L2 PUP (L2 Puppis). Directly below the triangle of bright stars to the south of Sirius lies Pi Puppis (of the constellation of Puppis, the Stern of Argo, the Ship), and almost immediately south of Pi lies curious L2 Pup. The Roman-letter name harkens back to Bayer's practice of following his Greek letter names by lower case Roman letters, followed by upper case Roman. Bayer's maps of Argo are wildly in error, however, because the stars could not be seen from mid and northern Europe. He had to rely on other reports, and did not himself name fairly bright L2 (Nicolas de Lacaille getting into the act). The number is used to separate this class M (M5) giant from more ordinary and unconnected fifth magnitude (4.88) class A (A0-peculiar) L1 Pup, which lies immediately to the south and is at least 10 light years away from L2. L2 Pup is not only one of the brightest variable stars in the sky, it is one of the brightest pulsating Mira-type variables, and more, one that can be followed through its whole cycle with the naked eye. Though actually classed as a "semi-regular" variable (technically an "SRb" star), such stars (or at least part of the sample) are now thought to be just low-amplitude Miras. To the eye, L2 varies between near-second magnitude (as bright as 2.6) to sixth (6.2) over a 141- day cycle, meaning that sometimes it is a prominent part of its parent constellation, other times not (indeed, like Mira itself). At a distance of 200 light years, this red giant star shines at a luminosity that is somewhere (depending upon the research adopted) between 1500 and 2400 times that of our Sun, radiated from a surface with a temperature of 3400 Kelvin. The problem is that most of the star's light is emitted in the infrared rather than in the optical where we directly see it. The star -- with a mass between 1 and 3 solar -- is probably in the early stages of dying with a dead carbon-oxygen core, though other scenarios (a dead helium core for example) cannot be ruled out. Typical of such stars, it is losing mass at a rate estimated between 3 ten-millionths and 5 hundred millionths of a solar mass per year (which while it seems low, is a million or so times the loss rate of the solar wind). The mass of dust gas around the star produces a silicon monoxide "maser," a natural radio version of the common "laser." The mass loss rate will someday increase, to reveal the old nuclear burning core, which will die as a small white dwarf. A curiosity is the low wind velocity of only a few kilometers per second, again suggesting early stages of Mira-type behavior. L2 Pup has been classed as a double star with a 10th magnitude companion about a minute of arc away. Over the past century, however, the two stars have separated far more than would be expected, showing that the seeming neighbor is just a line-of-sight coincidence.
Written by Jim Kaler. Return to STARS.