Depending on its initial mass , every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy.
Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves. The Hertzsprung-Russell diagram the various stages of stellar evolution. By far the most prominent feature is the main sequence grey , which runs from the upper left hot, luminous stars to the bottom right cool, faint stars of the diagram. The giant branch and supergiant stars lie above the main sequence, and white dwarfs are found below it.
Credit : R. This Hertzsprung-Russell diagram shows a group of stars in various stages of their evolution. When you look up at the sky on a dark night, you see many points of light. At first glance you might think they're all the same: far-away white stars on a black background.
The magnitude system for measuring brightnesses
But if you look closely, you might notice that some are a bit brighter, some a bit dimmer, some are a little redder, some bluer. If you've listened to "Doppler Shifting" you may think that those that appear redder are red-shifted, moving away from us, and those that appear bluer are moving toward us. And common experience might lead you to conclude that the bright ones are close and the dim ones are far. But that's not necessarily the case. There are a variety of star types in our Galaxy. Some are hot and bright and burn for a short time relatively speaking, of course! Some are cool and dim and burn a long time.
Some have incredibly strong stellar winds that cause the star to lose a large percentage of its mass over its lifetime. Some are the cooling cinders of stars, and some are just being born from nebulae of gas.
First, let's talk about how bright stars are. When you look into the sky and see a bright star, there may be several reasons that it looks bright.
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The first and most obvious is that it's close by. But it also might be that it's intrinsically brighter or more luminous. We assign a luminosity number or magnitude to stars to say how bright they are. We can assign an apparent magnitude - that's how bright they appear to be from Earth. But it's scientifically much more useful to calculate an Absolute Magnitude, which is the brightness a star would have if it were at a distance of 10 parsecs.
A parsec is equal to 3. Then the Absolute Magnitude is directly related to the star's luminosity. In the magnitude system, the lower the number, the brighter the star.
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In , two astronomers independently determined that there was a link between the luminosities or absolute magnitudes of stars and their spectral class based on temperature. A plot of this relationship is called a Hertzsprung-Russell or H-R diagram and is named after these two astronomers.
By plotting stars' absolute magnitude against their spectral type, one can see that most stars fall along a diagonal strip from high temperature, high luminosity stars to low temperature, low luminosity stars. These are the main sequence stars. Our star is one of them. There are a few stars that are not in this diagonal strip. There are some low temperature, high luminosity stars - these are called giants and supergiants. The reason they are so luminous while being relatively cool is because they're so big 50 times more massive than our Sun. Another group of stars are in the high temperature, low luminosity corner of the diagram.
Since these stars are hot, but not very luminous, they must be very small, so they're called white dwarfs. The star called Antares is a supergiant and is almost times the radius of Proxima Centauri, a red dwarf in the Alpha Centauri system.
O stars and B stars are very hot 50, Kelvin. Our Sun is a G star. It's about Kelvin and is in the middle of the main sequence and "an average star in our Milky Way".
Stars like Barnard's Star and Wolf are M dwarfs. They're on the cool Kelvin and dim end of the spectrum of main sequence stars. You are welcome to come up with a different mnemonic yourself! Stars in the main sequence can be classified by their spectral type. Our Sun is a "G" star, right in the middle of the main sequence. It will have a lifetime of about 10 billion years. Stars that are very luminous and very hot are called O or B stars. They are large and fairly young.