OMEGA-1 AND OMEGA-2 SC0 (Omega-1 and Omega-2 Scorpii), another
two-for-one special. In Scorpius, seven and
a half degrees northwest of Antares and
just one degree south-southeast of Beta
Sco (the northernmost star in the trio that makes the Scorpion's
head), lies a prominent duo of stars, fourth magnitude (3.96) Omega-1
Scorpii (the western of the pair) and fainter-fourth magnitude (4.32)
Omega-2. Only a quarter of a degree apart, at first they seem like a
naked-eye double. But once
again the eye is fooled, as they have no relation to
each other except chance alignment, as Omega-1 is 471 light years away
(give or take 18), Omega-2 60 percent as far, at 291 light years (plus
or minus 8). They are radically different physically as well. Omega-2
is a yellowish class G (G3, though classed as cool as G8) ageing giant, whereas Omega-1 is a blue,
hot class B (B1) youthful dwarf. Unfortunately Omega-1's
true color does not come through as a result of interstellar dust absorption and
reddening that makes it appear more white. If the path to the star were
clear, Omega-1 would be just over a magnitude brighter, shine at a nice
third magnitude of 2.94, and perhaps even have a proper name. Distance
and a temperature of 26,530 Kelvin (from which we calculate a lot of
ultraviolet radiation) gives a
whopping luminosity of 11,700 Suns, a radius
5.13 time that of the Sun, and a mass of just under 12 Suns (which agrees
well with a published value of 11.1), the star roughly 80 to 100 million
years old, well along to the cessation of core hydrogen fusion. A
projected equatorial rotation speed of 105 kilometers per second
(nowhere near the limit) yields a rotation period of under 2.5 days.
It may be a subtle "Beta Cephei" variable
with a period of 0.667 days but of unknown amplitude. Omega-1 Sco is
part of the expanding Scorpius OB association of young luminous
stars, specifically belonging to the "Upper Scorpius" subgroup along
with Antares, Sigma, Pi, and Nu Scorpii,
Chi and Rho
Ophiuchi, and 48 Librae. The average
distance to the subgroup is 145 light years, which places Omega-1
pretty much in the middle. Its mass of a dozen Suns is enough to someday
send it over the edge. An iron core derived from advanced nuclear fusion
will collapse to a neutron
star, blowing the outer envelope away in a grand supernova, the event creating
all the elements of the periodic table, which will mix with the gases
of interstellar space from which new generations of stars will be born.
Nowhere near as dramatic, Omega-2 is an ordinary helium fusing giant. Including some infrared radiation from its 5300
Kelvin surface, the star radiates at a rate of 141 Suns, which gives
a radius 14.1 times solar and a mass of 3 Suns. There is little if any
intervening interstellar dust. An interferometer measure of radius in
the infrared gives 15.6 times solar, which is decent agreement given
that such large stars do not have sharply defined surfaces. Instead
of exploding, Omega-2 will slough off its outer envelope. The inner
portion will turn into a planetary nebula
illuminated by the exposed core, which in turn will cool forever as
a white dwarf of around 0.7
solar masses.
Written byJim Kaler 09/09/16. Return to STARS.