Once you know the luminosity and temperature (or color) of a star, you can plot the star as a point on the H-R diagram. Plot the luminosity on the y-axis with brighter stars going toward the top. Since brighter stars have lower magnitudes, if you choose to plot magnitude on the y-axis, the values will decrease as you go up! That’s OK – just remember that the luminosity of the star is increasing.
Plot temperature on the x-axis. However, since we can’t know a star’s real temperature, you should instead plot color (either the traditional b-v or SDSS’s g-r) on the x-axis. Traditionally, hotter stars have been placed at the left of the chart and cooler stars to the right. Even though the temperature decreases as you move the right, the b-v and g-r values will increase. So you should make your H-R diagram with b-v or g-r increasing to the right.
The first H-R diagram you should try is a diagram for the brightest stars in the sky. The table below shows the 26 brightest stars in the sky. Instead of plotting their luminosities (which are so large that they’re hard to visualize), plot the stars’ absolute magnitudes. Absolute magnitude is defined as the magnitude that a star would have if you saw it from a distance of 10 parsecs (about 32 light-years). Stars with higher luminosities put out more light, so they are brighter – they have lower apparent magnitudes. Stars with lower luminosities put out less light, so they are dimmer – they have higher absolute magnitudes.
The table below shows the 26 brightest stars, giving their names, apparent magnitudes, absolute magnitudes, and b-v colors.
| Star Name | Apparent Magnitude | Absolute Magnitude | b-v |
|---|---|---|---|
| Sun | -26.8 | 4.8 | 0.63 |
| Sirius | -1.46 | 1.4 | 0.0 |
| Canopus | -0.72 | -2.5 | 0.15 |
| Arcturus | -0.04 | 0.2 | 1.23 |
| Alpha Centauri | -0.01 | 4.4 | 0.71 |
| Vega | 0.00 | 0.6 | 0.0 |
| Capella | 0.08 | 0.4 | 0.08 |
| Rigel | 0.12 | -8.1 | -0.03 |
| Procyon | 0.38 | 2.6 | 0.42 |
| Betelgeuse | 0.41 | -7.2 | 1.85 |
| Achernar | 0.46 | -1.3 | -0.16 |
| Hadar | 0.63 | -4.4 | -0.23 |
| Acrux | 0.76 | -4.6 | -0.24 |
| Altair | 0.77 | 2.3 | 0.22 |
| Aldebaran | 0.85 | -0.3 | 1.54 |
| Antares | 0.92 | -5.2 | 1.83 |
| Spica | 1.00 | -3.2 | -0.23 |
| Pollux | 1.14 | 0.7 | 1.0 |
| Formalhaut | 1.16 | 2.0 | 0.09 |
| Becrux | 1.20 | -4.7 | -0.23 |
| Deneb | 1.25 | -7.2 | 0.09 |
| Regulus | 1.35 | -0.3 | -0.11 |
| Adhara | 1.50 | -4.8 | -0.21 |
| Shaula | 1.60 | -3.5 | -0.22 |
| Gacrux | 1.63 | -1.2 | 1.59 |
| Castor | 1.98 | 0.5 | 0.03 |
These data are also available in a SkyServer workbook, which you can use in the Exercise below.
Exercise 1. Make an H-R diagram for the brightest stars by graphing b-v and absolute magnitude for the 26 stars above. Use a graphing program such as Microsoft Excel to make your diagram. You may use this SkyServer workbook to store the data. If you don’t have a graphing program, you can download a free program such as Open Office.
For help on how to make a graph using Microsoft Excel, see SkyServer’s Graphing tutorial.
Question 1. Do you see any groups of stars that appear to have something in common? Sketch a box around those groups.
Question 2. The stars in the upper right of the diagram are very bright but are also very cool. If the stars are cool, why do you think they are so bright?
Question 3. Where does our sun plot on this diagram? Is it hotter or cooler than average? Does it emit more or less light than average?
Question 4. Do you think your diagram constitutes a good random sample of stars? Why or why not?