Have astronomers just found Terminus? Much of Isaac Asimov's influential series is set on a planet so remote from the rest of the galaxy there are almost no visible stars in the sky. Now astronomers have detected two stars so utterly remote it is hard to believe they truly belong to our galaxy.
ULAS J0744+25 is 775,000 light years from Earth, while ULAS J0015+01 is 900,000 light years away. For comparison, the Large Magellanic Cloud, the nearest dwarf galaxy to ourselves is just 163,000 light years distant. Nevertheless, Dr John Bochanski of Haverford College says they represent the furthest known outposts of the galactic halo.
"The distances to these two stars are almost too large to comprehend," says Bochanski "To put it in perspective, when the light from ULAS J0015+01 left the star, our early human ancestors were just starting to make fires here on Earth."
Bochanski went searching to see how much further he could push this by looking for cool red giants, stars bright enough to be seen at such a distance. Unfortunately, red giants are the same color as red dwarfs, the most common stars in the galaxy.
"It really is like looking for a needle in a haystack," Bochanski says. "Except our haystack is made up of millions of red dwarf stars." Using images from the Infrared Deep Sky Survey and the Sloan Digital Sky Survey and filters for the preferred part of the spectrum Bochanski identified ULAS J0744+25 and ULAS J0015+01 as candidates, before confirming both at using the giant telescope at the MMT Observatory in Arizona.
Estimating the distance to stars is difficult, but in the Astrophysical Journal Letters Bochanski reveals that several techniques produced the same result, giving confidence the figures are broadly correct. It is thought that halo stars may be the products of interactions between the Milky Way and smaller galaxies, although Bochanski notes, “Most models don’t predict many stars at these distances. If more distant red giants are discovered, the models may need to be revised.”
The answer to the initial question, by the way, is no. Red giants are changing their output too rapidly to host a planet that could support life, so we'll have to look for an even harder to find star if we want to detect the galaxy's most distant habitable planet.