THE STARS
can reach down to at least 28. On the other end of the scale,
there are few stars with zero or even negative magnitudes;
Sirius is 1.46, while on the same scale the Sun would be
26.8. The scale is logarithmic, and a star of magnitude
1.0 is exactly a hundred times as bright as a star of 6.0.
Note that the apparent magnitude of a star has nothing
directly to do with its real luminosity. A star may look
bright either because it is very close on the cosmic scale,
or because it is genuinely very large and powerful, or a
combination of both. The two brightest stars are Sirius
(1.46) and Canopus (0.73), so that Sirius is over half a
magnitude the more brilliant of the two; yet Sirius is
‘only’ 26 times as luminous as the Sun, while the much
more remote Canopus could match 15,000 Suns.
Appearances can often be deceptive.
There is one curious anomaly. It is customary to refer
to the 21 brightest stars as being of the first magnitude;
they range from Sirius down to Regulus in Leo, whose
magnitude is 1.35. Next in order comes Adhara in Canis
Major; its magnitude is 1.50, but even so it is not included
among the élite.
The stars are not genuinely fixed in space. They are
moving about in all sorts of directions at all sorts of speeds,
but they are so far away that their individual or proper
motions are very slight; the result is that the constellation
patterns do not change appreciably over periods of many
lifetimes and they look virtually the same today as they
must have done in the time of Julius Caesar or even the
builders of the Pyramids. It is only our nearer neighbours,
the members of the Solar System, which move about from
one constellation into another. The nearest star beyond the
Sun lies at a distance of 4.2 light-years, a light-year being
the distance travelled by a ray of light in one year – over 9
million million kilometres (around 6 million million miles).This, of course, is why the stars appear relatively small
and dim; no normal telescope will show a star as anything
but a point of light. Yet some stars are huge; Betelgeux
in Orion is so large that it could contain the entire orbit
of the Earth round the Sun. Other stars are much smaller
than the Sun, or even the Earth, but the differences in mass
are not so great as might be expected, because small stars
are denser than large ones. It is rather like balancing a
cream puff against a lead pellet.
There is a tremendous range in luminosity. We know
of stars which are more than a million times as powerful as
the Sun, while others have only a tiny fraction of the Sun’s
power. The colours, too, are not the same; our Sun is
yellow, while other stars may be bluish, white, orange or
red. These differences are due to real differences in surface
temperature. The hottest known stars have temperatures of
up to 80,000 degrees C while the coolest are so dim that
they barely shine at all.
Many stars, such as the Sun, are single. Others are dou-
ble or members of multiple systems. There are stars which
are variable in light; there are clusters of stars, and there are
vast clouds of dust and gas which are termed nebulae. Our
star system or Galaxy contains about 100,000 million stars,
and beyond we come to other galaxies, so remote that their
light takes millions, hundreds of millions or even thousands
of millions of years to reach us. Look at these distant sys-
tems today, and you see them not as they are now, but as
they used to be when the universe was young – long before
the Earth or even the Sun came into existence. We now
know that many stars are the centres of planetary systems,
and in 2005 the light from two planets of other stars was
measured by astronomers using instruments on a space
telescope (the Spitzer Telescope). There is little doubt that
many Earth-like planets exist. It is a sobering thought.Crux Australis, the
most famous southern
constellation, is shaped
like a kite rather than
an X. Two of the stars in
the main pattern are of
the first magnitude, a third
11 ⁄ 2 and the fourth just
above 3. Two more brilliant
stars, Alpha and Beta
Centauri, point to it. Of
the four chief stars in Crux,
three are hot and bluish
white; the fourth (Gamma
Crucis) orange-red. Crux
also includes a famous
dark nebula, the Coal Sack,
and the lovely Jewel Box
open cluster. These three
photographs (Ursa Major,
Orion, Crux) were taken by
the author with the same
camera and exposure. The
camera was not guided, so
that the stars show up as
very short trails.152-191 Atl of Univ Phil'05 7/6/05 1:22 pm Page 167