A HR diagram(abbreviation of Hertzsprung Russel diagram) is a graph of stars' surface temperatures(x axis) versus their luminosities(y axis). Basically, what we do is observe a lot of stars, find each star's temperature and luminosity and put them all there on the graph. This graph is important in understanding stellar evolution due to a theorem called ergodic theorem. Let us see how.
When a star is born, it has a particular luminosity and temperature. As it lives its life, it's luminosity and temperature keeps changing, and finally it finishes it life. Basically, what I mean is that you take a star when it's born, find its temperature and luminosity, put that on a graph that reads luminosity versus temperature for y and x axis respectively, wait a few million years, see the star again, find it's then temperature and luminosity, put that on that graph, and keep doing it till the star dies. What you get then is a graph that tells you how the star's luminosity and temperature changed as it lived it's life. With luminosity and temperature, you can calculate all other stuff about the star and write down it's biography! Do that with all stars, and you get loads of biographies of different stars, and you become a master of stellar evolution! But wait, there is an issue here...a star typically lives it's life in the order of a billion years. We humans evolved one million years back, we discovered telescopes four hundred years back, and a typical human lives a hundred years, how will we understand stars with such little time?! The answer is HR diagram!!
Now back to ergodic theorem, it says that seeing a thousand stars as they appear to us now and finding their temperature and luminosity and then putting it on HR graph is same as following a star all it's life! Essentially a shortcut to understanding stars! That's the big advantage of HR diagram in studying stars... of course, the focus of my answer was the importance of ergodicity in studying stars but not explaining in detail the concept of ergodicity. That you can find in any statistical mechanics text book or maybe I can explain that somewhere in answers.com soon! Cheers, hope my answer helped!:)
The temperature and luminosity of stars.
The location on the Hertzsprung-Russell (HR) diagram where most stars lie is known as the main sequence. The HR diagram is a plot of stellar luminosity against surface temperature. The main sequence is a prominent band that extends diagonally across the HR diagram from high temperature and high luminosity to low temperature and low luminosity. The majority of stars, approximately 90% of all stars, are situated along the main sequence on the HR diagram. These stars are often referred to as main-sequence stars. They exhibit a smooth relationship between surface temperature and luminosity, with varying sizes and masses but sharing this common characteristic of lying on the diagonal band from the upper left to the lower right of the HR diagram. Source: Teach Astronomy - The Hertzsprung-Russell Diagram
The HR diagram compares the luminosity (brightness) of stars against their surface temperature or spectral type. This plot helps astronomers classify stars based on their intrinsic characteristics and evolutionary stages.
It shows certain key characteristics (brightness, and temperature) of stars.
I don't know ok
The HR diagram contains only stars - so everywhere.
Of course they are on the HR diagram. They are simply not on the main sequence.
All stars.
main-sequence stars
stars there called stars
stars there called stars
The red dwarves.
The colors of the HR Diagram are:BlueBlue-WhiteYellowOrangeRed-OrangeRed
The main reason that the HR Diagram is so useful and important to scientists is, you can tell the size of the star by plotting it on the HR Diagram. The different sizes of stars form a pattern on the HR diagram.
They are very hot stars.
The temperature and luminosity of stars.
The curve that currently contains most stars on the HR diagram is called the "main sequence". It consists of those stars that fuse hydrogen-1, converting it into helium-4.