January 2011 Mock: Question 17

This is a simple Hertzsprung-Russel diagram:

The Hertzsprung-Russel diagram is essentially a scatter graph of observable stars' luminosity vs. their temperature, which can be assessed by looking at the colours of light being emitted from the star. The letters along the x-axis are the categoric spectral types of stars, which are not essential. Luminosity is increasing with the positive y direction, and temperature increasing with the negative x direction. Both variables are on logarithmic scales, i.e. they do not increase in a linear or direct way, but by increasingly large amounts.

When the observable stars are plotted in this way, a pattern emerges. The most prominent feature is the 'Main sequence' which arcs across the diagram from top left to bottom right, with redder, cooler, dimmer stars on the bottom right end, working up through average yellow stars like our Sun all the way up to hot, bright, blue stars on the other end in the top left. In the diagram above, the Sun can be placed approximately between the 'Main sequence' and 'Giants' labels.

The other main features of the diagram are the clouds in the upper right and bottom left. In the top right are the giants (and supergiants)—massive, very bright, but relatively cool stars—and in the bottom left are the white dwarfs—remains of burnt out stars which are still extremely hot, but not very bright since they are no longer undergoing nuclear fusion.

The units on the axes can vary. In this diagram, luminosity is measured in solar luminosities i.e. 100Lo = 100 x the luminosity of the Sun. The units of temperature are Kelvins, which is a system of units whose individual sizes are equivalent to ºC, but for which 0K is absolute zero (~-273ºC).

For a sketching question like this, the main features to include are the shapes and positions of the main sequence and the clouds of white dwarfs and giants; and the labelling of the axes, with the direction in which they are increasing.