OMI SGR (Omicron Sagittarii). A small configuration of modestly bright stars
in Sagittarius about five degrees north of the "Little Milk Dipper" (and just barely north of the ecliptic) makes the lesser-known
"Teaspoon," which consists largely of Pi Sgr("Albaldah"), at magnitude 2.89 the luminary, fourth
magnitude (3.77) Omicron, which is about a degree and a half southwest of
Pi, Xi-1 Sagittarii at 2000 light years the most distant of the set), and
Xi-2 Sagittarii along with a small bunch of fainter Flamsteed stars. In spite of its
location within the Milky Way, Omicron Sagittarii is
close enough (just 142 light years, give or take 2) that there is little interstellar dust absorption along the line
of sight, and we'll ignore it. A class G (G8) giant with a temperature of 4844 Kelvin
(thus producing some infrared light
that must be accounted for), Omi Sgr shines with the light of 66 Suns, really rather modest for a "giant," especially
when noting that the star's radius is but 11.6 times that of the Sun. Theory
suggests a mass of about 2.7 Suns, though oddly other studies yield a smaller
mass of 2.0. Otherwise the star is a normal "clump" giant (so-called because
its luminosity and temperature are similar to so many others of its kind.
As a result, clump stars actually make a good distance indicator for other
galaxies. But back to Omicron
itself. It has a purported fourteenth magnitude companion 38.4 or so seconds of arc away.
Given the density of faint Milky Way stars, a physical relation seems unlikely
and the pairing is probably just a line of sight coincidence. If not, however,
the "companion" would be at least 1700 AU from the bright star and take at
least 42,000 years to make a full revolution around Omi Sgr A. It would also
be a red dwarf of little consequence
down toward the bottom of the "main sequence" of stars
that don't get hot enough inside to run thermonuclear fusion. As feeble as
they are such red dwarfs lead to even lower mass "brown dwarfs" and then on to planets. Stars
are usually thought of as being made from "the top down", from collapse of
dense bloblets in interstelllar clouds, while planets are commonly defined
as being built from the bottom up, by accumulation of dust in circumstellar
disks. Is there an overlap? Probably, but no one yet knows.
Written byJim Kaler 9/01/17. Return to STARS.