The Life of a Star

We all think of stars as being "permanent" don't we?  They were there when we were born, and most of them will likely be there when we die, so it is easy to understand why we perceive them as eternal.  But like all things stars do age and eventually, like us, they will all die.  So this section is devoted to an analysis of the different types of star, and to how they age and die in slightly different ways.  You will be reading about exotic events such as supernova explosions and planetary nebula ejections, and strange objects such as neutron stars and black holes.  I'll try to make these explanations as simple and easy to understand as possible.

 

Why Do Stars Grow Old?
Stars form from the condensation, or gathering together of huge clouds of hydrogen gas.  As these clouds draw together by gravitational attraction, the temperatures and pressures in the centre of the cloud become high enough for a process called hydrogen fusion to begin.  This process involves the conversion of four hydrogen nuclei into one helium nucleus, a process which releases two elementary particles called neutrinos plus a quantity of energy.  The energy produced heats the interior of the star to very high temperatures - millions or even hundreds of millions of degrees - and this is the primary nuclear engine which makes the star shine.  The high temperature has three main effects:

  • It creates inside the star a pressure, and it is this pressure which prevents the star from collapsing any further due to the gravitational forces which pulled it together in the first place

  • It maintains the right conditions to enable the nuclear reactions to continue - if it didn't the star would "go out"

  • It causes thermal energy (heat) to flow from the interior of the star to its surface, making the star hot and also making it shine

 

So the definition of a star is a body which shines by generating its own light, radiating that light and other forms of energy out into space.  As an example of how much energy is produced, the sun's brightness is equivalent to four trillion, trillion 100 watt light bulbs all burning at once.  A star which is burning hydrogen in its core is known as a Main Sequence star, and there is a diagram called the "Hertzsprung-Russell" diagram which shows that main sequence stars can all be plotted on a curve in the diagram.

How Does a Main Sequence Star Evolve?
Now, we've said that the hydrogen fusion reactions produce helium and so, over a period of time the concentration of helium in any given star will increase.  Physics dictates that this helium will migrate to the core and as more and more helium is produced it becomes compressed in the core which becomes hotter and hotter.  Eventually a point is reached when the helium itself is able to begin burning and this burning takes place via another nuclear reaction which requires these higher temperatures in order to proceed.  This reaction produces carbon and oxygen and all the time this process is going on hydrogen from the star also continues to burn in a thin shell which surrounds the helium core.  The rate of energy generation is now much greater than it was when the star was only burning hydrogen and so the star is correspondingly brighter.  More energy is also flowing from the core to the surface, and this increased pressure causes the outer layers of the star to swell, sometimes to an enormous size.  In spite of the very high internal temperatures this swelling process causes the surface temperature of the star to become very cool (in stellar terms perhaps as "cool" as 5,000 degrees F) and there is a tendency for the star to become red in colour.

 

  •  Perhaps it would be useful to note here that the colour of a star is related to it's surface temperature, much in the way that a piece of metal changes from red hot to yellow to white hot as it is heated in a fire.  So red means cool, white or blue means hot.

 

Another important factor here is that stars which have reached this stage in their development are no longer on the "main sequence" of the Hertzsprung-Russell diagram.  They have migrated off the main sequence and have become red giants or yellow or red supergiants.  So, what determines whether a star becomes a giant or a supergiant?  As we will see mentioned many times in this article, the mass of a star is critical to determining how it will evolve and we know that stars which have a modest mass (about that of our sun or a little larger) will produce red giants.  Larger mass stars will produce supergiants, which are even larger than giants.  The amount of mass with which the star starts out will also determine what happens when the helium in it's core has all been burned and is exhausted.

 

The Importance of Mass
If a star originally had less than about eight times the sun's mass at the start of it's life, it will follow a very different evolutionary path from one which started out with more than eight times our sun's mass.

  • Stars with initial masses of less than about eight solar masses finish their lives by ejecting planetary nebulae, leaving behind a white dwarf star in the centre.

  • Stars with initial masses of more than about eight solar masses end their lives in a most spectacular way, by exploding with enormous violence in something called a supernova.  This explosion leaves behind a very different remnant - either a neutron star or a black hole.

The reasons for the very different behavior for stars of different mass are associated with the nuclear reactions taking place inside them, and we will look a little more closely into this.

