The central and brightest of the three stars in Orion's Belt and
the fourth brightest in the whole of Orion. Alnilam
is a blue-white supergiant that for many years has served as a standard
star against which to compare others. Its relatively simple spectrum
also provides a useful background against which to study the gases of
the intervening interstellar medium. Like most supergiants,
Alnilam is rapidly shedding mass: a powerful stellar wind
blowing from its surface at speeds up to 2,000 km/s and carrying way
about two millionths of a solar mass per year (20 million times the rate
lost by the Sun). Though only about four million years old,
Alnilam is already fusing heavy elements in its core and doomed
to explode, in the next million years or so, as a supernova.
