Later On

A blog written for those whose interests more or less match mine.

Posts Tagged ‘astronomy

10 great astronomy photos of 2007

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With explanations. Go see and read.

Written by Leisureguy

13 December 2007 at 1:29 pm

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A complete solar cycle

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A spectacular set of photos from SOHO showing a complete solar cycle. Astonishing.

Written by Leisureguy

3 December 2007 at 12:07 pm

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The Eight-Burst Nebula

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Take a look. It’s beautiful and strange.

Written by Leisureguy

14 October 2007 at 9:08 am

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Black holes

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The latest New Scientist has a cover article on black holes, and I find that there’s much less that’s certain about black holes than I thought. Indeed, there’s not certainty that the “event horizon” even exists: because the concentrated mass of a black hole distorts space-time so greatly, the slowing of time could mean that, as particles approach the black hole, they never get to the event horizon:

According to relativity, time slows down for an object, from the point of view of an outside observer, as it accelerates close to the speed of light. Anything falling into a black hole approaches that velocity as it crosses the event horizon. So while someone riding a spaceship across the horizon would feel that he or she was moving at a terrific speed, someone watching from outside would see the ship slow and eventually stop at the horizon, never quite falling in.

If that’s true, the same argument should apply to gas, stars or whatever was collapsing to form the black hole in the first place. To an external observer, it would take infinitely long for the black hole to come into being. This is actually a long-standing problem that has never been fully addressed. “People have just assumed it’s one of those weird general relativity things and don’t discuss it very much,” says Krauss. When you add in quantum mechanics, which says that black holes actually radiate particles, the problem becomes even more acute. “If quantum theory says black holes must evaporate in finite time,” says Krauss, “and general relativity says they take an infinite time to form, you’ve got something disappearing before it exists.”

Krauss’s work, which will appear in the journal Physical Review D, began as an exercise in particle physics (www.arxiv.org/abs/gr-qc/0609024). Researchers had suggested that experiments at the Large Hadron Collider at CERN, the European particle physics lab near Geneva, Switzerland, scheduled to go online next year, could produce energy densities great enough to create a quantum black hole, much tinier than the tiniest subatomic particle. The smaller a black hole, the more quickly it should radiate away its mass, and Krauss and his colleagues were trying to figure out what that radiation might look like.

According to their model, quantum black holes should emit light, X-rays and other electromagnetic radiation at a rate so high that they never fully form in the first place. This piqued the researchers’ interest, so they tried the same calculations for cosmic black holes. They found that as a spherical shell of mass collapses inward, its gravity disrupts the quantum vacuum, giving rise to radiation similar to the quantum black hole’s. It, too, leaks so much energy that the mass never gets dense enough to form a black hole with an event horizon. Instead it forms what the researchers call a “black star”, which never completely swallows any surrounding matter from an external observer’s point of view.

That doesn’t mean the model is right, of course. “We’ve discussed it at lots of colloquia and seminars, and there has been lots of interest. The first reaction is incredulity; people are sceptical, but nobody’s poked a hole in it yet,” says Krauss. “For now I’m happy just that it’s spurring debate.”

The astrophysical community has been receptive but lukewarm. “When people ask me if I think black holes exist,” says Broderick, “it really depends on what you mean by the term. A black hole is not just this thing inside an event horizon, it’s an entire region of curved space-time. So I prefer to talk about ‘black hole space-time’ rather than black holes.” Still, Broderick thinks it’s going to be very difficult for one of these objects not to have an event horizon. “The only way to know for sure is to drop an undergraduate in and see whether his cellphone signal cuts off,” he says.

More at the link.

Written by Leisureguy

7 October 2007 at 10:24 am

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2,000,000 galaxies

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Two million galaxies

From Astronomy Picture of the Day: 2,000,000 galaxies, each galaxy with millions of stars. We live in a very, very large universe. The region above is only 100º across—that is, this is only a small segment of the universe. Bright regions indicate more galaxies, while bluer colors denote larger average galaxies. Dark ellipses have been cut away where bright local stars dominate the sky.

Written by Leisureguy

7 October 2007 at 9:43 am

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Video of the sun snipping off a comet’s tail

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Watch it happen.

Written by Leisureguy

5 October 2007 at 11:10 am

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