# HR Diagram Exercise

Here is an HR Diagram of all 15 of the brightest stars in the sky visible in the United States, as well as other interesting stars. Star luminosities (plotted vertically) are given in terms of the sun, as are the lines of constant radius (slanted downward across the diagram). In case you are not seeing this in color, the blue stars are toward the left, followed by white, yellow, orange, and red on the right. Temperatures are given on the top scale, so hotter stars are to the left and cooler to the right. The bottom scales, list the spectral types, with ten implied subcategories. For example, an A5 star is about halfway between an A0 and F0 star. The dark curving line is the Main Sequence of stars, Giants refer to stars about 100 times as bright as the sun (rather than as large), and supergiants refer to those about 10,000 times as luminous. All the information you need to do this exercise can be found by reading this paragraph and the diagram itself. In every case, when the most "most" (or "least", etc.) is used, it means of the stars on the diagram which are numbered. Some stars are the right answer to more than one question.

1. The Sun. (Hint: it has Luminosity of 1 and Radius of 1.)
2. Mira, the reddest star, being a long period variable giant.
3. Procyon, an F5 star slightly above the main sequence.
4. Rigel, the most luminous star listed.
5. Altair, a white (A7) star right on the main sequence.
6. Capella, a yellow giant.
7. The three belt stars of Orion being the hottest and bluest stars.
8. Aldebaran, a "red giant" that is really orange, being spectral type K5, and brighter and larger than other orange giants.
9. Mirfak (head of Perseus), being an F-type supergiant.
10. Fomalhaut, the nearest in size to the sun.
11. Delta Cepheid, the classic Cepheid, being a G0 supergiant.
12. Polaris, also a cepheid, but with a shorter period than Delta Cephei, and hence near it in the diagram but slightly less luminous.
13. Sirius, the nearest star included, an A0 star just below the main sequence.
14. Betelgeuse, the largest red supergiant.
15. Alcyone, brightest of the Seven Sisters (Pleiades) is a B7 giant, about 10 times the radius of the sun and 1,000 times as luminous.
16. Deneb, the most distant star first magnitude star, second in luminosity only to Rigel.
17. Arcturus, the middle of the three orange giants numbered, both in size and luminosity.
18. The star closest to having exactly 100 times the radius of the sun.
19. Regulus, type B7, being the bluest star falling exactly on the main sequence.
20. Spica, bluer than Regulus, a B1 main sequence star.
21. Antares, red supergiant second in radius only to Betelgeuse.
22. Vega, an A0 star exactly on the main sequence.
23. A star greater than 1,000 times the radius of the sun.
24. Pollux, the smallest of the three orange giants.
25. The smallest star listed.