Astronomy 124

Glossary of Terms

Absorption Lines: Dark lines superimposed over a bright continuous spectrum background, created when a cooler gas absorbs photons from a hotter source.

Accretion Disk: A disk of gas which accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.

Active Galaxy: Active Galaxies are galaxies characterized by certain properties: (1) High Luminosity, (2) Nonthermal Spectra that do not look like the sum of many stellar spectra, (3) Most of the luminosity is in a region of the spectrum other than optical (e.g., radio, UV, Infrared), (4) bright, star-like nucleus, (5) strong emission lines (most), (6) rapid variability, and sometimes (7) radio jets.

Active Galactic Nucleus (AGN): The central region of an active galaxy where the energtic activity is concentrated. The active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies.

Angular Momentum: Angular momentum is a measure of the rotational property of motion. It is defined in terms of the motion of a body with respect to some point in space and the angle between the direction of the motion and the direction toward that defining point. An important principle of physics is the "conservation of angular momentum" which means that the angular momentum of a system (the momentum of rotation about a point) remains the same as long as no external torque acts.

Apparent Magnitude: The brightness of a star as it appears to the eye or to the telescope, as measured in units of magnitude. The symbol used for apparent magnitude is the lower case letter m.

Absolute Magnitude: A measure of the intrinsic brightness (hence absolute) of a star. Defined to be equal to the apparent magnitude of a star if viewed from the standard distance of 10 parsecs. The difference between the observed apparent magnitude and the intrinsic absolute magnitude (assuming this is known from some other means) provides the distance to the star, through a formula known as the distance modulus. The symbol used for Absolute magnitude is the upper case letter M.

Asymptotic Giant Branch: The part of the HR diagram (in the upper right hand corner) where stars move to after Helium burning ceases in their cores. The carbon core of the star shrinks, the outer layers expand, and the star becomes a large red giant.

Big Bang: The state of extremely high (classically, infinite) density and temperature from which the universe began expanding. The beginning point of time and space for the universe.

Blackbody: An ideal object that is a perfect absorber of light (hence the name since it would appear completely black if it were cold), and also a perfect emitter of light. Light is emitted by solid objects because those objects are composed of atoms and molecules which can emit and absorb light. They emit light because they are wiggling around due to their heat content (thermal energy). So a blackbody emits a certain spectrum of light that depends only on its temperature. The higher the temperature, the more light energy is emitted and the higher the frequency (shorter the wavelength) of the peak of the spectrum.

Black Hole: An object, predicted to exist by the properties of the theory of general relativity, which is maximally gravitationally collapsed, and from which not even light can escape.

BL Lac Object (also Blazar): A type of active galaxy characterized by very rapid (day to day) variability by large percentages in total luminosity, no emission lines, strong nonthermal radiation, and starlike appearance. A BL Lac object is a radio galaxy aligned so that we are looking down the jet into the very heart of the system, right into the nucleus. Since we are looking right along the jet we see very rapid, highly luminous radiation.

Bolometric Magnitude: The magnitude that a star would have if all of its energetic emissions were included in the measurement. For example when you look at a star and observe its brightness with your eye, your eye is only detecting the brightness from the visible portion of the spectrum. If your eye could see into the ultraviolet the star might appear much brighter. The bolometric magnitude is the estimate of how bright the star would be if all of its light, from the entire spectrum, was observed.

Brightness: Like flux brightness is energy per unit time per unit area (e.g. ergs per second per square centimeter). How bright something appears to you depends on how much energy (light) it is giving off per second, and how spread out it is over your viewing area. A certain amount of light energy will appear much brighter if concentrated into a small region of emission than when spread out over a large emission region. A tiny lightbulb can seem very bright, even when its total light is small. The apparent brightness of a star is called the apparent magnitude and that is what is measured by a telescope: how much energy does the star put into the telescope's collecting area per second.

Bulge, Galactic: A thick region around the center of the Galaxy that spheroidal in shape, containing warm gas and metal-rich older stars.

Cepheid Variable Stars: A type of luminous giant star whose luminosity varies in a periodic fashion. Cepheids are characterized by a rapid rise in luminosity followed by a slow decline. The period of the cycle is related to the luminosity of the Cepheid by the Period-Luminosity relationship. The more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. Cepheids come in two types, Type I which are metal rich and Type II which are metal poor. Type I Cepheids are more luminous than Type II.

Chandrasekhar Mass: The maximum mass, approximately 1.4 solar masses above which an object has too much mass to support itself against collapse by electron degeneracy pressure. Hence, the maximum mass of a white dwarf.

Chromosphere: A layer of the Sun's atmosphere lying above the photosphere with a width of about 2000 km. It has a lower gas density than the photosphere but a higher temperature. (In fact the temperature continues to rise with altitude, into the corona lying above the chromosphere.) The temperature is sufficiently high to ionize hydrogen gas and produce emission lines, notably the red Balmer line that gives the chromosphere a pinkish color (hence the name, chromo="color").

Closed Universe: A model of the universe that has spherical geometry (hence is finite in space) and which will eventually stop expanding and recollapse (and hence is finite in time as well).

CNO cycle: A series of nuclear reactions that convert 4 hydrogen into 1 helium nucleus. The process starts with the capture of a proton (hydrogen nucleus) onto Carbon turning it into Nitrogen. Eventually further reactions turn the Nitrogen into an Oxygen, and the process is completed by the ejection of an alpha particle (helium nucleus) and the return to Carbon. Thus, carbon acts as a catalyst - it is neither destroyed nor created but simply facilitates the H --> He process. The CNO cycle is important only in stars more massive than the Sun.

Color Index: The difference in a star's brightness (magnitude) as measured in two different wavelength bands. For example, suppose one of the two bands were centered on red and the other on blue. Suppose blue was larger: taking the difference of blue minus red brightness would give you a number that quickly shows that the star is blue. Since color indices are measured in units of magnitudes their use can be somewhat confusing since the larger the magnitude the fainter the brightness.

Compact Radio Source: An object emitting radio wavelength emission from a small unresolved region. An example would be the very core of a radio galaxy.

