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Stars: The Hertzsprung-Russell diagram
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What is true about the stars found in area "B" in this picture?
The night sky is covered with millions of stars. What we see from Earth is just a fraction of all the stars in the universe. To us they all appear as tiny bright spots in the sky, but actually their differences are just as vast as the distances between them. How do people manage to study and understand them? This is Ejnar Hertzsprung, a Danish astronomer.
It’s the beginning of the 20th century, and Hertzsprung is busy comparing stars he can see from the Earth. He is trying to find a relationship between how bright the stars are - their luminosity - and their temperature. Independently in America, Henry Norris Russell is studying the same subject and comes to similar conclusions. The result of their studies is a diagram we now call the Hertzsprung-Russell diagram, to honour the two scientists. The graph looks like this: The horizontal axis represents the surface temperatures of the stars, marked with colours.
The hottest stars are blue, and we find them on the left side of the diagram. Stars with medium temperatures are white and yellow, and they fall in the centre. The coldest stars are red, and we find them on the right side of the diagram. The vertical axis shows the luminosity of a star - how bright the star really is. The most luminous stars are at the top and the least luminous stars are at the bottom.
Most of the stars on the diagram happen to be close to this diagonal line that runs from the top-left corner to the bottom-right corner. This is the MAIN SEQUENCE of the diagram. Both luminosity and temperature are also related to a star’s mass, so if we look at the main sequence we are able to compare the masses of different stars! Stars in the lower-right corner are not only dimmer and cooler, but they also have a much lower mass than those in the middle or upper-left corner. The small, cool, and dim stars in the very bottom-right corner even have their own name.
We call them RED DWARFS. But the stars that belong to the main sequence are not all the stars we know. There are also a few stars that are both very hot and extremely bright, much brighter than most of the stars. These are called BLUE GIANTS and BLUE SUPERGIANTS. They are in the upper-left corner of the diagram, slightly outside the main sequence.
Blue giants are also very heavy, even heavier than much bigger stars that belong to the upper-right corner of the diagram. The stars in the upper-right corner are cooler, but they still shine very brightly. This is because they have a huge surface area that emits light. We call them RED GIANTS and RED SUPERGIANTS. And finally, here in the bottom-left corner we have extremely heavy and dense cores of stars.
They are very hot, but they are so small that they appear very dim. We call them WHITE DWARFS. What about a star like our Sun? Where do you think it is on this diagram? With its medium brightness and moderate surface temperature, the Sun is in the middle of the main sequence, a little bit to the right!
The Hertzsprung-Russell diagram organises stars based on their luminosity and temperature. It allows us to compare different stars and observe patterns and differences between them. Knowing a star’s place in the diagram, scientists can make guesses and draw conclusions about its mass, size, and even its lifespan, evolution, and fate. In this way, the Hertzsprung-Russell diagram has been helping astronomers and astrophysicists study stars for over a hundred years!