Friday, November 18, 2011

Hubble Burning Questions

From HubbleCast and the incomparable Dr. J. The episode features answers to questions submitted by the public. What is the most empty spot of space you have ever seen? What's the longest single-shot exposure ever recorded of any object or area of space by Hubble? What are the farthest objects discovered by Hubble?
Three questions, just one answer. In 2003, Hubble was pointed at a part of sky which is, by normal standards at least, pretty empty. In particular, there are no bright stars in this area.

Now Hubble observed this field, which is only about a tenth the size of the full moon, for almost a million seconds. That's around 11.3 days' worth of total exposure time. The result is an image we call the Hubble Ultra Deep Field, and it is in fact the deepest optical image of the Universe that humanity has ever produced.

This is galaxy UDFj-39546284. Boring name, I know, but the point is that this is probably the most distant object ever discovered. Now its distance isn't 100% confirmed yet, but it's believed to be so far away that the light took 13.2 billion years to reach us. That's about 96% of the age of the Universe.

When galaxies collide and incorporate each other, what happens to the black holes? Do they eventually merge into one giant black hole? Yup, that's pretty much what happens.

As Hubble helped us discover in the 1990s, we think that almost all massive galaxies contain a central, supermassive black hole.

In addition, galaxy collisions are very common: they happen all the time and again, Hubble has showed us lots of great images of these collisions.

Now, eventually the two galaxies merge and settle into a single bigger new galaxy, and during this process, the same thing happens with their supermassive black holes. They merge into a single, even bigger, supermassive black hole at the centre of the new galaxy.

Now astronomers have made computer simulations of how this process works, but we also have some pretty good observational evidence that this process really does take place.

After watching the 49th episode, I was wondering whether there's more dynamics that Hubble could help identify, like gravity lens effects, rotating objects or clusters, collisions and so on.

Over the past 20 years, Hubble has been able to capture the change or the motion of a number of other phenomena and objects. Now some of these videos have been morphed together using computer software to smooth out the motion, but everything you are about to see is based on real Hubble images.

Nearby objects within the solar system show the most impressive movement in Hubble pictures. Planets rotate, and their satellites move around their orbits.

Like the Northern Lights here on Earth, Saturn has aurorae, and Hubble has watched them dance. Comets and asteroids sweep around the Sun, and sometimes even break up.

But there are also objects further away that we can see move. Fomalhaut b was the first planet outside the solar system to be directly imaged in visible light, and images taken 21 months apart show it inching along its orbit.

Hubble has also imaged a flash of light propagating through the dust surrounding the star V838 Monocerotis. The distances are so huge that this sequence took 4 years to film even though it's moving at the speed of light.

Cassiopeia A, a cloud of debris left over from a supernova that exploded three centuries ago, is still expanding, and Hubble observations 9 months apart show the material moving.

One of the most distant objects that Hubble has been able to watch change over time is Supernova 1987a - the explosion of a star in the Large Magellanic Cloud that happened in 1987. Over the past 20 years, Hubble has watched the shockwave spread out and light up the gas surrounding the star.

Now Hubble is really good at this type of observation because, a) its images are very detailed - so it can spot even very subtle motion - and b) it's been in operation for so long, almost 22 years now.

Can Hubble detect potential supernovae, and if so are we likely to see one from the surface of the Earth, and can we know when it's likely to occur?

Predicting supernovae is a bit like predicting earthquakes - we can spot which stars are likely to explode, but we can't tell when exactly the explosion is going to happen.

One of the supernova candidates which is closest to Earth is the star Eta Carinae, which is about 7 to 8000 light years away.

Now this star nearly exploded already in the 19th century, and when Hubble came to image the star in the 1990s, the huge gas cloud that was ejected during that failed supernova was clearly visible.

Now again, we can't predict exactly when Eta Carinae is going to explode - it could be tomorrow, it could be a million years from now. But of course in astronomical terms, that's just any minute now!

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