Conduction: Conduction is a process of heat transport through the physical collisions of the particles making up a substance. Similar to electrical conductivity, substances have heat conductivity. Substances with large heat conductivity can transfer heat rapidly (e.g. a hot metal plate). Some substances have low conductivity; they are insulators (e.g., an insulating blanket of foam). Conductivity is an important heat transfer mechanism within white dwarf stars, but not in stars such as the Sun.

Contact Binary: Two stars in a binary system that are so close together that they share a common envelope of gas. There gravitational fields overlap and the stars form a "peanut" shaped object, like a double-cored, "figure 8" object.

Continuous Spectrum: A smooth spectrum of emitted light with all wavelengths present overs some broad range of wavelengths. Blackbodies give off continuous spectra. The rainbow of colors seen when sunlight passes through a prism is an example of a continuous spectrum. In contrast, if some discrete lines are missing one is observing an absorption spectrum. If only discrete lines are present one is observing an emission spectrum.

Convection: A process of heat transfer in which hot material physically moves from a hot region to a cooler region (and cool material moves into the hot region). An example of convection is boiling where hot water, heated from below, rises to the surface to cool, while cooler surface water sinks to the bottom to be heated. The cycle continues with a net transport of heat from the bottom to the top. Convection is an important mechanism for heat transport within some types of stars and within certain regions of other types of stars. In the Sun convection is important in the top layers.

Core: The center region of a star where the temperature, pressure and density are the highest. In main sequence stars the core is where nuclear reactions take place. White dwarf stars are the left over cores of stars that have ejected their outer layers.

Core Collapse: Catastrophic gravitational infall of the center of a star when it no longer can generate sufficient pressure to maintain hydrostatic equilibrium.

Corona: The atmosphere of the Sun composed of hot, very thin gas, extending out away from the Sun for a substantial distance. This gas emits light but normally that light can't be seen against the direct glare of the Sun. During a total eclipse this direct light is blocked by the moon and the ghostly white glow of the corona becomes visible.

Cosmic Background Radiation: The blackbody radiation, now mostly in the microwave band, which consists of relic photons left over from the very hot, early phase of the Big Bang.

Cosmic Rays: Cosmic rays are very high energy atomic nuclei (mostly protons) traveling through space at high speeds close to that of light. When they hit atoms in the upper atmosphere of the Earth they generate short-lived exotic particles, in much the same way that experimental particle accelerators like CERN or Fermilab work.

Cosmological Principle: The principle that there is no center to the universe, that the universe is the same in all directions and the same everywhere, when considered on the largest scales. This principle means that what we observe of the universe from our specific location will be representative of the true nature of the universe.

Critical Density: The mass density of the universe which just stops the expansion of space, after infinite cosmic time has elapsed. The critical density is the boundary value between universe models that expand forever (open models) and those that recollapse (closed models). A measurement of the actual density of the universe could be compared to the critical density which would then, in principle, indicate the fate of the cosmos.

Dark Matter: Term used to describe any astronomical mass that does not produce significant light and hence is hard to observe. Examples of dark matter include planets, black holes, white dwarfs (because they are low luminosity) and more exotic things like weakly interacting particles (WIMPs).

Density: The density of an object is equal to the mass of that object divided by its volume. Substances (like lead, water, iron, granite) have a certain density under normal pressures. In such cases the density of a substance can also be used to determine how much mass will be present given a certain volume of the substance. For example, water has a density of 1 gram per cubic centimeter (gm/cm3) so a cube of water 10 centimeters on a side weighs 1000 gm (1 kilogram). Some substances (like gases) are compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible (and hot!) gases and is much denser at its center than near its surface.

Detached Binary: An "ordinary" binary star system where the two stars are well separated from each other. Each star evolves on its own, in a manner similar to an isolated star.

Disk, galactic: The flattened, rotating portion of the Galaxy, centered on the galactic nucleus, containing much dust and gas as well as newly formed stars. Galactic disks are found in sprial galaxies and often exhibit prominent spiral arms.

Distance Ladder: The series of techniques employed by astronomers to obtain distances to progressively more distant astronomical objects.

Distance Modulus: The formula for obtaining the distance to a star using the difference between the absolute and apparent magnitude of a star, (m-M). The formula is (m-M) = 5 log(d/10), where d is the distance as measured in parsecs. If a star has a distance modulus of m-M = 0 then it is exactly 10 pc away because the apparent and absolute magnitudes are equal. A positive value of the distance modulus means the apparent magnitude is larger than the absolute, i.e., the star is fainter than it would appear at a distance of 10 pc.

Doppler Shift: The change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened. Things moving away have their emitted wavelengths lengthened.

Dust: Tiny grains of stuff, e.g., carbon grains (soot) and silicate grains (sand) that are about 0.1-1.0 micron in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction. Dust scatters incident starlight, particularly the blue wavelengths of light (blue light has a wavelength comparable to the dust grain's size) causing interstellar reddening. The dust itself is cold, and cools even further by giving off infrared emission.

Eclipsing Binary: A binary star system where one star passes in front of the other at some point in their orbits, as we observe the system. This makes the total light from the system seem to vary with time---it is dimmer during eclipse and brighter when the system is out of eclipse. The way that the light changes as a function of time (such a graph of light versus time is known as a "light curve") can give direct information about the size of the stars and the orientation and size of the orbit.

Electron: An elementary particle (of the type known as a lepton) with a negative charge. One of the components of atoms, the electrons orbit around the nucleus, and the distribution and number of electrons determine the chemical properties of an element.

Electron Degeneracy Pressure: Quantum mechanics restricts the number of electrons that can have low energy. Basically, each electron must occupy its own energy state. When electrons are packed together, as they are in a white dwarf, the number of available low energy states is too small and many electrons are forced into high energy states. These high energy electrons make a significant contribution to the pressure. Because the pressure arises from this quantum mechanical effect, it is insensitive to temperature, i.e., the pressure doesn't go down as the star cools.

