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:
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.
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?