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This is a glossary of astronomical terms used in the lectures and on the website.
It's not intended to be a general dictionary – if you are not a native
English speaker, you will need to use a standard dictionary for non-astronomical
words you haven't seen before.
If there are any words you feel are missing from this glossary, please let me
know.
- absolute magnitude
-
The magnitude that an object would have if it were at a standard
reference distance of 10 parsecs. This is a measure of the
luminosity of the object.
- absorption line spectrum
-
Spectrum consisting of dark lines at discrete wavelengths, superimposed on a
background continuum spectrum. This is caused by cooler,
low-density gas lying between the continuum source and the observer. Most stars
have this type of spectrum, because there is a cooler atmosphere overlying the
star's visible surface or photosphere. The pattern of lines can be used to
determine the surface temperature and chemical composition of the star (note that
for stars, temperature has a much bigger effect than composition), and is
the basis of spectral classification.
- active galaxy, active galactic nucleus (AGN)
-
A galaxy whose total power output is not
dominated by the light from
its constituent stars. We believe that AGN are powered by accretion of
matter on to a central supermassive black hole. They are
more common at high redshift (i.e. when the universe was younger than it is now),
which is evidence against the Steady State theory of
cosmology. Types of AGN include quasars and radio galaxies.
- alpha particle, α
-
Bound state of two protons and two neutrons, nucleus of an atom of helium-4.
Produced in some radioactive decays, e.g. of uranium, and also very copiously in
big-bang nucleosynthesis.
- alpha-process element
-
Element whose most common isotope can be made up of a combination of alpha particles,
i.e. it has 2N protons and 2N neutrons, where N is some number. Examples include
carbon-12, oxygen-16, neon-20, magnesium-24. Alpha-process elements are made
mostly in core-collapse supernovae, and to a lesser extent in
helium-fusing stars.
- apparent magnitude
-
The magnitude that an object has as observed from Earth. This
depends both on the luminosity of the object and on its
distance from us.
- arcminute (')
-
1/60 of a degree. Occasionally used in astronomy, though arcseconds are much
more common, because angles in astronomy tend to be very small.
- arcsecond (")
-
1/3600 of a degree, or 1/206265 of a radian. The standard measure of (small!)
angles in astronomy.
- asymptotic giant branch (AGB)
-
Branch of the Hertzsprung-Russell diagram running from the
horizontal branch up and right towards the
red giant branch. Contains stars with an inert carbon core,
generating energy by fusing helium (or, later in their lives, hydrogen) in a
spherical shell around the core. This is the last stage in the life of
a Sun-like star, and ends when the star loses its
outer envelope as a planetary nebula
and becomes a white dwarf.
- B – V
-
A common colour index: blue magnitude minus visual
magnitude. The Sun has B – V = 0.65: smaller values indicate white or
blue stars, with high surface temperatures, while larger values belong to orange
or red stars with lower surface temperatures.
- beta decay
-
Radioactive decay of an isotope by conversion of a neutron to a proton, with
emission of an electron (and an antineutrino). Beta decay occurs when an
isotope has too many neutrons (relative to protons) to be stable.
(Strictly, this is
β– decay – the less common β+ decay
converts a proton to a neutron, with emission of a positron (and a neutrino).
Most isotopes which would be unstable to β+ decay instead
decay by electron capture, which requires less energy.)
- Big Bang model
-
Cosmological theory in which the universe expands from an initial superhot,
superdense state. The expansion is of space itself, not galaxies rushing apart
into pre-existing space. The mathematics of the model was worked out by
Friedman and (independently) Lemaitre in the 1920s, but in its present form
it has developed from work done by George Gamow and his group in the late 1940s.
This is the currently accepted model of cosmology, as it is supported by evidence
including big-bang nucleosythesis, the change in the appearance of the universe
with redshift, and especially the blackbody spectrum of the cosmic microwave
background.
- big-bang nucleosynthesis
-
The production of the light isotopes H-2 (deuterium), He-3, He-4
and Li-7 in the early universe, a few minutes after the Big Bang. This occurs
during a critical period when the universe is hot enough to provide the necessary
energy to make nuclei collide, but not so hot that the deuterium – the first
step in the chain – is broken down by energetic photons. Calculations of
the predicted abundance of deuterium and helium-4 can be compared with
observations to determine the density of ordinary matter in the universe. The fact
that the result agrees with the density derived from the cosmic
microwave background, and from direct observations, is strong evidence in
favour of the Big Bang cosmological model.
This process does not make elements heavier than Li-7 because there is no stable
isotope with mass 5 (so adding a proton or a neutron to He-4 doesn't work) or mass 8
(so trying to stick to He-4 nuclei together doesn't work either). Heavier elements
have to be made in stellar interiors, where the density is much higher and so
the triple-alpha process, which relies on having two collisions
occur almost simultaneously, will operate.
- binary star
-
A system consisting of two stars in orbit around each other (strictly, around their
common centre of mass). About half of all stars are in binary or multiple systems.
Binary stars are extremely important in astronomy because they provide the only
way of measuring the masses of stars. See
Vik Dhillon's seminar for more information.
- binding energy
-
The energy which has to be added to a bound state in order to unbind it (or,
conversely, the energy which was released when it was created from its components).
For example, the binding energy of the electron in a hydrogen atom is 13.6 eV
(the ionisation energy of hydrogen – i.e., the energy that has to be
given to the bound electron in order to unbind it). In nuclear physics, as
a result of E = mc2, the binding energy is observed
as a difference in mass – the bound state has less mass than the sum of its
components. For example, the mass of the proton is 1.0073 atomic mass units (u),
the neutron has mass 1.0087 u, but the bound state of a neutron and a proton
(a deuteron, the nucleus of deuterium) has
mass 2.0136 u. The difference, (1.0087+1.0073) – 2.0136 = 0.0023 u,
corresponds to the binding energy of the deuteron, 2.2 MeV. In determining the
relative stability of nuclei, the important variable is the binding
energy per nucleon, i.e. the total binding energy divided by the
total number of protons and neutrons, which is 1.1 MeV for the deuteron and
8.8 MeV for iron-56 (the most tightly bound nucleus).
- blackbody, blackbody radiation, blackbody spectrum
-
A blackbody is a theoretical object which is perfectly efficient at emitting
and absorbing radiation of all wavelengths (it is so called because at low
temperatures such an object would appear black). When heated to above the
temperature of its surroundings, a blackbody emits light (electromagnetic
radiation, to be precise – not necessarily visible light) with a
continuous spectrum described by the
Planck function.
