HR 6535 SCO (HR 6535 Scorpii), or just HR 6535, as HR numbers in the
Yale Bright Star Catalogue
are sequential from west to east (counting from the Vernal Equinox) and ignore the constellations. A seemingly innocuous sixth
magnitude star (though at 5.70 on the bright side) buried in the Milky Way, HR 6535 was clearly too faint to merit
a Greek letter and at 33 degrees
south of the celestial equator really
too far down in Scorpius for Flamsteed. A closer look reveals
some remarkable properties. First, HR6535 is almost exactly on the
Galactic Equator, the great circle that divides the Milky Way exactly
in two (or as exact as we can get it). The star has a galactic latitude
of just 0.0493 degrees (2 minutes, 57.5 seconds) north. For
completeness, with a galactic longitude of 355.667 degrees, it's not
that far off the galactic center either,
defined by Sagittarius A*, the supermassive black hole and source of radio
emission at the Galaxy's core.
Second, like 10 Monocerotis HR 6535 sits
squarely on, and is the most luminous member of, an open cluster, NGC 6383.
The luminous binary class O star (both O7 dwarfs) HR 6535 overwhelms
the rest of the stars in the rather sparse open cluster NGC 6383. (STScI
Digitized Sky
Survey.)
Third, it's not just one star, but two nearly identical ones in
close orbit and, to top it off, they are not just any old stars, but
two of rare class O dwarfs,
both originally thought to be 06, but now widely accepted as O7 (which
is not that much of a difference). The distance is a real problem as
the uncertainty in the parallax is almost as big as
the parallax itself, which for what it's worth yields a distance of
4240 light years. The better-determined distance of the cluster
considered as a whole, however, is a somewhat supportive 3200 light
years, so for further argument take an average of 3750 light years.
(It really doesn't matter all that much.) At that distance and in the
star's central direction, we might expect significant dimming by interstellar dust, and we are not
disappointed, as the visual extinction of light is (calculated from
the star color, which is that of a class A2 star) 1.22 magnitudes, which
raises the visual magnitude (if the view were clear) to a nice fourth
magnitude (4.48). Assuming the stars to be equal yields a visual
magnitude for each of 5.23. The temperatures are given as 39,000 and
38,000 Kelvin, which we'll use even though inconsistent with equality
(the other uncertainties certainly worse). Adding in the amounts of
ultraviolet light calculated from the temperatures, we find
luminosities of 260,000 and 220,000 Suns, radii of 11.2
and 11.4 times that of the Sun, and masses of 38 and 35 times solar,
similar to other analyses. Still fusing hydrogen in their cores, the stars are
only two to three million years old,
as is the cluster. No matter what the details, these are two
whopping stars. Taking an average projected equatorial rotation
velocity of 155 kilometers per second, we get a rotation period of under
3.66 days, which is the same as the measured orbital period of 3.66767
days, showing the pair to be in synchronous rotation, the same sides
of each perpetually facing each other, and the orbital plane to be
nearly in the line of sight. Kepler's third law then gives a
separation of 0.19 Astronomical Units, half Mercury's distance from
the Sun, 40 solar radii, 3.5 stellar radii. Mutual tides and rotation somewhat distort the
stars, making the pair into a non-eclipsing "ellipsoidal
variable" that changes over the cycle by a few hundredths of a
magnitude. Winds that blow from each star at a speed of some 2900
kilometers per second and at a rate of three hundred-thousandth of a
solar mass per year, 600 million times that of the solar wind, collide
to make the star a relatively strong source of X-rays. Several small companions
are listed for HR 6535, but that is not surprising either given that
it's a member of a cluster. The future of the stars is unclear. As they
evolve, they will encroach upon each other, perhaps merge, perhaps not.
We can be assured of at least one supernova explosion and
perhaps a resulting black
hole. Maybe even two of them that will eventually merge to create
gravity waves like those just discovered. (Stellar data are taken in
part from M. De Becker et al. in Astronomy and Astrophysics, 416, 233,
2004.)
