The life of a star is brilliant, furious and explosive. Although the countless twinkles in the sky may look one and the same, their journeys vary considerably depending on their mass. Today we will recount the various paths of these stars from birth until death.
The birth of a star is a wonderful process, whose mother is gravity. Stellar life begins with the gravitational collapse of giant molecular clouds a.k.a nebulae. As the clouds collapse they break into smaller fragments and each of these fragments releases its gravitational potential energy as heat. The temperature of the body thus increases, along with the pressure due to the decreasing volume, as the clumps coalesce, finally forming what is known as a protostar.
The protostar continues to gather mass from the molecular cloud from which it initially formed. This process is formally known as accretion, the sucking in of gas and dust from the parent cloud, as the protostar grows and grows. The star continues to do this for (taking the case of a star of one-solar mass i.e. our sun) 1,000,000 years! That’s a rather long first stage of life but finally at the end of this, our star has successfully completed its adolescence and it now known as a pre-main-sequences star as it achieves its final mass.
Note: the mass of stars is often conveniently given in solar-masses, i.e. how many times the mass of our sun the star is. Our sun is after all our local star and thus we use it as a unit against which to measure and compare the others.
Now protostars whose mass are less than 0.08 times the mass of our sun, or (1.6 x 10^29 kg to be precise) do not go on to do much of excitement. These objects are called Brown dwarfs, these stars shine very dimly and then progress to die a slow uneventful death as they cool over hundreds of millions of years.
For protostars whose mass is greater than the above given, they form what is known as Main sequence stars. Their core temperature exceeds 10 million kelvin (water boils at 373 kelvin for a reference point) and this initiates the fusion of hydrogen i.e where four protons (or hydrogen nuclei) come together to form a helium nucleus. Now this fusion creates a delicate balance – the energy released by the core of the star during fusion creates a high gas pressure pushing outwards. This outwards force counteracts the inward force of gravity, felt between all the atoms in the star which compels the star to collapse in on itself. These two forces, outward and inward, balance creating what astrophysicists call- hydrostatic equilibrium. The star thus lives in this stable state for millions of years as it uses up all the hydrogen in its core for fusion.
Smaller Main sequences stars burn their fuel much slower and can remain in this stable state for hundreds of billions of years. Whereas massive Main sequence stars have a higher core temperature and thus the rate of hydrogen fusion is much greater causing their lifetime in the steady state to be just a few million years. In the case of our sun, its stable lifetime is 10 billion years and luckily for us we are existing in the middle of its steady years.
However, all good things must come to an end and the eventually the core of the star will run out of hydrogen to burn. The resulting loss of outward pressure causes the force balance to be lost with gravity taking control. The core of the star begins to contract and thus increases in temperature once more. Imagine a room filled with people, if all the people have to huddle into one tiny corner of the room (i.e. contracted) the temperature in that part of the room increases. This is the same thing for the core of the star but replace the people with atoms. So as the temperature increases there is enough energy for helium fusion to occur in the core, whereby helium atoms fuse together to form even larger molecules. Secondly, it raises the temperature of the outer layers so that hydrogen fusion can occur here too. Eventually when fusion ceases, the star swells up and at the same time cools (think of people spreading out in the room again). Because the energy of the star is now spread over a wider surface area the star, resulting in a lower surface temperature, the shines at a lower luminosity (now emitting in infrared wavelength ranges instead of the visible in the electromagnetic spectrum) a.k.a the star is dimmer because the light appears redder. These stars can have diameters between 10 and 100 times that of our Sun and are known as Red Giants. When the eventual times comes this will be the next stage of our sun and what a terrifying imaging it would be seeing the sun swell up in the sky before our eyes, creating scorching temperatures on our planet’s surface. My pessimism, current global warming statistics and the new US president however makes me think that our species will not see that day.
Elderly Years and Death
The outer layers which have now drifted so far from the star’s core loose the gravitational pull from the core and eventually separate off completely. As in the great cosmological circle of life, these outer layers form the planetary nebula for new stars in the universe. But, what of the core? Well again, this depends on the mass in question of the initial star.
Case 1: Peaceful
If the star is about the same mass, or less, than our Sun, it will turn into a white dwarf star. The core collapse of the star continued until what is called a ‘electron-degeneracy’ is reached, this is when the star simply cannot collapse any further due to the outward pressure existing between the atoms themselves. They are the shrunken remnants of normal stars with an extremely high density. Their diameter is roughly 1% of the current diameter of the sun and one spoon of the substance would weight over several tonnes! These stars have no source of energy and thus eventually radiate all their existing energy such that they completely cool and when this happens they will no longer emit heat or light and become known as cool black dwarfs. There is little more to say on these except the mass limit that exists for this type of star – approximately 1.44 times the mass of the sun, this is known in fancy terms as the Chandrasekhar limit.
Case 2: Destructive
If the star is heavier than 1.44 times the mass of the sun the electron degeneracy pressure will be unable to supports its weight against the force of gravity as in the case of the white dwarf. The core will then undergo a violent collapse to form what is known as a neutron star. During this collapse further fusion can occur along with the production of a surge of neutrinos, head to ‘Rise of the neutrinos‘ to learn more about this little fellas. This in turn creates a shock wave which explodes outwards most of the in-falling core layers in a tremendous, gigantic explosion. This explosive event causes such an immense brightness it is akin to the brightness of 100 million suns for a short time. The fusion elements are then spewed out into the universe, for building blocks of later life. In fact this supernovae stage is the only stage in the universe where such high level fusion can occur to produce heavy molecules such as iron. If it weren’t for supernovae we’d live in a much less exciting universe, in fact we wouldn’t live at all. We are built from the products of supernovae, we are indeed star dust.
The inner core remnant, the neutron star, is extremely tiny, roughly 10km only and supported a pressure known as the ‘neutron degeneracy pressure’ this time. If you thought white dwarfs were dense, neutron stars will take it to another level completely. These stars are a little more active than the white dwarfs and rotate. Those that rapid rotate and have their magnetic poles aligned with the Earth we call pulsars. This is because we can detect a pulse of radiation during each revolution.
Case 3: Mysterious
Finally, for those stars that are even larger still the neutron degeneracy pressure will be insufficient still to prevent collapse. The mass of which a star is needed to be for this to occur is thought to be over 2 times the mass of our sun. In this case the supernovae occur but the remnant neutron star continues to collapse until it reaches what is known as its Schwarzchild radius and is thus known as a Black Hole. Every object has a Schwarzchild radius, it is the radius of a sphere such that if all the mass of the object were to compressed within this size sphere the escape velocity from the surface would be equal to the speed of light. Black holes are special creatures because all their mass is already contained within its Schwarzchild radius. It is called a black hole because light can no longer escape the object and it is thus no longer directly visible. The density of a black hole cannot be quantified and theoretical physicists have the resulting quandary that their calculations spit the value out to be infinite at the centre. Black holes are extremely curious beings, and a favourite topic of mine to write on here at RTU as many of you will know. Our understanding of them could reveal answers to many of great mysteries of this universe. What we can say for sure is that they are the final stage of the heaviest stars in our universe, though they are by no means a point of death. What lives inside a black hole, we can only image…