Thursday, 29 November 2018

School question time!

So I just had some questions on twitter and via email from school children about astronomy. So rather than just replying to the people in question I thought I'd put the answers in a blogpost so everyone can read them.

Set one:

Q1 "What would happen if two red giants collided?" - context is discussions about neutron stars colliding and forming a black hole

This happens sometimes in binary stars systems where two stars orbit around one another. If their masses are similar enough they can both become red giants at the same time, grow in radius and eventually touch. To work out what will happen we need to think about the structure of a red giant. In the center is the star's helium core, the remnants of it fusing hydrogen to helium over it's main sequence lifetime (i.e. the phase our Sun is in). The core is then surrounded by the hydrogen envelope that is the material that wasns't hot enough to burn during the main sequence. 

Then in a binary when the two red giants touch one of two things will happen, either they will merge, losing some of the hydrogen and making a supersized helium core. Or all the hydrogen will be removed and the two helium cores will orbit each other in a much shorter orbit. If these are close enough they may eventually merge as they emit gravitational radiation and explode in some way.

If the helium cores were big enough then supernovae might happen and the stars could eventually become neutron stars or black holes and then eventually also merge via gravitational radiation to cause a GW event. Especially in the case where the two stars touching results in the orbit shrinking so the stars are closer together. If they're closer together they'll merge more quickly.

It's also worth noting it's very rare for the two stars to both become red giants at the same time. More typically one star will become a red giant while the other in still a main sequence star. But we're pretty sure they must touch and interact strongly to get close enough to form GW events.

Finally two single stars might me moving around a star cluster and just bump into each other. The same two possible results apply but this would be even rarer as space is big and stars are small.

Q2 - " Is Betelgeuse the only red giant we can see in the night sky? How soon can we expect to see a supernova?"

There are many red giants/supergiants we can see in the sky, for example: Betelgeuse, Aldebaran, Antares and Arcturus. Also a star atlas will definitely contain many more. It's worth noting that many red giants won't go supernova, only those that were more massive than 8 times that of the Sun will explode. Betelgeuse is likely to be above that mass but we really can be sure about when it'll explode as we can't see inside it. It could explode tomorrow or in a few 100,000 years, so we just need to play a waiting game!

We do see many red supergiants explode in other nearby galaxies and they're 60% of all supernovae we see. It's just within our own Galaxy we can only expect 1 supernova every 50 to 100 years.

Q3 - Could the stuff thrown off a supernova go and form another star?

Yes, that's where all the iron in your blood game from as well as many (but not all) the elements in your body that aren't hydrogen came from. The iron comes mostly from a specific type of SN, type Ia which is from exploding white dwarf stars in a binary star. 

What happens is the material formed in the explosion gets through out to get mixed with the material surrounding the star making it richer in heavier elements. We can actually see in more and more distant galaxies that there are less and less heavy elements showing us in reverse how stars have built up all the elements over 13.7 billion years.

Set two:

1. Do you believe we will get to Mars by 2030?

We've been there since the 1960s with space probes so I guess you mean humans. My only answer is maybe. It depends on whether there is the political will to spend that large amount of money that such a large project will require. We could get there before 2030 if we wanted to. Also the other question is whether it is a one-way trip or a return journey. The latter is more tricky.

2. Do you believe there is currently life on Mars?

It's certainly possible but unlikely. Although life on Earth has been found in increasingly extreme environments. This is why it's key to go there and at least check whether there is or ever has been life there.

3. Do you believe we could possibly live on Mars in the future?

It would be hard, recent research has suggested it may be even more difficult to terraform Mars than we thought. There is only a very sparse atmosphere so people will need to use spacesuits and pressure domes to live in and will not be able to walk through the open air on Mars.

Then there is also the problem that the planet has no magnetic field so people would have to live underground rather than on the surface. This is to avoid the solar wind and cosmic rates that are dangerous to life.

So again it's certainly possible just really, really difficult. 

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