This depends only on the temperature of the body: hotter objects are brighter
(total power radiated ∝ T4) and bluer (wavelength of peak
emission ∝ 1/T) than cooler objects.
Hot dense objects, such as stars and planets, behave approximately as blackbodies,
so stars which appear blue have higher surface temperatures than those which
appear red.
The spectrum of light emitted by a blackbody is called a blackbody spectrum, or
blackbody radiation. Because it depends on temperature, it is also sometimes
called thermal radiation, or a thermal spectrum.
- black hole
-
An object whose gravity is strong enough that even light cannot escape.
These are believed to form when very massive stars collapse at the end of their
lives. There is evidence for stellar-mass black holes, in the form of
binary systems in which one component is invisible, despite
having a mass appropriate to a very bright star, and which are associated with
the emission of X-rays (caused by gas heating up to very high temperatures as
it falls into the black hole). Supermassive black holes
with masses of millions or even billions of solar masses are found in the centres
of galaxies.
- bolometric
-
Applied to a flux,
luminosity or magnitude, means
"integrated over the full range of wavelengths (or frequencies) from 0 to infinity".
- brown dwarf
-
An astronomical body too massive to be a planet but not massive enough to be
a star. In principle, a brown dwarf forms like a star, from a collapsing gas cloud,
but does not achieve the minimum mass needed to attain a central temperature high
enough for hydrogen fusion (about 10000000 K). As the formation mechanism is
not obvious in retrospect, the IAU Working Group on Extrasolar Planets
recommended
a definition such that brown dwarfs are objects which achieve the required
temperature for deuterium fusion but not for hydrogen fusion –
roughly, between 13 and 80 Jupiter masses (0.013 to 0.080 solar masses).
- Chandrasekhar limit
-
The limiting mass (about 1.4 solar masses) above which an object supported by
electron
degeneracy pressure will collapse under its own gravity. Important in the
evolution of stars, especially the collapses of compact objects (the iron core of
an evolved massive star, or a white dwarf) which create
supernovae. Named after
Subrahmanyan
Chandrasekhar, who first worked it out.
- CNO cycle
-
Process by which hydrogen is fused to helium in stars more massive than the Sun.
This process uses carbon-12 as a catalyst. Protons are added to the initial
C-12 nucleus successively, with two intervening β+
decays (of nitrogen-13 and oxygen-15) which convert protons to neutrons.
Eventually, adding a proton to nitrogen-15 produces a highly excited nucleus which
spits out an α-particle, hence converting the four
added protons to a helium nucleus and getting back the original carbon-12.
The net reaction is exactly the same as in the pp chain, and
so is the amount of energy generated, but the CNO cycle is much faster than
the pp chain at high temperatures. More massive stars have higher central
temperatures (to balance their greater gravitational force), explaining why
stars more massive than the Sun mostly use CNO, while stars of the Sun's mass
and lower run pp.
Note that, as carbon is itself made in stars, the very first stars ever formed
(so-called Population III) would have contained no carbon at
all, and could not have run CNO even though we believe they would have been
very massive. However, the amount of carbon needed is tiny, and all stars
observed today – even the lowest-metallicity globular cluster stars
– can run CNO if they are massive enough. Red giant stars
have hotter interiors (though cooler surfaces) than main-sequence stars, so
even solar-mass red giants run CNO.
- colour index
-
The difference between the magnitudes
of an object measured using two different
filters – e.g. B – V, blue magnitude minus
visual (yellow-green) magnitude. The longer-wavelength magnitude is always
subtracted from the shorter, so – because magnitudes go backwards,
a smaller colour index indicates an object with bluer colour. Colour index
is a measure of effective temperature, and nowadays is
normally used as the x-axis of the Hertzsprung-Russell diagram.
- colour-magnitude diagram
- Another name for the Hertzsprung-Russell diagram in the
case where the x-axis is colour index and the y-axis is absolute (or, for clusters,
apparent) magnitude.
- continuous spectrum
-
A continuous spectrum or continuum is one in which the light
is spread over a broad range of wavelengths rather than being concentrated at a ]
few specific wavelengths. The most common form of continuous spectrum is
the thermal or (approximately)
blackbody spectrum emitted by hot dense objects, e.g. stars,
but there are other forms of emission that produce continuous spectra (e.g.
bremsstrahlung and synchrotron emission, which are important
in the study of active galaxies).
- cosmic microwave background (CMB)
-
Blackbody radiation at a temperature of 2.7 K, observed to
emanate from the entire sky with extreme uniformity (one half of the sky is
slightly hotter than the other half, because we are moving and the radiation has
a Doppler shift, but apart from that it's uniform to a few
parts in 100000). In the Big Bang model, the radiation
was produced in the early universe when it was hot, dense and opaque (ideal
conditions for producing a blackbody spectrum) and subsequently released to
travel through the universe when the charged protons and electrons combined to
form neutral hydrogen (neutral gas is much more transparent to light than
charged plasma). At this time the temperature of the radiation would have been
about 3000 K, but the expansion of the universe has redshifted it down to its
present value.
The very tiny temperature variations in the CMB are sensitive to properties of the
universe such as its average density, its expansion rate, the amount of ordinary
and dark matter, the cosmological constant
and so on. Much of our present understanding of these properties comes from
analysing the CMB.
- cosmological constant (Λ)
-
A term introduced into the equations of General Relativity by Einstein so that
he could produce a static (non-expanding) cosmological model (in 1918, before
the expansion of the universe was discovered). Physically, it corresponds to
a non-zero energy of empty space (a vacuum energy). After the discovery of
Hubble's law, the cosmological constant was long assumed
to be zero, but observational data from Type Ia supernova in
the late 1990s caused its reintroduction – ironically now driving an
accelerating expansion of the universe, rather than preventing any expansion at all!
