ANTARES (Alpha Scorpii). A brilliant jewel set within the Milky Way, Antares guides us to one of the great
constellations of the sky, the Zodiac's Scorpius (or Scorpio), the celestial scorpion, one of
the few constellations that actually looks like what it represents.
Antares, a class M (M1.5) red supergiant gleaming redly at
the scorpion's heart, has a color similar to Mars. Since it is
found within the Zodiac, which contains the apparent path of the
Sun and planets, it is commonly mistaken for the red planet, a fact
shown by its name, Antares, or "Ant-Ares," which means "like Mars,"
"Ares" being the Greek name for the god of war. This magnificent
first magnitude (typically 0.96) star, shining opposite Betelgeuse, its counterpart in Orion, is ranked the 15th brightest in the
sky. It is, however, a semi-regular variable that can change by
several tenths of magnitude over a period of years. Its great
distance of 550 light years (second Hipparcos
reduction) reveals that it is truly luminous, to
the eye almost 10,000 times brighter than the Sun. Because it is cool, only about 3600
degrees Kelvin at its surface, it radiates a considerable amount of
its light in the invisible infrared. When that is taken into
account, the star becomes some 60,000 times brighter than the Sun
(with considerable uncertainty). A low temperature coupled with
high luminosity tells us that the star must be huge, luminosity and
temperature giving a radius of about 3 Astronomical Units. It is
so big that astronomers can easily detect and measure the size of
its apparent disk, which gives an even bigger radius of 3.4 AU, 65
percent the size of the orbit of Jupiter. The difference is caused
by uncertainties in distance, temperature, the state of pulsation,
and the actual location of the mass-losing surface, as the star is
slowly evaporating under a fierce wind that has encased it in a gas
cloud, or nebula, that shines by light scattered from the
ultraluminous star within. The amount of dimming by interstellar
dust is not well known. If as high as half a magnitude, Antares
could be as bright as 90,000 Suns, pretty much taking care of the
radius discrepancy. Buried within the wind is a fifth magnitude
(5.5) hot class B (B2.5) companion star (only 3 seconds of arc
away) that hides within Antares' bright glare. The two are
separated by roughly 550 AU and take perhaps 2500 years to orbit
each other. The companion hollows out a small ionized region
within the wind, and although blue-white, has the reputation of
appearing green as a result of a contrast effect with its brilliant
reddish mate. Antares, with an uncertain mass of 15 to 18 solar
masses, probably does not have much time left to it. It is massive
enough someday to develop an iron core and eventually to explode as
a brilliant supernova. The
event may be a million years off, an astronomical blink of an eye;
or it may occur tonight, so keep a watch on one of the great stars
of the nighttime sky. The companion, however, at around 7 to 8
solar masses, seems to be just below the supernova limit and will
probably die as a massive white dwarf. See the Moon
visit Antares.
Written by Jim Kaler 6/05/98. Last updated
6/26/09. Return to STARS.