Leonard E Parker

Center for Gravitation, Cosmology & Astrophysics

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Leonard E Parker Center for Gravitation, Cosmology and Astrophysics

White Dwarfs Are Cool: CGCA Astronomer Involved in Detecting One of the Coolest, Dimmest Stellar Corpses Yet Seen

Posted by Alex Urban on June 24, 2014

A team of astronomers -- including CGCA professor David Kaplan -- may have identified the coldest and dimmest specimen yet discovered among a class of objects called white dwarfs. If true, this remnant of a stellar death would probably be made of crystallized carbon, amounting basically to a gargantuan diamond floating in the cosmos.

What did David have to say about this devastatingly cool catch? “It’s a really remarkable object! These things should be out there, but because they are so dim they are very hard to find.”

To put this in perspective, it helps to start with a profound question: just what happens when old stars die? If they're of the vanilla variety and not terribly more massive than the Sun, they eventually run out of fuel, release their outer layers and leave behind an ultra-dense ball of carbon and oxygen where there was once a hotbed of long-sustained nuclear fusion. Such a remnant is called a "white dwarf," because astronomers are very clear in their naming conventions: to look at one with an optical telescope is to see an object that shines white and really is small potatoes compared to the inconceivably huge sizes of middle-aged stars like the Sun. (It brings to mind the old refrain from Douglas Adams: "Space is big. Really big.")

The white dwarf was discovered, oddly enough, as a friend (or "companion") to a pulsar dubbed PSR J2222-0137 using the Green Bank Telescope in Green Bank, WV. The first observations dramatically revealed that the pulsar was rapidly spinning more than 30 times per second, with strong evidence of a gravitationally bound companion orbiting once every 2.45 days. From its location on the sky (in the constellation Aquarius) and distance from Earth, the astronomers were able to very precisely and delicately time the pulses of light with the GBT, allowing them to directly measure crucial features of the orbit. For instance, the pulsar is 1.2 times as massive as the Sun and its companion -- the frankly wicked-cool white dwarf -- 1.05 times as massive as the Sun. But interestingly, these data rule out the possibility of a second pulsar, making the identity of the companion most likely the signature of one very cool white dwarf.

But how can astronomers possibly know its surface temperature, if it is a white dwarf? Well, knowing its location to such high precision and knowing how bright a white dwarf ought to appear at that distance tells you how bright it intrinsically is. If you know that, and if you're clever enough to also know how its light is distributed across frequencies, reading off the temperature is simple enough -- as ever, it is not enough to see. One must also observe.

However, don't start packing for the ultimate ski trip just yet. From all this, the astronomers inferred that what could be the coldest white dwarf yet detected can be no hotter than a frigid 4,940 degrees Farenheit!

For more, please see the National Radio Astronomy Observatory's press release..

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