April 7, 2012
Milady continued her work researching the meanings behind all of the data that has been gathered so far. We were interested in the types of stars that the planets orbit. Below are some of the resources that she was able to find starting with an excerpt of Nick Strobel's Astronomy Notes. Nick has offered his documents and line drawings to the net as a resource in astronomy education but requests you visit his site www.astronomynotes.com for the updated and corrected version:
Stars are divided into groups called spectral types (also called spectral classes) which are based on the strength of the hydrogen absorption lines. The A-type stars have the strongest (darkest) hydrogen lines, B-type next strongest, F-type next, etc. Originally there was the whole alphabet of types, based on hydrogen line strengths, but then astronomers discovered that the line strengths depended on the temperature. Also, the discussion in the previous section and the figure
above show that more than just the hydrogen lines must be used because a very hot star and a cool star can have the same hydrogen lines strength. The presence of other atomic or ion lines are used in conjunction with the hydrogen spectrum to determine the particular temperature of the star.
After some rearranging and merging of some classes, the spectral type sequence is now OBAFGKM when ordered by temperature. The O-type stars are the hottest stars and the M-type stars are the coolest. Each spectral type is subdivided into 10 intervals, e.g., G2 or F5, with 0 hotter than 1, 1 hotter than 2, etc. About 90% of the stars are called main sequence stars. The other 10% are either red giants, supergiants, white dwarfs, proto-stars, neutron stars, or black holes. The characteristics of these types of stars will be explored in the following chapters. The table below gives some basic characteristics of the different spectral classes of main sequence stars. Notice the trends in the table: as the temperature of the main sequence star increases, the mass and size increase. Also, because of the relation between luminosity and the size and temperature of a star, hotter main sequence stars are more luminous than cooler main sequence stars. However, there are limits to how hot a star will be, or how massive and large it can be. Understanding why the constraints exist is the key to understanding how stars work.
|Color||Class||solar masses||solar diameters||Temperature||Prominent Lines|
|bluest||O||20 - 100||12 - 25||40,000||ionized helium|
|bluish||B||4 - 20||4 - 12||18,000||neutral helium, neutral hydrogen|
|blue-white||A||2 - 4||1.5 - 4||10,000||neutral hydrogen|
|white||F||1.05 - 2||1.1 - 1.5||7,000||neutral hydrogen, ionized calcium|
|yellow-white||G||0.8 - 1.05||0.85 - 1.1||5,500||neutral hydrogen, strongest ionized calcium|
|orange||K||0.5 - 0.8||0.6 - 0.85||4,000||neutral metals (calcium, iron), ionized calcium|
|red||M||0.08 - 0.5||0.1 - 0.6||3,000||molecules and neutral metals|
Red giants can get up to about 50 times the size of the Sun. Supergiants are between 20 times the size of the Sun for the B0 supergiants and 1000 times the size of the Sun for the M0 supergiants. Despite the tremendous size of some stars, even the largest supergiant is only 1/7000 light years across. Since stars are severallight years from each other, they do not collide with each other (even the fat ones!).
This table gave us a good sense of the properties of each star including its color, classification, mass, diameter, temperature, and properties. All of which could lead to properties we may want to visualize. Further research lead her to the University of Oregon web page which showed visualizations of stars based on the above properties. They are known as Hertzsprung-Russell Diagrams stemming from the "discovery that the luminosity of a star is related to its surface temperature."