How did the HR diagram got its name? How scientists know the different sizes, temperature and color of stars?

Answer 1

They know via spectroscopy. ~

Answer 2

By the two scientists, danish astronomer Ejnar Hertzsprung and american astronomer Henry Norris Russel, that independently of each other began charting stars in the same way. What they did was to plot the absolute visual magnitude against the spectral class for stars that they knew the distance to. Distance was found by using the parallax method. Basically trigonometry and it only works on nearby stars. The temperature of stars is found by looking at its color. It is called a black body spectrum. Stars follow closely the curve of the hypothetichal black body which says that red is cold and blue is hot. White is in between with the star emitting light of blue red and green in kind of a balance. Those colors combined gives white light. When Russel mapped these stars he found the fell into a band. This is now known as the main sequence, the most stable portion of a stars life. But some stars didn´t fit into the chart. They were off the main sequence. How could a red star shine more than a blue star? Shouldn´t the star shining blue hot be brighter? The answer is that the red star is a giant star. More surface area means more light. Now we know that red giant stars are old stars on the edge of death. They have a very characteristic spectrum. So some stars are kind of working as mile markers. You take a look at the star, analyze the spectrum and compare the brightness we see with the brightness that star truly has. And so we can find the distance for stars that are too far away to be measured through the parallax method.

Answer 3

It is named after the two astronomers that developed it, about 100 years ago: Ejnar Hertzsprung and Henry Norris Russell.

For the color, it is relatively easy: you just look. Spectroscopy and photographic records would help refine the maximum of emittivity, and color is a direct relation to temperature (black body radiation).
If one knows how far a star is, then measuring how bright it is allows to determine how much energy it puts out. Enter stellar physics that balances fusion reaction rate with mass, and mass and size are thus derived.

Answer 4

It is named after its creators Hertzsprung & Russell. Color is measured by the intensity in different filters.
Size is not directly displayed , however it can be inferred from a combination of luminosity and surface temperature.