What is the life-cycle of a star?

Answer 1

SIMPLE EXPLANATION:

Step 1: The dust and gas from a nebula group together.
Step 2: When this group is big enough, energy is created - this group starts to shine.
Step 3: Then it reaches the size of a star.

Step 4: This star becomes a Red Giant - about 500 or more times as big as the Sun now.
Step 5: When the Red Giant runs out of its energy, it explodes as a supernova.
Step 6: This supernova leaves behind a little (50 km across) and dens star - white dwarf.
Step 7: If the Red Giant weighed at least twice as much as our sun, this white dwarf turns into a neutron star and then into a black hole. This whole step occurs in less than a minute.
(Step 8: If a really huge and dens star gets sucked into a black hole, a quasar might have formed)
Note 1: The lifecycle of a star lasts about 10-15 billions of years.
Note 2: Step 8 only may never occur in the lifecycle.

DETAILED EXPLANATION:

Step 1 - Grouping up the gas from Nebula: A nebula is a huge cloud of dust and gas. When two atoms of this dust/gas meet each other, one of them may rip out (wrest) an electron from the other one (this electron would be excess). Let's call this atom 'Atom A'. When Atom A meets another atom that lacks an electron (as far as I know, hydrogen is one of these kinds of atoms), the electron may jump off to Atom B (atom that lacks an electron). However, this very electron may still be following the line of one of the Atom A's shells. This simply means that Atom A and Atom B share the electron, so that they are stuck together. Over a time, a group of dust and gas will be created. According to another theory, the most popular one, the nebula groups up the gas and dust using the force of its own gravity.

Step 2 - Forming a protostar: When the gravity of the group of dust and gas is strong enough, it makes some smaller groups orbit itself. While orbiting is in process, the smaller groups are caused to either create a friction on themselves or bump into the bigger group. In both cases, thermal (heat) and light energy are created. (Basically, GPE - Gravitational Potential Energy is converted into KE - Kinetic Energy (when gravity pulls down molecules to make them move), which is then converted into heat and light (when the movement of molecules causes them to create friction/bump into the bigger gas and dust group and emit light and heat).) While the 'star' is getting energy, it is no longer just a group of dust and gas - it is a protostar.


Step 3 - Becoming a main sequence star: The protostar reaches the size of a star and the gravity gets really strong. In this stage, the protostar becomes a 'main sequence star'. Once the star is hot enough, nuclear fusions occur. That happens because when molecules are moving pretty fast, they can overcome the interatomic force and make two nuclei join together (That happens, for example, when two molecules are collected by gravity to create a mixture and the temperature causes them to move. Then they 'bump into each other' to form a single molecule. All atoms in those two molecules are now joint together). Interatomic force is made up from a huge amount of energy, and once it's overcome, all that energy is released. The hydrogen molecules that the star is made of now 'react with their selves' to form denser elements like helium. That energy creates a press from outside of the core (and heat and light), which does not let the gravity to make the star shrink. This means that gravity has the same amount of energy as the press from nuclear fusions. (If the star is not hot enough to fuse hydrogen, it forms a brown dwarf. It is a failed star that does not produce much energy (does not emit light). If Jupiter was at least 3 times bigger, it would be a brown dwarf.)

Step 4 - Swelling up to a Red Giant: As the star runs out of it's hydrogen fuel from the core, it starts to use up hydrogen from outer layers (also, if the star is big enough, it starts to fuse up fused up elements like helium. Really massive stars may fuse it up to iron. This makes the core denser). The gravity there is not as strong as in the core, but the amount of energy produced by nuclei fusions remains the same. This causes the star to swell up. The diameter, usually, becomes about 400-500 times bigger than it was as a main sequence star. So far, the star is a 'Red Giant'. More massive stars form Red Supergiants. Red Giants have a huge volume and small density. One of the biggest Red Supergiants ever found is VY Canis Majoris. It is a billion times bigger, than our Sun.

Step 5 - A supernova explosion: When a Red Supergiant completely runs out of its energy (all of the gas to turn into denser materials is used up), the press from inside of the core is not created. Also, the Red Giant uses up all the helium in the core and fuses it up into much denser materials. Just heat and and gravity are not enough to do it, so the star takes some energy from the outer layers' nuclear fusions and from the outer layers' kinetic energy. This causes the layers to stop expanding. Instead, gravity pulls them down and makes the layers travel almost as fast as the speed of light. When they finally meet with the core, all of that kinetic energy produced is now converted into light, heat, gamma rays and so on. This energy causes a massive explosion called supernova. Smaller stars cannot fuse up helium, so supernova is not created. The hydrogen from outer layers continues to expand to form a planetary nebula.

