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# Astrophysics: Absorption Lines, Spectral Classes, and the Sun

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1. Explain why the absorption lines of an element have the same wavelengths as the emission lines of that element.

2. Make up your own mnemonic to help you remember the sequence of spectral classes.

3. Why does convection develop about 70% of the way from the Sun's center to its surface?

4. What do we mean when we say that the Sun has a temperature of 5800K?

5. What evidence do we have that there is a relationship between sunspots and the temperature of the Earth?

6. What are radio and infrared radiation more useful than visible light in studying molecular clouds?

7. What evidence do we have that young stars are rounded by dusty disks rather than complete spherical shells of dust?

8. Why can't hydrogen fusion and helium fusion go on at the same time at the center of a star?

9. Suppose two single stars form at the same time. They have the same masses and chemical compositions. What can be said about the evolution of the two stars from that point forward?

10. In cluster 1, the main sequence extends from spectral class O to spectral class K. In cluster 2, there are no main sequence stars cooler than spectral class G. Which cluster is older and how do we know?

##### Solution Summary

Fundamental question on the sun and its relation to stars and the earth are answered.

##### Solution Preview

1. Let us take the case of Hydrogen atom. It contains only one electron. The spectral lines are formed due to the transition of electron from an upper level to the lower level or vice versa. The single electron in the hydrogen atom gets a quantum energy and jumps to an upper level means an absorption line occured in that spectrum. After some time it will emit the same quanta of energy and comes back to the earlier energy level. Another thing to remember is that the energy between two levels is constant for a particular element. The energy levels of an electron around a nucleus are given by:

E_n=−hcR_∞Z^2/n^2

where is R∞ the Rydberg constant, Z is the Atomic number, n is the principal quantum
number, h is Planck's constant, and c is the speed of light. The Rydberg levels depend
only on the principal quantum number n.

2. The classic mnemonic for the spectral sequence O B A F G K M is Oh Be A Fine Girl, Kiss Me. Other favorite mnemonics for the spectral sequence include: Old, Bald, And Fat Generals Keep Mistresses , and
Oh Boy, An F Grade Kills Me.

3. For stars, the preferred way of energy transportation from the interior to the surface depends on many factors, such as temperature, pressure and the detailed chemical composition of the stellar matter. In the outer layers of the Sun, energy is primarily transported by convection, while radiation is the dominant form of energy transport in the
interior regions. Stars with a mass only slightly larger than that of the Sun (~10% larger) have convective cores and only a shallow outer convective layer. These ...

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