When people talk about the temperatures of stars, we usually are referring to the surface temperature. The surface temperature is way cooler than the believed temperature of the core. Its believed/calculated because no one is ever going to be able to get inside a star and measure one. The temperature at the core of our star, the Sun can reach to about 25 million degrees. Temperatures inside stars could reach four times.
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Does size matter in star temperatures?
Being a large start does not mean it is the hottest star, The largest known star is UY Scuti which is a massive 1,708 times times larger than our Sun but is cooler than our Sun. UY Scuti is only 3,365 K compared to our sun at 5,778 K. The reason for the difference is that the Suns' gravity is holding the Sun in and thereby causing the Sun's atoms to hot up.
Smaller stars are not necessarily hotter, the closest star to our solar system, Proxima Centauri is a red small red dwarf star whose temperature is about 3,042K, not far off the temperature of UY Scuti.
Colours of Stars
The temperatures of the stars follow the colours of the rainbow, (Red, Orange, Yellow, Green, Blue, Indigo, Violet). The coldest stars are red and the hottest stars are blue.
Green Stars
You might what might be a mistake with the diagram, there's green and there's no green stars. There are technically green stars but we can't see them. Stars give off a range of colours rather than one specific colour. Green being slap bang in the middle will be polluted by other colours and not appear green, in fact they'll appear white because of light pollution. Our Sun although it looks yellow in the sky, it looks white in space could really be a green star if it only gave off line in one colour.
Why not all stars are white is because for example, blue stars are on the edge of the spectrum so there's not much to pollute and affect the colour. The same goes with red stars as they are on the end of the scale. There are no invisible stars such as Infra-red or Ultra-violet because they're polluted by other colours to be visible.
Life of a Hot Star
The hotter a star is, the shorter it lives. A star that is cool and small is more energy efficient and will live many times longer than a hotter larger star. Proxima Centauri will still around long after our star has exploded in a supernova explosion. Our Star will around for longer than say Regulus which is a large hot blue star.
Hottest Star Candidates
WR 102
WR 102 is a Wolf-Rayet star, an extremely hot star that is nearing its death. It has passed the main sequence of its life. The Main Sequence is the part of a stars life in which it has finished burning/converting hydrogen into helium. A Wolf-Rayet star has blown off its hydrogen atmosphere and progresing to the next stage. wolf-rayet stars are rare, there are only two stars that can be seen by the naked eye, Regor and Theta Muscae. All the others are too far to be visible including WR 102.
WR102 is about 210,000 Kelvin temperature. It is one of three stars that have a temperature of 200,000 or more. The other being WR 142 in Cygnus and LMC 195-1 in The Large Magellanic Cloud, a satellite galaxy. All three are Wolf-Rayet stars.
Neutron Star
When a star has died, it will either become a black hole or a neutron star depending on its size and mass before it exploded. A Neutron star is the result of the death of a small - medium star whereas anything larger could create a black hole. Neutrons are atoms without electrical charge. A Neutron Star could be nothing larger than the size of Manhatten Island but a teaspon of the stuff would weigh the same amount as a house. Neutron stars can reach temperatures of over 100 billion to a trillon. Quora
Hadar (Beta Centauri)
Based on the Hertzsprung-Russel diagram that you can find elsewhere, one of the hottest stars out there is Hadar which is also known as Beta Centauri in the constellation of Centaurus. It has an estimated temperature in excess of 30,000 K, which is about five times more than our own Sun.
Although Alpha Centauri is the second closest star to us, Hadar is a long way away, it is 525 light years from us compared to that of Alpha Centauri which is a mere 4.32 light years. The only heat we get is from our own Sun.
Interesting Sun's Temperature Fact
The temperature of the Sun is about 5.7K but the corona of the Sun reaches to a few million degree less 500 miles from the photosphere. Scientists have yet to work out the reason why this happens.
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