HD 140283 (HD 140283 Librae). And now (once again) for something
completely different. Excluding its nuclear-burning core, the Sun is composed of 92 percent hydrogen atoms and
8 percent helium atoms. That's 100 percent. All the other elements
(loosely called "metals") sneak in at a mere 0.15 or so percent, so
they don't show up in rounded-off H and He abundances. Oxygen leads
the pack, followed by carbon, neon, nitrogen, and so on down to
uranium, which must be there but is too sparse to be detected. The
hydrogen and most of the helium atoms come from the Big Bang, the H primordial,
the He from nuclear reactions that took place within the first three
minutes of the life of the Universe. All the other elements (and
a bit of helium) were formed in the nuclear cauldrons of massive stars
and supernovae that,
through their winds and explosions, blew them into interstellar space. The metal
content of the stellar birth
clouds therefore increases with time, so that young stars have
higher metal abundances than old stars. The very FIRST stars,
however, those formed right after the Big Bang, should have had no
metals at all. We can't find them. Presumably, the first stars were
all massive and therefore exploded, so there are none left to see.
How far back in metal content, and thus toward the Galaxy's birth, indeed toward
that of the Universe, can we go? Not visible to the naked eye, but
a clear binocular object, seventh magnitude (7.21) HD 140283 (in the
Henry Draper spectral catalogue) is certainly a stop
along the way. Only 190 (give or take under 2) light years away in
Libra, HD 140283 is the closest of the
really low-metal stars. With a temperature of 5580 Kelvin, the iron
abundance relative to hydrogen is 0.0040 that found in the Sun. The
oxygen value is higher, but still only 2.1 percent solar. The star
is classified as "sdF3," the "sd" standing for "subdwarf." Subdwarfs were
once thought to be dwarfs that are sub-luminous for their classes.
Because of their low metals, however, they are actually hotter and
smaller for their luminosities, the low metal abundances giving them
classes that are anomalously warm. (All are of class F or cooler.
Don't confuse them with similarly named class O and B subdwarfs, which
are highly evolved and are on their way to becoming white dwarfs.)
Because they are ancient, subdwarfs such as these all have low masses,
below about 80 or 90 percent that of the Sun; those of higher initial
mass have long since burned away. HD 140283, however, once a real
subdwarf, is actually a subgiant that is turning into
a giant with a dead helium
core. From its temperature, luminosity (3.8 times that of the Sun),
chemical composition, and theory, HD 140283 seems almost as old as
the Universe itself, which dates at 13.8 billion years. Whatever
the final word, the star is certainly among the oldest there are.
While close to us now, HD 140283 is a visitor from the ancient Galactic halo, and is just
passing through the orderly disk at a speed of 361 kilometers per
second relative to the Sun, just a bit more than the true subdwarf
Groombridge 1830. Whatever the
details, HD 140283 is among the oldest things you can see other than
the Universe itself that surrounds you. (Thanks to H. E. Bond et
al. in the "Astrophysical Journal Letters," 765:L12, 2013.)
Written byJim Kaler 8/15/14. Return to STARS.