 it continues to produce stunning images and observations although its future is in doubt.
Hubble’s Law
The relationship discovered by Edwin Hubble in the 1920s that shows a linear relationship bewteen the reshift of an non-local galaxy and its recessional velocity. This means that the more distant a galaxy is from us, the faster it is moving away from us . This was a key piece of evidence suggesting an expanding Universe and supporting a Big Bang model.
The most abundant element in the Universe, its most common isotope has a single electron orbiting a proton.. Atomic hydrogen formed from the decoupling of electrons about 300,000 years after the Big Bang. It is the prime fuel for nucleosynthesis in stars. Its fusion into helium is the source of energy for main sequence stars such as our Sun.
hydrogen burning
The fusion of hydrogen (protons) to helium in the core or shells of stars. Main sequence stars fuse hydrogen through the proton-proton chain or the CNO cycle. It is a nuclear reaction and not 'burning' or combustion with oxygen. a chemical reaction.
Instability strip
A region on the HR Diagram occupied by pulsating variable stars such as Cepheids.
Technique where light is collected by two or more separated collectors such as radio antennas or telescope mirrors and then correlated. It is generally used to improve the resolution of a source.
A German scientist (1571-1630) who used Tycho Brahe's observations of planetary motions to devise three empirical laws now known as Kepler's Laws of Planetary Motion.
Kepler’s Laws
Kepler proposed three laws of planetary motion based on his analysis of Tycho Brahe's long and detailed observations of the orbits of the planets. The laws are:
  1. The planets orbit the Sun in elliptical orbits, with the Sun as one common focus.
  2. The line between a planet and the Sun (the radius vector) sweeps out equal areas in equal periods of time (sometimes called the Law of Equal Areas).
  3. The square of a planet's period, T, is directly proportional to the cube of its average distance from the Sun, r:
    T2r3. This law, the Law of Periods or Harmonic Law also applies to other orbital systems from the moons of Jupiter through to binary star systems. Astronomers use it to calculate the masses of stars in binary systems.
light year
The distance light travels in a year: about 9,460,000 million kilometres or 9.46 x 1015 m. (Light travels at 300 000 kilometres a second.) 1 parsec = 3.26 light years.
line spectra
Also called absorption spectra, these arise when photons of specific frequency are absorbed by cooler gas, leaving a darker line on a background continuum region. Most stellar spectra, including that from our Sun, are absorption spectra.
luminosity L
This is the total power output of an object such as a star. The Sun’s luminosity, LS = 4.0x1026W.
luminosity class
Stars of the same spectral class may vary widely in luminosity. This difference is indicated by a roman letter after the spectral class; Ia and Ib- bright supergiant and supergiant, II-bright giant, III-giant, IV-subgiant, V-main sequence (dwarfs), VI-subdwarf and VII or wd-white dwarf. The Sun is thus a G2 V star.
The main grouping of stars on the Hertzsprung-Russell Diagram. Main sequence stars are characterised by core fusion of hydrogen to helium. The Sun is on the main sequence. Stars spend the bulk of their lifespan on it.
In astronomy a metal is any element heavier than Helium. This includes elements such as Carbon and Oxygen. Stars with an abundance of metals are said to have high metallicity.
The path made when a small particle of interplanetary dust entering our atmsophere burns up.
These are different. Meteors are small particles, usually smaller than grains of sand, which travel through space: they become visible as ‘shooting stars’ when they enter the Earth's atmosphere and burn up as a result of friction. Meteorites, on the other hand, are large enough to reach the ground without being destroyed. Big ones produce craters.
Milky Way
The bright band of stars stretching across the night sky. It is actually what we can see of our own Galaxy (we are looking at it from the inside, of course.) Our Galaxy is sometimes called the Milky Way Galaxy. It is a barred spiral galaxy.
An area of dust and gas in space. A nearby star can make a nebula shine, either through reflected starlight, such as in the Pleiades, or because energy from the star makes the nebula itself glow as in M42, the Great nebula in Orion. Other types of nebulae are dark or absorption nebula such as the Coalsack, planetary nebulae that are actually the ejected outer layers of a dying star and supernova remnants.
