BDanielMayfield wrote: Golganfinchian wrote:
Ann wrote:In reality, the Sun is white.
I thought the Sun is actually green.
So, what does this make the sun really, a white dwarf??? or a green dwarf??? Really now, why should a middle of the main sequence middle aged star be called a dwarf simply because it's smaller than over the hill bloated behemoths? Therefore, I say, down with dwarfism in astronomy! Replanitize Pluto! Dedwarf the Sun!
The terminology could actually kill you!!!
Stars are called "dwarfs" when they are on the so called main sequence
. When stars are on the main sequence, they fuse hydrogen to helium in their cores. There are "yellow dwarfs" like G-type Alpha Centauri A and the Sun (even though both Alpha Centauri A and the Sun are white), "orange dwarfs" like Alpha Centauri B (the K-type yellow star which was recently found to have a planet) and "red dwarfs" like tiny Proxima Centauri, the tiny M-type yellow-orange star which is at present the star other than the Sun which is closest to the Earth. You can see the relative sizes and colors of the Sun, Alpha Centauri A, Alpha Centauri B and Proxima Centauri here
We may even talk about "blue dwarfs". Regulus, the B-type blue-white alpha star of constellation Leo can be described as a blue dwarf, because it is still on the main sequence and fuses hydrogen to helium in its core. Regulus is of spectral class B7V (the "V" means "main sequence"). A bigger, hotter star which is also on the main sequence is Achernar, alpha star of the constellation Eridanus. Achernar is of spectral class B3V. Interestingly, both Regulus and Achernar are highly oblate due to very fast rotation. You can see a comparison between the Sun, A-type dwarf Altair and B-type dwarfs Regulus and Achernar here
All stars as massive as the Sun, and probably all stars as massive as Alpha Centauri B, are destined to eventually use up the hydrogen in their cores. Then they can't fuse hydrogen to helium in their cores any more, and then they start fusing hydrogen to helium in a shell around their cores instead. Then they are no longer dwarfs. Instead, they have become become giants
Okay. You can have red, orange, yellow and blue dwarfs. You can also have red, orange, yellow, white or blue giants. But what about the main sequence stars which are technically described as white, even though they are blue-white? What about Vega? It is a main sequence star of spectral class A0. Since it belongs to spectral class A, it may technically be described as "white", in the same way that the Sun is described as "yellow". And since it fuses hydrogen to helium in its core, it is a dwarf. So Vega is a "white dwarf", then?
Sadly, no! Because "white dwarf" is a term that refers to the tiny exposed cores of dead stars. White dwarfs start out as central stars of planetary nebulae. Check out this picture
of a planetary nebula. Note the very blue color of the central star. This very blue star is a white dwarf!!!