Philips Atlas of the Universe

GLOSSARY

C

Caldwell catalogue: A list of 109 bright nebular

objects, none of which are included in the Messier

catalogue.

Carbon-nitrogen cycle: The stars are not ‘burning’ in

the usual sense of the word; they are producing their

energy by converting hydrogen into helium, with

release of radiation and loss of mass. One way in which

this conversion takes place is by a whole series of

reactions, involving carbon and nitrogen as catalysts. It

used to be thought that the Sun shone because of this

process, but modern work has shown that another cycle,

the so-called proton-proton reaction, is more important

in stars of solar type. The only stars which do not shine

because of the hydrogen-into-helium process are those

at a very early or relatively late stage in their evolution.

Cassegrain reflector: A type of reflecting telescope

(see reflector)in which the light from the object under

study is reflected from the main mirror to a convex

secondary, and thence back to the eyepiece through a

hole in the main mirror.

Celestial sphere: An imaginary sphere surrounding

the Earth, concentric with the Earth’s centre. The

Earth’s axis indicates the positions of the celestial

poles; the projection of the Earth’s equator on to

the celestial sphere marks the celestial equator.

Centrifuge: A motor-driven apparatus with long arm,

at the end of which is a cage. When people (or animals)

are put into the cage, and revolved and rotated at high

speeds, it is possible to study effects comparable with

the accelerations experienced in spacecraft. Astronauts

are given tests in a centrifuge during training.

Cepheid: An important type of variable star, Cepheids

have short periods of from a few days to a few weeks,

and are regular in their behaviour. It has been found

that the period of a Cepheid is linked with its real

luminosity: the longer the period, the more luminous

the star. From this it follows that once a Cepheid’s

period has been measured, its distance can be worked

out. Cepheids are luminous stars, and may be seen

over great distances; they are found not only in

our Galaxy, but also in external galaxies. The name

comes from Delta Cephei, the brightest and most

famous member of the class.

Charge-Coupled Device (CCD): An electronic

imaging device which is far more sensitive than a

photographic plate, and is now replacing photography

for most branches of astronomical research.

Chromatic aberration: A defect found in all lenses,

resulting in the production of ‘false colour’. It is due to

the fact that light of all wavelengths is not bent or

refracted equally; for example, blue light is refracted

more strongly than red, and so is brought to focus

nearer the lens. With an astronomical telescope, the

object-glass is made up of several lenses composed of

different kinds of glass. In this way chromatic aberration

may be reduced, although it can never be entirely cured.

Chromosphere: The part of the Sun’s atmosphere

lying above the bright surface or photosphere, and

below the outercorona. It is visible with the naked eye

only during total solar eclipses,when the Moon hides

the photosphere; but by means of special instruments

it may be studied at any time.

Circular velocity: The velocity with which an object

must move, in the absence of air resistance, in order

to describe a circular orbit around its primary.

Circumpolar star: A star which never sets, but merely

circles the celestial pole and remains above the horizon.

Clusters, stellar: A collection of stars which are

genuinely associated. An open cluster may contain

several hundred stars, usually together with gas and

dust; there is no particular shape to the cluster.

Globular clusters contain thousands of stars, and are

regular in shape; they are very remote, and lie near the

edge of the Galaxy. Both open and globular clusters are

also known in external galaxies. Moving clusters are

made up of widely separated stars moving through

space in the same direction and at the same velocity.

(For example, five of the seven bright stars in the Great

Bear are members of the same moving cluster.)

Collimator:An optical arrangement for collecting

light from a source into a parallel beam.

Colour index:A measure of a star’s colour and hence

of its surface temperature. The ordinary or visual

magnitude of a star is a measure of the apparent

brightness as seen with the naked eye; the

photographic magnitude is obtained by measuring the

apparent size of a star’s image on a photographic plate.

The two magnitudes will not generally be the same,

because in the old standard plates red stars will seem

less prominent than they appear to the eye. The

difference between visual and photographic magnitude

is known as the colour index. The scale is adjusted so

that for a white star, such as Sirius, colour index 
0. A

blue star will have negative colour index; a yellow or

red star will have positive colour index.

Colures:Great circles on the celestial sphere. The

equinoctial colure, for example, is the great circle

which passes through both celestial poles and also

the First Point of Aries (vernal equinox), i.e. the

point where the ecliptic intersects the celestial equator.

Coma:(1) The hazy-looking patch surrounding the

nucleus of a comet.(2) The blurred haze surrounding

the images of stars on a photographic plate, due to

optical defects in the equipment.

Comet: A member of the Solar System, moving around

the Sun in an orbit which is generally highly eccentric. It

is made up of relatively small particles (mainly ices)

together with tenuous gas: the most substantial part of

the comet is the nucleus, which may be several

kilometres in diameter. A comet’s tail always points

more or less away from the Sun, due to the effects of

solar wind.There are many comets with short periods,

all of which are relatively faint; the only bright comet

with a period of less than a century is Halley’s. The

most brilliant comets have periods so long that their

return cannot be predicted. See also sun-grazers.

Conjunction: The apparent close approach of a planet

to a star or to another planet; it is purely a line-of-sight

effect, since the planet is very much closer to us than

the star. An inferior conjunction,for Mercury and

Venus, is the position when the planet has the same

right ascensionas the Sun (see inferior planets.)

A superior conjunction is the position of a planet when

it is on the far side of the Sun with respect to the Earth.

Constellation: A group of stars named after a living

or a mythological character, or an inanimate object.

The names are highly imaginative, and have no real

significance. Neither is a constellation made up of

stars that are genuinely associated with one another;

the individual stars lie at very different distances

from the Earth, and merely happen to be in roughly

the same direction in space. The International

Astronomical Union currently recognizes 88 separate

constellations.

Corona: The outermost part of the Sun’s atmosphere;

it is made up of very tenuous gas at a very high

temperature, and is of great extent. It is visible to

the naked eye only during total solar eclipses.

Coronagraph: A type of telescope designed to

view the solar corona in ordinary daylight; ordinary

telescopes are unable to do this, partly because of

the sunlight scattered across the sky by the Earth’s

atmosphere, and partly because of light which is

scattered inside the telescope – mainly by particles

of dust. The coronagraph was invented by the French

astronomer B. Lyot.

Cosmic rays: High-velocity particles reaching

the Earth from outer space. The heavy cosmic-ray

primaries are broken up when they enter the top

part of the Earth’s atmosphere, and only the secondary

particles reach ground-level. There is still much doubt

whether cosmic radiation will prove to be a major

hazard in long-term spaceflights.

Cosmology: The study of the universe as a whole;

its nature, origin, evolution, and the relations between

its various parts.

Counterglow: SeeGegenschein.

Crab Nebula: The remnant of a supernova observed

in 1054; an expanding cloud of gas, approximately 6000

light-years away, according to recent measurements.

It is important because it emits not only visible light

but also radio waves and X-rays. Much of the radio

emission is due to synchrotron radiation(that is,

the acceleration of charged particles in a strong

magnetic field). The Crab Nebula contains a pulsar,

the first to be identified with an optical object.

Culmination: The time when a star or other celestial

body reaches the observer’s meridian,so that it is

at its highest point (upper culmination). If the body is

circumpolar, it may be observed to cross the meridian

again 12 hours later (lower culmination). With a

non-circumpolar object, lower culmination cannot

be observed as, at that point, the object is then below

the horizon.

Cybernetics: The study of methods of communication

and control that are common both to machines and

also to living organisms.

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