How is hr diagram used

In general, the taller a person is, the greater their mass but as with many other characteristics of humans there is a large variation. Some people are tall and skinny, others shorter but higher mass. There are, however, real physical limitations on both the height and mass of people.

We do not expect to find a 3. One of the most useful and powerful plots in astrophysics is the Hertzsprung-Russell diagram hereafter called the H-R diagram. It originated in when the Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their colour hence effective temperature. Independently in the American astronomer Henry Norris Russell used spectral class against absolute magnitude. Their resultant plots showed that the relationship between temperature and luminosity of a star was not random but instead appeared to fall into distinct groups.

These are seen in the H-R diagram below. It has a few specific stars included in the plot but otherwise just shows the main regions. The majority of stars, including our Sun, are found along a region called the Main Sequence.

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.

These diagrams , called the Hertzsprung-Russell or HR diagrams , plot luminosity in solar units on the Y axis and stellar temperature on the X axis, as shown below. Notice that the scales are not linear. Hot stars inhabit the left hand side of the diagram , cool stars the right hand side.

The Sun is found on the main sequence with a luminosity of 1 and a temperature of around 5, Kelvin. Astronomers generally use the HR diagram to either summarise the evolution of stars, or to investigate the properties of a collection of stars. The Colour Magnitude Diagram or CMD is a plot of observational data see Figure 1 which shows how a population of stars can be plotted in terms of their brightness or luminosity and colour or surface temperature.

Such a plot is now known as a Hertzsprung-Russell or H-R diagram. The Hertzsprung - Russell Diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage.

Stars in the stable phase of hydrogen burning lie along the Main Sequence according to their mass. What color are the hottest stars? How are stars born? Stars are born when large gas clouds collapse under gravity. They form hot cores that gather more and more gas and dust until a protostar is formed. If only a small amount of gas is around, then only a small star will form; if a large amount of gas is present, then a massive star will form.

What is the hottest star on the HR diagram? The majority of these stars when plotted on an H-R diagram go down from left to right in a diagonal line. The temperature scale along the bottom axis goes from coolest on the right to hottest on the left. This is contrary to the normal convention, where values increase going left to right on an axis. Where are stars born? Stars are born within the clouds of dust and scattered throughout most galaxies.

A familiar example of such as a dust cloud is the Orion Nebula. For stars with mass less than around 1. That moves the star back down and to the left in the HR-diagram, leaving a kind of "cusp" in the path through the HR diagram. The location of that cusp is called the "tip of the red giant branch". Because we know the luminosity and color of that tip, when we find it on an HR diagram of stars in a galaxy, we can work out a lot of information about the galaxy redshift, distance, etc.

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