The physics of Λ is emphatically not yet understood – we don't even
know if it really is "constant", or varies with time.
- critical density
-
The value of the density of the universe for which (in the absence of a
cosmological constant) expansion will just
continue forever, but at an ever-decreasing rate (expansion rate tends to zero as
time tends to infinity). This is the value of the density for which the geometry
of space-time is "flat" (angles in a triangle add up to 180°, parallel
lines remain parallel). To the best of our ability to measure it, the geometry of
our universe is indeed flat.
- curvature (k)
-
The overall geometry of space-time. In General Relativity, space can have a
positive curvature (k > 0), analogous to the surface of a sphere (angles of a
triangle add up to more than 180°, parallel lines converge to meet at the poles),
a negative curvature (angles of a triangle add up to less than 180°,
parallel lines diverge), or zero curvature ("flat"). The
curvature depends on the density parameter: if
Ω > 1, k > 0 and vice versa. Our universe appears to have
Ω = 1 (exactly, to within our experimental error), and thus a flat
geometry; this is unexpected in the standard big bang
model, but is explained by the inclusion of inflation.
- dark energy
-
Another word for the cosmological constant, avoiding the
implication that it really is a constant (which we haven't yet either confirmed
or refuted). Not to be confused with dark matter: they are two entirely different
things (the similarity in name is yet another example in a long list of badly
chosen astronomical terminology – sorry about that, not my fault!).
- dark matter
-
Matter in the universe which cannot be seen, but is detected indirectly because it
exerts gravitational force – for example, the Sun's velocity around the
Galactic centre is too fast to be explained by the visible mass of our Galaxy, but
would make sense of the Galaxy were much more massive than the sum of its stars.
Data from various sources, including clusters of galaxies, big
bang nucleosynthesis, the CMB and galaxy formation,
indicate that this unseen material is not ordinary atoms, but is mostly composed
of a new and unknown type of particle. See
Ed
Daw's seminar for more information.
- density parameter (Ω)
-
The average density of the universe, expressed as a fraction of the
critical density. In the absence of a cosmological constant,
a universe with Ω > 1 will eventually recollapse in a Big Crunch,
whereas universes with Ω ≥ 1 expand forever (note that if Λ > 1,
even universes with Ω > 1 usually expand forever – the repulsive
effect of vacuum energy changes the rules). Our universe seems to have
Ω = 1 to high precision.
- deuterium
-
"Heavy hydrogen" – the isotope of hydrogen whose nucleus consists of a
proton and a neutron bound together, instead of just a proton as with ordinary
hydrogen. Stable (just), but very rare in the cosmos – made only in the
early universe, and destroyed in stars. Its abundance is a
good test of the Big Bang cosmological model. (The still
heavier isotope of hydrogen with two neutrons, H-3 or tritium, is
radioactive, decaying to He-3 by beta-decay.)
- distance modulus
-
The difference between apparent magnitude m and
absolute magnitude M, usually given the symbol μ.
A measure of distance: after correction for the effects of interstellar dust,
m – M = μ = 5 log(d/10), where d is the distance in parsecs and the
log is base 10.
- Doppler shift
-
The shift in the wavelength of light caused by the motion of the light source
towards or away from us. For speeds small compared to the speed of light,
the fractional change in wavelength Δλ/λ = v/c where
v is the velocity towards or away from us (away from us has positive sign) and c
is the speed of light. For galaxies (not for stars),
the Doppler shift is a measure of the galaxy's distance, according to
Hubble's law, v = Hd where d is the distance in Mpc and
H is Hubble's constant. For stars, Doppler shift can be used to study
binary stars and extrasolar planets.
- eccentricity
-
Parameter describing the shape of an ellipse, ranging from 0 (perfect circle) to
1 (straight line). Numerically, it is the ratio of the distance between the
foci of the ellipse to the length of its major axis – see for example
this diagram
in Wikimedia Commons. The elliptical orbits of the planets in our solar system
have small eccentricities (of the 8 major planets, the largest eccentricity is
0.2, for Mercury), and are therefore nearly circular, but the orbits of many
binary stars and – surprisingly –
extrasolar planets have quite large eccentricities and
are noticeably oval.
- effective temperature
-
A measure of the surface temperature of an object, defined as
the temperature of a blackbody with the same surface area
and total power output as the object in question. Quoted surface temperatures
of stars and other astronomical bodies are generally effective temperatures.
(Note that, since a blackbody is a perfectly efficient emitter of radiation
whereas real objects are not, the effective temperature is always less than the
true physical temperature of the object.)
- electron capture
-
Radioactive decay in which an isotope captures one of its own bound electrons into
the nucleus, combining it with a proton to convert it into a neutron (with emission
of a neutrino). Electron capture decay occurs when an isotope has too many
protons (compared to its number of neutrons) to be stable.
- emission line spectrum
-
Spectrum in which the light is emitted at a small number of discrete wavelengths,
instead of being spread over a range of wavelengths like a rainbow. Emission line
spectra are characteristic of hot, low-density gas – astronomical objects
with emission line spectra include planetary nebulae and
H II regions. Many streetlights, especially mercury-vapour
lights and the older, dark yellow, type of sodium light, have emission line spectra,
as do neon advertising signs.
- envelope
-
The cooler outer layers of a star, generally the part of the star that is too cool to
undergo any fusion processes.
- extrasolar planet, exoplanet
-
A planet orbiting a star other than the Sun.
See the Extrasolar Planets Encyclopaedia for
an up-to-date list.
- flux
-
Power per unit area. The SI unit is W m–2 for
bolometric flux, W m–2 nm–1
for monochromatic flux at a specified wavelength, and
W m–2 Hz–1
for monochromatic flux at a specified frequency.
- fusion
-
Nuclear process which makes heavier nuclei by combining lighter ones. This generates
energy for elements up to iron, but not for elements beyond iron (where
binding energy per nucleon decreases again).
The power source of stars.
- galactic cluster, open cluster
-
Open clusters are small, irregularly shaped clusters
containing a few hundred stars. In the Milky Way, most open clusters are
fairly young objects (a few hundred million years); unlike globular clusters,
they are associated with the disc of the Galaxy and appear close to the Milky
Way in the night sky.
