MIRA (Omicron Ceti). Mira, its very name telling us that we should take strong
notice, Mira "the amazing one," the word coming from the same root
as "miracle," Mira the only proper-named star in the sky that for
a time is too faint to be seen with the naked eye. Few of the
stars in its resident constellation Cetus, the Whale, are prominent; only the
Alpha and
Beta
stars are of the second magnitude. Mira itself was relegated
by Johannes Bayer to be the "Omicron" (the 15th letter in the
Greek
alphabet) star. In 1572, Tycho Brahe studied a "new star" so
bright that for a time it was visible in daylight. Not new at all,
Tycho's star is now known to have been an old star that exploded
and disappeared forever. Two dozen years later, David Fabricius
thought he might have found another, though much fainter one, in
Cetus. This star, however, returned, and has been doing so for
over 400 years. Mira is the brightest of the red class M "long
period variables," thousands of which are now known. Our star
varies from about third magnitude (though sometimes it can reach
second) way down to tenth, 40 or so times fainter than the human
eye can see alone, and then back again over a 330 day period. As
a result it is sometimes a part of its constellation, sometimes
not. Mira, with a temperature just above 2000 degrees Kelvin, is
one of the coolest stars in the sky. From its distance of 420
light years, we calculate an average luminosity (that includes
invisible infrared radiation) 15,000 times that of the
Sun. The
star is approaching the last stages of its life. Long ago, the
hydrogen fusion that powered its core ran out, and then the by-
product of that fusion, helium, fused to carbon and oxygen, and now
the helium has also run out. The result of these internal changes
is a hugely distended, very luminous star that is double the size
of the orbit of Mars. Hubble Space Telescope observations show
that the star is so unstable that it is not even round. The light
variations are caused by pulsation, changes in size that also
affect the star's temperature. Mira's great size and instability
promote a wind that will soon blow away its outer envelope, the
inner nuclear burning portions condensing into a burnt-out "white
dwarf," a tiny star the size of Earth, the rest of the star lost to
interstellar space. These long period variables help enrich the
interstellar gases, out of which new stars condense, with chemical
elements formed in their nuclear cauldrons. Most of the carbon in
the Universe seems to have come from them. Mira has a white dwarf
companion to which all these events have already happened. Many
billions of years from now, the same will happen to our Sun.