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.