A yellow dwarf is a small, main sequence star. Our Sun is a yellow dwarf. And like most members of this class is a Type G. They have typical surface temperatures of between 5 000°C and 7 500°C. Their life expectancy in this stable state is between 4 billion to 17 billion years. Typical G-type stars have solar masses of 0.84 to 1.15. They all convert hydrogen to helium in their cores and will evolve into red giants when their fuel runs out. Their final stage is to shrink to a white dwarf.
Such candidates in a binary system, on the other hand, will begin sucking materials from its giant neighbour and finally self-destruct in a supernova.
The orange dwarf is an intermediary between yellow and red. They are smaller than a yellow dwarf and remain main sequence stars for much longer, typically 30 billion years.
A red dwarf is a small, cool, very faint main sequence star whose surface temperature is under 3 500°C. It is a low-luminosity star, and typically one twelfth the mass of our sun. Red dwarfs are the longest surviving, up to 14 trillion years old. If the theory that the universe will die by fading into a cold void is true, then red dwarfs will be the last remnants of heat and light. The universe may end this way in 100 trillion years.
A “flare star” is a faint, cool red-dwarf that displays sudden short-lived increases in luminosity caused by extremely powerful flares that occur above its surface.
Once red dwarfs come out of their main sequence stage several trillions of years later, their brightness will increase, not by increasing in size but by increasing their surface temperature. This is a theoretical state, and only holds for those red dwarfs that have a mass of no more than a quarter of our Sun. As the theory goes, if their size remains stable they will become brighter and hotter as their surface temperature increases, creating a blue dwarf.
A blue dwarf is entirely theoretical. It is theorised that this is the end stage of a red dwarf. But since red dwarfs last trillions of years astronomers have yet to find one.
A white dwarf is a small, very dense, hot star that is made up mostly of carbon. These faint stars are what remains after an intermediate-to-low-mass star (a red giant) loses its outer layers. They eventually become a black dwarf.
It is of similar size to Earth but is the equivalent of one solar mass, so it is very dense. The star will continue to shine through the fusion of helium nuclei in the triple alpha process. Further contraction is prevented by the repulsion of electrons in the core. If the star has little mass, it may end its life here, throwing off its outer layers, creating a planetary nebula out of its atmosphere, and a hot, dense “white dwarf” out of its core. The white dwarf shines only by residual left-over heat and will eventually fade into a mere cinder (a black dwarf).
White dwarfs have evolved from stars with an initial mass of up to three or four solar masses or possibly higher, and have no shrunk to below 1.4 solar masses. They are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star—which would require much more mass, say, 3 solar masses.
The Red Spider Nebula in the constellation Sagittarius contains the hottest white dwarf ever observed, believed to be as much as 250 000°C, some even suggesting higher.
A brown dwarf is a failed protostar. This is the fate of any object starting out at less than 0.08 solar masses , where the pressure and temperature at the core are not high enough to initiate nuclear reactions. A brown dwarf is not very luminous.
Astronomers use Jupiter as a yardstick, determining that any object with a mass of between 15 times and 75 times that of Jupiter is classified as a brown dwarf. And the analogy goes further. With a temperature of below 1 650°C, the atmosphere of a brown dwarf is cool enough to have heat, convection and condensation, and therefore air currents. It is thought to have similar cloud structure to a body such as Jupiter. Material rises into the atmosphere and then condenses into iron vapour, precipitating out as iron rain.
So far about 200 brown dwarfs have been identified and may indeed account for a lot of the ordinary matter in the universe. They produce a low, dim light—hence their name. Their gravity is 300 times that of Earth.
Gliese 229B with a surface temperature of about 745°C in the constellation Lepus is one notable example. Another brown dwarf, thought to have at least 48 times the mass of Jupiter, orbits the sun-like star 15 Sagittae. It was identified in 2002. There is much debate as to whether an object of this type is a planet or a brown dwarf. Astronomers are trying to categorise them in terms of how they are formed. One suggestion gaining acceptance is that a brown dwarf is a substellar object with the mass equal to between 13 and 80 Jupiters. Because they are small such objects cannot fuse hydrogen like stars. However they may have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Below the Jupiter mass of 13 this is not possible, and it is further suggested that only such smaller candidates would be defined as planets.
Like the blue variety this is also theoretical and believed to be the result of a white dwarf cooling to such a low temperature that it emits no detectable light. However, there has not been enough time since the origin of the universe for any star to cool down enough to become a black star.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Saturday, 20 March 2021