Red Dwarf Stars :
Red dwarfs are very low mass stars. Consequently they have relatively low temperatures in their cores and energy is generated at a slow rate through fusion.Hence these stars emit little light, sometimes as little as 1⁄10,000 that of the Sun. Even the largest red dwarfs have only about 10% of the sun’s luminosity.
Red Dwarf stars are smaller than our sun. And since they are smaller, they also have less mass. Because of their small size, these stars burn their fuel very slowly, which allows them to live a very long time. This also causes these stars to not shine as brightly as others. Some red dwarf stars will live trillions of years before they run out of fuel.
Red dwarf stars only burn a little bit of fuel at a time, they are not very hot compared to other stars. Think of a fire. The coolest part of the fire is at the top of the flame where it glows red, the hotter part in the middle glows yellow, and the hottest part near the fuel glows blue. Stars work the same way. Their temperature determines what color they are. Thus, we can determine how hot a star is just by its color.Very few stars that you see in the sky are red dwarfs. This is because they are so small and make very little light,that they are masked by brighter stars.They look red because they are less luminous.
Yellow Stars :
Like the Sun, these medium-sized stars are yellow because they have a medium temperature.Their higher temperature causes them to burn their fuel faster. This means they will not live as long, only about 10 billion years or so. Near the end of their lives these medium-sized stars swell up, becoming very large. When this happens to the Sun, it will grow large enough to engulf even the Earth. Eventually they shrink again, leaving behind most of their gas. This gas forms a beautiful cloud around the star called a Planetary Nebula.
The sun is only about 5 billion years old. It still has another 5 billion years or so before it will expand and turn into a planetary nebula.
The Sun is so hot that when it dies, it will take a long time to cool off. The Sun will die in about 5 billion years, but it will still glow for many billions of years after that. As it cools, it will be what is called a White Dwarf Star. Eventually, after billions, maybe even trillions of years, it will stop glowing. At that point it will be what we call a Black Dwarf Star. Because the process for a star to become a black dwarf takes such a long time, it is believed there are still no black dwarf stars in the universe.
White Dwarfs :
A white dwarf is what stars like the Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, this type of star expels most of its outer material, creating a Planetary nebula. Only the hot core of the star remains. This core becomes a very hot white dwarf, with a temperature exceeding 100,000 Kelvin. Unless it is accreting matter from a nearby star, the white dwarf cools down over the next billion years or so. Many nearby, young white dwarfs have been detected as sources of soft, or lower-energy, X-rays.A white dwarf’s mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a Neutron star.White dwarfs form as the outer layers of a low-mass Red Giant Star puff out to make a planetary nebula. Since the lower mass stars make the white dwarfs, this type of remnant is the most common endpoint for stellar evolution. If the remaining mass of the core is less than 1.4 solar masses, the pressure from the degenerate electrons (called electron degeneracy pressure) is enough to prevent further collapse to form neutron star.
Red Giants :
When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remains, initiating fusion of hydrogen to helium in the shell. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star’s luminosity by a factor of 1,000–10,000. The outer layers of the star then expand greatly, beginning the red giant phase of the star’s life.
Due to the expansion of the outer layers of the star, the energy produced in the core of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star’s visible light output towards the red – hence red giant, even though the color usually is orange.
Blue Giant Stars :
Blue stars are large and compact, this causes them to burn their fuel quickly which in turn makes their temperature very hot. These stars often run out of fuel in only 10,000 – 100,000 years.
A blue giant is extremely bright. Like a lighthouse, they shine across a great distance. Even though blue giant stars are rare, they make up many of the stars we see at night because they shine so brightly.
Blue giant stars die in a spectacular way. They grow larger just like the sun-sized stars, but then instead of shrinking and forming a planetary nebula, they explode in what is called a Supernova. Supernova explosions can be brighter than an entire galaxy, and can be seen from very far away.
Giant and Super Giant Stars :
As a sun-sized star gets old, it starts to run out of its hydrogen fuel. When the process of burning hydrogen in the star’s core begins to slow down, the core gets more compact and dense. This means all the stuff in the middle of the star gets really close together. As the center gets smaller and smaller it starts to heat up again. When it gets hot enough it will start to burn a new fuel called helium.
Once ignited, helium burns much hotter than hydrogen. The additional heat pushes the outer layer of the star out much further than it used to be, making the star much larger. Imagine a hot air balloon. As the air inside the balloon gets hotter, it stretches the balloon out further and further. As the giant star gets hotter, its outside stretches out further and further. When our own sun begins to stretch into a giant star, it will engulf Mercury, Venus, Earth and Mars.
The only difference between Giant Stars and Super Giant Stars is their size. Super Giant Stars are much bigger. If the Sun were replaced by a super giant star, it would extend from the center of our Solar System almost all the way out to Uranus.
Neutron Star :
If the core mass is between 1.4 and 3.2 solar masses, the compression from the star’s gravity will be so great the protons fuse with the electrons to form neutrons. The core becomes a super-dense ball of neutrons. Only the rare, massive stars will form these remnants in a supernova explosion. Neutrons can be packed much closer together than electrons so even though a neutron star is more massive than a white dwarf, it is only about the size of a city. The neutrons are degenerate and their pressure (called neutron degeneracy pressure) prevents further collapse.If the core remnant has a mass greater than 3 solar masses, then not even the super-compressed degenerate neutrons can hold the core up against its own gravity. Gravity finally wins and compresses everything to a mathematical point at the center. The point mass is a Black Hole. Only the most massive, very rare stars (greater than 10 solar masses) will form a black hole when they die.
In general, compact stars of less than 1.44 solar masses – the Chandrasekhar limit – are white dwarfs, and above 2 to 3 solar masses (the Tolman–Oppenheimer–Volkoff limit), a quark star might be created; however, this is uncertain.Gravitational collapse will usually occur on any compact star between 10 and 25 solar masses and produce a black hole. Some neutron stars rotate very rapidly and emit beams of electromagnetic radiation as pulsars.