Glossary
of Astronomical Terms
Astronomy
| Topics
Absorption
nebula A cloud of dust and gas which blocks, or absorbs, radiation
from stars or galaxies
behind it as seen from Earth.
Black
body A theoretical object that is both a perfect absorber and emitter
of radiation. A star approximates closely to this ideal. The distribution
of energy (as a function of wavelength)
from a blackbody depends only on the temperature
of the object.
Bound
electron An electron whose total energy is negative. This meanthat
energy must be supplied to this electron in order to
remove it from, eg. its parent atom.
Emission
nebula A nebula which emits radiation, often in the form ofvisible
light, due to the excitation of gas and dust by stars
within the nebula itself (eg. Orion, Lagoon, Trifid,
Eagle nebulae).
Excited
atom When an atom gains energy, possibly due to being "hit"by a
bombarding particle or due to absorption of a photon,
the electrons within that atom move to energy levels
above the Ground State. The atom is then said to
be "excited".
eV
The ELECTRON VOLT. This is a convenient, atomic-sized unit
of energy. 1 eV = 1.6 x 10-19J.
Fusion
A nuclear process in which smaller nuclei join (or fuse) together
to form a more stable nucleus, with the consequent
release of energy. The process that powers the
stars.
Ground
state The most energetically stable state in an atom or molecule.
Electrons in the ground state have their minimum
total energy.
H
I Region A volume of space primarily composed of neutral (atomic)Hydrogen
atoms.
H
II Region A volume of space where many of the Hydrogen atoms havebeen
ionised due to interactions between high energy photons
and neutral H atoms.
Ionised
An
atom has the same number of protons and electrons. If
an atom gains or loses electrons it becomes a charged ION.
It is then said to be in an "ionised" state.
K
The KELVIN, the SI unit of thermodynamic temperature. One
Kelvin is the same magnitude as the degree Celsius, but The
zero point on the Kelvin scale commences at Absolute Zero
(approx. - 273° C).
Light
year The distance travelled in one year by an object moving at thespeed
of light:
approximately 9 x 1015m.
Luminosity
A measure of the brightness of an astronomical object. Measured
in Joules. The luminosity of the Sun is about 2
x 1030J.
Molecule
A structure consisting of two or more atoms.
Nebula
A nebula is a cloud of dust and gas in space. Nebulae can be very
large: tens of light years across. The Orion nebula is a region
of star formation. Other types of nebula can be
produced when a star is at an advanced stage
in its evolution .(Planetary nebulae,
such as the Dumbell nebula) or when a star explodes as a supernova (a supernova
remnant, such as the Crab nebula).
NGC
Refers to the New General Catalogue, a database mainly consisting
of information on nebulae and galaxies.
nm
The
nanometre, a small unit of distance. 1 nm = 10-9m.A typical
atom is about 0.1-0.2nm in diameter.
Orion
A
constellation, or grouping of stars, prominent in the Southern
sky during the winter months. Contains two very Bright
stars: Rigel and Betelgeuse. Also well known for the
"belt of Orion", beneath which lies the
"sword of Orion" which contains the giant
Orion nebula.
Orion
complex A large area of the sky centred on the constellation ofOrion,
containing large amounts of nebulosity and many hot,
young stars.
Photon
The
elementary package or quantum of energy in which light
(or other EM radiation) is emitted and absorbed. The
energy of a photon can be calculated using the Planck-Einstein
formula, E = hf.
Radiation
A general term referring to energy travelling in the form of waves,
such as (a)electromagnetic waves, (b) particles,such as those emitted during
radioactive decay (a and b ).
Reflection
nebula A nebula which is visible mainly due to the scatteringradiation
(often blue light) from particles of gas and dust
within the nebula itself.
Spectral
class Stars are grouped into different "spectral classes" inAccordance
with their surface temperature. TheHottest stars are class O and B, the
coolest are class S. The Sun, with a
surface temperature of about 6000K, is
class G.
Supernova
The
end result of the evolution of the most massive stars.A supernova is a
massive explosion, where the contents of the
star are thrown into space at speeds approaching 0.1c. The
energy released over a period of a few weeks can be
similar to that released by the Sun in its entire (approx. 10
billion year) lifetime. The result of a supernova can be the
creation of a neutron star, or even a black hole.
Transition
The
movement, or change, from one state to another.
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