Hertzsprung-Russell Diagrams

A Hertzsprung-Russell Diagram is a type of scatterplot that plots the temperatures of stars(or their spectral classifications) against the absolute magnitude of their luminosities(how much electromagnetic energy they radiate). These plots mainly serve to visualize the relationship between the luminosities and temperatures of stars. Using the data from H-R Diagrams, we can make inferences on the other properties of stars.

As you can notice, there are three distinct "regions" in the diagram: the main sequence, the upper right, and the lower left. During the main sequence, stars, including the Sun, are in the process of fusing hydrogen in their cores. To the upper right are where supergiants and red giants lie. It may seem weird why these stars have peculiar placement on the plot because in general, during the main sequence, the greater the temperature of the star, the greater its luminosity. However, these stars are specifically supergiants and red giants, which expanded because they burnt out their hydrogen fuel, causing their inner cores to contract and their outer cores to expand. Since giant stars don't burn as much hydrogen, they aren't outputting as much energy, causing them to be generally cooler than other stars. On the lower left side of the plot, one will mainly see white dwarfs. The reason why white dwarfs are so hot is because of the way they are formed(which is expanded upon on the page corresponding to white dwarfs). However, this doesn't make them luminous as they don't have an active energy source so they slowly cool over time.

For stars in the main sequence, we can make the inference that the lower a star is on the main sequence(less luminous and temperature), the lower its mass will be. Conversely, if a star is higher up on the main sequence(more luminous and hot), it'll have a greater mass. This means that the main sequence is essentially a diagonal line of stellar mass(going up and to the right). Why is this so? Well, if a star has a lot of mass, its self-gravity will cause there to be more pressure towards the inner core. This means that the pressure exerted by nuclear fusion must be greater if it's being pushed by a greater self-gravitation. If the pressure exerted by nuclear fusion is greater, then there must be more nuclear fusion occurring in general. This increase in nuclear fusion increases the brightness of the main sequence star on the outside along with the surface temperature.