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.