A fundamental particle. A neutral lepton, there are three, one each associated with the elesctron, muon and tau particles. The mass of neutrinos is thought to be either extremely small or zero. The interact extremely weakly with normal matter. They are produced in fusion reactions in stellar interiors. The first detection of neutrinos associated with an object from beyond our Sun was from Supernova 1987A.
A neutral particle found in the nucleus of atoms heavier than hydrogen. The rest mass of a neutron is slightly heavier than a proton. Free neutrons are unstable and undergo beta decay to produce a proton, an electron and an antineutrino with a mean lifetime of about 14 minutes. Neutrons are not fundamental but are baryons. A neutron is composed of one up and two down quarks, giving a net charge of zero.
neutron star
A giant ball of neutrons (particles found in the nuclei of atoms). Neutron stars are very dense, only ten or twenty kilometres across, but more about 1.4 to 3 × the mass of our Sun. They are formed in supernova explosions.
These are a type of cataclysmic variable caused charactrised by a brightening of up to 10 magnitudes within several days. They are caused by a thermonuclear eplosion of material that has accreted onto a white dwarf in a close binary system. Unlike a supernova explosion the white dwarf remains after the explosion and can accrete more material from its companion so that the process is repeated after many thouands of years.
Nucleosynthesis is the production of nuclei from lighter nuclei. Big bang nucleosynthesis in the first few minutes of the Universe formed deuterium, helium and traces of lithium nuclei from fusion of protons (hyrdrogen nuclei). Stellar nucleosynthesis takes place in stars and is reponsible for production of elements heavier than helium including carbon, oxygen and iron. It also Very heavy nuclei are formed in the late evolutionary stages of more massive stars including the final supernova explosion.
O spectral class
O-class stars are very hot with effective temperatures in the range of 28,000 - 50,000 K and thus appear blue. Their spectra are characterised by weak ionised He+ lines and a strong UV continuum. Main sequence O stars are high mass, 20 - 60 & times; the at of the Sun and 9 - 15 × its radius. They are extremely luminous, ranging from 90,000 - 800,000 × the Sun's luminosity but have extremely short main sequence lifespans of only 1 - 10 million years. Examples include Alnitak and Mintaka in Orion.
open cluster
An open cluster is a group of stars that are thought to have formed from a common cloud of dust and gas. Open clusters are also known as galactic clusters as they are generally located along the disk of spiral galaxies where star formation is more common. Open clusters have a few dozen up to a few thousand stars in a vloume up to 20 parsecs or so wide. They contain Population I stars, young and high in metallicity. Open clisters are not gravitationally bound so cluster members disperse into the general disk population of stars. Examples include the Pleiades, Hyades, both in Taurus and the Jewel Box in Crux.
parsec pc
The distance in space at which an object would subtend a parallax angle of 1 arc second from Earth. This corresponds to a distance of 3.08 x 1016 m or about 3.26 light years.
period-luminosity relationship
Several classes of pulsating variable stars such as Type I and II Cepheids and RR-Lyrae stars exhibit a period-luminosity relatinship. The longer the period of variability, the greater the intrinsic luminosity of the star. Astronomers apply this relatinship to determine the ditance to such a star and hence also the cluster or galaxy it is found in. Stars that obey a period-luminosity relationship can be used as standard candles.
photographic magnitude
This is the apparent magnitude of a star as recoreded on a photographic plate. As most photographic emulsions are more sensitive to blue light than red light cooler, redder stars appear less bright when photographed than a blue star. The photographic magnitude these days is ususally replaced by the B magnitude which is the magnitude as recoreded through a B or blue filter.