Elliptical Galaxy: A galaxy classification in the Hubble scheme, ellipticals get their names from their overall shape. The ellipticals are subclassified by their degree of ellipticity as they appear to the observer. E0 types are completely spherical, and E7 types are very elliptical (i.e., elongated). E1 through E7 have increasing degrees of ellipticity. They are smooth and structureless, and contain mainly old Pop. II type stars. Ellipticals range in size from the relatively rare Giant Ellipticals, which can be as big as a Megaparsec across with a trillion stars, to the very common dwarf ellipticals which can be as small as a kiloparsec across with a million stars.

Emission Lines: The bright lines seen against a darker background, created when a hot gas emits photons characteristic of the elements of which the gas is composed.

Emission Nebula: A glowing cloud of hot interstellar gas. The gas is energized by nearby or embedded hot young stars. The gas is mainly hydrogen and the light mainly hydrogen emission, but other elements (e.g. nitrogen, oxygen) are also present and also give off their own emission lines.

Energy: Energy is usually defined as "the capacity to do work" but just what does that mean? Work is defined in physics as the exertion of a force over some distance, e.g., lifting a rock up against the gravity of the Earth. You probably have a pretty good colloquial grasp of the idea of "work" as something that takes effort. Energy is also something that is "conserved" within a closed system. This means that it is neither created nor destroyed but simply moved about (possibly changing from one form of energy to another). Light is basically a form of energy, one that radiates through space. So the Sun can release nuclear energy, creating light which travels through space to the Earth, where it can be absorbed by, say, a photocell, which in turn permits a motor to run propelling a solar-powered car forward.

Envelope: The outer, less dense portion of a star, surrounding the hot, dense core. The outer 3/4 or so of the radius of the Sun is considered to be the envelope.

Equilibrium: A balance in the rates of opposing processes, such as emission and absorption of photons, creation and destruction of matter, etc. so that there is no net change.

Escape Velocity: The outward velocity required to leave the surface of a body mass M and radius R and escape to infinity (not fall back). The formula for the escape velocity is (2GM/R) 1/2.

Event Horizon: A boundary dividing space into a region that can be seen from one that cannot. In the case of a black hole, it is that surface surround the region out of which light itself cannot escape. No signal or information from within the event horizon can reach the outside universe.

Evolutionary Track: As a star ages and evolves, its location on the HR diagram changes. If you trace out all the locations of a given star during its lifetime you get an evolutionary track on the HR diagram for that star. Such tracks are a compact and convenient way to show how a star changes over its lifetime.

Expansion Factor: The amount by which the universe has scaled up in size since an earlier time due to the expansion of space. The scale factor is equal to 1+z where z is the cosmological redshift.

Fission, Nuclear: The release of nuclear energy by the breaking apart of large, heavy elements (e.g. Uranium) into two or more smaller atoms. Nuclear fission is the basis for so-called A bombs, and for nuclear power reactors.

Flux: A flux is the rate at which something is transferred through a surface, like 10 flies per minute through the busted screen door. In astronomy we use flux to express the amount of energy radiated per second across an area like a square centimeter.

Frequency: This is a property of a wave, and it is the number of wave crests that pass a given point per second. Frequency is is measured in units of inverse time (e.g., ``cycles per second''). A cycle per second is the unit of frequency and it is known as a ``Hertz.'' Since light moves at the constant speed of light, the frequency of a light wave is related to the wavelength: the frequency is given by the number of wavelengths that go by per second at the speed of light, hence frequency is wavelength (distance) divided by speed (c). The higher the frequency of light the greater its energy.

Fusion, Nuclear: The release of nuclear energy by the fusing or joining together of light elements to form a heavier element. The Sun obtains its central power from the fusing of four hydrogen atoms into one helium atom. Nuclear fusion is a main source of energy in H-bombs. Nuclear fusion is being studied as a possible controlled power source, but this has not yet proven to be feasible.

Giant Molecular Cloud: A region of dense interstellar medium that is sufficiently cold that molecules can form. They are very cold (10-20K) with relatively high densities (trillion particles per cubic meter), and huge. Even though the temperatures are very cold the molecules in these molecular clouds emit radio radiation which can be detected on Earth. These regions are believed to be where new stars can form.

Globular Cluster: A dense, rich, spherical cluster of stars, held together by their own mutual gravity, and containing up to hundreds of thousands of stars within a diameter of order 100 pc. Globular clusters are generally found in the halo of the Galaxy, and contain old Population II type stars.

Gravitational Lens: A massive object which causes light to bend and focus. This occurs because light falls in a gravitational field.

Gravitational Radiation: The theory of general relativity predicts that if one changes the distributions of masses (which generate gravitational fields) in certain ways one can get propagating waves of gravity in a manner analogous to the propagating waves of electric and magnetic fields (i.e., light) in the theory of electromagnetism. Gravitational radiation carries energy and travels at the speed of light.

Gravity, Surface: A spherical object of Mass M and radius R produces a downward gravitational acceleration at its surface (the surface gravity) equal to GM/R 2. Increasing the mass or decreasing the radius will cause the surface gravity to go up. The surface gravity of the Earth (called one "g") is equal to 9.8 meters per second squared.

Hadron: A class of particles which participate in the strong interaction (the force that binds atomic nuclei together). Hadrons consist of those particles (baryons, mesons) which are composed of quarks

Halo, Galactic: An extended region surrounding a galaxy. The halo contains globular clusters and other old stars. The halo apparently has considerable mass but relatively low luminosity, suggesting that a lot of dark matter must be present in the halo.

Helium Flash: In certain low-mass stars when they become red giants their cores are supported by electron-degeneracy pressure. When helium burning begins by the triple-alpha reaction the temperature in the core rises, but the pressure doesn't increase because electron degeneracy pressure is relatively insensitive to pressure. The nuclear reaction rate increases with temperature and a sort of mini explosion occurs. This runaway nuclear reaction finally causes the core to expand a bit, lowering the temperature, leading to a core supported by ordinary pressure, and kept hot by slower, more stable triple-alpha reactions.

Helium Shell Flash: Helium burning in the shell of an triple alpha reaction. The temperature in the shell increases, the burning begins and runs away before the shell can readjust. The shell expands, cutting off the burning entirely. It can never settle down to steady burning. These shell flashes buffet the outer layers of the star and eventually blow those layers away from the star to create a planetary nebula.

