Philips Atlas of the Universe

ATLAS OF THE UNIVERSE

The Lives of the Stars

W

hen we try to work out the life-story of a star, we

are faced with the initial difficulty that we cannot –

usually – see a star change its condition as we watch, just

as an observer in a city street will not notice a boy chang-

ing into a man. All we can do is decide which stars are

young and which are old, after which we can do our best

to trace the sequence of events. Earlier theorists, through

no fault of their own, picked wrong.

The mistake lay in a faulty interpretation of the HR

Diagram. It was supposed that a star began as a very large,

cool red giant such as Betelgeux; that it heated up and

joined the Main Sequence at the upper left of the Diagram,

and then slid down towards the lower right corner, cooling

steadily and becoming a dim red dwarf before fading out.

Certainly this would explain the giant-and-dwarf divisions,

but we now know that red giants such as Betelgeux are

not young at all; they are far advanced in their evolution,

so that they rank as stellar old-age pensioners.

According to current theory, a star begins by condens-

ing out of the tenuous material making up a nebula. Chance

condensations lead to the appearance of non-luminous

masses called globules, many of which can be seen in neb-

ulae because they blot out the light of stars beyond. Gravity

causes the mass to shrink, and as it does so it heats up near

its centre. When the temperature has risen sufficiently, the

mass begins to glow and turns into a protostar.

M27 (NGC 6853) –

a planetary nebula in

Vulpecula always known

as the Dumbbell Nebula.

It is just under 1000 light-

years away. This image

was taken in October 1998

with Antu, the first unit

of the VLT (Very Large

Telescope) at Cerro Paranal

in Chile. The Antu mirror

is 8.2 metres (323 inches)

in diameter.

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