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The Question
(Submitted June 16, 1997)
I have just became aware of the enormity of Betelgeuse.
Do we know of stars that are larger than this?
If so, approximately how many are of this scale are larger?
Does a star have to be of this size to be a candidate for a black hole?
The Answer
Betelgeuse (also known as alpha ori) is a very large star, an M
supergiant. This is because it has evolved far from the state in which
stars spend most of their lives, known as the main sequence. For stars on the
main sequence, which includes our Sun, there is simple proportionality
between size and mass, and also a simple scaling for luminosity. For
evolved stars the situation is less simple. Betelgeuse is more than
1000 times larger than the Sun, and 50000
times as luminous, but only about 20 times as massive. Most of the light
from Betelgeuse comes out in the infrared, however, which is very
different from the Sun. One consequence of the advanced evolutionary state
of Betelgeuse is that it probably was much more massive when it was on the
main sequence, and has already lost a significant fraction of its mass
(probably more than half) in a stellar wind.
There are many stars that are as massive as Betelgeuse is now, and
probably many that are as massive as Betelgeuse was when it was
on the main sequence. Of the 100,000,000,000 (100 billion=10^11) or
so stars in our galaxy, it is estimated that approximately 1% have main
sequence masses greater than 30 times that of the Sun, which is where Betelgeuse
may have started out. A very crude estimate is that such stars spend
1% of their lives as supergiants, which would suggest 10,000,000 stars
similar to Betelgeuse in our galaxy.
In spite of this fact, there are very few stars which are visible to the
naked eye which are as large as Betelgeuse. This is simply a consequence
of the fact that we can distinguish bright stars in only a small fraction
of the galaxy. Another one is Mira, in the constellation Cetus. Mira
is probably larger than Betelgeuse, so large that it is thought that the
outer layers of the star are barely held together by gravity. Mira is
known to pulsate and eject its outer layers, probably in large part
because of its weak gravity. Possibly the most massive known star is
eta carina, which may have been 150 times as massive as the Sun when it
first formed, and may be 50 - 60 times as massive as the Sun currently.
In the 1830s eta carina underwent a tremendous outburst during which it
became a brilliant naked eye object and ejected an amount of gas with
mass approximately equal to the mass of the Sun.
It is likely that the minimum main sequence
mass for a star which will eventually make a black hole is 8 - 10 times
the mass of our Sun. This is quite a bit less than Betelgeuse had when
it was on the main sequence, and there are many such stars in our galaxy.
I hope this helps!
Tim Kallman for Ask an Astrophysicist
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