HII Region: "HII" (pronounced H two) is ionized hydrogen. (The roman numeral refers to the ionization state. I means neutral, II means one electron ionized, III means two electrons ionized, etc. Of course Hydrogen only has one electron, so HII is as high as it gets.) An HII region then is a region of ionized hydrogen in space. When electrons recombine with the hydrogen nuclei, the emit photons, hence an HII region corresponds to an emission nebula.

Horizontal Branch: Stars that are burning Helium in their core lie along a nearly horizontal line in the HR diagram referred to as the Horizontal Branch. It is like the main sequence (which is the line of stars that are burning hydrogen in their cores).

H-R Diagram: A graph that uses two stellar properties, such as luminosity versus surface temperature, as its axes. Individual stars are positioned on the graph according to their properties (Absolute Magnitude, and Spectral type correspond to luminosity and temperature). The resulting graph reveals relationships between a stars luminosity and temperature which in turn allows us to determine derived properties such as the age of the star, its evolutionary phase, its radius, etc.

Hubble Constant: The constant of proportionality (designated H) between recession velocity and distance in the Hubble law. It is a constant of proportionality but not a constant in time, because it can change over the history of the universe.

Hubble Law: A linear relationship between the distance to a galaxy (R) and the velocity with which that galaxy is receeding from us (v) due to the overall expansion of the universe. The relation is v = Ho R where Ho is the constant of proportionality known as Hubble's constant. The present "best" value of the Hubble constant is about 70 kilometers per second per Megaparsec.

Hubble Time: The inverse of the Hubble constant. The Hubble time, also called the Hubble age or the Hubble period, provides an estimate for the age of the universe by presuming that the universe has always expanded at the same rate as it is expanding today.

Hydrostatic Equilibrium: This refers to the balancing of forces in a fluid (fluid=hydro, static=stationary, equilibrium=balance). Stars exhibit hydrostatic equilibrium because while they have enormous self-gravitational forces pulling them together, there is a substantial pressure force pushing up, preventing the star's collapse. If hydrostatic equilibrium is lost the star will expand or contract depending upon which force is larger.

Initial Mass Function (IMF): The distribution of masses created by the process of star formation. This function, developed from observation of many stars, tells you how many stars of a given mass there should be in a population of stars. Basically there are few massive stars and many low mass stars.

Interference, Wave: A wave is something that moves along and has high points (crests) and low points (troughs). If two (or more) different wave trains pass over one another the crests and troughs can add together to make bigger crests and troughs, and a crest and a trough can add together to produce zero. So if light is a wave phenomenon, then two light sources produce waves that in some places produce large amplitudes and other places produce zero. When to point sources of light are projected onto a screen this wave interference effect produces alternating light and dark spots. This demonstrates the wavelike nature of light.

Interstellar Extinction: As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, causing the total intensity of the light to diminish. The more dust, the dimmer the star will appear. It is important to take this effect into account when measuring the apparent magnitude of stars. The dark bands running across portions of the milky way in the sky are due to extinction by copious amounts of dust in the plane of our galaxy.

Interstellar Reddening: As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, preferentially the short wavelength (blue) components. The spectrum of the light that remains is increasingly dominated by the long wavelength (red) end of the spectrum, hence the light is "reddened" as it travels through space. It is important to take this effect into account when measuring the color indices of stars.

Interstellar Medium: The name given to the stuff that floats in space between the stars. It consists of gas (mostly hydrogen) and dust. Even at its densest the interstellar medium is emptier than the best vacuum humanity can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.

Irregular Galaxy: A galaxy type from the Hubble classification scheme. These galaxies tend to smaller than others, containing 100 million to 10 billion stars, with an irregular overall shape.

Irregular Galaxy Cluster: Clusters of galaxies that are not too centrally condensed, with a somewhat nonspherical overall shape, containing a few galaxies up to hundreds of galaxies. Our local group is an example of an irregular cluster of galaxies. Clusters of galaxies contain all types of galaxies; despite the name, this type of cluster contains more than "irregular galaxies."

Jets, Radio: Narrow, collimated beams of plasma that are producing radio (synchrotron) emission. These jets emerge from the cores of radio galaxies and can extend outward across regions of space larger than the size of the galaxy itself. These jets are believed to be powered and launched from an accretion disk orbiting a supermassive black hole at the galaxy's core.

Kelvin scale: This is the temperature scale which uses the same size of degree as the Celsius or Centigrade system, but which begins at absolute zero, the coldest temperature possible corresponding to the lowest possible energy state of a system. Temperature in degrees Kelvin gives a measure of the average energy of a system.

Kinetic Energy: The energy associated with macroscopic motion. In nonrelativistic physics the kinetic energy is equal to one half the mass times the velocity squared, i.e., 1/2 mv2.

Kirchhoff's Laws: These are a set of rules for remembering when and why one should observe a continuous spectrum of light, an emission spectrum, and/or an absorption spectrum. Solid bodies or dense gases or liquids give off continuous spectra. Thin cool gas will absorb certain wavelengths of a continuous spectrum of light producing an absorption spectrum. Warm gas emits certain wavelengths producing an emission spectrum. The specific wavelengths emitted or absorbed is uniquely determined by the chemical makeup of the gas.

Lepton: A member of a class of particles which do not participate in the strong interaction (the force that binds atomic nuclei togeter). The best-known lepton is the electron. Another example is the neutrino.

Light Curve: A plot of the amount of light detected from an object (i.e. the apparent magnitude) as a function of time. Light curves provide evidence of eclipsing binaries, variable stars, and track the progress of nova and supernova explosions.

Lookback Time: The time required for light to travel from an emitting object to the receiver. Hence when we look at a distant object we are "looking back" in time.

Luminosity: Total amount of energy radiated per second. It has units of energy per second (e.g. ergs per second). Since many astronomical objects radiate away energy this is an important characteristic. We compare luminosity of an object to the solar luminosity, the total energy given off per second by the sun. One solar luminosity is 4 × 1033 ergs per second. Luminosity has the same units as Power, e.g. energy per second. The Watt is the familiar unit of power. For comparison, a 400 Watt light bulb is 10-24 solar luminosities.

