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  • Have you ever wondered why Hubble can make detailed images of galaxies, but stars appear

  • as featureless blobs?

  • What the most distant object ever observed is?

  • Who gets to use Hubble?

  • Or what Hubble's oddest discovery is?

  • Then stay tuned.

  • Last time we asked you to send your astronomy-related questions, and over the last month or so,

  • you've sent us hundreds of really good questions.

  • 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.

  • Almost every object you see in this image is in fact a very distant galaxy. In fact,

  • let's have a look at this guy over here.

  • 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.

  • How do you prioritize what Hubble photographs?

  • Now once a year, all the astronomers who want to use Hubble apply for observing time with

  • Hubble by submitting proposals that contain detailed information on the scientific questions

  • they want to address and the data they need.

  • Now the total amount of observing time requested by all of the proposals is always much greater

  • than the total amount of time that is actually available.

  • And so there's a committee of astronomers that looks at all the proposals and ranks

  • them according to their scientific merit. And it's only the best 10-15% that actually

  • get executed

  • If Hubble can zoom into distant galaxies with striking detail, why can't it point the same

  • cameras to a nearby star and map its surface in recognizable detail?

  • This is the star Betelgeuse. It's a very big star, and quite close to us, only a few hundred

  • light years away.

  • This is the galaxy Arp 273, which is about 500 000 times farther away than Betelgeuse.

  • But at the same time, it's also a billion times bigger.

  • Which means that its apparent size on the sky is still about 2000 times larger than

  • that of Betelgeuse.

  • Although stars are very close to us, they're just too small, so that being able to see

  • details on the surface of a star is beyond the capabilities even of Hubble.

  • 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.

  • In episode 49, we looked at so-called Herbig-Haro objects, which are jets of matter that are

  • shot out by newborn stars. Now Hubble was able to 'film' the motion of these jets over

  • a time period of about14 years.

  • And it is indeed true that 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!

  • What is the most odd thing you guys have discovered with Hubble?

  • Well, one thing's for sure, although this came up a lot in the questions, it's not little

  • green men, and it's not planet X.

  • More seriously though, you might have heard that the 2011 Nobel prize for physics was

  • awarded for the discovery that the expansion of the Universe is accelerating. Hubble played

  • a part in that discovery, and it came as a complete surprise to everyone. Now, such revolutionary

  • and completely unforeseen discoveries are of course very

  • rare.

  • But from time to time, Hubble does send us images that at least look surprising. And

  • I'll leave you with a collection of these.

  • This is Dr J signing off for the Hubblecast. Once again, and for the fiftieth time, nature

  • has surprised us beyond our wildest imagination.

Have you ever wondered why Hubble can make detailed images of galaxies, but stars appear

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