Lecture: Doomed Massive stars: Evolution from birth to supernovae and Black holes.

 

Lecturer: Dr. Raman Prinja from the Department of Physics and Astronomy, University City of London. 

We left school at around about 5.45pm and did a detour via Starbucks on our way to the station in the midst of a pouring downpour. We left and the train got stuck halfway there so we ended up arriving to the lecture ten minutes late. It wasn’t too bad though since we got rather good viewing (and uncomfortable seats) on aisle of the theatre.

Anyways back to the notes:

The first thing we learnt was that one flair of the sun can give enough energy to light Europe for a 1000+ years.

Then he went on to tell us about the Sagittarius Star Cloud.

Not all the stars in the Sagittarius cloud are the same star. It contains many massive stars that are 20-100 times bigger than the sun and they are up to a million times more luminous. This means they are a brighter at a further distance.

Sagittarius stars have a much more wider influence than any other stellar type and the centre of our galaxy are near their cluster.

Starburst galaxies are the violent formation of massive stars at the earliest times in the universe.

 

 A starburst Galaxy

 

Orion’s belt and sword

In the sword of Orion, there is a stellar nursery with enough raw materials to create 300 solar massive stars. Stars are formed with a disk of material.

30 Dor is an example of a small dwarf galaxy pushing into ours and has massive stars forming.

 3 dor

If you disturb a star whilst it is forming, for example by colliding with it, then it will first create shockwaves within its self before rotating and collapsing. When you have rotating gas and gravity, the material is squashed into a disk.

The stages the star goes through to form are:

1. Molecular cloud
2. Polar wind and disk
3. Newly formed star

 Star Formation slide

 Star formation slide 2

Planet formation here is completely expected as star formation and planet formation are linked.

There are more than a 150 extra solar planets found so far. The cat telescope in Hawaii is used to discover them.

 

The Cat Telescope

 

To understand why stars shine, we first need to understand why the sun shines.

The core pressure in the sun is a billion earth atmosphere and the core temperature is 15 million degrees. This results in nuclear fusion (hydrogen burning). Seven hundred million tons of hydrogen is turned into six hundred and ninety five million tons of Helium every second. The missing 5 million is turned into energy. Gamma radiation is created by sunlight. This energy is finite, as the sun will die 4.6 billion years. This therefore means stars are not eternal.

 

 

The life story of a star is gravity versus the pressure of hot gases inside the star.

 

Hydro static equilibrium - where forces balance.

This is called hydrostatic equilibrium, when forces balance. To explain this further, we can use a fish as an example.

When the fish is in the water, the pressure from above pushes down on the fish and the weight of the fish pushes it down in the water, however the pressure from the water below the fish balances it out, keeping it in hydrostatic equilibrium.

 

 

The stars are not eternal as there has been stellar evolution over billions of years. Another thing to say is all stars are not the same. They all have different stellar paths over the years. This therefore means there are different groups of stars within the universe. Examples of these groups are dwarfs (sun), giants and super giants.

 

A Hertzsprung  - Russell (H-R) diagram

This diagram shows the patterns of luminosity. The surface temperature and the radius of the star.

The birth mass of a star is very important, as this will determine its end state. The higher the mass, the stronger the gravity and the greater pressure to keep the star stable. This means there is a greater interior temperature as there is a more rapid energy production (fusion) and greater luminosity. This overall leads to a shorter life.

 

 

The lifetime of a star depends on the available fuel supply divided by the rate of consumption. This can also be written as mass divided by luminosity. As I have said before, the end state depends on the initial mass of the star at birth. An example of this would be the sun. As it is the lightest, there will be a nova or supernova at its death.

 

 

A planet nebula signals the death of sun like stars.

 

Planet Nebula

They appear as disks like planets. When the sun dies, the outer layer will be expelled and then you will be left with a white dwarf. This means the sun will be a planet nebula in roughly 5 billion years. This will also cause Earth’s orbit to move out slightly as the sun’s forces are now weaker. This means that Earth will be too hot to live on.

 

 

 

The big bang

When the big bang occurred, it made 75% hydrogen and 25% helium. The stars made the other elements by fusion. For example hydrogen fusion made helium and then helium fusion made carbon. These heavy elements are made only in the core of massive stars, these include life-giving elements. The sun can only make up to carbon, as it is not a massive star.

 

 

A doomed star is not a supernova. It has 100,000-km/hr winds and losses mass by radiation.  An example of this would be Eta Carina, which is a doomed star, and radiates 5 million times the power of the sun.

Eta Carina

It is a site of an outburst, a 150 years ago. It is a 100 solar mass star and has chemically processed material (C, N, O). Its wind power is a 100 million times solar wind. 

Stars run out of energy and form a supernova, which undoes billions of years of stellar evolution in under a few seconds.

When a star dies, the gravity causes it to collapse in on the core and shockwaves pushes outer layers away.

 

When the first supernova happened, we saw the light 16 years later light up the sky. It was a hundred million times the sun output energy and had a 20 solar mass. The supernova was the end state for the middleweight stars.

Black holes were once very massive stars and have no known force. They can withstand crushes. The are not a cosmic vacuum. Only inside the horizon is matter pulled inwards. Far way from a black hole, gravity is no different than for another object of the same mass. If the black hole where to replace the sun, the orbit of the planet would be no different though it may be a bit more colder.

To detect a black hole, one must look for a massive star orbiting around an ‘invisible object.’

A black hole

A hypernova is the demise of the most massive stars in the universe.  There is a candidate for a hypernova in M101, which is 25 million light years away, and roughly 850 light years across. It is the largest expanding shell ever discovered and its gamma ray burst rivals that of the supernova. When a star dies, it changes into a hypernova which is an especially energetic supernova.

 

M101 - a candidate of a  hypernova

What is a burst? It is the abrupt and violent transition between the death of a massive star and the resulting birth of a black hole. A massive star causes a hypernova, which then creates a gamma ray burst and then a black hole.

Gamma Ray Burst 

Our planet is not made of pristine explosions, but in fact used gas in a repeated cycle.

 

How do we get heavier material such as uranium? In a supernova.

A neutron star is a blue star and is very hot. To find its size we have to times the temperature by the radius. It is a tightly band star with a super conductive crust. If the crust cracks slightly, then it crates a strong magnetic field.

 

 

A neutron Star

When a white dwarf dies, it becomes very hot and then cools down after a million years. It then becomes a black dwarf, which is also known as black matter.

A white dwarf is the hottest star at 150 000 million degrees Celsius.

A white dwarf