X SGR (X Sagittarii). This wonderful fifth magnitude (typically 4.5, near fourth magnitude) star has a several things going for it, in part as a result of its name. "X" denotes the "unknown," even "mystery." Well, not in this case, as the name simply tells us that it is variable, the seventh variable discovered within Sagittarius (the sequence starting with "R"). "X" also "marks the spot," X Sagittarii doing that quite well, as it is the nearest naked-eye star to the direction to the Galaxy's Center (Galactic longitude and latitude both zero degrees), and acts rather like a Polaris, which marks the North Celestial Pole. Perhaps then it deserves a Latin proper name: "Stella Centri Galaxiae" perhaps, or "Stella Mediogalactica" (which erroneously implies that the star is AT the center), or for a real mouthful "Stella iter ad Centrum Galaxiae Indicans." Best, rather obviously, to stick with "X," which in "mystery mode" guides our eye to toward the three-million- solar-mass black hole that lies at the Galaxy's center 25,000 light years away (and thus so obscured by interstellar dust that it is visible only in the infrared and radio parts of the spectrum). At a distance of 1075 light years (as determined by parallax), X Sgr falls far short of the actual Center. A nominal class F (F7) bright giant, X Sgr is one of the sky's few naked-eye Cepheid variables, like better-known Delta Cephei, Mekbuda, Eta Aquilae, and by odd coincidence, that most famous of other indicators, Polaris (which is a distinctly odd case). Changing like clockwork between magnitudes 4.2 and 4.9 and back every 7.01283 days, its class ranges between about F5 and G9 with a G2 average. Lying deep in Sagittarius (oddly only five degrees northwest of another bright Cepheid, W Sgr), X Sgr is afflicted by a fair degree of interstellar dust absorption, best estimated at about 0.75 magnitudes. Without the obscuration, the star would at its brightest reach close to third magnitude. Using the famed Cepheid period-luminosity relation to calculate the absolute visual brightness, and comparing that to the apparent magnitude, we find a distance of 1075 light years, exactly that determined through direct parallax. With a typical temperature near 5300 Kelvin, there is not much infrared or ultraviolet to deal with, giving us a total luminosity of 3100 times that of the Sun, which in turn gives us a radius of 66 times solar. A slow projected equatorial rotation speed of 24 kilometers per second then leads to a ponderous rotation period of less than 138 days. Direct measure of angular diameter coupled with distance yields a smaller radius 53 times that of the Sun, but that is in infrared light, implying that the size of the star depends on the color with which you examine it (as is the case for many large stars). A mass of 7 or 6 solar masses respectively depends on whether the star is cooling with a dead helium core or heating as a core helium-burner. X Sgr is unsual in that pairs of shock waves (rather like sonic booms) have been observed moving through its outer layers during the pulsation cycle, the only Cepheid for which they have been seen. Having started life as a hot class B dwarf, X Sgr's age falls between 43 and 65 million years. Its fate is to make a massive white dwarf of between 0.9 and 1.0 solar masses, all the rest of the mass ejected back into interstellar space from which it came. (Thanks to Latin scholar David Bright for the wonderful names.)
Written by Jim Kaler 10/31/08. Return to STARS.