The smallest ‘unit’ of electromagnetic radiation. The energy of a photon is a function of its frequency and can be calculated from the expression E = hf where h is Planck’s Constant (= 6.626 x 10-34 Js)
A relatively small, speherical body that orbits a star , for instance, the Earth. Note the definition of what constitutes a planet is currently under heated discussion by astronomers and planetary scientists. For instance should Pluto be regraded as a planet or be labelled as a Trans-Neptunian Object? If Pluto is a planet should the recently discovered object 2003 UB313 that is larger than Pluto but much further out also be regarded as a planet? (See solar system.)
planetary nebula
A planetary nebula is the nebula formed in the AGB stage of a star's life when it ejects its outer layers of gas. The exposed inner region of the star left behind is initially so hot that the intense ultraviolet radiation it emits ionises the expanding, ejected shell. This results in the cloud glowing, similar to an emission nebula. Such objects are called planetary nebulae after their initial description by Herschel in the 18th century. Through small telescopes they appear as faint discs, like a dim planet though they are not related. Planetary nebulae typically contain 0.1 - 0.2 solar masses at densities equivalent to a vacuum on Earth. Spectacular images by modern telescopes including the HST reveal a wide range of shapes that pose interesting problems for theorists to explain.
Pre-main sequence stars
Stars not yet hot enough in their cores for hydrogen fusion to begin. Pre-Main Sequence stars are protostars collapsing under gravity and heating up. They would appear on a Hayashi track on the HR diagram.
population I star
Stars found in the spiral arms of galaxies, including our Sun, are generally younger and have high metallicities. They are referred to as Population I stars.
population II star
Population II stars are older, red stars with lower metallicities and are typically located in globular clusters in galactic halos, in elliptical galaxies and near the galactic centre of spiral galaxies.
proper motion
This is the shift of a star's (or other celestial object's) position across the celestial sphere. Proper motion is measured in units of arcseconds per year, ("/yr).
A proton belongs to the hadron family of particles. It is composed of two up quarks and one down quark, thus is a baryon. The charge on a proton is the same magnitude as that of an electron but is positive. The rest mass of a proton is 1.6726 × 10-27kg, about 1836 &times that of the electron. Hydrogen nuclei are protons. The antiparticle of a proton is the antiproton.
A neutron star that puts out pulses of radio energy. A pulsar is a neutron star that is emitting a beam of radio waves, and spinning, and so the effect is like that of a lighthouse. Pulsars keep exceptionally good time. Most of them ‘pulse’ at a rate between ten times a second and once every three seconds; some spin around hundreds of times a second. Pulsars have a mass of about 1.4 to 3 solar masses compacted into a sphere about 10 km across.
The smallest ‘unit’ of energy. A quantum of light is called a photon.
Quarks are fundamental particles from which hadrons are formed. There are six types of quarks; up, down, charm, strange, top and bottom. Each has a spin of ½ and a charge of -1/3 or +2/3. Quarks are bound together through the exchange of gluons. Each quark has a corresponding antiquark as its antimatter particle.
Extremely distant, incredibly luminous objects. They are now generally accepted to be the exceedingly bright centres of certain galaxies, tiny central cores that put out the power of perhaps a thousand ordinary galaxies. The centre of a quasar is thought to be a supermassive black hole of up to a billion solar masses in a volume no larger than our solar system.
The apparent magnitude of a celestial object such as a star when measured through an R filter. An R filter lets red light through.
The word ‘radiation’ has two different meanings, which can be confusing. First, there is electromagnetic radiation. This is thought of as travelling as ‘waves’ through space. Light, radio waves, X-rays, infrared and ultraviolet rays are all kinds of electromagnetic radiation. This is what astronomers are usually talking about when they refer to ‘radiation’. Second, there is ‘ionising radiation’ – the radiation that we associate with radioactive materials and nuclear power. Much of this is actually sub-atomic particles: neutrons, protons and electrons. Gamma rays, which are also produced by nuclear processes, can be thought of either as ‘ionising radiation’ or as a very short wavelength kind of electromagnetic radiation.
