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.