PEACOCK (Alpha Pavonis). What a curious name, one derived not from Greek, Latin, Arabic, or even someone's name spelled backwards, but one in straightforward English. Peacock, the luminary of Pavo, the Peacock, hardly needs translation. Many star names directly reflect their constellations, and clearly so does this one. To be in English, it must be modern, and so is the constellation. Far too deep in the southern hemisphere for the Greek sky-watchers, Pavo was invented by two Dutch explorers around the year 1600 and included on Bayer's famed Uranometria. One of the brighter stars in the sky, Peacock shines at mid-second magnitude (1.94), but is hardly known in the north as it is not visible much north of 32 degrees north latitude. One of the hotter stars in its magnitude rank, Peacock is a hot blue class B (B2) star that has been taken both as a hydrogen-fusing dwarf and as a subgiant (which implies that evolution toward death is beginning to take place). Its temperature of 18,700 Kelvin, however, is notably low for a B2 dwarf, and implies more of a giant status, which it certainly does not have. From its distance of 180 light years, we calculate a visual luminosity 450 times that of the Sun. Much of a hot star's radiation is in the invisible ultraviolet, however, and when that is taken into account, the total luminosity climbs to 2100 solar, which together with temperature yields a radius 4.4 times solar. Direct measure of angular radius gives very nearly the same dimension. With a mass somewhere around 5 to 6 times that of the Sun, the star will - - anomalies in temperature and class aside -- surely die as a massive white dwarf. Peacock is also a close double (found by analysis of the spectrum) with a short period of only 11.8 days, implying a separation of only 0.21 astronomical units (about half Mercury's distance from the Sun). Analysis of the orbital velocities obtained from the star's spectrum, however, gives a separation of only 0.008 AU. The reason is that the orbit is nearly in the plane of the sky, so that we sense only a part of the orbital speed, the difference in the two numbers showing an orbital tilt of only a few degrees. That conclusion is consistent with the low rotation speed of only 39 kilometers per second. The star must be spinning with its rotation axis directed nearly at us, which in turn is consistent with the star having no particular chemical anomalies, as true slow rotators so often do (as a result of chemical separation in a quiet atmosphere). Peacock's one great claim to astronomical fame is a measure of an upper limit to its deuterium (a heavy form of hydrogen) abundance. Deuterium was made in the Big Bang, the event that created our present Universe, and its abundance is a test for theory and a determinant of the Universe's nature. The low abundance in Peacock suggests that stars may "burn" their deuterium and that corrections must be taken into account.
Written by Jim Kaler. Return to STARS.