radio telescope
A device for collecting the radio waves that are put out by objects in space (such as individual stars, whole galaxies, supernova remnants, etc). Most radio telescopes look like satellite communication dishes. You don’t look through a radio telescope; instead, the radio waves are collected, turned into electrical signals and then processed by computers. The end result is usually a picture: like a photograph, but made with radio waves instead of light.
rapid process (r-process)
During the final destruction of a high-mass star in a supernova explosion, a high flux of neutrons is released as iron nuclei are ripped apart. These neutrons can be captured by many of the heavy nuclei to produce other nuclei in a method known as the r-process (r for rapid). The high number of neutrons available in these few seconds can be captured by unstable nuclei before the nuclei have had a chance to decay. In this way nuclei of elements such as lead, gold and all the way up to uranium can be synthesised.
red giant
Red giants are post-main sequence stars on the Red Giant Branch of the HR diagram. They have low effective temperatures, ~ 3,000 K (K or M spectral class) so appear orange or reddish but have high luminosity, 102 to 103 × that of the Sun with luminosity class II or III. Their radius is typically 100 × that of the earlier main sequence stage. Red giants are have hydrogen shell burning and eventually helium core burning. Aldebaran is a red giant.
red shift
When an object, such as a galaxy, is going away from you, the light it puts out appears to become longer in wavelength – that is, shifted towards the red (long) end of the spectrum. The faster the object is travelling away, the greater degree of redshift. All galaxies (beyond our local group of galaxies) show redshift, which indicates that the Universe as a whole is expanding. Redshift applies to radio waves, and other wavelengths, as well as to light. It is a special case of the Doppler shift.
reflection nebula
This type of nebula is associated with hot, young stars, typically B2 or hotter. Dust in the cloud scatters light from the star so that it appears blueish to an observer. The classic example of a reflection nebula is that surrounding the Pleiades cluster.
The higher the resolution of a telescope, the more details we can see from the images obtained on it. Technically we are referring here to the spatial or angular resolution of a telescope. For circular apertures, such as in telescopes, where the light rays from a source are parallel, as is the case for distant point sources of light such as stars, the light will be diffracted so as to form an Airy disc. The circular diffraction pattern formed contains 84% of the light in the central bright spot with decreasing percentages in the surrounding bright rings. The first diffraction ring should have less than 2% of the light in the central Airy disc. Two objects are said to be resolved if their Airy discs are sufficiently separated to be seen as distinct.
resolving power
The ability of a telescope to distinguish between close, but separate, objects or features.
Sensitivity is a measure of the minimum signal that a telescope can distinguish above the random background noise. All other things being equal, a telescope of larger primary mirror or lens is more sensitive than one with a smaller primary.
Solar System
The Sun and other bodies associated with it: the nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) and their moons, as well as comets, asteroids and dust and gas.
spectral class
Stars can be classified according to key characteristics in their spectra, mainly the presence and strength of spectral lines. The main spectral classes, in sequence from hottest to coolest are: O, B, A, F, G, K, M. Each class is further subdivided into ten subdivisions from 0(hottest) to 9 (coolest). Our Sun is a G2 star.
spectral lines
Molecules (and individual atoms) can exist in different energy states. When a molecule jumps from a high energy state to a lower one, it puts out energy. This energy takes the form of a burst of light or radio waves or other kinds of radiation. The size of the jump determines the frequency of the radiation. Each molecule can only make jumps of certain sizes; as a result, it puts out radiation only of certain frequencies. These frequencies are called spectral lines. Each kind of molecule can be recognised by its own particular patterns of spectral lines – they form a ‘signature’. By looking for spectral lines, astronomers have found many different kinds of molecules (such as water and many carbon-based molecules) in space.
