1 AND 4 CEN (1 and 4 Centauri), another two-for one special. The stars
of Centaurus (the Centaur) have an odd
naming history. The letters look as if Bayer had done his usual job
in his great atlas, the Uranometria, Greek letters first then lower then
upper case Roman. However, he could not see most of the constellation, nor for that matter could Tycho
Brahe, on whose observations the atlas is based. Instead the letters
were re-applied using Greek then upper and lower case Roman (reversed
from Bayer's scheme) by Nicolas de Lacaille, who explored the southern
heavens. Working from England, Flamsteed could see no further down than
35 or so degrees south of the celestial
equator, so he measured the positions of only four stars near the
constellation's northern boundary with Hydra (the stars just south of Hydra's tail).
These four were then later numbered from east to west. Of them 3 Centauri (actually 3 Cen A, a young B5 dwarf) is the best known, really
famed, as it's one of the very few stars whose spectrum reveals the presence of the light
isotope of helium, He-3 (of which there is practically none on Earth;
all our helium comes from the decay of uranium and thorium and is He-4).
2 Cen is a fine red, slightly-variable, class
M5.5 giant. So now we round off
the quartet with 1 Centauri, a class F (F3) subgiant, and 4 Cen, classed a
B4 subgiant but (see below) really a dwarf, all giving us a nice range
of color. With a well-known temperature of 6790 Kelvin and distance
of 63.8 light years (give or take just 0.3), 1 Cen radiates almost all
its energy in the visual spectrum, shining with a luminosity of 6.0
Suns, which leads to a radius 1.8 times solar.
A projected equatorial rotation velocity of 75 kilometers per second
gives a rotation period under 1.2 days (the star just slightly warmer
than the "rotation break" at F5 above which stars lose their outer
convective layers and speed up; or rather don't slow down). Theory then
reveals a mass of 1.4 Suns and shows the star indeed to be either a
subgiant (whose core is exhausted of hydrogen fuel)or an older dwarf
soon to become one, the age just under three billion years. By contrast,
blue-white 4 Cen's temperature of about 16,400 Kelvin is not
well-determined. Factoring in a lot of ultraviolet radiation and the
distance of 637 light years (give or take 87), 4 Cen's total luminosity
is around 1225 times that of the Sun, its radius 4.3 times solar. A
rotation speed of at least 27 kilometers per second gives a rotation
period less than 8.1 days. Since there are no obvious abundance
anomalies caused by element separation in a quiet atmosphere, the star
is probably rotating much faster (so as to keep things stirred up) with
its rotation pole pointed more or less at us. With a mass of 5.5 Suns,
4 Cen appears to be an older dwarf closing in on its dwarf lifetime
of 65 million years. Both 1 and 4 Cen have spectroscopically-detected
companions with respective periods
of 9.945 and 6.927 days, which, assuming they are of low mass, give
from Kepler's laws a
separation from their parent stars of 0.30 and 0.12 Astronomical Units.
1 Cen may be a Delta Scuti type of variable
with a short period of 0.02 days, while 4 Cen has a line-of-sight 15th
magnitude neighbor currently 15 seconds of arc away. Both 1 and 4 Cen
will eventually slough off their outer layers and die as white dwarfs with respective
masses of 0.6 and 0.9 times that of the Sun. If nothing else, the set
of four stars shows the wondrous variety of the starry sky. (Thanks
to Jerry Diekmann, who suggested these stars.)
Written byJim Kaler 4/22/16. Return to STARS.