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Have you ever wondered why Hubble can make detailed observations of galaxies

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but stars appear as featureless blobs?

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What the most distant object ever observed is?

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Who gets to use Hubble?

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Or what Hubble’s oddest discovery is?

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Then stay tuned.

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Episode 50: Q&A with Dr J

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Presented by Dr J, aka Dr Joe Liske

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Hello and welcome to the Hubblecast.

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Believe it or not this is in fact our 50th episode.

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To mark the occasion we’ve decided to do something a little different today.

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Last time we asked you to send your astronomy-related questions

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and over the last month or so, you’ve sent us hundreds of really good questions.

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Now unfortunately, there’s no way we can discuss them all.

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And so what we’ve done is, we’ve picked the ones we like best

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and we’ll try to answer them in today’s episode.

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What is the most empty spot of space you have ever seen?

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What’s the longest single-shot exposure ever recorded of any object or area of space by Hubble?

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What are the farthest objects discovered by Hubble?

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Three questions, just one answer.

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In 2003, Hubble was pointed at a part of the sky that is

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by normal standards at least, pretty empty.

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In particular, there are no bright stars in this area.

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Now Hubble observed this field,

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which is only about a tenth the size of the full moon,

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for almost a million seconds.

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That’s around 11.3 days’ worth of total exposure time.

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The result is an image that we call the Hubble Ultra Deep Field

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and it is in fact the deepest optical image of the Universe that humanity has ever produced.

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Almost every object you see in this image

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is in fact a very distant galaxy.

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In fact, let’s have a look at this guy over here.

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This is galaxy UDFj-39546284.

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Boring name, I know,

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but the point is that this is probably the most distant object ever discovered. 

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Now its distance isn’t 100% confirmed yet,

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but it’s believed to be so far away that the light took 13.2 billion years to reach us.

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That’s about 96% of the age of the Universe.

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How do you prioritise what Hubble photographs?

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Now once a year, all the astronomers who want to use Hubble

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apply for observing time by submitting proposals 

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that contain detailed descriptions of the scientific questions they want to address

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and the data they need.

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Now the total amount of observing time requested by all of the proposals

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is always much greater than the total amount of time that is actually available.

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And so there’s a committee of astronomers that looks at all the proposals

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and ranks them according to their scientific merit.

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And it’s only the best 10-15% of the proposals that actually get executed.

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If Hubble can zoom into distant galaxies with striking detail,

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why can’t it point the same cameras to a nearby star and map its surface in recognisable detail?

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This is the star Betelgeuse. It’s a very big star, and it's quite close to us

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only a few hundred light years away.

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This is the galaxy Arp 273

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which is about 500 000 times farther away than Betelgeuse.

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But at the same time, it’s about a billion times bigger.

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Which means that its apparent size on the sky

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is still about 2000 times larger than that of Betelgeuse.

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Although stars are very close to us

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 they’re just too small

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so that being able to see details on the surface of a star 

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 is beyond the capabilities even of Hubble.

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When galaxies collide and incorporate each other, what happens to the black holes?

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Do they eventually merge into one giant black hole?

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Yup, that’s pretty much what happens.

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As Hubble helped us discover in the 1990s

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we think that almost all giant galaxies contain a central, supermassive black hole.

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In addition, galaxy collisions are very common, they happen all the time

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and again, Hubble has showed us lots of great images of these collisions.

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Now, eventually the two galaxies merge and settle into a single bigger new galaxy

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and during this process, the same thing happens with the supermassive black holes.

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They merge into a single, even bigger, supermassive black hole

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at the centre of the new galaxy.

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Now astronomers have made computer simulations of how this process works

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but we also have some pretty good observational evidence 

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that this process really does take place.

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After watching the 49th episode, I was wondering whether there’s more dynamics

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that Hubble could help identify, like gravity lens effects, 

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rotating objects or clusters, collisions and so on.

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In episode 49, we looked at so-called Herbig-Haro objects

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which are jets of matter that are shot out by newborn stars.

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Now Hubble was able to film the motion of these jets

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 over a time period of about 14 years.

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And it is indeed true that over the past 20 years,

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Hubble has been able to capture the change or the motion 

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of a number of other phenomena and objects.

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Now some of these videos have been morphed together 

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using computer software to smooth out the motion,

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but everything you are about to see is based on real Hubble images.

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Nearby objects within the solar system 

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show the most impressive movement in Hubble pictures.

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Planets rotate, and their satellites move around their orbits.

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Like the Northern Lights here on Earth, 

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Saturn has aurorae, and Hubble has watched them dance.

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Comets and asteroids sweep around the Sun, and sometimes even break up.

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But there are also objects further away that we can see move.

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Fomalhaut b was the first planet outside the solar system

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Fomalhaut b was the first planet outside the solar system to be directly imaged in visible light

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and images taken 21 months apart show it inching along its orbit.

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Hubble has also imaged a flash of light

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propagating through the dust surrounding the star V838 Monocerotis.

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The distances are so huge that this sequence took 4 years to film

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even though it’s moving at the speed of light.

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Cassiopeia A

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a cloud of debris left over from a supernova that exploded three centuries ago

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is still expanding, and Hubble observations 9 months apart show the material moving.

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One of the most distant objects that Hubble has been able to watch

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change over time is Supernova 1987a

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 the explosion of a star in the Large Magellanic Cloud that happened in 1987.

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Over the past 20 years, Hubble has watched the shockwave spread out

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and light up the gas surrounding the star.

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Now Hubble is really good at this type of observation because

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a) its images are very detailed – so it can spot even very subtle motion

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and b) it’s been in operation for so long, almost 22 years now.

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Can Hubble detect potential supernovae, 

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and if so are we likely to see one from the surface of the Earth,

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and can we know when it’s likely to occur?

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Predicting supernovae is a bit like predicting earthquakes

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we can spot which stars are likely to explode

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 but we can’t tell when exactly the explosion is going to happen.

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One of the supernova candidates which is closest to Earth

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is the star Eta Carinae, which is about 7 to 8000 light years away.

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Now this star nearly exploded already in the 19th century

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and when Hubble came to image it in the 1990s

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the huge gas cloud that was ejected

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during that failed supernova was clearly visible.

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Now again, we can’t predict exactly when Eta Carinae is going to explode

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it could be tomorrow, or it could be a million years from now.

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But of course in astronomical terms, that’s just any minute now!

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

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Well, one thing’s for sure, although this came up a lot in the questions,

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 it’s not little green men, and it’s not planet X.

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More seriously though

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you might have heard that the 2011 Nobel prize for physics was awarded

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for the discovery that the expansion of the Universe is accelerating.

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Hubble played a part in that discovery, and it came as a complete surprise to everyone.

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Now, such revolutionary and completely unforeseen discoveries 

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are of course very, very rare.

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

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And I’ll leave you with a collection of these.

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This is Dr J signing off for the Hubblecast.

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Once again, and for the fiftieth time, nature has surprised us 

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beyond our wildest imagination.

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Thanks for all the questions and keep in touch!

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www.facebook.com/hubbleesa www.twitter.com/hubble_space

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Transcribed by ESA/Hubble