A crash course in stellar evolution
Although all stars form in a very similar fashion as a collapsing ball of gas, there is a very wide range of stages stars go through. The exact stages a star will go through depend heavily on the composition and mass of the star, as well as its surrounding environment.
All stars start out as part of a massive interstellar gas cloud. For stars to form, the gas needs to be cold enough to collapse under its own gravitational forces. When the gas starts to collapse some of the regions will be denser than others. These over-densities will start separating from the rest of the gas cloud and slowly collapse into gas balls that will eventually be stars. Depending on the size of the cloud, anywhere from a dozen, to several thousand stars can be formed.
The matter in the over densities continue to clump together, now separate from the surrounding cloud, spiraling in towards the centre of gravity of the system. Once these clumps have been separated from the main gas cloud they continue collapsing until the gasses are so close that cores start to heat up, and radiate energy. These are protostars. Not all of the gas in the clump will get eaten up by the protostar, some of it will get pulled into an orbit around the spinning core, and eventually flatten out into the disk which may later form planets.
Once the protostar has collapsed enough, the pressure and temperature in the centre of the core will become just right for hydrogen fusion. At this point it is now a star. Hydrogen fusion is often referred to by astronomers as hydrogen burning, as the process converts hydrogen to helium. If the protostar never gets hot enough in the core to start fusion, is can still continue life as a brown dwarf: a star which doesn’t shine brightly, but does radiate until some outside force disrupts it.
While hydrogen fusion is still occurring in the core of the star, the star is called a main sequence star. The term main sequence refers to the place where it sits on the Hertzsprung-Russell diagram, a tool used by astronomers to classify all stars. The main sequence is the line containing all stars still converting hydrogen to helium in the cores, regardless of age or metalicity (how metal-rich it is).
The energy generated by the fusion process is immense and is enough to create stability with the gravitational forces pulling the outer shells of gas in towards the core. The outer layers will get heated by the radiation coming from the core, and then in turn emit its own radiation. This continues up through the layers, until eventually escaping the star as the radiation we see. The process of going up through the layers of gas reduces the energy, as the levels get much cooler towards the surface. As a comparison, the core will be upward of 10 million degrees, while the surface is a mere two to forty thousand degrees.
Through the next 80+% of the star's lifetime, they will remain on the main sequence, converting the hydrogen in their core into helium. Eventually, there is not enough hydrogen left to create the energy needed to withstand the gravitational pressures, and this is when it stops being a main sequence star.
This main sequence life time varies widely, and is dependant on the mass of the star. A star like the Sun will spend around 10 billion years on the main sequence (it’s already half-way through), while in contrast a star with half its mass will take 80 billion years to get to the same point. Massive stars have a much shorter life time, as a star with 9 times the Sun's mass would burn out in only 25 million years (40 times quicker than the Sun!).
Once the star stops hydrogen fusing in the core, there is no longer enough energy to prevent the gravitational forces from crushing the core. As the core collapses, it becomes even hotter, and it’s possible to start fusion reactions with heavier elements (first helium, then carbon and oxygen, and so on).
Before the core starts burning helium, it will collapse under gravitational pressure. The helium will be packed so tight together that the electrons in each atom will be as close as they can get to each other. This will drag the gas surrounding with it in towards the centre. The pressure on the gas will be great enough to make the right conditions to begin hydrogen fusion. This process is called hydrogen shell burning, and provides more helium for the core. The additional helium will allow the core to compact even more, until there’s enough pressure for the fusion process to begin.
Helium fusion produces much more heat than hydrogen fusion, and this causes the core to swell up, and create equilibrium with the gravitational forces. This continues until the core is depleted of helium.
The more massive stars will skip the inert helium core phase, and almost simultaneously start helium and hydrogen fusion reactions in the core and shell respectively. Eventually, these super massive stars will have fusion reactions between heavier elements, like carbon and oxygen, possibly up to iron.
Stars in the helium burning phase are called giants (or supergiants for the very massive stars). These stars, as the name suggests, are much bigger and brighter than there previous states, and as such stand out brightly. However, even though there is more energy coming from the star, the temperatures on the surface are actually lower than when the star was on the main sequence. This is due to the fact that there is much more surface area for the heat and light to escape through.
Once again, the stars in similar points of their lifetime will occupy a very distinct place on the Hertzsprung-Russell diagram. The Giants and Supergiants sit above the main sequence, in terms of brightness and size, but, as mentioned earlier, to the cooler side.
