What is the significance of the hertzsprung russell diagram

Main Sequence stars tend to be in the beginning or middle of their lives, when the pressure outwards from the heat of nuclear fusion in their cores still balances out the inward force of gravity. Red Giants and Supergiants are older small- and medium- size stars that have bloated, and are therefore more luminous even though they aren't very hot.

White dwarfs have blown off their outer layers and are super hot, but not very luminous because they are so small. Blue giants are just massive, and don't live very long. At the time start of the 20th century the only two observable features of stars were their brightness and colour, which was associated with the temperature.

Plotting these against each other yielded nothing:. Their genius was to realise that the effect of distance was dominant, and that what was needed was to work out the absolute brightness I.

Then suddenly the correlation appeared and the field of astrophysics took off - generalisations were possible. What is the Hertzsprung-Russell diagram and why is it so important to astronomy research? Jun 15, Explanation: The Hertzprung-Russell diagram is a graph relating stars' temperature to their luminosity. Main Sequence stars vary widely in effective temperature but the hotter they are, the more luminous they are, hence the main sequence tends to follow a band going from the bottom right of the diagram to the top left.

These stars are fusing hydrogen to helium in their cores. Stars spend the bulk of their existence as main sequence stars. Other major groups of stars found on the H-R diagram are the giants and supergiants; luminous stars that have evolved off the main sequence, and the white dwarfs.

Whilst each of these types is discussed in detail in later pages we can use their positions on the H-R diagram to infer some of their properties. Let us look at the cool M-class stars as an example. If we look at the H-R diagram below we can see that in fact there are three main groups of these stars. At the bottom-right of the diagram we can see two named stars, Proxima Centauri and Barnard's Star. Following the broad band straight up we come across Mira, also cool but much more luminous.

Travelling further up we come across Antares and Betelgeuse. Why do these three groups differ so much in luminosity? The answer to this question depends upon the Stefan-Boltzmann relationship. You may recall from equation 4.

If two stars have the same effective temperature they each have the same power output per square metre of surface area. As the H-R diagram however shows that one is much more luminous than the other it must have a greater total power output therefore must have a much greater surface area - the more luminous star is bigger. We can see this from the full expression for luminosity in equation 4.

The difference between the three groups of M-class stars is thus a difference in size. This is acknowledged by the names given to each of the groups. The most luminous ones are called supergiants luminosity classes I and II , the luminous ones are called giants luminosity class III and the dim ones are part of the main sequence luminosity class V though historically the term dwarf stars was applied to this group.

If we look at the vertical band on the H-R diagram for hotter stars around type A spectral class we see a similar pattern:. In this case the supergiants Rigel and Deneb have the same effective temperature as Sirius but have extremely high luminosities. They have large radii than Sirius hence greater surface areas and higher luminosities. Sirius is a main sequence star but because it is hotter than the red main sequence Barnard's Star it is much more luminous than it.

If you follow the pink band for hot stars down to the bottom of the H-R diagram you will notice that it intersects another group of stars that includes Procyon B. These are the white dwarfs. They are very hot about 10, K or hotter therefore emit a lot of energy per second for each square metre of their surface. H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar properties including diameter, mass , age, and evolutionary state.

Our understanding of the processes at work in the stars depends on knowing these parameters, so H-R diagrams have been essential tools in twentieth-century astronomical research. Figure 1. The spectral classes and corresponding surface temperatures are given at bottom, while the luminosities are given at left.