And the temperature of a star comes from its mass. The more massive a star, the hotter it's going to be. Stars don't get more more massive or hot than blue
Blue Giants Are Very Hot
That's about 172 times hotter than the Earth, but still quite cool compared to blue supergiants. These stars are some of the hottest and can have surface temperatures in excess of 40,000K – about four times hotter than the Sun.
A star's color tells us about its temperature and mass, and blue stars are the hottest and most massive of all. Any star that has three or more times as much mass as the sun will tend to look blue to our eyes.
Red supergiants have low surface temperatures, below 4,100 K. This is very cool for a star and makes them shine with a red colour. The star Betelgeuse in the constellation of Orion is a red supergiant. Blue supergiants are much hotter.
The temperature of a blue supergiant would be somewhere around 20,000 K. So the temperature range of a white dwarf would be from 4 K up to about 100,000 K. To cover all supergiants you would be looking at 4000K to say 50,000K.
The hottest: WR 102
These stars not only burn incredibly hot and bright, but their stellar winds also blast much of their potential fuel into space. The hottest known star, WR 102, is one such Wolf-Rayet, sporting a surface temperature more than 35 times hotter than the Sun.
Green and purple stars do exist. The color of stars depends on their temperatures, and they emit radiation throughout the visible spectrum.
In astronomy, a green star is a white or blueish star that appears greenish in some viewing conditions (see § Psychology below). Under typical viewing conditions, there are no greenish stars, because the color of a star is more or less given by a black-body spectrum.
Red stars are the coolest. Yellow stars are hotter than red stars. White stars are hotter than red and yellow. Blue stars are the hottest stars of all.
What causes the different colours of stars? The reason stars look different colours - some are red, some are blue - is all to do with their temperatures. It's the opposite to what you might think: red stars like Betelgeuse are actually cooler than blue stars like Rigel.
You see, the color of a star is defined by its temperature; the coolest stars are red, while the hottest ones appear blue. And the temperature of a star comes from its mass. The more massive a star, the hotter it's going to be. Stars don't get more more massive or hot than blue giant stars.
A white dwarf can be 100,000,000 times denser than a blue giant.
They Don't Live Very Long. If a star has become a blue giant, it's already reached at least the middle age of its lifespan. Because these stars burn through hydrogen so quickly, it doesn't take long (from a stellar standpoint) to get there. Our Sun is approximately 4.5 billion years old, about halfway through its life.
Because these stars' energy is spread across such a large area, their surface temperatures are actually relatively cool, reaching only 4,000 to 5,800 degrees Fahrenheit (2,200 to 3,200 degrees Celsius), a little over half as hot as the sun.
As the star begins to expand it becomes a subgiant and then a red giant. At the bloated out surface, the increased amount of energy is spread out over a larger area so each square inch will be cooler.
Think of the color wheel as a clock where every hour marks a new color family. Absolutely warm and cool colors can be found at 0 (red – the warmest color) and 180 (cyan – the coolest color) degrees.
The dead star at the center of the Red Spider Nebula has a surface temperature of 250,000 degrees F, which is 25 times the temperature of the Sun's surface. This white dwarf may, indeed, be the hottest object in the universe.
The smallest stars around are the tiny red dwarfs. These are stars with 50% the mass of the Sun and smaller. In fact, the least massive red dwarf has 7.5% the mass of the Sun.
Intense ultraviolet radiation from newly formed stars can ionise surrounding hydrogen gas, stripping away electrons and causing the gas to emit a faint pinkish glow.
According to the researchers who conceptualized such objects, Fred Adams and Gregory Laughlin, frozen stars will not form for trillions upon trillions of years. Some of these stars may come from collisions between sub-stellar objects called brown dwarfs, which are larger than planets but too small to ignite into stars.
Violet stars are of two temperature ranges: those whose Planckian peak wavelength lies between 380 and 450 nm, or 6700-7900 K temperature and those above the violet range in the ultraviolet that appear violet to blue in color. For example, A spectral type stars range in temperature from 7600 to 11,500 K.
Every night the star puts on a light show when the same turbulence that causes it to twinkle refracts its light waves to shine in different shades. Its multicolored nature has earned Sirius the nickname "Rainbow Star."
It's a flashy rainbow star
Although white to blue-white in color, Sirius might be called a rainbow star, as it often flickers with many colors. The flickering colors are especially easy to notice when you spot Sirius low in the sky.
O-type stars are rare but luminous, so they are easy to detect and there are a number of naked eye examples.