Luminosity Class: Stars are classified by luminous they are. The various luminosity classes correspond to regions on the HR diagram. The luminosity classes are I - Supergiants, II - Bright giant, III - Giant, IV - Subgiant, V - Main Sequence. The luminosity class was originally based on the width of the observed spectral lines within a given spectral type. Width provides a measure of how fast the atoms are moving (doppler shift) and this in turn provides a measure of the strength of the gravitational field of the star, which then determines whether it is a Giant, Supergiant, etc. The main thing is that there are fundamentally different types of stars and the luminosity class helps us to categorize these different types.

Luminosity Function: The relative number of astronomical objects that have a certain luminosity. In the case of stars, the low luminosity stars are the most abundant, and the number of stars declines rapidly with increasing luminosity.

MACHO: Massive Compact Halo Object. Any object such as a white dwarf, neutron star, or black hole that could account for some or all of the dark matter in the halos of galaxies.

Magnitude: An astronomical unit of brightness. Originally corresponding to the eye's response to starlight, the magnitude system is logarithmic, with 5 magnitudes corresponding to a factor of 100 in brightness. To further confuse things larger magnitudes correspond to fainter objects.

Main Sequence: The name given for the line in the H-R diagram along which stars lie that are burning hydrogen in their cores. The majority of a star's lifetime is spent as a main sequence star. Main sequence stars are luminosity class V.

Main Sequence Turnoff: When stars age, they run out of hydrogen in their cores. When that happens they begin to change and they move off the main sequence toward the red giant branch. When a cluster of stars forms, all the stars will be on the main sequence. The more massive stars evolve faster and leave the main sequence. The point on the main sequence where stars have just left (defining the end of the main sequence for that cluster) is the main sequence turnoff point.

Mass: The measure of how much "stuff" something has, mass determines the inertia of an object (its resistance to being accelerated by a force) and how much gravitational force it exerts on another object. In pre-Einsteinian physics mass was conserved, neither created nor destroyed. Einstein discovered that mass can be converted into energy and vice versa. The conservation of mass is still a good approximation since mass-energy conversions generally involve relatively small amounts of mass. The mass of astronomical objects is often measure in terms of the Sun's mass. The solar mass is 2 × 1033 grams.

Mass Luminosity Relation: A main sequence star's luminosity is roughly proportional to its mass to the 3.5 power: L ~ M3.5. This relationship was derived from the observations of the masses of various types of main sequence stars, but it has also been demonstrated by the calculation of stellar models of different massed zero age main sequence stars.

Metals: Astronomers refer to all elements other than hydrogen and helium as "metals" (even though these elements aren't all metals as defined by chemists).

Nucleus, Galactic: The central region of a galaxy characterized by high densities of stars. The nucleus may also contain a supermassive black hole and may be the source of considerable high-energy, nonstellar luminosity.

Neutrino: Any of three species of very weakly-interacting lepton with an extremely small, possibly zero, mass. Electron neutrinos are generated in the interior of the Sun (and other stars). Generally such neutrinos do not interact with matter and stream out through the Sun. A very few of these many neutrinos can be detected in sophisticated detectors here on Earth, giving us a "window" into the interior of the Sun. In 1987 neutrinos from a Supernova in the Large Magellanic Cloud were detected in terrestrial neutrino experiments.

Neutron: A charge-neutral particle (of the hadron type) which is one of the two particles that make up the nuclei of atoms. Neutrons are unstable outside the nucleus, but stable within it. The number of protons in the nucleus determines what element that nucleus is. Different isotopes of a given element have different numbers of neutrons in the nucleus. The total number of neutrons and protons affects properties such as radioactivity or stability, the types of nuclear reactions, if any, in which the isotope will participate, and so forth.

Neutron Degeneracy Pressure: Quantum mechanics restricts the number of neutrons that can have low energy. Each neutron must occupy its own energy state. When neutrons are packed together, as they are in a neutron star, the number of available low energy states is too small and many neutrons are forced into high energy states. These high energy neutrons make up the entire pressure supporting the neutron star. Because the pressure arises from this quantum mechanical effect, it is insensitive to temperature, i.e., the pressure doesn't go down as the star cools. Similar to electron degeneracy pressure but, because the neutron is much more massive than the electron, neutron degeneracy pressure is much larger and can support stars more massive than the Chandrasekhar mass limit.

Neutron Star: A dead ``star'' supported by neutron degeneracy pressure. A neutron star is the core remnant left over after a supernova explosion.

Nova: A star that experiences an abrupt increase in brightness by a factor of a million (in contrast to the much brighter supernova). A nova is produced in a semidetached binary system where hydrogen-rich matter is being transferred onto a white dwarf. As more and more hydrogen builds up on the surface the temperature rises. The material is degenerate so when the temperature becomes high enough for nuclear burning to take place it does so explosively producing a nova.

Nucleosynthesis: The process by which nuclear reactions produce the various elements of the periodic table.

Omega: The ratio of the actual density of the universe to the critical density. A value greater than one indicates that the universe is denser than the critical value and this corresponds to a closed universe. A value less than one is an open universe.

Opacity: The property of a substance that determines how hard it is for radiation to get through that substance (hence how "opaque" that substance is). The atmosphere has low opacity to light. Fog has a much higher opacity and light cannot stream through a fog very far before being scattered. The opacity of a substance determines how well radiation can transport heat by radiative transport.

Open Cluster: An open cluster is a somewhat loose, irregular grouping of several hundred stars, around 10 pc across and generally found in the disk of the Galaxy. Open clusters consist of Population I stars and represent stars formed relatively recently at about the same time. Examples include the Pleiades and the Hyades.

Open Universe: A model of the universe which expands forever and is infinite in space and time, although it begins with a Big Bang. The total mass density of the universe is too small to cause recollapse.

Parallax: Generally speaking, parallax is the apparent shift in the direction to an object as seen from two different locations. This shift can be used to determine distances (through "triangulation"). Stellar parallax occurs as the Earth orbits the Sun and our line of sight to a nearby star varies. The effect is to make the star appear to shift position over the course of the year. In reality, stellar distances are so great that parallax shifts are less than an arc second, completely unobservable to the unaided eye.

