Philips Atlas of the Universe

Introduction to the Stars

H

ow many stars can you see with the naked eye on a

clear, dark night? Many people will say, ‘Millions’, but

this is quite wrong. There are roughly 5800 stars within

naked-eye range. Only half these will be above the horizon

at any one time, and faint stars which are low down will

probably not be seen. This means that if you can see a

grand total of 2500 stars, you are doing very well.

The ancients divided up the stars into groups of constel-

lations, which were named in various ways. The Egyptians

had one method, the Chinese another, and so on; the con-

stellations we use today are those of the Greeks (admittedly

with Latin names) and if we had used one of the other sys-

tems our sky-maps would look very different, though the

stars themselves would be exactly the same. In fact, a con-

stellation pattern has no real significance, because the stars

are at very different distances from us, and we are dealing

with nothing more than line of sight effects.

Ptolemy, last of the great astronomers of Classical

times, listed 48 constellations, all of which are still given

modern maps even though they have been modified in

places. Some of the groups were named after mythological

characters, such as Orion and Perseus; others after animals

or birds, such as Cygnus (the Swan) and Ursa Major (the

Great Bear), and there are a few inanimate objects, such as

Triangulum (the Triangle). Other constellations have been

added since, notably those in the far south of the sky

which never rose above the horizon in Egypt, where

Ptolemy seems to have spent the whole of his life.

Some of the new groups have modern-sounding

names, such as Telescopium (the Telescope) and Octans

(the Octant). During the 17th century various astronomers

compiled star catalogues, usually by stealing stars from

older groups. Some of the additions have survived (includ-

ing Crux Australis, the Southern Cross), while others

have been mercifully deleted; our maps no longer show

constellations such as Globus Aerostaticus (the Balloon),

Officina Typographica (the Printing Press) and Sceptrum

Brandenburgicum (the Sceptre of Brandenburg). Today

we recognize a total of 88 constellations. They are very

unequal in size and importance, and some of them are so

obscure that they seem to have little claim to separate

identity. One can sympathize with Sir John Herschel, who

once commented that the constellation patterns seemed to

have been drawn up so as to cause the maximum possible

inconvenience and confusion.

Very bright stars such as Sirius, Canopus, Betelgeux

and Rigel have individual names, most of which are

Arabic, but in other cases a different system is used. In

1603 Johann Bayer, a German amateur astronomer, drew

up a star catalogue in which he took each constellation and

gave its stars Greek letters, starting with Alpha for the

brightest star and working through to Omega. This proved

to be very satisfactory, and Bayer’s letters are still in use,

though in many cases the proper alphabetical sequence has

not been followed; thus in Sagittarius (the Archer), the

brightest stars are Epsilon, Sigma and Zeta, with Alpha and

Beta Sagittarii very much ‘also rans’. Later in the century

John Flamsteed, the first Astronomer Royal, gave numbers

to the stars, and these too are still in use; thus Sirius, in

Canis Major (the Great Dog) is not only Alpha Canis

Majoris but also 19 Canis Majoris.

The stars are divided into classes or magnitudes

depending upon their apparent brilliance. The scheme

works rather like a golfer’s handicap, with the more bril-

liant performers having the lower values; thus magnitude 1

is brighter than 2, 2 brighter than 3, and so on. The faintest

stars normally visible with the naked eye are of magnitude

6, though modern telescopes using electronic equipment

ATLAS OF THE UNIVERSE

THE GREEK

ALPHABET

· Alpha

‚ Beta

Á Gamma

‰ Delta

 Epsilon

̇ Zeta

Ë Eta

ı Theta

È Iota

Î Kappa

Ï Lambda

Ì Mu

Ó Nu

Í Xi

Ô Omicron

 Pi

Ú Rho

Û Sigma

Ù Tau

̆ Upsilon

Ê Phi

̄ Chi

„ Psi

ˆ Omega

Ursa Major (The Great

Bear). This is the most

famous of all the northern

constellations. Seven main

stars make up the ‘Plough’

or ‘Dipper’ pattern. Six

of them are white; the

seventh, Dubhe, is orange.

Orion. This brilliant

constellation is crossed by

the celestial equator, and can

therefore be seen from every

inhabited country. Its pattern

is unmistakable; two of the

stars (Rigel and Betelgeux)

are of the first magnitude,

and the remainder between

11 ⁄ 2 and 2. Like Ursa Major,

Orion is a ‘guide’ to the

constellations, though it is

out of view for part of the

year when it is close to the

Sun and above the horizon

only during daylight.

152-191 Atl of Univ Phil'05 7/6/05 1:21 pm Page 166