 

Evolution of Smaller Stars
When stars with initial masses of less than about eight times that of the sun reach a point in their evolution that enough helium has been produced in the core, they start to burn that helium to produce oxygen and carbon, as we have seen in the foregoing text.  During this process they gradually expand to become red giants and the star brightens by a factor of between 1,000 and 10,000 times.  As the red giant continues to grow in size it can grow as large as the orbit of our Earth or possibly even as far out as Mars.  This is the ultimate fate for our sun, but long before the sun has grown this large the temperatures on our world will have soared to levels which will extinguish all life.  However there is no need to worry unduly yet about all this, because the process will not even begin for a few billion years, and my guess is that by then we will either have "extincted" ourselves in some way.  It is more likely that our remote ancestors will be roaming the galaxy looking for new worlds to colonize and pollute.  I sincerely hope that the fate of our species is the one which takes us out into space as a roaming species and I also hope that by that time we will have found better ways to live with nature rather than use it with little thought for the future.  Anyway, what about our stars, and what about our sun, which by now is a red giant?  What's next?  Well, when a star reaches this stage it becomes very distended and begins to lose its outer layers of hydrogen, stripped off by a strong solar wind which blows outwards from the helium burning core.  In this process, most of the remaining hydrogen envelope is stripped off and carried away in the "wind", and during this phase the star starts to pulsate, expanding and contracting in cycles, almost like a death agony.  The pulsation period is quite long - several months to more than a year - and the star is called a "Long Period Variable".  We do see many examples of long period variables in the sky and have been able to learn a lot about the exact composition of these objects, which is one of the ways which has been used to develop a complete understanding of the evolution process.  Meantime, a lot of ejected material has been left surrounding the star.  This material is known as a planetary nebula, and we can see numerous planetary nebulae in our immediate galactic neighbourhood.  Typically these nebulae have masses of about 20% of a solar mass, although some are much larger, and they expand outwards from the star, driven by the solar wind at about 10 to 20 miles per second.  About 1,500 planetary nebulae have been identified in our Milky Way galaxy so far, and it is estimated that one new planetary nebula comes into existence per year.  These nebulae are illuminated by their central star, and they can have incredible shapes and colors as the solar winds distort them.  Here is an excellent example of a planetary nebula.

This image of the Messier 57 planetary nebula in the constellation of Lyra was taken at our Spanish observatory using a 14 inch diameter telescope and a CCD camera.  Left behind in the centre of the nebula is the dying star, which is a very interesting object, but which does not have a very exciting future.  With all (or most) of the hydrogen gone we are left with a star consisting of carbon and oxygen - the product of helium core burning - surrounded by a thin layer of helium - the product of hydrogen burning in the shell.  The typical mass of the remaining star ranges from perhaps less than half of one solar mass to more than one solar mass, but due to compressive forces the star is very small, close to that of Earth - about 8,000 miles in diameter.  So, to understate the point, our star has now become "rather compact".  In fact a spoonful of matter would weigh between 10 and 100 tons, so it is indeed highly compressed.  A point is eventually reached when all nuclear reactions cease, and the star begins to cool.  It has become what we know as a White Dwarf.  White Dwarfs are burned out stellar cinders and as they continue to cool over many billions of years they become virtually undetectable, even if they are quite close to us.  So that's what happens to "smaller" stars.  What about the big boys?

Evolution of Larger Stars
If a star begins its life with a significant amount of hydrogen - perhaps twenty solar masses, something very different happens to it.  You will recall that all stars start by burning hydrogen to make helium, and that they then burn that helium to make carbon and oxygen.  Well, if our star is sufficiently massive when all the helium has been converted to carbon and oxygen further compression can take place and the core temperature and pressure rise sufficiently for the carbon-oxygen core to ignite, converting the core to elements such as neon, magnesium, silicon and sulphur.  Now the star is running on borrowed time.  You will remember that what keeps a star from collapsing under it's own gravity are the nuclear reactions taking place inside it and each of the new reactions to which our star has turned has been a producer of energy.  Next the star reverts to burning the silicon and sulphur, which produces iron, nickel and other similar molecular weight elements, which start to build up in the core.  At this point we have various levels of conversion taking place in skins, one laid over another, and the structure of our star resembles that of an onion, with a central core of iron surrounded by a shell of burning silicon and sulphur, then oxygen and carbon, then helium and finally hydrogen.  Now we have arrived at "Crunch Time!"  The key difference now from anything which has happened before is that the burning of iron is a net consumer of energy.  Up until now, as we said above, conversion of lighter elements to heavier ones always produced energy, but any reaction from this point to convert iron to heavier elements actually consumes energy, and this causes sudden and catastrophic changes in the star.  As the mass of the iron core approaches 1.4 solar masses, due to continued silicon and sulphur burning in the thin shell overlying the iron core, a point is reached when the amount of energy being produced by the star is insufficient to balance the gravitational forces and the iron core collapses.  This happens very suddenly, and in less than a second the core collapses from a diameter of about 5,000 miles to only about 12 miles.  The amount of energy produced in this collapse is gigantic - and in less than a second energy equivalent to that produced by 100 stars like our sun during their entire lifetime of 10 billion years is released.  This sudden and explosive release of energy sends out a massive pulse of neutrinos (elementary atomic particles), which take a large part of the released energy with them, surging out into space.  However, the energy deposited in the lower layers of the star creates a shock wave which runs quickly through the star to the surface, heating the various layers as it goes and inducing explosive nuclear burning.  The resultant explosion ejects a major proportion of the stellar materials into space at a speed of thousands of miles per second, and the surface brightens rapidly until the star becomes as bright as a billion suns.  It is at this point, if the star is not very remote from us, that we here on Earth suddenly see a "new" star in the sky.  Some have even been visible in the daytime if the supernova explosion is sufficiently close to us, but let's hope that one does not occur too close by!  If a supernova explosion did take place even a few light years from Earth, it could have sufficiently disastrous effects as to render Earth uninhabitable, so delicate is the balance of our ecology.  Luckily, no nearby candidates for supernova seem to exist, but 20 or 30 per year are discovered by astronomers observing galaxies beyond the Milky Way.

