In short, Dwarf Stars are small stars compared to the giants of the universe such as UY Scuti. The term Dwarf Star was created by Ejnar Hertzsprung to distinguish between the two different types of Red Stars, those that are giant stars such as Antares and those that are much smaller such as Proxima Centauri. Hertzsprung who along with Henry Norris Russell created the famous Hertzsprung-Russell star size map.
The star map shows the Sun as being in the middle, the sun is classed as a Yellow Dwarf star because although it is one of the larger stars in the galaxy, it is tiny to compared to UY Scuti, the currently recognised largest star in the milky way which when you discover that UY Scuti is over 1,700 times as big as the Sun, you can see why the Sun is called a Yellow Dwarf star.
When it comes to size of a star, size matters, it is best to be small rather than large because the smaller stars convert their hydrogen to helium at a slower rate than larger stars and will live longer. Our Sun is expected to die in about 5 billion years, that's so long in the future, we don't need to worry about. When we move and if we ever do move to another planet, we should find a planet orbiting a red dwarf as they will last for possibly trillions of years.
When stars use up their hydrogen, you'd think that they'd begin to shrink, instead the opposite is true. As a star uses up it fuel, the outward pressure overpowers gravity and they expand outwards. Betelgeuse was probably a normal star until it finished its main sequence and then expanded outwards to the size it is today.
In addition to being the name of the space ship in the television series of the same name, a Red Dwarf star is amongst the smallest stars out there in the universe. Whilst difficult to see from the Earth, they are very common in the universe and are said to make up at least three quarters of the stars in the Milky Way. The nearest and most well known is as mentioned earlier Proxima Centauri, which is also the nearest star to our own.
Red Dwarfs are known to have Extrasolar Planets (Exoplanets) orbiting them such as Gliese 581b and Proxima Centauri. Compared to other types of stars, it is easier to spot planets round Red Dwarf stars as they exert more of an influence on the star that a planet does to a larger star.
A problem with living on a red dwarf's orbiting planet is that the planet could be tidally locked, meaning that only one side of the planet would ever have access to the stars ligh. The planet would have to be nearer to the sun proportionally compared to what our planet is to our star.
The nearest star to us excluding the Sun is a Red Dwarf star which was only discovered in 1915 by Robert T.A. Innes according to Google. The star although being our nearest neighbour, is not bright enough to be seen here on Earth.
Red Dwarf stars are cool stars compared to the blue-hot stars such as Spica which is about ten times the temperature of Proxima Centauri.
As mentioned earlier, our own star is a yellow dwarf star and it is the star by which other stars are measured by. For example, when talking about Luminosity, the Sun has a Luminosity of 1 and the other star's Luminosity is a multiple of the Sun. Other yellow dwarfs have been known to have planets orbiting them. The first exoplanet to be discovered, Peg 51b was found to be orbiting a Yellow Dwarf star in 1995.
Orange Dwarf stars are also known as K-Class stars which are slightly cooler and smaller than our own star. These orange stars last longer than our Sun and are cooler also so planets could exist round them. A notable orange dwarf is Sadira or as it officially known Ran in the constellation of Eridanus. At least 2 exoplanets orbiting round Ran. As they are older and live longer than most other stars such as our own, intelligent alien life forms could have built up a complex civilisation and these are therefore of great interest to those searching for extra-terrestrial life forms. We have not yet received any signals from there yet. Ran is about 10 light years from us, a neighbour.
When a star dies, it will either become a neutron star, black hole if it has high mass otherwise a nebula. At the heart of the nebula will be a neutron or a white dwarf star. What dictates how a star will become either a white dwarf or a neutron is the following :-1, 2
- A White Dwarf will be caused by the death of any star that is less than 10 times the mass of our Sun.
- A Black Hole will be generated by the death of a star that is larger than 10-15 times the mass of The Sun.
- A neutron star will be created by the death of a larger star.
- A white dwarf is supported by electron degeneracy pressure, a neutron star by neutron degeneracy pressure
- A white dwarf has a larger radius --about 600 times
- A neutron star has a stronger gravitational field -about 400,000 times
- A neutron star will have higher temperatures at birth, spin faster, and have stronger magnetic fields, among other things.
A white dwarf will exist for many more years as a white dwarf than as it did as a main sequence star. After an unimaginably long time, it will turn into a black dwarf having run out of energy to shine. Whilst no black dwarf exists yet, there are a number of white dwarf stars out there, the nearest being a mere 8.6 light years away as the companion star to Sirius.
Black Dwarf stars are hypothetical stars that don't yet exist but are predicted to in a future long away. When a star dies and turns into a white dwarf star, it will emit light for an unimaginable length of time but once it has run out of all energy, it will considerably and no longer produce light. When it has stopped producing it light, it becomes black and no longer exists.
A black dwarf is the ultimate end of many stars including our own Sun. Because of the length of time it takes to go from a white dwarf to a black dwarf, the universe is too young for any to exist as yet. After an unfathomable amount of time, the black dwarf may well decline and disappear into nothing.
Brown Dwarf stars are giant balls of gas, too big to be classed as a gas planet and too small to start nuclear fusion. If Jupiter, the biggest planet in our solar system had bigger mass then it would not be a gas giant but a brown dwarf.
Teide 1, a star in the Pleiades, a star cluster in the constellation of Taurus is the first brown dwarf to have been discovered in the middle of the nineties. Its spectral type is M8 which implies it is a red dwarf, the difference is that it was identified as a brown dwarf after its spectral type was recorded by Hipparcos.
There is a theory that our Sun has a companion brown star called nemesis that when it comes closer to the Oort Cloud that a comet is sent in our direction and can cause a mass extinctions such as the one that caused the end of the dinosaurs and Prehistoric Animals 65 million years ago. It is only hypothesis and one that has not been proven.
The oort cloud is reckoned to be about one light year away from our solar system and any object sent on its way to Earth would take a long time to get here. As Brown Dwarfs are dark and don't give off much light, it is hard to spot Nemesis, that is if it is out there.
The current nearest Brown Dwarf star so far discovered is WISE J085510.83-071442.5 which is a mere 7.5 light years away, still too far to travel to and back given the speed of our satellites and space ships currently. The star is in the constellation of Hydra.
At the other end of the spectrum are the giants, the supergiants and hypergiant stars which are fewer in number than the dwarf stars. These giant stars are normally at the end of their lives such as Betelgeuse which will one day end its life in a supernova. Betelgeuse is predicted to explode soon in astronomical terms not as in everyday soon.