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.