The Hertzprung–Russell (H–R) diagram plots the real brightness ofeach star against its surface temperature. How is this graph interpreted?

Question

What will be an ideal response?

Answer 1

The first thing to note on the H–R diagram is that moststars appear in a band running from the bottom right tothe upper left. In the bottom right corner are stars thatare reddish, which means they have lower surface temperatures,emit less energy, and are smaller. In the upper left corner are stars that are blue, which means their surfacetemperatures are very high, they emit more energy intotal, and have a lot of mass. Rigel, at one of the cornersof the constellation of Orion, is an example of a blue star.This band of stars running diagonally from the top left tothe bottom right corners of the diagram is known to astronomersas the main sequence. All the stars on the mainsequence are mature stars, doing what most stars do mostof the time: fusing protons into helium nuclei in their cores.Their position on the sequence depends on one thing: howmuch mass they contain, because more mass means higherdensities at their cores and hotter temperatures. So, asonemoves leftward and upward along the main sequenceone finds stars that are more massive, as well as hotter andbrighter. Those with high surface temperatures appear hotbecause they are hot, and they are hot because they have alot of mass so they create a lot of pressure in their cores.Within the main sequence, surface temperature and realbrightness are correlated because they both depend on thestar’s mass. Thesun appears roughly in the middle of thesequence; it is a medium-sized star, perhaps just slightlylarger than average.

But not all stars appear on the main sequence. In theupper right corner there are stars that are emitting a hugeamount of energy, which means they are very large. Yettheir surface temperatures are relatively low, so they appearas red stars. These are known as red giants. A famousexample is Betelgeuse, the large red star at one corner ofthe constellation of Orion. It can easily be seen on a clearnight with the naked eye. At the bottom of the graph, onthe other hand, are stars that have hot surfaces even thoughthey seem quite small. These are known as white dwarfs.Sirius B, the companion star to Sirius, the brightest star inthe night sky, is a white dwarf. Both red giants and whitedwarfs are behaving oddly because they are at the end oftheir lives. They have begun to run out of hydrogen nuclei,the protons that sustain stars throughout most of their lives.