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Dwarf star, any star of average or low luminosity, mass, and size. Important subclasses of dwarf stars are white dwarfs and red dwarfs. Dwarf stars include so-called main-sequence stars, among which is the Sun. The color of dwarf stars can range from blue to red, the corresponding temperature varying from high (above 10,000 K) to low (a few thousand K).
White Dwarfs are the corpses of stars. 97% of the stars in the Universe will become White Dwarfs. Once the star is completely out of fuel it can use, and so it puffs out its outer layers, revealing the hot carbon core; the leftover material from this last fusion reaction. The star is now a white dwarf. It starts out hot, the temperature that the star’s core was, but then it starts to cool down over time. Eventually, after billions and even trillions of years time, the white dwarf will cool down to the background temperature of the Universe.
Binary white dwarf stars are white dwarf stars that orbit each other. White dwarf binary stars in general are extreme systems that radiate gravitational waves as they orbit each other. To balance the loss of this energy, the stars gradually come closer together until eventually they merge. Many white dwarf binaries will explode as supernovae when they merge, but if it is too small to trigger such an explosion, it will probably start fusing its helium atoms, and when it does it will shine like a normal star again.
A black dwarf is a white dwarf that has cooled down to the temperature of the cosmic microwave background, and so is invisible. Unlike red dwarfs, brown dwarfs, and white dwarfs, black dwarfs are entirely hypothetical. It is entirely hypothetical because the universe is not old enough for white dwarfs to cool down into black dwarfs.
The Universe is filled with red dwarf stars. Astronomers categorize a red dwarf as any star less than half the mass of the Sun, down to about 7.5% the mass of the Sun. Red dwarfs can’t get less massive than 0.075 times the mass of the Sun because then they’d be too small to sustain nuclear fusion in their cores. Red dwarfs generate as little as 1/10,000th the energy of the Sun. The largest known red dwarf has only 10% the luminosity of the Sun. Red dwarf stars with 10% the mass of the Sun are through to live 10 trillion years. Our own Sun will only last about 12 billion or so. This is because red dwarf stars use up every last drop of hydrogen while our Sun will die when it uses up just the hydrogen in its core.
You may have heard about a yellow dwarf star. The Sun is a yellow dwarf star. Yellow dwarf stars are not true dwarf stars, at least not in the sense that red or white dwarf stars are dwarfs. Often used imprecisely, if not erroneously, the term “yellow dwarf” refers to stars of the “G” spectral class on the main sequence, with such stars usually having a mass of about 0.8 to 1.2 times that of the Sun, and surface temperatures of between 5,300K, and 6,000K.
Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star. In fact, most astronomers would classify any object with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be a brown dwarf. Given that range of masses, the object would not have been able to sustain the fusion of hydrogen like a regular star; thus, many scientists have dubbed brown dwarfs as "failed stars". All of the brown dwarfs discovered so far are part of a binary system. These dwarfs can also be pretty cold; there was one found not too long ago that was only as warm as a cup of coffee.
The Editors of Encyclopaedia Britannica, Britannica. Dwarf Star. 9 Mar. 2007, www.britannica.com/science/dwarf-star.
Cain, Fraser. “White Dwarf Stars.” Universe Today, 25 Dec. 2015, www.universetoday.com/24681/white-dwarf-stars/.
Tate, Jean. “Black Dwarf.” Universe Today, 25 Dec. 2015, www.universetoday.com/41096/black-dwarf/.
“Binary White Dwarf Stars.” Www.cfa.harvard.edu/, 3 Oct. 2013, www.cfa.harvard.edu/news/su201117.
Cain, Fraser. “Red Dwarf Stars.” Universe Today, 25 Dec. 2015, www.universetoday.com/24670/red-dwarf-stars/.
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Christoforou, Peter. “10 Interesting Facts about Yellow Dwarf Stars.” Home Page -, 24 July 2017, www.astronomytrek.com/10-interesting-facts-about-yellow-dwarf-stars/.