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Young stellar objects;
Newborn stars technically, which are still surrounded by their birth cloud; this class includes T Tauri stars, the first of their type stars, and Herbig-Haro objects, they are actually just gas that are sent in opposite directions from a new star, they are usually hidden in the birth cloud. New stars are usually created in HII regions, which act as a nusery for stars, like the Orion Nebula, which is noted to be a birth place for stars over the last 1 or 2 million years or so.





Main sequence stars;
These are stars that have shed their birth clouds and shine through nuclear fusion. The smallest of these is the red dwarfs, which have little mass but are common amongst the stars, however popular they may be, you cannot see even the closest one (without telescopic aid), Proxima Centauri, the nearest known star, because even that is beyond the sun. As previously mentioned although the red dwarfs are dim, they will outlast the life cycle of our sun.






Next are the red giants, which are the opposite in the fact that they are much larger than our sun, around the size of the orbit of earth, 938,900,000 km. A red giant doesn't have hydrogen burning at it's core, it burns it in a particular region just outside it, aptly named the hydrogen-burning shell, it does this because it has already turned all it's core hydrogen into helium through nuclear fusion.

There are some stars that are alot bigger than a red giant, which are called red supergiants, which can typically be around 1000 or 2000 times bigger than our sun.

Then there is the End states of stellar evolution, which are central stars of planetary nebulae, which are little stars at the center of some specific larger nebula, visible in the planetary nebula of NGC 2392 in Gemini. (image)
The central stars of the planetary nebula turn into white dwarfs, as they are the remains of sun-like stars. The actual nebuala, composed of stars made from gas which a star expelled, fade and blow away. This leaves behind the stars which become white dwarfs.




White dwarfs; which while they are called 'white' dwarfs, they have a range of keeping to white, yellow or possibly red, as the colour of the stars depends on how hot they burn. White dwarfs are the remains of sun-like stars, which never really die, they just fade away.

The essence of a white dwarf is whilst it still gives off heat, it doesn't burn anymore. They are the second second most common stars, after red dwarfs, dispite the fact that even the closest one is still too far away to see without proper equipment. Although the white dwarfs may be small and compact, they have the same mass as the sun,1.989E30 kg , despite the fact that they only occupy the space of the earth.

Supernovas, are enormous explosions which can destroy stars. These are the main varieties.
The first is called Type 2, which is an explosion of a larger, brighter star (bigger than our sun), the star would have been a red supergiant. (possibly burning bright enough to be a blue one.) When it explodes it leaves behind a neutron star, it mayimplode leaving behind a black hole.



The second type is type 1a, which is brighter, hence it is used to measure the expansion of the universe. (Which is only growing faster.) This type of supernova also explodes, but in a reliable manner, they explode due to their 'binary systems', where gas from one star flows dowm to another, which builds up an outer hot layer; which then reaches critical and explodes. (without the build up reaching 'critical' there is no explosion, and no, there is no event when the build up passes critical.)

The 'binary systems', as far as we know, come from two stars; one a white dwarf and the other like the sun, with the dwarf taking gas from the larger. Although there is evidence that some Type 1a supernovas are eased by the combination of two white dwarfs in the system instead.

Then there are Neutron stars, which are stars, that whilst they outweight (via mass) the white dwarfs, they are tiny in comparison. Whilst they may be small, at around 1 or 2 dozen miles across but has a mass that could beat that of the sun.

There are some stars called 'pulsars', which are highly magnetized, spinning neutron star that produces radiation; these beams of radiation sometimes pass satalies which cause brief spurts, which are called 'pulsars', hence the name. The 'pulse' of radiation can be used as a tell of how fast the star spins (ranging a few hundred times per second or once every few seconds.)

And finally there are black holes which are dense, compact objects with matter cram packed into them, meaning the amount of gravity is so strong that it prevents anything from escaping, (including light) some physicists theorize that objects within have left our universe. Black holes are detected by the effects they have on surrounding objects, that is that the matter surrounding them becomes hot and moves quickly, although it never gets organised. It should also be noted that some matter is shot out at a fraction of the speed of light, (186,000 miles per second in a vacuum.) from the matter that swirls within a black hole. There is also the fact that stars orbit around the black hole due to the gravity it omits.

There are three basic kinds of black holes;
Stellar mass black holes, which have the mass of a star; from three times the size of the suns up to around a hundred times that, (although non of that mass have yet been discovered.) They are around the size of a neutron star. They form in supernova explosions, although other means are possible.

Supermassive black hole;
These have masses of hundreds of thousands to more than 20 billion times the mass of the sun. These supermassive black holes are located, generally, at the core of galaxies; either they 'grow' there or the galaxy forms around them. Astronimers believe that there is one in every galaxy; or at least in every full size galacy, as they are unsure about the dwarf galaxies.
The size of these, which the diameter of the event horizon that is given, that is the spherical surface around the black hole, which is where the velocity needed to escape the hole is equal to the velocity of light, 299,792,458 metres per second. When outside of the horizon, the velocity needed to escape is smaller, hence light and high-speed matter can leave.

Intermediate mass black hole;
These are black holes that aren't really very well known to us. Hence most imformation on them is guess work and theories; they have estimated masses of a few hundred to ten thousands the mass of the sun. As it is more massive than any star, hence it probably didn't form from the collapse of a single star. although they haven't been found in the central regions of galaxy, hence we don't really have any idea where they come from or how they are formed.