60-second Astro News: White Dwarfs Still Burn, Black Holes Merge

White Dwarfs Still Burn

When stars near the end of their lives, they stop burning. Without nuclear fusion to fend off the press of gravity, most stars will shed their outer layers before collapsing into white dwarfs.

These crushed remnants, no larger than Earth, support themselves against further collapse via the exotic physics of electron degeneracy. But they have no source of energy — or light. They should slowly cool and dim, or so the theory goes.

Now, ultraviolet Hubble Space Telescope observations of two ancient clusters show that the steady cooling of white dwarfs isn't so steady after all. And that's because some of them do, in fact, still burn.

The globular clusters M3 and M13 are both about 13 billion years old. But despite the clusters' similar age and appearance, Jianxing Chen (University of Bologna, Italy) and colleagues find that M13 has extra bright white dwarfs. That abundance, they show, originates in the cluster's relatively larger number of weensy stars, those with less than about half the Sun's mass. Even after they collapse, these stars will retain an envelope of hydrogen for later burning — in effect, a security blanket of thermonuclear fusion that keeps them warm over the ages.

About 70% of the white dwarfs in M13 are of the slow-burn variety, still fusing hydrogen on their surfaces. The finding upsets the notion of white dwarfs as inert, forever-cooling embers. Read more in the Hubble Space Telescope press release and in Nature Astronomy.

X-ray Magnifying Glass

The Chandra X-ray Observatory has detected 24 X-ray photons that have traveled 12 billion years from a supermassive black hole . In fact, we'd see even less than that if not for an intervening galaxy, which acted as a cosmic magnifying glass and redirected some photons toward Earth.

Yet with just two-dozen photons, Daniel Schwartz (Center for Astrophysics, Harvard & Smithsonian) and colleagues have sussed out the nature of the X-ray-emitting system, known as MGB 2016+112. The most likely scenario is that two galaxies have come together some 2 billion years after the Big Bang, and their respective supermassive black holes are in the process of merging. They emit copious amounts of X-rays, many of which are absorbed in swirling gas surrounding the pair. The black holes currently orbit each other 650 light-years apart. Eventually, this hefty duo will unite, radiating gravitational waves in the process.

It's possible that what the astronomers are seeing is actually a single black hole and the very beginning of its thousand-light-year-long jet, rather than a second black hole. Follow-up spectroscopy will help distinguish between these two scenarios.

More information on this system is available in the Chandra press release and the study preprint.

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