The Story of Astronomy (33 page)

The space age has provided many new directions in which to take astronomy. The skies are being mapped in almost every frequency of the spectrum and in more detail than ever before. Objects once studied only through theory, such as black holes and planetary formations, are now the focus of intense observational study. We have close-up images of planetary surfaces, moons, and asteroids and comets on their journeys around the Sun. There are pulsars like the one in the Crab Nebula, the remains of an exploding supernova where the heavy elements are synthesized. There are countless galaxies from the nearby Andromeda Galaxy to distant active galaxies harboring at their core black holes with a mass of over a billion suns. We can now observe so far back in the history of the universe that we will soon be able to see the first galaxies shortly after they formed, and we can map the distribution of galaxies throughout the firmament into the giant clusters and superclusters, themselves gathered to form the large-scale “walls” around the void.

Humans have been on Earth for a few million years, and our recorded observations of the universe go back only a few thousand years. We have seen how humans have always looked with wonder at the skies. We have seen how the uncovering of the secrets of the universe has gradually taken place. This knowledge has been passed
on to future generations. We are the fortunate people that now inherit this knowledge, and we are able to appreciate the origins of the universe far better than our ancestors. We know that there is far more for us to discover in the skies, and that every generation will add new knowledge. It is important to remember also that we are the custodians of the Earth and we must look after it. For all we know, our fragile world may be unique in the universe.

Words in
SMALL CAPITALS
refer to other entries in the Glossary.

absolute magnitude
The magnitude a star would have if it were located at ten
PARSECS
from the Earth.

accretion
The gradual accumulation of matter by an astronomical body, usually by gravitation.

active galactic nucleus
A
GALACTIC NUCLEUS
giving out strong emissions in the
ELECTROMAGNETIC SPECTRUM
.

active galaxy
A very luminous
GALAXY
, usually containing an
ACTIVE GALACTIC NUCLEUS
.

aphelion
The point of a planet's or comet's orbit at which it is farthest from the Sun. Opposite to
PERIHELION
.

apogee
The point in the orbit of the Moon, or of any planet, at which it is at its greatest distance from the Earth; also the greatest distance of the Sun from the Earth.

apparent magnitude
A measure of the brightness of a stellar object as seen from Earth.

asteroid
A rocky object over a few hundred meters in diameter orbiting the Sun.

atmosphere
The sphere of gases surrounding the Earth or any celestial body.

aurora
Light radiated by atoms and
IONS
in the Earth's upper atmosphere.

azimuth
An arc of the heavens extending from the
ZENITH
to the horizon, which cuts it at right angles; the quadrant of a great circle of the sphere passing through the zenith and
NADIR
.

barred spiral galaxy
A
SPIRAL GALAXY
but with the spiral arms attached to a bar running through the nuclear bulge.

Big Bang
The event that created the universe about 13 billion years ago, creating space, time, energy and matter.

binary star
Sometimes called a double star. Two
STARS
that revolve around each other. They are held together by the force of their mutual
GRAVITY
.

black hole
A body with such a strong gravitational field that light cannot escape from it.

blazar
A type of
ACTIVE GALAXY
with very powerful emissions.

celestial object
Any object visible in the night sky.

celestial sphere
The whole of the night sky mapped onto a sphere.

cepheid variable star
A pulsating yellow
SUPERGIANT
star used to calculate stellar distances.

cluster of galaxies
A collection of a few hundred to a few thousand
GALAXIES
held together by their own gravity.

coma
A spherical diffuse cloud of gas seen around the nucleus of a
COMET
near the Sun.

comet
A small body of ice and dust in orbit around the Sun. The ice vaporizes near the sun giving rise to a characteristic tail.

conjunction
The lining-up of three or more bodies. For example Earth–Venus–Sun, which gives rise to the transit of Venus across the Sun.

cosmic background radiation (CMR)
The radiation from the primordial fireball known as the B
IG
B
ANG
that fills all space.

cosmic ray
High-speed particles traveling through space.

dark energy
A repulsive gravitational effect causing the
UNIVERSE
to expand outward.

dark matter
Undetected missing matter from the universe with as yet unknown properties.

declination
The angular distance of a heavenly body (north or south) from the celestial equator, measured
on a meridian passing through the body. It corresponds to latitude on the Earth.

dwarf star
Any star smaller than a giant, e.g.
MAIN SEQUENCE
stars and
WHITE DWARFS
.

eclipse
The blocking of all (total eclipse) or part (partial eclipse) of the light from one celestial body by another.

ecliptic
The plane of the Earth's orbit extended to infinity from the Sun. So called because eclipses can happen only when the Moon is on or very near this plane.

electromagnetic radiation
A very wide range of radiation including gamma rays, X-rays, the optical spectrum, microwaves and radio waves.

electromagnetic spectrum
The whole array of possible electromagnetic emissions.

electron
An atomic particle with negative charge, usually found orbiting an atom.

elliptical galaxy
A galaxy that is elliptical in shape with no spiral arms.

emission nebula
A gaseous
NEBULA
, glowing by the light from a nearby star.

ether
A medium believed to occupy the whole of space, carrying light as a wave motion.