A spectrograph is an instrument used to obtain and record an astronomical spectrum. The spectrograph splits or disperses the light from an object into its component wavelengths so that it can be recorded then analysed. Modern multifibre spectrographs as the 2dF on the AAT allow hundreds of individual spectra from stars or galaxies to be obtained simultaneously.
spectroscopic binary
A binary system detected by Doppler shifts in its spectral lines is called a spectroscopic binary. Analysis of the spectral line shifts versus time reveals information about the radial velocities of the component stars. In spectroscopic binaries the component stars are often very close and may in fact exchange material due to tidal interactions. Orbital periods range from a few hours to months, with separations of much less than an AU in many cases. The first spectroscopic binary detected was Mizar A.
spectroscopic parallax
Spectroscopic parallax uses the spectral classification and luminosity of a star (obtained from photometric observations) to place it on an HR diagram. By then comparing the apparent magnitude with the expected absolute magnitude a distance to the star can be estimated.
standard candle
A large ball of gas, mostly hydrogen, held together by gravity. Stars produce their energy through nuclear reactions: in normal stars, hydrogen is being converted to helium, and this process releases energy. Our Sun is a very ordinary kind of star: it is about five thousand million years old, and will keep going for about another five thousand million years.
stellar evolution
Standard candles are celestial objects with known luminosities. This allows them to be used as distance indicators as their distance can be inferred by comparing their apparemnt and absolute magnitudes. Examples include Type Ia supernovae and Cepheids.
super giant
These are the largest, brightest stars known. Of luminosity classes I, they are found along the top of the HR diagram and have evolved from high-mass main sequence stars (> about 11 solar masses). They are short-lived and will eventually explode as a supernova. Examples include Rigel, Deneb and Betelgeuse.
The explosion of a star at the end of its life . Not all stars explode, only those about ten to twenty times more massive than our Sun. A supernova creates the elements that are heavier than iron (such as gold, silver and most of the other metals).
supernova remnant
The debris left by a supernova. Supernova remnants can often be seen by radio telescopes. Some are detectable thousands of years after the original star exploded.
surface temperature
(see effective temperature)
T Tauri stars
T Tauri stars are young, irregular, variable stars. They are protostars of 3 solar masses or less, still collapsing and undergoing mass-loss. This material is ejected into space at speeds up to 500,000 km h-1 at a rate of 10-7 to 10-8 solar masses per year. The T Tauri phase may last 10 million years during which a mass equal to the Sun is ejected. Spectral analysis shows that these stars rotate rapidly and have high abundances of lithium, an element that is consumed early in a star's life. They are located just to the right of the main sequence on an HR diagram.
trigonometric parallax
Nearby celestial objects exhibiti a perioidic shift in position relative to background stars due to the earth's motion around the Sun. By measuring this stellar parallax and applying trigonometry the distance to the nearby star can be determined. This method is used to define the parsec.
Part of the electromagnetic spectrum between the visible and X-ray regions. Ultraviolet radiation is absorbed in our atmosphere so UV telescopes such as FUSE need to be placed in space.
variable star
A variable star is simply one whose brightness (or other physical property such as radius or spectral type) changes over time. Intrinsic variables are those in which the change in brightness is due to some change within the star itself such as in pulsating stars like the Cepheids. Extrinsic variables are those in which the light output changes due to some process external to the star itself. The most common example of these are the eclipsing binaries.
visual binary
A binary system in which component star can be visually resolved through a telescope. Examples include α Cen and β Cru.
visual magnitude
The magnitude of an object as measured through a V filter. This approximates the peak response of the human eye thus is often referred to a visual.
white dwarf
White dwarfs (luminosity class wd) are the hot, compact, collapsed remains of stars that have exhausted their core fusion. They are about the size of the Earth but with the mass of the Sun hence are extremely dense. Electron degeneracy pressure prevents further collapse.

White dwarfs are very hot but dim due to their small surface area.

X-ray radiation is part of the electromagnetic spectrum between the UV and gamma-ray components. As X-rays are blocked by our atmosphere X-ray astronomy is only possible from space such as with the Chandra telescope. X-ray emissions are associated with high-energy astrophysical events such as accretion onto neutron stars and black holes.
zero-age main sequence
This is the position of a zero-age star when it arrives on the main sequence. It is used to infer the age of cluster stars.