Parsec: A unit of distance used to describe the vast scales of the cosmos, the parsec is equal to about 3.262 lightyears, or 3.09 × 1016 meters. A star that is one parsec away would produce a parallax angle of one second of arc. A star that has a parallax shift of 0.1 arcseconds would be at a distance of 10 parsecs, and so forth.

Peculiar Velocity: Any velocity a galaxy has with respect to us that is not a Hubble law velocity due to the expansion of space. Peculiar velocities are due to the gravitaional influences of nearby galaxies, for example, if a galaxy is orbiting in a cluster of galaxies. Peculiar velocites add or subtract an additional component to the observed redshift, confusing somewhat the determination of the Hubble constant.

Period-Luminosity Relation: A relationship between the pulsation period of a variable star (e.g. a Cepheid) and its luminosity (or absolute magnitude). Generally the more luminous the star the longer the pulsation period. The relationship permits distances to be measured. One determineds the pulsation period and uses the relationship to get the absolute magnitude. The apparent magnitude of the star then gives you the distance modulus.

Photodisintegration: The process by which atomic nuclei are broken apart into their constituent protons and neutrons by the impact of high energy gamma rays (photons). Photodisintegration takes place during the core collapse phase of a Type II supernova explosion.

Photoelectric Effect: There is a phenomenon called the photoelectric effect wherein light incident upon certain metals can cause currents to flow (this is the basis of photocells). What happens is that the light causes electrons to be knocked loose from the surface. However certain experimental data was difficult to explain using the standard wave picture of light:

  1. The light had to be above a certain threshold frequency or no current would flow, regardless of the intensity.
  2. If the light was above the threshold frequency the current would flow no matter how low the intensity of the light.
Thus whether or not electrons were knocked out depended on frequency not on intensity. OK, you might say, maybe you need a certain amount of energy to knock loose an electron, but why couldn't you get that energy with lots of low energy (low frequency) light?

Here is an analogy. Suppose we have a soda machine that only accepts dollar bills. Do you have enough money for a soda? Suppose you have a penny. Not enough. A friend starts giving you more pennies. But no matter how many pennies they give you, you will never get a soda because you need a whole dollar in one bill to get one soda to be ejected from the machine. (In this analogy the soda can is the electron, and the coins represent photons of discrete values, i.e., energies.)

In the photoelectric effect the electron needs the threshold energy in the form of one photon to be ejected. The electron doesn't store up lower energy photons until the threshold is reached. It needs that energy all in one go. Thus, Einstein reasoned, the properties of the photoelectric effect are consistent with light coming in discrete packets, called photons. The subsequent history of twentieth century physics amply confirms this picture.

Photometry: The measurement of light. Specifically refers to the procedure of highly accurate measuring of the apparent magnitudes of astronomical objects. In general, astronomers measure only a portion of the wavelength spectrum when they do photometry. Different types of photometry are defined by the portion of the wavelength that they examine. For example "UBV Photometry" measures the light within three standard regions defined by filters. These are Ultraviolet, Blue and Visual (hence UBV). There are many different photometry systems and standards.

Photon: Experiments have shown that light of a given energy (frequency) is not something that can be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy hf where h is Planck's constant and f is the frequency. These discrete bundles of light are known as photons. It is often useful to think of light as a bunch of particle photons. Other times it is useful to think of light as a wave. Astronomers do both as needed.

Photosphere: The surface layer of the sun where the continuous blackbody-type spectrum is produced that we directly observe when we look at the Sun. The Sun doesn't have a "surface" like we usually think of one, since it is a ball of gas, but the photosphere looks like a surface because it is the point where light from the hot gas of the Sun escapes into space without further scattering. It is as far into the Sun as we can directly see.

Planetary Nebula: At the end of the life of a lower mass star it is on the asymptotic giant branch as a huge red giant. The core is composed of carbon and oxygen; helium burning has ceased. Helium Shell Flashes take place in the region around the core and their energy causes the outer layers to be ejected. The expanding ejected nebula composed of the dying star's outer envelope is called a planetary nebula.

Population I Stars: Relatively young stars, containing a larger fraction of metals, found mainly in the disk of the Galaxy.

Population II Stars: Relatively old stars, containing a smaller fraction of metals, found mainly in the halo of the Galaxy and in Globular Clusters.

Proper Motion: The motion that an object has in the plane of the sky. The direction is in the plane perpendicular to the radial line (see radial velocity). Because the stars are so very far away, their proper motions on the sky are small. One needs to observe for a long time (years) to see proper motions in even relatively nearby stars.

Proton: A particle of the hadron family which is one of the two particles that make up atomic nuclei. The proton has a postive electrical charge.

Proton-proton chain: The series of nuclear fusion steps by which the sun converts four hydrogen nuclei into one helium nucleus and thereby generates energy in its core.

Protostar: A forming star, prior to settling down to the main sequence and burning hydrogen in its core.

Pulsar: A rotating magnetized neutron star that produces regular pulses of radiation when observed from a distance. A pulse is produced every time the rotation brings the magnetic pole region of the neutron star into view. In this way the pulsar acts much as a light house does, sweeping a beam of radiation through space.

Pulsar Glitch: A sudden change in the pulsar period due to a sudden shift in the crust of the neutron star (a "starquake").

Pulsar, Millisecond: As the name implies, these are pulsars with periods measured in terms of milliseconds (thousandths of a second). The shortest have a period of about one and two milliseconds. Millisecond pulsar periods are very constant. They don't slow down much implying a weak magnetic field. Most millisecond pulsars are found in binary systems. It is believed that millisecond pulsars are old pulsars that have had their spin rates increased through the accretion of angular momentum-containing matter from the other star in the binary. Mass transfer has spun up these pulsars back to very fast spin rates. The idea that the millisecond pulsars are actually old systems gains support from the presence of millisecond pulsars in globular clusters.

Quasar: From quasi-stellar object, a star-like (i.e. unresolved) object that has a very large luminosity and is located at very large distances from us (as indicated by their high cosmological redshifts). Although technically the term quasar refers to objects that are highly luminous in the radio band, the term tends to be used for both radio-loud and radio-quiet objects high-redshift unresolved objects. Quasars are believed to be powered by supermassive black holes in the centers of galaxies in the process of formation early in the history of the universe.