Here is a picture of the remnants of a supernova explosion which occurred in the year 1054AD and which was witnessed and documented by the Chinese.  It is in the constellation of Taurus and is known as the Crab Nebula, because of the tendrils which have been produced.  This picture of the Crab Nebula was taken by us in Spain.

Unlike a planetary nebula ejection, the material from a supernova ploughs into the surrounding interstellar medium at great speed, compressing it, mixing with it and enriching it with freshly synthesized elements which the star had been creating over all those millennia.  Supernova remnants remain visible for hundreds of thousands of years as they spread ever farther from their point of origin.  Those of you who have read the section of this web page which deals with "Where Did We Come From?" will remember I told you that the carbon, magnesium and all the other elements in our bodies came originally from stars - that this is the "Star-Stuff" of which we are made?  Well, this is where a lot of it comes from, and this process is one way in which gets distributed throughout the galaxy.

Stellar Remnant
Remember that a white dwarf was left at the centre of a planetary nebula when a smaller star aged and ejected it's outer envelope?  So, what kind of object is left behind after a supernova?  As you might expect, following such an incredible event, a rather incredible object is left behind.  Basically what is left is a ball of neutrons, and the object is called a Neutron Star.  The neutrons are pressed so closely together that the star is only about 12 miles across, yet it still contains a mass equivalent to 1.4 solar masses.  So, we have an object with 40% more mass than our sun, occupying a space the size of a city.  As if that's not enough, the star is spinning rapidly because, as the iron core collapsed conservation of angular momentum required it to start spinning as it reduced in size, rather like a skater spins faster if they draw in their arms during a spin.  This collapse also bunches the magnetic field lines running through the core of the star, greatly amplifying the strength of the magnetic field (shortly after the supernova event, the magnetic field of the neutron star can be one trillion times larger than that of Earth).  These intense magnetic field lines "somehow" direct beams of radiation out along the magnetic poles (see, there are still a lot of things which we don't know).  In a final twist, if these beams of radiation are not aligned accurately with the star's rotation axis, then a beam of energetic radiation sweeps through space as the star rotates, not unlike a beam of light from a lighthouse.  We can detect these pulses of energy using radio telescopes, and the objects which produce them are called Pulsars.  The speed of rotation varies from supernova remnant to supernova remnant, and can range from 30 times a second in the case of the pulsar at the center of the Crab nebula (above), to thousands of times a second.  Imagine if you can, an object the size of a city, with a mass greater than the sun's rotating that fast.  Not easy to imagine, hey?

 

Black Holes
There's one final step in the stellar evolution cycle which we haven't mentioned, and it's one which fascinates people - Black Holes.  One of the predictions of Einstein was that light could be "bent" in it's course by gravity.  This was proved when scientists observed a star at a known position in space which was about to be occulted by our sun.  The theory said, if Einstein was right, that light coming towards us from the star would be bent by the sun's gravity as it passed by, and so the star should remain visible for a short time longer than calculations based on its known position said it should.  Effectively we would be looking "around" the sun.  This is exactly what was observed, and the effect of large gravitational fields on light is now well documented.  There is even a phenomenon called Gravitational Lensing, which occurs when our direct view of far distant galaxies is blocked by a nearer massive object, like a large cluster of nearer galaxies.  The effects of the large gravitational pull of the intervening galaxy cluster causes light from the distant object to be bent around the cluster, making it possible to see the distant galaxy even though the other objects are in the way.  But what's this all got to do with black holes I hear you cry.  Well, objects such as neutron stars have a lot of mass in a very small space, as explained above.  So their gravity is very intense, which means that they have the ability to noticeably bend light.  If our supernova explosion created a stellar remnant with a mass greater than 1.4 solar masses, then its gravitational field will be so intense that it bends any light emitted by the star so much that it travels in an arc around the star, and this prevents the light from ever escaping.  Because we see no light escaping the object is known as a "black hole".

There is a separate section in these pages on black holes which deals with them in rather more detail, so if you would like to know more about them, please visit that page.  Meantime, this slightly simplistic description of how stars are born and evolve should have given you a pretty reasonable understanding of the subject.  The bottom line is that nothing lasts forever, not even a star!