The Pleiades are one of the
best-known open clusters; here is their HR diagram.
- galaxy
-
A large concentration of stars, gas and dark matter held
together by its own gravity. Galaxies are generally larger and more massive than
star clusters, contain dark matter (which star clusters
generally don't), and normally include stars with a range of ages and
heavy-element content. They are classified into elliptical,
spiral, barred spiral and irregular categories according to the
Hubble tuning fork.
Our own galaxy, the Milky Way, is also known as the Galaxy with a
capital 'G'; external galaxies have a small 'g'. (The word "galaxy" itself
simply comes from the Greek for "milky way".)
- globular cluster
-
Very massive star cluster containing tens or hundreds of
thousands of stars, pulled by its own gravity into a roughly spherical shape
(hence the name). In the Milky Way, globular clusters are found far from the
plane of the Galactic disc, and are composed of very old stars, low or very low in
heavy elements. They are believe to date from the earliest stages of the formation
of our Galaxy.
The Messier object M3 is a fine
example of a globular cluster;
here is its HR diagram.
- H II region [pronounced "H two"]
-
Roughly spherical region of ionised hydrogen surrounding newly formed OB (massive)
stars. The ultraviolet radiation from the hot stars ionises the surrounding hydrogen,
and when the electrons subsequently recombine with the protons an
emission line spectrum is produced. H II regions are
typically red in colour, owing to a prominent hydrogen emission line (Hα)
at 656 nm.
- habitable zone
-
The region around a star in which an Earth-like planet could support liquid water
oceans on its surface (we suspect that water may be essential for life, hence
the name). Note that whether a planet in the habitable zone actually does
have surface water depends strongly on the planet – Mars is probably inside
the Sun's habitable zone, but has no surface water because it was too small to
keep much of its atmosphere.
- heavy-element content
-
The proportion of a star's outer layers (i.e. of its initial composition,
before any fusion processes) made up of elements heavier than helium.
Usually expressed as a fraction by mass (i.e. not by number of atoms) and
given the symbol Z. The Sun has Z = 0.02, and is regarded as high in heavy elements:
in some old stars, the heavy-element content is at the level of parts per million.
- Hertzsprung-Russell diagram (HR diagram)
-
A plot of absolute magnitude
(or log(luminosity)) of stars
against their colour index
(or log(effective temperature); occasionally
spectral class), used to investigate the properties and
evolution of stars. [Note that for star clusters,
apparent magnitude can be used instead of absolute
magnitude, because the stars are all at essentially the same distance from
us. In this case the y-axis will be offset by the distance
modulus of the cluster.)
The HR diagram is named after
Ejnar
Hertzsprung and
Henry
Norris Russell, who both independently came up with the idea in the early
20th century.
- horizontal branch (HB)
-
Branch of the Hertzsprung-Russell diagram running horizontally
left from the red giant branch at about
absolute magnitude +1. Contains stars fusing helium to
carbon in their central core. The length of the HB is a measure of the
heavy-element content of the stars plotted: stars low in
heavy elements tend to be bluer on the HB than
high-metallicity stars. In
stars of solar (i.e. high) heavy-element content, the HB is so short that it
just forms a red clump of stars lying on top of the
red giant branch.
- hot Jupiter, hot Neptune
-
A class of extrasolar planets with masses comparable to
Jupiter (about 300 Earth masses) or Neptune (about 20 Earth masses), but orbiting
very close to their host stars (orbital periods typically a few days). These
are the easiest planets to discover by Doppler shift and
transit methods, so are over-represented in our sample,
but hot Jupiters in particular seem to be a well-defined category (masses from
0.2 to 5 Jupiter masses, periods from 0.8 to 10 days). It is believed that
these are gas giant planets which formed far from the star (about where our
Jupiter is) and then migrated inward because of friction with the
protoplanetary disc.
- Hubble's law
-
The observation that the redshifts
of distant galaxies are proportional to
our distance from them, v = Hd where v is the "recession
velocity" calculated from the redshift of the galaxy's spectral lines, d is
its distance in Mpc and H is Hubble's constant (which has a value of about
70 km s–1 Mpc–1). This effect is caused by the
expansion of the Universe: it's important to note that the galaxies are not
really moving through space, rather space is expanding and carrying the galaxies
with it.
- Hubble tuning fork
-
the standard classification system for galaxies, which divides them into
elliptical galaxies, lenticular galaxies, spiral and barred spiral galaxies,
and irregular galaxies. The split between spiral and barred spiral galaxies
causes the diagram of the system to have two parallel arms like a tuning fork,
hence the name. See the
Wikipedia article (or the
lecture summary) for more detail.
- inflation
-
A hypothetical period of extremely rapid (exponential) expansion
very early in the history of the universe (starting at about
10–35 s after the big bang and lasting for a very small
fraction of a second). Because inflation involves expansion much faster
than light (note that although objects cannot travel through space
faster than light, the fabric of space itself can expand as fast as it wants),
it has the effect of making the post-inflation universe extremely flat
(any pre-existing curvature is diluted away) and extremely uniform (because
the presently visible universe expands from such a very tiny region of
pre-inflation space). This agrees with observations, and is otherwise very
hard to explain.
- infra-red
-
Electromagnetic radiation ("light", loosely speaking) with wavelength in the
range 700 nm to 1 mm (the longer part, from about 0.2 to 1 mm, is often split off as
submillimetre). Important in astronomy for a number of reasons:
- it can penetrate dust, so is ideal for observing dust-obscured regions such
as those in which star formation is taking place;
- for distant objects at redshifts of order 1 or more,
spectral lines etc. which are normally seen at visual wavelengths are shifted
into the infra-red;
- adaptive optics on modern telescopes can compensate for the effect of the
atmosphere more effectively at infra-red wavelengths, giving better images;
- for study of extrasolar planets, the brightness contrast
between the planet and the star is much less severe in the infra-red, so there
is more chance of observing planets directly.
Near infra-red (700–3000 nm) can be observed from the
ground, if you choose a high, dry site (e.g. the South Pole or the Atacama
Desert) and avoid the strong water absorption lines. Mid
infra-red (3–50 μm) and far infra-red
(50–1000 μm) require space-based instruments.
