Australian Sky & Telescope — November-December 2017

(Marcin) #1
 OUR SUN IS an average star about a
third of the way through its lifetime. For
most of a star’s existence, its colour and
brightness depend almost entirely on
how much hydrogen it was born with.
Stars much more massive than our Sun
are hot, bright and blue-tinted; stars less
massive than ours are comparatively
cooler, fainter and red-hued.
The plot of colour (temperature)
versus absolute brightness (luminosity)
above, shows that most stars lie along
a swath called the main sequence. (Our
Sun is about midway along this arc.)
Main-sequence stars are busy burning
hydrogen in their cores. Red giants and
supergiants, for their part, have stopped
hydrogen core-burning. Instead,
they host vast envelopes of gas that

surround nuclear shell-burning layers
around an inert, compact core.
Plots like that seen here are called
Hertzsprung-Russell diagrams after the
two astronomers — one Danish and
one American — who independently
developed them in the early part of
the last century. Mainstays of modern
astronomy, H-R diagrams have greatly
aided astronomers in teasing out the
secrets of stellar evolution.
Despite in many cases truly
astronomical longevity, all stars must
die, and their initial mass largely
determines their fate. Late in their lives,
stars that end up with cores of about
1.4 times the mass of our Sun or less
will, after using up the last of their fuel,
become white dwarfs. That’s our own

star’s destiny, far off in the future. By
then it will have shrunk to not much
larger than the Earth, but it will bear an
incredible density: One teaspoon of its
matter would weigh about a metric ton.
Those stars whose late-life cores
have between roughly 1.4 and 3 times
the Sun’s mass will, at some sudden
moment, cataclysmically explode in
a supernova and wind up as neutron
stars. The size and density of these
objects make white dwarfs seem huge
and practically porous in comparison:
A teaspoon of neutron star, which is
only 10 or 20 kilometres across, would
weigh a billion tonnes.
Finally, those stars with cores over 3
times our star’s mass will, after going
supernova, condense into black holes.

The lives of stars


LEAH TISCIONE /


S&T


Luminosity (Sun = 1)

Star sizes not to scale

CentauriAlpha
B

Procyon B

CentauriProxima

Sirius A Alpha Centauri A

Regulus

Sirius B

Rigel
Polaris

Spica

Eta Carinae

Aldebaran

Antares

Betelgeuse

Sun

White
Dwarfs

Horizontal
Branch

MAIN


SEQUENCE


Red
Dwarfs

Red
Giants

Blue
Supergiants

Red
Supergiants

10 -5

30,000 20,000 10,000 6,000 4,000 2,500

10 -4

10 -3

10 -^2

10 -1

1

10

10 2

10 3

10 4

10 5

10 6

Surface temperature (kelvins)

http://www.skyandtelescope.com.au 29
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