We have now come to the final part of a star's lifetime. Depending on the mass of the star, there will be a number of very different outcomes.
For low mass stars (those with less than 8 times the mass of the Sun), the pressure never gets large enough for carbon or oxygen fusion, so carbon and oxygen get built up in a non-reacting core. These cores have around half the mass of the Sun packed into an area the size of the Earth! This creates such a strong gravitational force that it pulls the shells of gas very close, causing lots of fusion, and blowing the outer shells out to very large radii. A star the size of the Sun will blow up so much that the outer layer will reach Earth! Eventually, the star looses its grip on these outer shells, and they float off into space. This is what astronomers call planetary nebulae.
Eventually, these gasses go off to mix with other gasses and form new stars, with more heavy elements.
After a few tens of thousands of years, the core will have finished its fusion reactions, and become a white dwarf. These are small stars, which contain between half and one and a half times the mass of the Sun, are very hot, but not very bright. This is due to the heat being emitted in non-visible radiation. If the white dwarf has a companion it may become a Type I supernova, otherwise, this star will cool off and become a dead stellar remnant.
Stars larger than 8 times bigger than the Sun will end in a much more exciting way. These heavier stars will fuse heavier and heavier elements together, all the way up to iron. Once a star starts to build up Iron in its core it can no longer generate the energy to support the outer layers of gas. This presses them onto the iron, and causes silicon to fuse to iron and increase the size of the core.
Once the core reaches about one and a half times the mass of the Sun, it can no longer support itself, and collapses from around the size of the Earth to about 10km in diameter! When the core collapses it releases all it potential energy, and the outer shells are “bounced” off the core by a shockwave driven from the core. This creates a Type II supernova, blowing the outer layers off the star in a violent explosion.
The core of the star left behind is bound so tightly that the electrons have been ejected from the atoms in the core, and the only thing keeping it stable is the fact that neutrons can only be so close. This core is known as a neutron star, which can be observed as a pulsar if has strong magnetic fields and is rotating.
Star which are large enough, and collect enough mass as they collapse will form a black hole. In this case, the gravitational force is much greater than the force acting on the neutrons, and the core effectively collapses into nothing, leaving behind a black hole event horizon.
All stars start out as part of a massive interstellar gas cloud. For stars to form, the gas needs to be cold enough to collapse under its own gravitational forces. When the gas starts to collapse some of the regions will be denser than others. These over-densities will start separating from the rest of the gas cloud and slowly collapse into gas balls that will eventually be stars. Depending on the size of the cloud, anywhere from a dozen, to several thousand stars can be formed.
The matter in the over densities continue to clump together, now separate from the surrounding cloud, spiraling in towards the centre of gravity of the system. Once these clumps have been separated from the main gas cloud they continue collapsing until the gasses are so close that cores start to heat up, and radiate energy. These are protostars. Not all of the gas in the clump will get eaten up by the protostar, some of it will get pulled into an orbit around the spinning core, and eventually flatten out into the disk which may later form planets.
Once the protostar has collapsed enough, the pressure and temperature in the centre of the core will become just right for hydrogen fusion. At this point it is now a star. Hydrogen fusion is often referred to by astronomers as hydrogen burning, as the process converts hydrogen to helium. If the protostar never gets hot enough in the core to start fusion, is can still continue life as a brown dwarf: a star which doesn’t shine brightly, but does radiate until some outside force disrupts it.
While hydrogen fusion is still occurring in the core of the star, the star is called a main sequence star. The term main sequence refers to the place where it sits on the Hertzsprung-Russell diagram, a tool used by astronomers to classify all stars. The main sequence is the line containing all stars still converting hydrogen to helium in the cores, regardless of age or metalicity (how metal-rich it is).
The energy generated by the fusion process is immense and is enough to create stability with the gravitational forces pulling the outer shells of gas in towards the core. The outer layers will get heated by the radiation coming from the core, and then in turn emit its own radiation. This continues up through the layers, until eventually escaping the star as the radiation we see. The process of going up through the layers of gas reduces the energy, as the levels get much cooler towards the surface. As a comparison, the core will be upward of 10 million degrees, while the surface is a mere two to forty thousand degrees.