- iron-peak elements
-
The elements around iron (roughly from vanadium to zinc), which have the highest
binding energy per nucleon and are therefore the most stable. These are made in
conditions of nuclear statistical equilibrium, requiring extreme
temperatures and densities, which occur in the central regions of
supernovae. The term "iron peak" is used because the existence
of this process makes these elements more common than those around them,
forming a peak in the
distribution
of elemental abundances.
- Local Group
-
The small group of galaxies of which the Milky Way is part, consisting of the
Andromeda Galaxy, M31, the
Triangulum Galaxy, M33 and
around 50 dwarf galaxies, among the largest of which are the two Magellanic Clouds
(irregular satellites of the Milky Way – the
Large Magellanic
Cloud is the fourth largest member of the group after M33) and the
elliptical satellites of
M31, M32 and NGC205.
- luminosity
-
The power output of an astronomical object such as a star. This usually means
the bolometric power output, but in some cases can refer to
the power output over a specific wavelength range (this will usually be
indicated by a subscript, such as LV for visual
luminosity).
- magnitude
-
A measure of the brightness or flux received from an object.
The definition is m = –2.5 log(F) + C, where m is the
(apparent) magnitude, F is the flux and C is a constant defined according to some
standard system, e.g. the one in which the magnitude of the bright star Vega is
defined to be zero. The log is base 10. The minus sign means that
larger magnitudes imply fainter objects: the Sun has an apparent
magnitude of –26.7, a bright star about 0, and a barely-visible star about
5 or 6. Absolute magnitudes use a standard distance of
10 parsecs, instead of the real distance, and are therefore a
measure of luminosity rather than flux
received. Magnitudes are usually measured over a specific wavelength range
defined by a standard filter, e.g. one of the UBVRI set.
The difference between magnitudes measured through two different filters is
called a colour index.
- main sequence
-
Main branch of the Hertzsprung-Russell diagram, running
from top left (bright and blue) to bottom right (faint and red), and containing
>90% of all stars. These are stars fusing hydrogen to helium in their
central cores. The main-sequence lifetime of a star is very sensitive to its
mass: the most massive stars will stay on the main sequence for only a million
years or so, whereas any star of less than about 0.9 solar masses has a
main-sequence lifetime greater than the age of the universe (so, all such stars
that have ever been born are still on the main sequence). The Sun's predicted
main-sequence lifetime is about 10 billion years – it's currently roughly
halfway through this.
The position of a star on the main sequence is determined by its mass: the more
massive that star is, the hotter and brighter it will be. Stars do not change
their properties very much while on the main sequence: it is by far the longest and
most stable stage in a star's life.
- massive star
-
Usually refers to a star of initial mass >8 solar masses, which will
eventually end its life as a supernova. These stars are
very luminous, blue while on the main sequence, and very
short-lived (a few million years, compared to 10 billion years for the Sun).
They are also extremely rare, comprising ≪1% of all stars.
- mass-luminosity relation
-
The observation, based on the measurement of stellar masses in
binary systems, that there is a very close correlation
between mass and luminosity for
main-sequence stars.
The relationship is a power law, L ∝ Mn,
where the luminosity L and the mass M are measured in terms of the Sun's
luminosity and mass. The value of n changes as we go from the lowest to the
highest mass stars, but a value between 3.5 and 4 is good for most of the
main sequence except for the very lowest (<0.5 solar masses) and highest
(>20 solar masses) mass stars. This is the basis of our understanding of
the relationship between mass and lifetime: if a 10-solar-mass star is 10000
times as luminous as the Sun, it will use up its core hydrogen 1000 times
faster, and will therefore have a main-sequence lifetime of only 10 million
years instead of 10 billion.
- metallicity
-
Another name for heavy-element content. Because metals are
easy to detect in stellar spectra (their outer electrons are loosely bound,
so can easily absorb photons of visible light), the original analyses of
spectra tended to focus on metallic elements. Hence astronomers have the
slightly weird habit of referring to all elements heavier than helium
as "metals"!
- monochromatic
-
Applied to a flux,
luminosity or magnitude, means
"measured at a specific wavelength or frequency". In practice, the
flux, luminosity or magnitude will actually be measured over a specified
range of wavelengths, which may be small ("narrowband")
or quite large ("broadband"). There are a number of standard astronomical
filter systems, e.g. UBVRI, which allow astronomers to
take measurements over standard, well-defined wavelength ranges.
- neutron star
-
The remnant of the collapsed iron core of a core-collapse
supernova. The protons and electrons from the original iron
combine to create neutrons (emitting neutrinos in the process), and the neutrons
are packed closely together into something that is effectively a giant atomic
nucleus, having a mass about 1.5 times that of the Sun but being only 20 km
across. Neutron stars emit very little light, but can sometimes be detected
as pulsars. Probably the most famous is the
Crab Nebula pulsar,
the last remnant of a star seen to explode in 1054 by Chinese and Japanese
astronomers.
- nucleosynthesis
-
Any mechanism for producing new elements (technically, new nuclei, i.e. new isotopes,
but I don't think a process that just made new isotopes of existing elements would
really count). In astrophysics, there are several types of nucleosynthesis:
- big bang nucleosynthesis, which occurs a few minutes after
the Big Bang and makes mostly helium-4 (with a little deuterium
and helium-3 left over, and a little lithium-7 produced);
- stellar fusion processes, which make the elements from
carbon to the iron peak –
most of which are also made in
- explosive fusion processes, which occur under conditions
of extreme heat and pressure in supernovae, and make primarily
iron peak elements and, in core-collapse supernovae,
alpha-process elements;
- s-process neutron capture, which occurs in helium-fusing
stars and makes those isotopes of elements from iron to bismuth which are closest
to the line of maximum stability;
- r-process neutron capture, which occurs in extremely
neutron-rich environments, possibly supernovae, and makes
very neutron-rich unstable isotopes which subsequently decay to slightly neutron-rich
stable isotopes from iron to uranium (and higher, but elements heavier than
uranium have short half-lives and do not survive);
- various types of p-processes, involving addition
of protons, removal of neutrons as a result of collisions with high-energy
photons, and conversion of neutrons to protons by absorption of neutrinos, all
of which produce rare proton-rich nuclei;
- spallation – knocking bits off pre-existing nuclei
by collision with energetic cosmic rays, which seems to be the only way to make the
rare light elements lithium-6, beryllium and boron (lithium-7 is also made in
big-bang nucleosynthesis).
