June 2007 Archives

Venus meets Saturn

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After the Moon - Saturn - Venus conjunction that happened earlier this month, the main event comes on this weekend: Saturn and Venus will be less than a degree apart, very clearly visible in the northwest sky this Sunday soon after sunset (and for a few hours afterwards). Plus, the bright star Regulus is not far away and should make the spectacle even better. It will look like this (click the image for a larger version):

Venus and Saturn

Clearly visible, that is, weather permitting. The forecast for Melbourne on Sunday is "a few showers", but since the forecast for Monday is "mainly fine", we can expect a decent weather on Sunday evening. We should have rain every day until then, which should help with the drought, so you can all stop feeling guilty about hoping for good weather on Sunday. If the sky is clear, this event will be visible even from brightly lit cities, and will be even better with binoculars.

Do black holes exist?

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A while ago (in 2005, I think), I took part in a discussion in a (not astronomy-related) mailing list on the subject of the existence of black holes. The argument of the guy pushing this position was interesting and, for me, very relevant: black holes can't exist because there was no time for them to form.

See, the story is like this: when a super-massive star dies, its core collapses due to gravity and gets progressively smaller very quickly; as it gets smaller, gravity at its surface increases, and so does its escape velocity. When surface gravity increases past a certain point, the escape velocity becomes higher than the speed of light, which means that nothing can escape that body anymore, not even light. Presto, a black hole. After that, the core continues to shrink until it becomes a single point, with infinite density and gravity: a singularity.

However, that's the story as told from the point of view of the shrinking core. As Einstein told us in the theory of general relativity, clocks run slower when in a gravitational field, and the stronger the gravity (or acceleration) the clock is subjected to, the slower it runs. The effect is that, as seen from the outside, the shrinking core actually shrinks progressively more slowly as its surface gravity increases (and, from the point of view of the core, the universe outside moves progressively faster as it - the core - shrinks), and the speed of "shrinkage" tends to zero as the escape velocity approaches the speed of light. Therefore, from the point of view of anyone outside a black hole, the black hole never finishes forming: the escape velocity never actually reaches the speed of light.

It's a good argument, and I couldn't think of a good response to it. It turns out that this is a scientifically interesting problem, and a recent post by Phil Plait, the Bad Astronomer, points to a paper by astronomers from the Case Western Reserve University that argues that not only black holes never finish forming, they can't finish forming because Hawking radiation makes them evaporate before the event horizon forms (from the point of view of an external observer).

Read Phil's article on the subject, as he explains the whole situation much better than I can, and the comments make for excellent reading as well. The original paper, I have to say, was a bit too technical for me...

Lunar encounters

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If you have a relatively clear view to the west and north-west, today is a good day to go outside after sunset and look to the sky. The Moon, Saturn and Venus will be very close to each other and very bright in the north-west sky (as seen from Melbourne) for a few hours following the sunset today. This is what it will look like just after 6pm (click on the image to see a larger version):

sky at 6pm 18/6/07

Tomorrow may be even more spectacular, with the Moon very, very close to Saturn (in fact, the further north you are, the closer it will be, and if you happen to be a bit north of Papua, you'll see the Moon moving in front of Saturn just before 6pm). It will look like this at 6pm:

sky at 6pm 19/6/07

In both days, the bright star Regulus also makes a good appearance and adds to the spectacle. The Moon will actually occult Venus, Saturn and Regulus over these two days, but none of the occultations will be visible from Australia. And, sadly, in both days the weather forecast for Melbourne is not very good for anyone looking at the sky. As I write this, at 2pm, the sky is very cloudy and the north-western sky is not visible at all. Let's wait and (try to) see...

Carnival of Space #7

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The 7th edition of the Carnival of Space is up at Star Stryder, Dr Pamela Gay's blog. For those who can't quite place the name, she is also one of the presenters of the Astronomy Cast podcast, with Universe Today's Fraser Cain.

A blog carnival is a series that points to interesting and recent blog posts about a specific subject; this one is, obviously, about space. Check it out; there are lots of links to interesting posts and, more importantly, to interesting blogs in the subject.

Recent news

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I haven't been updating this blog much lately... not for the lack of news to comment on, but more for a lack of free time. Now that I'm "back" and, hopefully, with a bit more of free time in the foreseeable future, we'll see more content showing up here. And I'll start with a quick rundown of what's been in the news recently:

Atlantis The shuttle was launched yesterday, finally, and it reached orbit without incident. However, examinations of the thermal protection made in orbit found a 4-inch (10 cm) gap in one of the thermal blankets protecting the underside of the ship. NASA does not seem to be worried about it, as it is not located in any of the areas the suffer most of the heat of re-entry. The shuttle is expected to land on 19 June (Florida time).

Gliese 581c It is not a transiting planet, after all (that is, it never moves between Earth and its star during the orbit). That limits what information can be discovered about this planet in the near future. Still, it looks more and more likely that this planet is not as terrestrial as we'd like it to be; it's more likely that it is more venusian than terrestrial, and the fact that it is tidally locked to its star can't help the weather. Which brings us to...

Gliese 581d This planet received much less attention from the media, but some scientists believe it has a better chance of harbouring life than its more famous brother. Gl581d is significantly farther away from its star (0.25AU, versus 0.073AU for Gl581c), has a longer orbital period (84.4 days) and is more massive (8 Earth-masses). It is also tidally locked to the host star, though. One recent paper argues that a planet of this size is likely to have a dense atmosphere, and certain types of atmosphere would put it safely inside the habitable zone. More relevant still...

Red dwarfs ...stars like Gliese 581 are very, very, very stable after they "mature" (conditions in their vicinity are probably not very friendly during their formative years, though). A planet like Gl581d might stay inside its habitable zone for many billions of years (as opposed to Earth, which will stay habitable for the next billion years or so), which gives it very good chances of eventually developing life. In a "good news, bad news" scenario, though, it is thought that the radiation emitted by the star in its first billion years of life might be strong enough to strip the atmosphere of any planet in the habitable zone without a strong magnetic field, and that any planet in the habitable zone would become tidally locked in its first 500 million years of existence and would lose its magnetic field, thus becoming uninhabitable. We'll have to wait for more advanced instruments to be able to gather more information about what these planets are like today...

New planets There were quite a few; in fact, there were 28 new planets discovered over the last few months. There is a very dense "super-Jupiter" (eight times as massive as Jupiter, but only slightly larger) in a very elliptical orbit around a F-type star (larger and a bit hotter than the Sun) in the constellation Hercules; there was another hot Jupiter in Monoceros, which was interesting mostly because it was the first planet discovered by the European orbiting observatory COROT; another transiting hot Jupiter running a very short (31 hours long) orbit around its host star; a transiting hot Neptune around red dwarf GI-436 (a hot Jupiter is a planet similar to Jupiter — that is, composed mostly of hydrogen — orbiting close to its star; a hot Neptune is similar, but with much more water in its composition); and many more, as exoplanets.org reports.

That's it for now. There are certainly many interesting events going on, and the last few months were very exciting in the field of astronomy; let's hope that the trend continues.

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This page is an archive of entries from June 2007 listed from newest to oldest.

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