Answer to Question 1
A survey on the family of stars could reveal the following characteristics:a. Taking a survey is difficult because you must be sure to get an honest sample. If you don't survey enough stars, or if you miss some types of stars, your results can be biased.b. M dwarfs and white dwarfs are so faint they are difficult to find even near Earth and may be undercounted in surveys.c. Luminous stars, although they are rare, are easily visible even at great distances. Typical nearby stars have lower luminosity than our Sun.
Answer to Question 2
The set of star types, called the spectral sequence, is important because it is a temperature sequence. The O stars are the hottest, and the temperature continues to decrease down to the M stars, the coolest. For further precision, astronomers divide each spectral class into 10 subclasses. For example, spectral class A consists of the subclasses A0, A1, A2, . . . A8, A9. Next come F0, F1, F2, and so on. These finer divisions define a star's temperature to a precision of about 5 percent.The study of spectral types is more than a century old, but astronomers continue to discover and define new types. The L dwarfs, found in 1998, are cooler and fainter than M stars. They are understood to be objects smaller than stars but larger than planets and are called brown dwarfs. The spectra of M stars contain bands produced by metal oxides such as titanium oxide (TiO), but L dwarf spectra contain bands produced by molecules such as iron hydride (FeH). The T dwarfs are an even cooler and fainter type of brown dwarf than L dwarfs. Their spectra show absorption by methane (CH4 ) and water vapor.
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