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from the there's-supposed-to-be-an-earth-shattering-kaboom dept.
There are a few known ways that black holes could be created. First way: start with a very massive star, watch it burn through all its nuclear fuel, and wait for the core to collapse in a violent supernova explosion, leaving a black hole behind. Second way: start with a neutron star, and let it accumulate enough mass to push it over the edge, either by accretion or by colliding with something like another neutron star. There was long theorised to be a third way: for a star to collapse directly into a black hole with no supernova. Astronomers using the Large Binocular Telescope, and the Hubble and Spitzer Space Telescopes have observed just such a thing happening to a star in the nearby galaxy NGC 6946, 22 million light years away. They saw a star of 25 solar masses, given the designation N6946-BH1, quietly wink out into a black hole.
Astronomers have watched as a massive, dying star was likely reborn as a black hole. It took the combined power of the Large Binocular Telescope (LBT), and NASA's Hubble and Spitzer space telescopes to go looking for remnants of the vanquished star, only to find that it disappeared out of sight.
It went out with a whimper instead of a bang.
The star, which was 25 times as massive as our sun, should have exploded in a very bright supernova. Instead, it fizzled out — and then left behind a black hole.
"Massive fails" like this one in a nearby galaxy could explain why astronomers rarely see supernovae from the most massive stars, said Christopher Kochanek, professor of astronomy at The Ohio State University and the Ohio Eminent Scholar in Observational Cosmology.
As many as 30 percent of such stars, it seems, may quietly collapse into black holes —no supernova required.
"The typical view is that a star can form a black hole only after it goes supernova," Kochanek explained. "If a star can fall short of a supernova and still make a black hole, that would help to explain why we don't see supernovae from the most massive stars."
He leads a team of astronomers who published their latest results in the Monthly Notices of the Royal Astronomical Society.
The original paper (paywalled) appears to be "The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star" (DOI: 10.1093/mnras/stx816) but it is not linked in the JPL press release. Also available as arXiv:1609.01283.
(Score: 2) by AthanasiusKircher on Tuesday May 30 2017, @06:19PM (2 children)
That depends on the type of supernova [wikipedia.org] and the mass of the star. Yes, iron cores develop in many stars before supernova events, but it's not required. There are various mechanisms that can lead to supernovas.
Just to clarify, the reason why iron formation generally causes the star to be "done" is because nuclear fusion only gives off energy up to a certain size of nucleus. Iron is basically the limit. So, fusion beyond iron doesn't actually produce a net energy output, and the star collapses under its own "weight." Heavier elements are frequently formed in supernovas because of the extreme pressures created during the collapse and explosion which force fusion of nuclei even when they no longer produce a net energy output.
(Score: 2) by DannyB on Tuesday May 30 2017, @06:59PM (1 child)
Thanks.
That would explain why the primates on rocky planets do find heaver elements in the crust of their planet's surface. Something beyond the normal everyday fusion process of the star had to produce those heavier elements.
Also, thanks for explaining why iron is the limit. I didn't know that.
What was shocking to me back in the 90's was the vast scale of it all. If only a tiny bit of the heavier elements flying out in the supernova ever forms a planet, then consider that tiny bit is enough to form something as large as the earth and other rocky planets in the same system.
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(Score: 2) by AthanasiusKircher on Tuesday May 30 2017, @07:55PM
No problem. One other thing to note, some heavy elements are also produced outside of supernovas, particularly in the so-called s-process [wikipedia.org] that happens in stars after iron forms. (These actually hasten the death of the star, by absorbing energy.) But you're right that the violence of supernovas is also conjectured to be a major source of heavier elements.