- Olbers' Paradox
-
The argument that the sky should not be dark at night. If the universe were
infinite in size (containing infinitely many stars),
infinitely old and not expanding, all lines of sight would
eventually hit a star, so you would expect the entire night sky to be at the
same temperature as the surface of the average star – about 3500 K or so!
The paradox is resolved if any one (or more) of its premises is false: if the
universe does not contain an infinite number of stars, many lines of sight may
not hit a star; if it is not infinitely old, the light from stars beyond a certain
distance away hasn't yet had time to reach us; if it is expanding, the
cosmological redshift will lengthen the wavelength of the
light from more distant stars such that they no longer appear bright. Therefore,
neither the Big Bang nor the Steady State
model has this paradox: both models are expanding, and the Big Bang is also not
infinitely old. (In fact, for stars, the finite age of the Big Bang is much
more important than the expansion – but you need the expansion to avoid
being fried by the cosmic microwave background, which started
out at 3000 K.)
Named for
Wilhelm
Olbers, who published a discussion of it in 1823, but recognised much
earlier – Kepler wrote about it in 1610.
- open cluster
- see galactic cluster.
- parallax
-
In this course, the shift in the apparent angular position of a nearby star over the
course of a year, caused by the Earth's movement around the Sun.
See this section of
Nick Strobel's Astronomy Notes for more detail.
- p-process, p-process isotope
-
Form of nucleosynthesis
which produces rare isotopes unusually rich in protons
(to the left of the s-process line). There are actually
several different processes that contribute to this, some of them involving
the addition of protons to a pre-existing nucleus and others the knocking out
of neutrons, e.g. by energetic photons. Believed to occur in
supernovae.
- parsec (pc)
-
The distance at which a star would have a parallax of
one arcsecond – about 3.26 light years, or
3.086×1016 m. The standard unit of distance in astronomy.
It takes SI prefixes: 1 kpc = 1000 pc, 1 Mpc = 1000000 pc, etc. The nearest
star is about 1.3 pc away, the centre of our Galaxy is about 8 kpc away, and the
nearest large galaxy (M31) is 0.7 Mpc away.
- planetary nebula (PN)
-
An expanding shell of gas produced when a star on the asymptotic
giant branch becomes unstable and loses its outer layers in a stellar wind.
The central carbon core of the star is left behind as a young white
dwarf: because it is extremely hot, it excites the electrons in the expelled
gas; as the electrons de-excite, they produce emission lines
which cause the planetary nebula to glow brightly. PNe frequently have a
distinctive green colour, caused by lines from ionised oxygen: as stars are
never green, this is an easy way to identify them in colour images.
- pp chain
-
Means by which hydrogen is fused to helium in stars of the Sun's mass and lower.
The usual series of reactions is p + p → 2H + e+
+ ν, followed by 2H + p → 3He and then
3He + 3He → 4He + 2p. The net reaction
is 4p → 4He + 2e+ + 2ν – you actually put
6 protons in, but you get two back at the end.
- protoplanetary disc
-
Disc of gas and dust around a young star or protostar.
It is believed that these discs eventually condense into planets, as the
dust grains collide and stick together. Sometimes abbreviated to
proplyd.
Do not confuse protoplanetary discs with planetary nebulae
– planetary nebulae have nothing whatsoever to do with planets! (This
is another example of poorly chosen astronomical names.)
- protostar
-
A star in the process of formation – the gravitational collapse of the
original gas cloud has proceeded far enough that there is a recognisable
dense object in the middle, but the star has not yet ignited hydrogen fusion and
joined the main sequence. Young protostars are shrouded by
gas and dust and can only be observed in the infra-red, but
protostars which have nearly reached the main sequence can be observed as
T Tauri stars.
- pulsar
-
A rotating neutron star, observed as a source of extremely regular
pulses of radio and/or X-ray emission (sometimes, as in the Crab pulsar, also
optical). This is caused by charged particles being trapped in the neutron star's
extremely large magnetic field, creating a beam of radio emission from the star's
magnetic poles. If these are not exactly aligned with the star's rotation poles,
this beam will sweep across the sky like a lighthouse beam, creating regular pulses
every time the beam hits your telescope.
The timing of pulsar pulses is so regular that they can be used as accurate clocks
(comparable to atomic clocks in precision!). Hence, pulsars in binary systems
have been used to test General Relativity, and planets around pulsars have been
detected by the minute variations that they create in the pulse timing.
- r-process, r-process isotope
-
Form of nucleosynthesis
involving the rapid addition of neutrons to
a pre-existing nucleus. This produces very unstable, super-neutron-rich nuclei
which undergo repeated beta-decays until they reach a stable
isotope. It is responsible for about half the isotopes heavier than iron, and tends
to produce relatively neutron-rich isotopes. Stable isotopes to the right of the
s-process line result from the r-process.
The r-process requires a very high density of neutrons to operate. It may occur
in supernovae, or possibly around very young neutron
stars. This is still an active field of research.
- red clump
-
A concentration of stars at about absolute magnitude +1
on the red giant branch of the
HR diagram of high-metallicity
star clusters. This is basically a very red, very short
horizontal branch, consisting of stars fusing helium to
carbon in their cores (unlike the "real" red giants, which are fusing hydrogen
in a shell around an inert, non-fusing, helium core). Because the red clump lies
on top of the red giant branch, it can be very difficult to work out whether
a particular star in this region of the HR diagram is fusing helium or hydrogen.
- red giant branch (RGB)
-
Branch of the Hertzsprung-Russell diagram running diagonally
up and right from the middle of the main sequence. Contains stars fusing hydrogen
to helium in a shell around an inert helium core. This is the stage in a star's
life immediately after the main sequence, and starts when the star runs out of
core hydrogen. It ends when the inert helium core becomes hot enough to initiate
the triple-alpha process by which helium fuses to carbon.