Radial Velocity: The velocity that an object possesses that is directly toward or away from the observer (hence on a radial line toward or away from the observer). The radial velocity can be determined using the doppler shift.

Radian: A unit of angle equal to about 57 degrees. The length along the arc of a circle covering by one radian is equal to the radius of the circle. The complete angle around the circle (360 degrees) is equal to 2 pi radians. The radian is particularly useful because if you know the distance to some object and you measure its apparent size as the angle it subtends in your field of view in radians, then the actual size is just that number of radians times the distance to the object. For example, a meter stick held up at a distance of 100 meters makes an apparent angle in your field of view of 0.01 radians.

Radiative Transport: The direct transport of energy via light (electromagnetic radiation). How fast radiation can carry heat through a star is determined by the opacity of the star. When you feel heat coming off a fire you are getting that heat through direct (infrared) radiation from the fire--an example of radiative transport.

Radio Galaxy: A galaxy that is emitting most of its energy in the form of radio waves rather than light in or near the visible bands where stars emit most of their radiation. This means that radio galaxies are dominated by some non-stellar process.

Radius: The radius of a star or planet is the distance from the center of the star or planet out to its surface. Radius is equal to half the diameter. Star sizes are often compared to the solar radius which is 7 × 1010 cm.

Red Giant: A star with low surface temperature (thus red) and large size (giant). These stars are found on the upper-right hand side of the HR diagram. The red giant phase in a star's life occurs after it has left the main sequence. The Sun will become a red giant in about 5 billion years.

Redshift, Cosmological: A redshift caused by the expansion of space. The wavelength of light increases as it traverses the expanding universe between its point of emission and its point of detection by the same amount that space has expanded during the crossing time.

Reflection Nebula: A nebula composed of dust particles that scatter and reflect incident light (rather than glowing from their own intrinsic emission). Dust preferentially scatters short wavelengths, so reflection nebulae have a characteristic blue appearance.

Regular Galaxy Cluster: These are great groupings of galaxies into huge spherical distributions that have large numbers of galaxies concentrated in their centers. The tend to contain thousands of galaxies and to have many bright elliptical and S0 type galaxies.

Relativity, General: Einstein's theory of relativity incorporating the force of gravity into the special theory of relativity. This theory incorporates gravity into the nature of space and time. It predicts, among other things, the existence of gravitational radiation and black holes.

Relativity, Special: The specific set of rules relating observations in one inertial frame of reference to the observations of the same phenomenon in another inertial frame of reference. Einstein's theory of special relativity ensures that the laws of physics for mechanicsl and for electromagnetism are the same for all such observers. It postulates that the speed of light is the same for all observers. One of its more famous consequences is the equivalence of matter and energy through the equation E = mc 2.

Roche Lobe: The region surrounding a star in a binary system inside of which the star's material is gravitationally bound to the star. If a star exceeds its Roche lobe it can become a semidetached binary. Two binary stars that share a Roche lobe constitute a contact binary.

Rotation Curve: A plot of the orbital velocity in the disk of a galaxy versus the radius from the center of the galaxy. This curve can then be used to obtain the mass within a given radius (by using Kepler's laws for orbital dynamics). Typically rotation curves suggest that galaxies have considerably more matter than that associated with the visible stars (see dark matter).

R-Process: The huge numbers of neutrons given off during a supernova explosion allow for the rapid (hence "r") absorption of neutrons by various elements which transforms them into elements higher up in the periodic table. This is an important step in the process of nucleosynthesis.

RR Lyrae Variables: A variable star that has a regularly varying luminosity. These stars all have about the same luminosity making them suitable for obtaining distances. They are not as useful as Cepheid variables, however, because they are not as luminous.

Schwarzschild Radius: The radius of the event horizon surrounding a nonrotating black hole. Its size is given by Rs = 2GM / c2. For a one solar mass star this is about 3 kilometers.

Semidetached Binary: A binary system where one star is too large and some of its outer layer transfers over to and falls onto its binary companion star. Semidetached binary stars can form accretion disks.

Seyfert Galaxy: Seyferts are spirals galaxies that have bright starlike cores. Seyferts have strong emission lines, and the emission lines are very broad, implying velocities from 500 to 4000 km/sec. Seyferts are classified into two types based on the width of their emission lines. Seyferts with very broad Hydrogen emission lines are called Type I, and Seyferts with more narrow Hydrogen emission lines are called Type II. Many Seyferts also have compact radio sources at their centers.

Shell Burning: In later stages of a stars life regions of the envelope become hot enough to begin nuclear burning. These burning regions lie in a shell surrounding the core. For example, helium burning might take place in the core (where the hydrogen has been exhausted) with a shell of hydrogen burning surrounding it. There can be more than one region of shell burning, each shell with its own nuclear reactions.

Singularity: In classical general relativity, a location at which physical quantities such as density become infinite. A singularity lies at the center of a black hole.

Spectral Type: A system of classification for stars based on the presence and strength of various types of emission lines in their spectrum. Basically the spectral type is a measure of the surface temperature of the star since the temperature determines which emission lines will be present and how strong they will be. From hottest to coolest stars are grouped into categories O, B, A, F, G, K, and M. Each letter is subdivided into 10 numbers, from hotter to cooler 0, 1, 2, 3, 4, etc.

Spectrum, Electromagnetic: The distribution of light separated in order of some varying characteristic such as wavelength or frequency. The "electromagnetic spectrum" refers to the full range of possible frequencies and wavelengths of light. If we "take a spectrum" of a star we analyze its light according to wavelength or frequency by, say, passing the light through a prism. A "spectral line" refers to emission or absorption at a particular wavelength of light.

Spectrum, Nonthermal: A continuous spectrum that is not being produced by ordinary thermal processes associated with dense, hot matter (e.g. blackbody radiation). Synchrotron radiation has a nonthermal spectrum.

Spectroscopic Binary: Binary stars whose binary nature is determined by observations of their spectra. Periodic doppler shifts in emission or absorption lines reveal that the stars are moving in orbits even when the separate stars cannot be resolved in direct observations.

