Topic 10

What is the life history of stars?

Stars and Galaxies

The reason that we are here is that we live on a planet that is just the right distance away to give us conditions that are suitable for life.  All the other planets in the solar system are too hot or too cold.  We won't go into why conditions on Earth are suitable for life here.

The Sun is a typical middle-sized star.  It is one of many billions of stars to be found throughout the Universe.  Stars come in all sizes.  Often they are found in pairs, each orbiting the other.  The Sun as a single star is in a minority.  In fact astronomers think that Jupiter is a failed star.

Astronomers believe that there are many stars that have solar systems, if not the majority.  They cannot see the planets; they are too far away, but they can detect the wobble caused by the gravitational pull of planets on a star.

Distances in space are big, and are measured in light years.  1 light year is about 10¹⁶ m.  The nearest star, Proxima Centauri, is 4 light years away.  That means that light, travelling at 300 million metres a second, takes 4 years to get from there to here.  The light arriving now left when you were in the First Year (Year 7).  If the Sun were represented by a marble 1 cm across, the nearest star would be 300 km away.  Flying at the speed of a supersonic plane (60 m/s) it would take 500 000 years to get there.

Stars are in huge clusters called galaxies.  Our galaxy is called the Milky Way and is about 100 000 light years across.  It contains many millions of stars.  The Sun is on one of the arms of the galaxy.

Be careful not to confuse galaxies with constellations.  Early astronomers with good imaginations could see patterns formed by stars.  These are helpful to our observations from the Earth.  However the stars in any given constellation are in no way related to each other, nor are they near each other.

Birth of a Star

Stars are formed from clouds of gas called nebulae.

Particles of dust and molecules of gas come slowly together under the force of gravity.

Question 1  What features of gravity make particles come together?  ANSWER

The process takes many millions of years to happen, although the shock waves from the explosion of a supernova may help the process.

In the case of the Solar System, it is thought that the Sun and the planets formed from a slowly spinning disc of gas and dust.

Smaller amounts of dust collect together to form planet sized objects.  If these are captured by the gravity of a star, they will orbit as a planet.  Smaller masses of material may get captured by gravity to form satellites (moons).

If sufficient gas and dust collects in one place, it comes together under the force of gravity.  The more mass there is, the stronger the gravity, therefore more material comes in.  As it comes together, the whole thing starts to heat up.  This is a protostar.

At this time the star is only giving off  radiation in the infra red and visible light regions.  If there is not sufficient material, then the whole mass will cool down again.

If there is enough material, it will get hot enough for fusion to occur.  The temperatures involved have to be many millions of degrees Celsius.  The star lights. 

Go back to Unit 2 Topic 10 if you need to revise fusion.

The process is shown in this picture:

Question 2  Write down an equation that sums up fusion.  ANSWER

Life of a Star

Scientists have set off fusion reactions that resulted in the devastating explosions of hydrogen bombs.  The amount of hydrogen involved is tiny, less than the volume of a party balloon.  In a star there are colossal amounts of hydrogen.  So there should be a huge explosion.

In fact the star goes into a stable state.  The force outwards due to the explosion is balanced by the force of gravity.

Question 3  What is meant by balanced forces?  ANSWER

The idea is shown in this picture:

The Sun has been in this stable state for about 4500 million years.  It is half way through its life now.  It is burning 4 million tonnes of hydrogen fuel every second.  It sounds a lot, but the Sun has sufficient reserves to keep on going for another 4500 million years.  We all be long gone and forgotten.

The reaction is nuclear, NOT chemical.  Hydrogen fuel is NOT being burned with oxygen.  This is a common bear trap.

The nuclear fusion reactions in the Sun are right at the very core.  Here there are extreme temperatures and all elements exist in a form in which all the electrons are stripped off.  This is called a plasma.

As well as hydrogen fusing to form helium, other elements are formed by fusion up to and including iron.  To form elements with nuclei bigger than iron needs even more extreme conditions, which we will see later.

Originally the only element in the universe was hydrogen.  All other elements have been made by fusion in stars or supernova explosions.

Old Age of a Star

Helium is a waste product of fusion.  Although some helium nuclei are used up in fusion to larger elements, helium starts to build up in the star.  This makes the star swell into a giant star.

When the Sun swells to a red giant, it would engulf the orbits of Mercury and possibly Venus. 

Question 4  What do you think will happen on Earth?  ANSWER

Red giants are so called because the surface temperature is lower than yellow stars like the Sun.  However they are still pretty high.

Death of a Star

After a while, a dying star starts to shrink under the effect of gravity and finally becomes a dwarf.  It gradually cools and ends up as a dead star.  Depending on its size, it can collapse into a very small space.  It is thought that the Sun will end up as a neutron star of extremely high density but no bigger than the Earth.

The Sun will go out with a whimper.

By contrast, stars that are bigger than the Sun can end their lives with something of a bang.  They swell to red supergiants.  Then the outward force of expansion comes less than the gravity.  Suddenly the star collapses in on itself.  This collapse may take only a few minutes.  The result is an extreme explosion called a supernova.  This is the most violent event that occurs in the Universe.

The extreme conditions that occur in supernovae produce heavy elements from iron upwards.  Some of the elements with big nuclei are very unstable and decay radioactively very quickly.  Others like uranium have half-lives of 4500 million years.  The heaviest stable element is lead.  Precious metals like gold and silver are all evidence of previous supernova explosions.

These elements are hurled out in the cloud of material that spreads out from the supernova explosion.

Supernova explosions often leave nebulae, clouds of gassy material.

This material can come together to start to form new stars and planets.

The remains of a supernova form a black hole.  This is a place where most of the mass of the star is concentrated into a planet sized space (or smaller).  The material is incredibly dense.  Gravity is so strong that even light cannot escape.  The laws of physics as we know them do not apply.  Black holes are the ultimate cosmic predators, gobbling up stars that they come across.

Although you can't see the black hole itself, you can see it gobbling up material from a nearby star.

Question 5 Why can you not see a black hole?  ANSWER

Now try the Crossword

Now try the Topic Quiz

Home                    Unit Index