- redshift
-
Doppler shift caused by motion away from observer
(motion towards observer being blueshift),
but particularly used in astronomy to refer to the effect of the expansion of
the universe (cosmological redshift). Cosmological redshift is
caused by the expansion of space, and is not strictly speaking a Doppler shift
(since nothing is moving through space – it's just that the space between
source and observer is expanding). It is usually given the symbol z (small z,
not to be confused with capital Z which is metallicity).
- s-process, s-process line, s-process isotope
-
Form of nucleosynthesis
involving the slow addition of neutrons to
a pre-existing nucleus. Individual neutron captures occur rarely, so if the
nucleus produced is unstable, it will have time to decay before another neutron
is captured. For this reason, the s-process proceeds along a well-defined line
on the chart of the nuclides: add
a neutron to a stable isotope; if result is stable, add another neutron, but
if result is not stable, allow it to decay first, then add another neutron.
All s-process isotopes are therefore "next to" another stable isotope, from
which they are made either just by adding a neutron, or by adding a neutron and then
allowing an unstable daughter nucleus to decay. The s-process terminates
at bismuth-209, because Bi-210 decays to polonium-210 which is unstable to
alpha-decay, producing lead-206. This bends the s-process
back on itself, forming a closed loop (Pb-206 → Pb-207 → Pb-208 →
Pb-209, which decays to Bi-209 and we're back where we started), so it can't get
any further. Elements beyond bismuth (thorium and uranium are the only ones
with half-lives long enough to be significant) must be produced by the
r-process.
The s-process runs in helium-fusing stars, especially AGB stars.
The neutrons are produced by reactions such as 13C + α →
16O + n.
- Schwarzschild radius
-
The radius of a black hole (specifically, a spherical, non-rotating
black hole). The equation for the Scwarzschild radius is
r = 2GM/c2, which works out to 3 km per solar mass.
Named for Karl
Schwarzschild, who worked out the relevant solution to Einstein's field
equations of General Relativity in 1916, only a year after Einstein published
them (and before Einstein himself had derived any exact solutions).
- spallation
-
In general, any process in which small fragments of an object are knocked off the
main body as a result of impact. In the context of
nucleosynthesis, refers to the production of light elements,
especially lithium, beryllium and boron, when cosmic rays collade with heavier
nuclei.
- spectral class
-
The spectral class of a star is defined by its
absorption lines. The underlying physical property is
the surface temperature, and hence (via the blackbody spectrum)
spectral class is strongly correlated to colour, but it is the line pattern and not
the colour which is used in the definition (spectral classification predates
colour images). The main classes (and their defining spectral lines) are O (ionised
helium), B (helium), A (hydrogen), F (hydrogen and ionised metals), G (ionised and
neutral metals), K (neutral metals) and M (molecules, especially TiO), running
from hottest to coolest – the odd ordering of the letters is caused by the
fact that the classes were defined before the physics was understood. The
standard mnemonic is "Oh, Be A Fine Guy/Girl, Kiss Me" – I have heard
others, but several of them weren't printable.
- standard candle
-
Any class of astronomical object whose absolute magnitude
can be deduced from some observable property, e.g. the period of variation of
a variable star. Extremely useful for measuring the distance of objects beyond
the range of parallax, because the calculated absolute
magnitude can be compared with the observed apparent magnitude
to get the distance modulus. Commonly used standard candles
include classical
Cepheid variables (range to about 20 Mpc, useful for nearby galaxies) and
Type Ia supernovae (range out to redshifts of 1, useful for
cosmology).
- star cluster
-
A gravitationally bound group of stars which formed together from the same
collapsing gas cloud, and therefore have the same age (give or take a million
years or so) and initial chemical composition. As clusters are fairly
compact, all the stars are also at about the same distance from us. These properties
make star clusters extremely useful for studying stellar evolution.
- Steady State model
-
A theory of cosmology, invented in 1948 by Hermann Bondi, Tommy Gold and Fred Hoyle,
in which the universe expands but
its average density and overall properties remain the same because new matter is
created to "fill in the gaps" left by the expansion. At the time of its conception,
the Steady State model agreed with observational data and had the advantage of
avoiding the unphysically young age of the universe predicted at the time by the
Big Bang theory (this came about because
Hubble's original value of the Hubble constant was much too large, leading to
an age of the Universe younger that the measured age of the Earth's crust!).
However, subsequent data, including the fact that active galaxies
are found preferentially at high redshift – the Steady State expects the
universe to look the same at all times, and hence at all redshifts – tended
to contradict the theory, and it was eventually killed by the discovery of the
cosmic microwave background, and in particular its
blackbody spectrum, in the mid 1960s.
- subgiant branch
-
Branch of the Hertzsprung-Russell diagram seen only in very old
populations of stars, e.g. globular clusters. Runs from
the turn-off point of the main sequence
to the bottom of the red giant branch, and contains stars in
transition from the MS to the RGB (they are fusing hydrogen to helium in a thick
shell around a small helium core).
More massive stars transition from the MS to the RGB very quickly, so catching
them in the act is very unlikely. Therefore, younger clusters exhibit a gap
between the MS and the RGB, known as the Hertzsprung gap,
rather than a subgiant branch.
- Sun-like star
-
A star of <8 solar masses, which will not get hot enough to fuse carbon and will
therefore end its life as a carbon white dwarf.
- super-Earth
-
A class of extrasolar planets with masses larger than the
Earth but less than any of the solar-system giant planets (<10 Earth masses,
compared to 14 Earth masses for Uranus and 17 for Neptune). As detection
techniques improve, more super-Earths are being discovered – it seems to
be quite a common class of planets, although there are none in our solar system.
Super-Earths are probably basically rocky, but some, e.g.
Kepler
11d, e and f have very low densities and must have extremely thick
atmospheres of light elements (H/He or water vapour) accounting for several
percent of the planet's total mass. This is very unlike the Earth – our
atmosphere contributes less than one-millionth of the planet's mass. These
low-density objects are sometimes called mini-Neptunes instead
of super-Earths, to emphasise their different structure.