Spectroscopic Parallax: Something of a misnomer, this process doesn't really involve parallax at all, but it is a way to get distances. One uses the observed spectrum of the star to obtain the spectral type and luminosity class. These allow you to determine the type of star that it is and to locate it in the HR diagram. From this you obtain the absolute magnitude. The absolute magnitude and the apparent magnitude constitute the distance modulus, from which the distance is obtained.

Spectroscopy: Spectroscopy is the study of the detailed features of a star's spectrum, done by measuring the intensity of the star's light at as many different wavelengths as possible. The resulting spectrum of light allows one to locate the emission and absorption lines, determine the composition of the star, its doppler shift, its spectral type, and its luminosity class.

Spiral Galaxy: A galaxy consisting of a flattened rotating disk of stars, a central bulge and a surrounding halo. The disk is prominent due to the presence of young, hot stars which are often arrayed in spiral patterns. The characteristic appearance of these bright spirals gives the galaxy type its name.

S-Process: The absorption of neutrons by elements in massive stars, causing them to transform to other isotopes, and, through subsequent nuclear decay, into other elements. The flux of neutrons is small enough that the process happens slowly (hence "s" process). An important part of the nucleosynthesis of the elements.

Standard Candle: Any astronomical object of known luminosity that can thus be used to obtain a distance. Cepheid variables, Main sequence stars, and type I supernovae have all be used as standard candles.

Stefan-Boltzmann Law: This is a law of blackbody radiation that states that the amount of energy given off by a blackbody per second per unit area (see flux) is proportional to the fourth power of the temperature of the blackbody. In practical terms this means that hotter objects give off a lot more energy than cooler objects (by the fourth power of the ratio of their temperatures to be exact).

Supermassive Black Hole: A black hole that has a million or as much as a billion solar masses. Such huge black holes lurk at the centers of many active galaxies.

Supernova: The explosion of a star. Supernovae come in two types: Type I is caused by sudden nuclear burning in a white dwarf star. Type II is caused by the collapse of the core of a supermassive star at the end of its nuclear-burning life. In either case, the star is destroyed and the light given off in its explosion briefly rivals the total light given off by a whole galaxy.

Supernova Remnant: The material blown off during a supernova, now seen as a great glowing cloud expanding into space.

Synchrotron Radiation: Synchrotron radiation is the name for the type of radiation emitted by electrons moving close to the speed of light in the presence of magnetic fields. Two things are required: high speed electrons and magnetic fields. The magnetic force causes the electron (which has a negative electrical charge) to follow a spiral course around the magnetic field. This is an acceleration, and accelerated charges produce electromagnetic radiation. This radiation is produced in pulsars and in active galaxies, and is observed mainly in the radio wavelengths.

Thermal Equilibrium: (1) The idea that any energy radiated away from an object (e.g. a star) is replaced by energy generation so that the temperatures remain constant. (2) A state in which energy is equally distributed among all particles, and all the statistical properties of the particles can be described by a single parameter, the temperature.

Thermal Pulse: A sudden increase in temperature caused by a dramatic increase in a nuclear burning rate. Although it falls short of what we would call an explosion, it does drive a dynamic readjustment in the star.

Triple Alpha Reaction: The process by which helium (also known as an alpha particle) is converted into carbon. When temperatures are high and the density of helium is large, three helium atoms can combine to form one Carbon atom. Hence the name: 3 helium reaction.

Tully-Fisher Relation: An empirical relationship between the width of the 21-cm line of hydrogen emissions from spiral galaxies, and the mass of the galaxy. The relationship arises because a larger mass increases the rotation rate, and a faster rotation causes a broader line; the precise calibration must be determined observationally.

21 Centimeter Emission: A radio wavelength emission that originates with a neutral hydrogen atom, i.e., a single proton, or hydrogen nucleus, with its accompanying electron. The proton and the electron each have a quantum "spin", which points either "up" or "down." They can both be "up" or "down" (Parallel spins), or one can be "up" and the other "down" (antiparallel spins). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a 21 cm radio photon is emitted. Thus, cold neutral hydrogen in space emits this radiation which can be detected using a radio telescope.

Visual Binary: A binary star system that can be resolved into separate stars in direct observations. In binary systems that are not visual, the two stars are so close that they can't be distinguished (resolved) in observations.

Wavelength: A wave is an oscillation in space, with peaks and troughs. The distance from one peak to the next is the wavelength. It is measured in units of distance. The wavelength of ocean water waves will be meters. The wavelengths of visible light corresponds to hundreds of nanometers (billionths of a meter). Wavelength is an important way to characterize a wave. For light, the shorter the wavelength, the higher the energy of the light wave.

White Dwarf: The remnant of a star, at the end of its life, consisting of a carbon and oxygen core supported by electron degeneracy pressure. The surface has a very high temperature and radiates mainly in the ultraviolet (hence white as in white hot), but it is only about the size of the Earth (hence dwarf). The maximum mass that can be supported by electron degeneracy pressure, and hence the maximum possible mass of a white dwarf, is known as the Chandrasekhar mass and is equal to 1.4 solar masses.

Wien's Law: This is the law of light that says for blackbody emission, the higher the temperature of the blackbody emitter, the higher the frequency (or shorter the wavelength) of the predominant light it emits. The specific relation is (Peak Wavelength) = 0.29/T where the wavelength is given in centimeters and T in degrees Kelvin.

WIMP: An acronym for a Weakly-Interacting Massive Particle. A particle with a nonzero mass which participates only in the weak nuclear interaction. Such particles (currently hypothetical) could fill space and provide gravitational force without any associated luminosity. As such they are a candidate for dark matter.

X-ray Burster: A semidetached binary system where matter is accreting onto a neutron star. As hydrogen accretes onto a neutron star (possibly producing a variable X-ray source) the hydrogen is promptly burned into helium. The helium accumulates until the temperature is high enough for a degenerate helium burning explosion.

Zero Age Main Sequence (ZAMS): The theoretical line of the main sequence which corresponds to the onset of hydrogen burning and the beginning (zero age) of a star's life.

Copyright © 1999 John F. Hawley. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 124 at the University of Virginia. Reproduction, distribution, and commercial uses are prohibited.