Exceedingly cool

In astronomy, the most interesting things are never the obvious ones. Most often they’re small and faint and easy to overlook. The world of the Zooniverse recently highlighted just such an object. Say hello to SDSS J110217.48+411315.4. And show it some respect. It’s unfathomably old. In fact J1102, as I’ll call it, is possibly the oldest white dwarf star ever discovered.

White dwarfs, much like pages in a book, are only white when they’re shiny and new. Bright, hot little orbs of carbon, oxygen and helium, they used to be the cores of stars. Now they’re little more than remnants. They don’t burn anymore, and any heat and light they give out is just an echo of their stellar days gone by. As they cool, their brightness starts to fade. J1102 here has been cooling for 12 billion years, and as a result, it’s not white anymore. Instead, as you can see from the SDSS image at the top of this post, it’s so cool by now that it’s decidedly yellow. While a freshly created white dwarf will be millions of degrees in temperature, J1102 is only around 3800 K. About the same temperature as most sunspots (the coldest part of the Sun). It’s so old and faint that even though it’s practically in our back yard, only 100 light years away, it’s still managed to evade the prying eyes of our telescopes for a long time.

Actually, white dwarfs are plentiful in the galaxy, because most of the stars that die leave one behind. A stellar ember, standing like a tombstone in the night. The image to the left is full of them. Look past the bright glittery stars and look at the smaller specks in the image. Several of those faint and unassuming spots of light are white dwarfs. In fact, with these dead stars being so numerous, they’ve even been seriously considered to have potentially habitable planets in tow.

Could J1102 be the cooling remains of one of the first stars ever born in the Universe? It’s not inconceivable. But one puzzle seems to remain. Most stars live for billions of years before dying as plantary nebulae and leaving behind white dwarfs. If J1102 has been cooling for so long, the star which created it must have lived an extremely short life. But stars with such brief lives usually die as supernovae and leave behind neutron stars. So how exactly did this paradoxical little thing come to be…?

A customary tip of the hat goes to those lovely people at the Galaxyzoo Forums for this one. There’s more on this star (and another, WD 0346+246) at Science Daily. For more technical reading, it’s covered by a paper in ApJ Letters, Kilic et al (2010).

Image credits:
Upper – Sloan Digital Sky Survey
Lower – NASA/H. Richter (University of British Columbia)

And no, you can’t call it a yellow dwarf star. The name’s already taken — The Sun is a yellow dwarf star. That would just make things confusing.
There is no rulebook for the how, what, and where of life. Even if there was, life probably wouldn’t follow it. Life is just like that.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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