Step 6 - Forming a white dwarf: A supernova leaves behind a small, dens star - exactly opposite to what a Red Giant is. After the explosion, only the core has been left. But now it is a bit different to what it was as a main sequence star or a Red Giant. It has been squashed in so that it is now about 50km in diameter. The mass of it though would be similar to the mass of the Sun. This object is called a 'White Dwarf'. In a White Dwarf, a cube of sugar would weigh hundreds of thousands of tons. (White dwarf will cool down in about 10 billion of years to form a black dwarf)

Step 7 - Denser stars: If the Red Giant has weighed at least 1.5 times more than the Sun, a supernova would leave behind a slightly different object. When the gravity becomes strong enough, it squashes all of the electrons, protons and neutrons together. This creates one gigantic nucleus. It is not that big though - just about 10 miles across. The size may increase the speed of rotating around the object's axis - it may spin more than 30 times a second around itself. If that's the case, it is a neutron star. Neutron stars can be different: when a supernova is really powerful, it wrests most of the electrons from the core (so do ionising radiations like gamma rays that are released by the supernova). This will mean that the core would only be made of ions. These ions will then be turned into a nucleus with an electric charge. As far as we all know, if an object with an electric charge moves, it creates an electromagnetic field. The faster it moves, the stronger the field is. But because a neutron star moves (spins) really quick, the electromagnetic field is unimaginably strong. It makes the neutron star easier to detect. This kind of neutron star is called a 'pulsar'.
But apart from the pulsar, there are a few other types of a neutron star. One of them is obvious - a 'quite neutron star'. It is the same as a pulsar, but it has no electromagnetic field (because the supernova was not as strong).
If a Red Giant weighed more than 3 times more than our Sun, a hypernova might have occurred. When the neutrons are under critical pressure, they can simply disappear. If they do disappear (turn into gravitational energy), then a black hole is formed. There is no matter inside a black hole. Even time and space get sucked in to it.
If one wanted to work out, how fast an object would be accelerating when being sucked into a black hole (according to the black hole's gravity), one would realize that the object would travel much faster than the speed of light (which is impossible). This means that space and time must not only be curved, but they must travel faster, than the speed of light (this is possible because space/time is not material). Also, if the black hole is spinning, it has to have a centre of rotation. Everything would be orbiting it. If an object would be travelling fast enough to overcome the black hole's gravity (travel above the speed of light), this object would be thrown away (as if a meteor was moving close to Earth fast enough to ignore Earth's gravity). But if there is nothing inside the black hole apart from the space-time structure (apart from space and time), only space and time should be orbiting the centre. As I said before, it must be travelling above the speed of light. This means that space and time must first be sucked in, then orbit the centre of rotation and finally be thrown away with a 'beyond the speed of light'.

Step 8 - Forming a quasar: This step will only be if a black hole is formed.
If a really massive object gets sucked in to a black hole, it will become as bright as all the stars in 100-150 galaxies added together. Why?
Well, imagine an object being sucked in to a black hole. When it gets closer to the black hole, the difference in gravitational pull in one meter would be nearly as much as the difference in gravity between the Sun and the Earth. Therefore, this object will be stretched to either become gas or dust. Because it will be travelling really fast, when meeting another object, it will create friction. In this process, the gas/dust will lose/gain a lot of electrons. This will turn the gas'/dust's atoms into ions. They will be moving really quickly. This movement will create such a strong electromagnetic field, that it will overcome the black hole's gravity. Besides the fact that it will curve the space-time (because this will make object travel above the speed of light), it will throw away all the gas and dust that was being sucked into a black hole. As this gas will travel with an unimaginable speed, it will create friction while bumping into another gas'/dust's molecules that are now being sucked in to a black hole travelling with a high speed as well. This friction will release such a huge amount of energy, that, as I have said, the gas/dust will become as bright as all the stars in 100-150 galaxies added together. This process will last about a billion of years - till all of the dust and gas is used up. It is called a quasar.

Step 9 - Black hole's vapour: This gas and dust will not only release visible light and heat - it will get rid of electrons accelerating them to the speed of light. When these electrons meet another atom, they bump into it releasing loads of energy, coming out in the form of gamma rays. When this kind of gamma rays reaches another atom, it turns into matter (and antimatter). (That's because E=mc2. Mass can become energy and energy can become mass (matter)). The matter follows the line of the electromagnetic field of the quasar (antimatter also follows the line of the same electromagnetic field, but it goes in opposite direction). As the matter (and antimatter) is gone, it takes a little part of the black hole's mass. Over a huge amount of time (about 1050 years), the black hole will weigh as much as our Sun. The amount of energy inside the black hole will be enough to escape from it. As it escapes, it tears out the black hole (as in supernova). The black hole vapours. (This is probably the reason why the Big Bang has occurred.)

Thanks very much for reading, and I hope I have explained what a lifecycle of a star is.

Answer 2

Nebulae (gas and dust) protostar (formed by immense pressure in nebulae) ^ becomes either a Brown dwarf (basicall dead :P) or a Main sequence star which becomes: A Red giant (when MSS runs out f hydrogen ) if red giant is small it becomes a white dwarf and does nothing or a planetary nebulae if it's a big one it either becomes a Neutron star or a Black hole