Sunday, 12 March 2017

Talk : Things That Go Bang In The Night (Sky)

The University of Nottingham Science Public Lecture Series had their February talk presented by Julian Onions on the subject of Things That Go Bang In The Night (Sky). @Gav Squires was there and has kindly written this guest post summarising the event, with a few additions from NSB who was also at the event.

Julian Onions

Of everything in the universe what would go off with the biggest bang? At that size, scales are in the region of billions of years - astronomy is a slow science. And it's big, we talk about things in terms of solar masses. A solar mass is approximately 2x10^30kg.

First a bit of solar theory. A big ball of gas collapses down and the pressure makes it get hotter and hotter and in the centre nuclear fusion happens. So there is gravity pushing inwards and energy pushing out and at some point these two forces reach equilibrium. The sun is around 15million degrees Kelvin at the centre. A red dwarf is around half a solar mass and it like a boiling pot. It last for a long time and it very efficient. The sun is a boiling mass at the edge but from around a third of the way out from the centre it is being held up by radiative pressure (light). Giant stars, from around five solar masses, are being held up by just light.

When the sun runs out of fuel, the outward pressure will stop and gravity will win. It will contract down to around the size of the Earth and will become a white dwarf. Then it will glow for hundreds of billions of years. In fact, no white dwarf formed since the beginning of the universe has gone out yet.

Nova are new stars - they are things that suddenly brighten in the night sky. White dwarfs start to steal material from a fellow binary star, building up a hydrogen shell. This warms the star up and can cause the nuclear reactions to start again. It burns very fiercely and this is almost instantaneous and we get a burst of light. It happens for between 25 and 80 days and then it dims. This can happen several times.

A white dwarf is around the size of the Earth and is less than 1.5 solar masses. A neutron star is between 1.5 and 5 solar masses and is around 20km wide. A black hole is larger than 5 solar mases and has an even horizon that is around 30km across. Is there anything between a neutron star and a black hole? In a white dwarf matter goes into a fiery, squashed state called degenerate matter, which is very odd stuff. If you add more matter, it gets smaller. In a regular start matter is around 0.1kg/cm3. Degenerate matter is 10,000kg/cm3 while neutron star matter is 1014kg/cm3.
Masses of different star types compared

At this point it's time to introduce a new unit of measure, the foe. It's a unit of super nova power. One foe is equivalent to 10^44 Joules or around the same amount of energy in 186 Earths. To give that a little context in terms of "bang", the biggest hydrogen bomb we've ever developed is 10^17 Joules.

A kilonova is around 10 foe. It happen when 2 neutron stars are orbiting each other, losing energy through gravitational waves. There is a huge explosion when the two stars become one and this is one of the major ways that heaver elements in the universe are created. This creates short gamma ray bursts but because neutron stars are difficult to see these kilonovas are hard to track down.

There are several types of supernova - 1a/1ax/1b/1c, 2a/2p/2c/2b. Zwicky originally had other types but these have since been subsumed into one of these. Type 1a supernova detonate while 1b, 1c and all type 2 suffer from core collapse.

Comparing supernova types 

Type 1a is around 1 foe in energy and is very similar to the nova. However, the white dwarf is a bit more advanced. It still steals material from a companion but rather than just the outer layers burning off it all lights up. The temperature gets to around 100,000,000 degrres Kelvin and then it explodes. This happens at the same point in every white dwarf - when it reaches around 1.5 solar masses. These are a favourite of astronomers as they almost always give off the same amount of light so it is easy to measure distance. They are almost like a "standard candle" for measuring the universe.

SN1987a - a recent Supernova

Type 1ax was only discovered in 2013. It's where a white dwarf that has lost nearly all of its outer layer of hydrogen and helium goes supernova. Energy wise it's at maximum half a foe and probably around a third of supernova are of this type.

In types 1b, 1c and 2 the hydrogen in the centre of the star is burnt off. Then the star starts burning the less efficient helium. To give some context, the sun will burn for 10 billion years using its hydrogen but only for an extra billion by burning its helium. By the time it reaches silicon, the star is getting desperate and when it reaches iron, it is using more energy to burn it than is being given off. With the power off, there is nothing to counteract the force of gravity. The star contracts at a third of the speed of light. The whole thing then stops, shudders and explodes but no-one knows why. One theory is that the gravity creates neutrinos - most of the energy comes out in neutrinos rather than light. The centre that is left is now a white dwarf or a neutron star.

"Onion Burning"

In a type 1b, the star loses its outer layer of hydrogen so its surface is just helium. Type 1c loses its hydrogen and its helium so it has carbon and oxygen at its outer layers. Type 2l are between 5 and 100 foe but you don't see the actual explosion as it doesn't give out light. Then there is a Peak of luminosity, which slowly fades. Type 2p has a Peak of luminosity and then a plateau before the fade. Type 2n and 2b are all pretty much the same.

A hypernova is a much bigger star that explodes and these generate long gamma ray bursts. Again, they are caused by core collapse. For stats between 8 and 10 solar masses, electron capture takes away the power that was being used to support the star. The temperature gets up around 10^10 degrees Kelvin and then the whole thing catches fire. Between 10 and 140 solar masses, the star suffers from iron core collapse. Between 140 and 250 solar masses, the star suffers from pair instability. Very high energy gamma rays are produced and the energy coming out goes into creating matter rather than supporting the star. These are very rare. Over 250 solar masses and you get photodisintegration. The star turns in on itself and the iron is turned into helium. This then turns into a black hole. The size and the amount of heavy metal in a star determines its fate.

What of other "bangs" in the night sky? The recent detection of gravitation waves was caused by two black holes colliding. Three solar masses worth of energy were given off, around 5300 foe. So, what if two super massive black holes collided? These are 1,000,000 solar masses each and could happen when two galaxies collide. This is a very rare occurrence and it would also be quite a drawn out affair - the two super massive black holes would orbit each other for a billion years. Super massive black holes give off 10^9 foe of energy anyway, this is emitted constantly over millions of years.

Then of course there is the big one, literally. The big bang gave off 10^25 foe of energy. It took around 20 minutes and then the universe went into a decline for the next 300,000 years. That's a one off though and the likelihood of a local black hole collision is very low. So, the supernova is the winner. If a supernova goes off in our galaxy it will probably be visible during the day and Betelgeuse is a candidate to go off in the not too distant future.

So, which big bang are we most likely to see....

The Public Lecture series returns on the 16th of March where Dr Mandy Roshier and Dr Steve North will be talking about Bits & Bytes - When Horses Meet Computers. For more information visit the UoN website:

Image sources
All courtesy of Gav Squires from the talk

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