Through the next 80+% of the star's lifetime, they will remain on the main sequence, converting the hydrogen in their core into helium. Eventually, there is not enough hydrogen left to create the energy needed to withstand the gravitational pressures, and this is when it stops being a main sequence star.
This main sequence life time varies widely, and is dependant on the mass of the star. A star like the Sun will spend around 10 billion years on the main sequence (it’s already half-way through), while in contrast a star with half its mass will take 80 billion years to get to the same point. Massive stars have a much shorter life time, as a star with 9 times the Sun's mass would burn out in only 25 million years (40 times quicker than the Sun!).
Once the star stops hydrogen fusing in the core, there is no longer enough energy to prevent the gravitational forces from crushing the core. As the core collapses, it becomes even hotter, and it’s possible to start fusion reactions with heavier elements (first helium, then carbon and oxygen, and so on).
Before the core starts burning helium, it will collapse under gravitational pressure. The helium will be packed so tight together that the electrons in each atom will be as close as they can get to each other. This will drag the gas surrounding with it in towards the centre. The pressure on the gas will be great enough to make the right conditions to begin hydrogen fusion. This process is called hydrogen shell burning, and provides more helium for the core. The additional helium will allow the core to compact even more, until there’s enough pressure for the fusion process to begin.
Helium fusion produces much more heat than hydrogen fusion, and this causes the core to swell up, and create equilibrium with the gravitational forces. This continues until the core is depleted of helium.
The more massive stars will skip the inert helium core phase, and almost simultaneously start helium and hydrogen fusion reactions in the core and shell respectively. Eventually, these super massive stars will have fusion reactions between heavier elements, like carbon and oxygen, possibly up to iron.
Stars in the helium burning phase are called giants (or supergiants for the very massive stars). These stars, as the name suggests, are much bigger and brighter than there previous states, and as such stand out brightly. However, even though there is more energy coming from the star, the temperatures on the surface are actually lower than when the star was on the main sequence. This is due to the fact that there is much more surface area for the heat and light to escape through.
Once again, the stars in similar points of their lifetime will occupy a very distinct place on the Hertzsprung-Russell diagram. The Giants and Supergiants sit above the main sequence, in terms of brightness and size, but, as mentioned earlier, to the cooler side.
We have now come to the final part of a star's lifetime. Depending on the mass of the star, there will be a number of very different outcomes.
For low mass stars (those with less than 8 times the mass of the Sun), the pressure never gets large enough for carbon or oxygen fusion, so carbon and oxygen get built up in a non-reacting core. These cores have around half the mass of the Sun packed into an area the size of the Earth! This creates such a strong gravitational force that it pulls the shells of gas very close, causing lots of fusion, and blowing the outer shells out to very large radii. A star the size of the Sun will blow up so much that the outer layer will reach Earth! Eventually, the star looses its grip on these outer shells, and they float off into space. This is what astronomers call planetary nebulae.
Eventually, these gasses go off to mix with other gasses and form new stars, with more heavy elements.
After a few tens of thousands of years, the core will have finished its fusion reactions, and become a white dwarf. These are small stars, which contain between half and one and a half times the mass of the Sun, are very hot, but not very bright. This is due to the heat being emitted in non-visible radiation. If the white dwarf has a companion it may become a Type I supernova, otherwise, this star will cool off and become a dead stellar remnant.
Stars larger than 8 times bigger than the Sun will end in a much more exciting way. These heavier stars will fuse heavier and heavier elements together, all the way up to iron. Once a star starts to build up Iron in its core it can no longer generate the energy to support the outer layers of gas. This presses them onto the iron, and causes silicon to fuse to iron and increase the size of the core.
Once the core reaches about one and a half times the mass of the Sun, it can no longer support itself, and collapses from around the size of the Earth to about 10km in diameter! When the core collapses it releases all it potential energy, and the outer shells are “bounced” off the core by a shockwave driven from the core. This creates a Type II supernova, blowing the outer layers off the star in a violent explosion.
The core of the star left behind is bound so tightly that the electrons have been ejected from the atoms in the core, and the only thing keeping it stable is the fact that neutrons can only be so close. This core is known as a neutron star, which can be observed as a pulsar if has strong magnetic fields and is rotating.
Star which are large enough, and collect enough mass as they collapse will form a black hole. In this case, the gravitational force is much greater than the force acting on the neutrons, and the core effectively collapses into nothing, leaving behind a black hole event horizon.