Hertzsprung-Russell Diagram. The x-axis is labeled 'Surface Temperature' (in units of kelvins) with high temperatures on the left and low temperatures on the right. The points representing stars which appear furthest to the left are drawn in blue, those in the middle temperature range are drawn in yellow and orange, and those furthest to the right are drawn in red. The y-axis is labeled Luminosity (in units of solar luminosity), with low luminosities on the bottom and high luminosities on the top. A third parameter, Radius (in units of solar radii), is also labeled. Lines of constant radius extend from the upper-left to the lower-right, covering the whole space with a set of parallel lines. In the lower-left corner we find the line labeled 'R is equal to 0.001 solar radii'; successive lines are labeled 0.01, 0.1, 1, 10, 100, and 1,000 solar radii, with the line for 'R is equal to 1000 solar radii' being located in the upper-right corner. A series of blue, yellow, and orange points scattered along the 'R is equal to 0.01 solar radii' line is made up of white dwarf stars. A large set of red (and a few yellow and orange) points made up of giant stars appears between the 1 and 1,000 solar radii lines. The 1 solar radii line extends from the upper-left corner to the lower-right corner. The Main Sequence (a curved sequence of blue, yellow, orange, and red points) mainly follows the 0.1 - 10 solar radii lines; at the high luminosity end it curves up to slightly higher luminosities and at the low luminosity end it curves down to slightly lower luminosities.
[NMSU, N. Vogt]
T = 36,416K, L = 154141.17188 L<img src=pics/sun.gif alt=, R = 9.9593 RSolar"> T = 27,336K, L = 20045.33789 L<img src=pics/sun.gif alt=, R = 6.3736 RSolar"> T = 20,534K, L = 3269.94678 L<img src=pics/sun.gif alt=, R = 4.5621 RSolar"> T = 15,439K, L = 597.42877 L<img src=pics/sun.gif alt=, R = 3.4496 RSolar"> T = 11,622K, L = 73.41272 L<img src=pics/sun.gif alt=, R = 2.1341 RSolar"> T = 8,762K, L = 13.41272 L<img src=pics/sun.gif alt=, R = 1.6047 RSolar"> T = 6,620K, L = 2.45054 L<img src=pics/sun.gif alt=, R = 1.2016 RSolar"> T = 5,015K, L = 0.56162 L<img src=pics/sun.gif alt=, R = 1.0022 RSolar"> T = 3,813K, L = 0.01334 L<img src=pics/sun.gif alt=, R = 0.2672 RSolar"> T = 2,913K, L = 0.00019 L<img src=pics/sun.gif alt=, R = 0.0547 RSolar"> T = 28,949K, L = 0.06521 L<img src=pics/sun.gif alt=, R = 0.0103 RSolar"> T = 21,742K, L = 0.02352 L<img src=pics/sun.gif alt=, R = 0.0109 RSolar"> T = 16,344K, L = 0.00676 L<img src=pics/sun.gif alt=, R = 0.0104 RSolar"> T = 12,300K, L = 0.00194 L<img src=pics/sun.gif alt=, R = 0.0098 RSolar"> T = 9,270K, L = 0.00070 L<img src=pics/sun.gif alt=, R = 0.0104 RSolar"> T = 16,816K, L = 73791.88281 L<img src=pics/sun.gif alt=, R = 32.3156 RSolar"> T = 7,199K, L = 16911.71094 L<img src=pics/sun.gif alt=, R = 84.4097 RSolar"> T = 4,138K, L = 28162.04102 L<img src=pics/sun.gif alt=, R = 329.6468 RSolar"> T = 6,483K, L = 228.00356 L<img src=pics/sun.gif alt=, R = 12.0857 RSolar"> T = 3,736K, L = 533.41949 L<img src=pics/sun.gif alt=, R = 55.6514 RSolar">

Reading the figure:

Place the cursor on a region filled by stars on the figure above, and click with your mouse. A separate browser window will appear with information about stars found in that region of the Hertzsprung-Russell diagram. You should be able to estimate the luminosity, temperature, and radius of these stars by eye by reading the diagram. If you know the value of two of these three quantities exactly, you can calculate a specific value for the third as well.

You can read off the temperature of a star by tracing a line down from the cursor to the x-axis. (The x-axis is the line drawn on the bottom of the figure, running from 60,000 K (hot) on the left to 4,000 K (cold) on the right.) Find the point along this line which falls below the cursor: this will tell you the approximate temperature of the stars found in the region of the Hertzsprung-Russell diagram that you have selected.

Next, you can read off the luminosity (or brightness) of the star by tracing a line over to the left from the cursor to the y-axis. (The y-axis is the line drawn on the left hand side of the figure, running from 0.00001 LSolar – 100,000 times fainter than the Sun – on the bottom to 1,000,000 LSolar – one million times brighter than the Sun – on the top.) Find the point along this line which falls to the left the cursor: this will tell you the approximate luminosity of the stars found in the region of the Hertzsprung-Russell diagram that you have selected.

Finally, you can read off the radius (or size) of the star. There are eight diagonal lines running across the image, labeled R = 0.0001 RSolar (bottom left corner) to R = 1,000 RSolar (upper right corner). They run from ten-thousand times smaller than the Sun to one-thousand times larger in radius. Find the line which is nearest to the cursor: this will tell you the approximate size of the stars found in the region of the Hertzsprung-Russell diagram that you have selected.

Place your cursor along the Main Sequence, or in the region where giant stars or white dwarfs are found, and try to read the values of L, T, and R for yourself from the diagram. Then calculate specific values for the luminosity, temperature, and size of the star, by assuming values for two of the three quantities.