What is the significance of the hr diagram

HR Diagram. Stellar Classification. Latest Gallery Images. The H-R diagram is important because stars can be classified by their placement on it. Mark C. Oct 27, Just worth mentioning how much effect this diagram had on the field of astrophysics? Explanation: 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: From: 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.

Related questions Is it possible for the earth to be sucked into a black hole? If so, what would happen? Clearly the stars are not randomly distributed on this graph. They fall in several well-defined areas, with most stars on a narrow strip running from upper left to lower right. This graph, the H-R diagram, was a fundamental advance in astronomy. 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:.