- supergiant star
-
A massive star that has left the main
sequence. Unlike Sun-like stars, which typically
become much more luminous as red giants or
AGB stars than they were on the main sequence, massive stars
typically do not change their luminosity much as they
evolve, but they change their size, and hence their surface temperature and
colour, a great deal, moving across the HR diagram from blue
to red and back again.
The bright star Betelgeuse
in Orion is a red supergiant; Deneb
in Cygnus and Rigel in Orion are
blue-white supergiants.
- supermassive black hole (SMBH)
-
A black hole with a mass of typically over a million times the Sun's, found in the
centre of most large galaxies. The mass of the hole is strongly correlated with the
mass of the galaxy's bulge: giant elliptical galaxies (which are effectively all
bulge) have the most massive SMBHs, up to several billion solar masses, whereas
Sc galaxies with very small bulges have low-mass SMBHs or even none at all.
The Milky Way is an Sbc galaxy, and has a fairly low-mass central black hole of
"only" 4 million or so solar masses; the Local Group Sc spiral
M33 appears to have no
central SMBH at all (at least none over a few thousand solar masses).
- supernova (SN)
-
The explosion of a star. There are two main types:
- core-collapse supernovae, caused when a massive star
creates an iron core which then collapses under gravity (fusion of iron
does not generate energy, so once the collapse starts it cannot be stopped
by a new fusion process), and
- Type Ia supernovae (sometimes called thermonuclear
supernovae), caused when a carbon white dwarf exceeds the
Chandrasekhar limit and collapses under its own gravity,
causing runaway carbon fusion which destroys the entire star.
Core-collapse supernovae are important because they release a large amount of
heavy elements into the interstellar gas, where they can be incorporated into
new stars (and planets). Type Ia supernovae are important because they can be
used as standard candles to probe the
Hubble law up to very large distances, allowing
cosmologists to study the cosmological constant.
All supernovae leave behind an expanding shell of ejected gas, similar to a
planetary nebula but much larger. Core-collapse supernovae
also leave the remains of the collapsed iron core, which becomes either a
neutron star or a black hole.
- supernova remnant (SNR)
-
The expanding cloud of gas left after a supernova explosion – similar to,
but much larger than, a planetary nebula. Usually produces
synchrotron radiation (radio emission from charged particles trapped in magnetic
fields), and X- and γ-rays (high energy photons produced by scattering off
energetic electrons) as well as emission lines. Responsible for disseminating
the products of explosive nucleosynthesis into the
interstellar gas, where they can be incorporated into new stars. The most
famous SNR is the Crab Nebula,
the remnant of SN1054.
- thermal spectrum, thermal radiation
-
Continuous spectrum of radiation produced by a dense body
which is hotter than its surroundings – (approximately) a
blackbody spectrum.
- transit
-
Passage of a planet across the face of a star, leading to a small drop in the
amount of light received. An increasingly important method of detecting
extrasolar planets, and the only current technology
capable of detecting planets as small as the Earth around stars like the Sun.
Unfortunately, transit detections only tell us the size of the detected
planet, and not its mass (except in rare cases where the gravitational effects of
multiple planets in one system cause shifts in the timing of transits that can
be analysed). As a result, it can be quite difficult to decide if a particular
signal really is a planet, and not, for example, a grazing eclipse by a
binary companion star:
the Kepler mission has 2740 planet
candidates (January 2013) but only 150 confirmed discoveries
(August 2013).
- triple-alpha process
-
Process by which helium-4 fuses to make carbon-12. As a three-body collision is
too improbable to be realistic, this is actually a two-step process going via
the extremely unstable isotope beryllium-8
(half-life about 0.07 femtoseconds!), which is formed by the collision
of two alpha particles and then converted to carbon-12
by adding a further alpha particle. This requires extremely high temperatures
(> 100,000,000 K, ten times what's needed for hydrogen fusion) and very high
densities (so that the two collisions can occur in sufficiently quick succession,
before the Be-8 falls apart).
- T Tauri star
-
A type of protostar – specifically, a relatively
low-mass Sun-like star that has not yet reached the main
sequence. Identified by variability and strong emission
lines from an active stellar atmosphere. Although they are not yet fusing
hydrogen, they are actually brighter than they will be once they reach the
main sequence: they are still contracting, and are powered
by conversion of gravitational potential energy into heat and radiation.
Many T Tauri stars have detectable protoplanetary discs
and will develop planetary systems in the future.
- turn-off point
-
In the HR diagram of a star cluster,
the highest point on the main sequence, representing the most
massive stars that are still capable of core hydrogen fusion.
As the main-sequence lifetime of a star depends strongly on its mass
(because of the mass-luminosity relation), the
position of the turn-off point can be used to determine the age of the cluster.
- UBVRI
-
A common set of standard broadband filters in astronomy. See
the Wikipedia article
for a list.
- visual
-
This can have several meanings:
- a standard filter, V, in the UBVRI system, covering
the wavelength range 500–600 nm (which corresponds roughly to the peak
sensitivity of the dark-adapted eye);
- more generally, the range of wavelengths to which the human eye is
sensitive, roughly 400–700 nm (also referred to as "optical" wavelengths);
- observed by eye, or more generally by imaging, as opposed to some other method
(e.g. "visual binary", one in which the two stars are observed as separate objects,
contrasted with "spectroscopic binary", one in which the presence of two stars is
deduced from periodic Doppler shifts in the spectral lines).
- white dwarf
-
A compact stellar remnant consisting of typically 0.6 to 1 solar mass of carbon
and oxygen, the exhausted core of a Sun-like star.
A typical white dwarf is about the size of a medium-sized planet – more massive
white dwarfs are smaller than less massive ones, because they have stronger gravity.
White dwarfs in binary systems may accrete material from
their companion star: this may subsequently be blown off in a nova
outburst (which won't harm the white dwarf) or, if it collects until the white dwarf's
mass exceeds the Chandrasekhar limit, may cause the entire
white dwarf to explode as a Type Ia supernova.
White dwarfs can be seen in the bottom left-hand corner of the
HR diagram, but as they are extremely faint will often not be
visible in real HR diagrams as they will be invisible to the telescope that
collected the data.
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