Euclidean geometry
The classical geometry of space as described by Euclid in the ancient world.

event horizon
The boundary of a
BLACK HOLE
.

false-color image
An image, usually outside the optical part of the spectrum, showing the radiation in false colors.

galactic disc
A disc of gas and dust surrounding the nucleus of a galaxy.

galactic nucleus
The central part of a galaxy inside the nuclear bulge.

galaxy
An assembly of young stars, gas and dust kept together by their mutual gravity.

gas giant
A star with a radius of 10 to a 100 times that of the Sun.

geocentric
The system of the world with the Earth as the center.

gravitational energy
The energy of a gravitational field such as is found around a
BLACK HOLE
.

gravity
The property that all matter has an attraction for all other matter in the universe.

heliocentric
The system of the universe with the Sun at the center.

Hertzsprung–Russell (H–R) diagram
A diagram plotting the absolute magnitude of stars against their surface temperature or spectral class.

Hubble constant (
H
₀
)
A constant relating the distance of a galaxy to its speed of recession. The reciprocal of
H
₀
determines the age of the universe.

infrared
Radiation with a wavelength greater than the red end of the visible spectrum.

intercluster medium
Gas and dust between the galaxies in a cluster.

intergalactic medium
Gas and dust between neighboring galaxies.

interplanetary medium
The gas and dust in the space between the planets.

ion
An atom becomes an ion when it loses one or more valence
ELECTRONS
and thus acquires a positive charge, or gains one or more electrons and thus acquires a negative charge.

kelvin (K)
Temperatures measured from absolute zero and referred to in units of kelvin, after Lord Kelvin (1824–1907).

Kuiper Belt
A ring of space beyond the planets that is the birthplace of
COMETS
in the
SOLAR SYSTEM
.

lenticular galaxy
A disc-shaped galaxy, but without the spiral arms.

light year
The distance traveled by light in a year. About 5.88×10
¹²
miles (9.46×10
¹²
km).

local group
The M
ILKY
W
AY
and its neighboring galaxies.

magnitude
A measure of the brightness of any object in the sky. Plotted on a logarithmic scale with the brighter stars as the lower magnitudes.

main sequence
A grouping of stars on the H
ERTZSPRUNG
–R
USSELL
D
IAGRAM
showing the hottest and brightest to the coolest and dimmest stars.

magellenic clouds
Two neighboring galaxies seen from the Southern Hemisphere.

mass
The mass is the amount of matter in a body. (This tautology was first put forward by Isaac Newton.)

Messier catalog
The catalog compiled by Charles Messier (1730–1817) consisting of all the nebulous objects in the sky.

meteor
A streak of light seen in the sky when any space debris is vaporized by the Earth's atmosphere.

meteorite
A fragment of space debris that survives the atmosphere to land on Earth.

meteroid
A fragment of space debris orbiting the Sun.

microwaves
E
LECTROMAGNETIC
RADIATION
with wavelengths of the order 0.001 to 1 meter.

Milky Way
The galaxy to which the Sun belongs.

nadir
The point of the sky directly below the point of observation. The opposite to a
ZENITH
.

nebula
A cloud of interstellar gas and dust.

neutron
An uncharged particle found in the nucleus of an atom.

neutron star
A very dense remnant of a collapsed star, consisting almost entirely of
NEUTRONS
.

nova
A new star. Usually a star in a binary system which suddenly emits powerful radiant energy.

nuclear fusion
Atomic energy generated by the fusing together of atomic nuclei.

Oort Cloud
A hollow spherical shell far out in the S
OLAR
S
YSTEM
, mainly populated by comets.

orbit
The path of an astronomical object moving round another such object.

parallax
The apparent displacement, or difference in the apparent position, of an object, caused by actual change (or difference) of the position of the point of observation. In astronomy there are two kinds of parallax, viz diurnal and annual, the former when a celestial object is observed from opposite points on the Earth's surface, the latter when observed from opposite points of the Earth's orbit. The horizontal parallax is the diurnal parallax of a heavenly body seen on the horizon.

parsec
A measure of distance based on
PARALLAX
; 3.26 light years.

perihelion
The point of a planet's or comet's orbit at which it is nearest to the Sun. Opposite to
APHELION
.

photon
A particle of light.

photosphere
The region in the Sun's atmosphere where most of the visible light originates.

planetary nebula
A luminous shell of gas ejected from an old low-mass star.

plasma
Hot ionized gas (see
ION
).

precession (of the equinoxes)
The earlier occurrence of the equinoxes in each successive sidereal year, due to the retrograde motion of the equinoctial points along the ecliptic, produced by the slow change of direction in space of the Earth's axis.

proton
A positively charged particle found in the nucleus of an atom.

protoplanetary disc
A disc of material surrounding a
PROTOSTAR
or a new star where planets could be created.

protostar
A sphere of gas still growing in mass until it is dense enough to shine and become a star.

pulsar
A pulsating radio source emitted from a rotating
NEUTRON STAR
.