When you look at the night sky you can see many beautiful stars. If you are out in the country or camping in the mountains or the desert away from the city lights, you may see thousands of them. You may even be able to see part of the Milky Way. In a town or city, you can't see nearly as many stars because the city lights create a glow in the sky masking many of them.
There are several different kinds of stars in the sky. Some are very big. A couple of stars have been found that are 100 to 200 times larger than the sun. Some very old stars are smaller than the Earth. Scientists study stars and place them in groups based on how they are alike and how they are different.
Click on a star type to learn more about each one.
This is an example of the Hertzsprung–Russell diagram (HR diagram), first used in 1912. Two astronomers, Ejnar Hertzsprung from Denmark and Henry Norris Russell from the Unites States, both discovered that the brightness of a star depends on the surface temperature of the star. They each made this discovery on their own separately. Together, they came up with this diagram that explains the brightness, temperature and classes of stars.
The scale on the left shows how bright a star is.
The letters across the bottom represent the spectral class of stars, or color of stars.
O – Blue
B – Blue/White
A – White
F - White/Yellow
G – Yellow
K – Orange
M - Red
The temperature of the stars measured across the bottom of the scale are measured in Kelvin. Zero Kelvin equals -273 degrees Celsius, -459 degrees Fahrenheit.
As you can see, there are only a few categories of stars. Most stars in our universe are main sequence stars, including our sun Sol. Notice how the biggest stars are the brightest but not the hottest. The white dwarf stars are near the end of their life and losing much of their brightness but they are very hot.
Can you find Sol? Can you find Antares? Can you find Wolf? What is the temperature of Beta Centauri? What class is Polaris? What class is Bellatrix? How bright is Vega? How bright is Deneb? How bright is Beta Sirius?