29:50 Modern Astronomy
Fall 2002
Lecture 10...September 20, 2002
Starlight - Part II

Begin with recollection of Kirchoff's Laws of Radiation:

1. A hot solid or liquid object, or dense opaque gas emits a continuous spectrum. This means some light at all wavelengths. (Sketch of spectrum).
2. A hot rarefied (low density) gas produces an emission spectrum. There is light at some wavelengths, no emission at most wavelengths. (Sketch of spectrum)
3. Light from a continuous spectrum which passes through a cool gas shows and absorption spectrum. (Sketch of spectrum).

Illustration via Figures 16-6, 7-6, demonstrations.

Continuous Radiation from Hot Objects (Blackbody radiation)

An addendum to Kirchoff's 1st Law is the following: as objects get hotter, they get brighter and bluer. This is described in Wien's Law and the Stefan-Boltzmann Law.

To describe changes with temperature, you need a temperature scale. In physics and astronomy, you use the Kelvin temperature scale. One degree K = one degree C. The zero of the Kelvin system corresponds to absolute zero. Room temperature is about 293K, water boils at 373K.

Wien's Law

Let be the wavelength at which a hot object is brightest, and T is the temperature. Wien's Law says:

Transparency with blackbody spectra

Let's work some examples
(1) The Earth has a surface temperature of 290K. At what wavelength is it brightest?

(2)What is the surface temperature of the Sun, given that it is brightest at a wavelength of 475 nanometers ( meters)?

This gives us a good estimate for the temperature of the Sun.

Let's look at some spectra of the Sun and other stars and see what we can figure out, using Wien's law and Kirchoff's laws.
Transparencies of stellar spectra

The Stefan-Boltzmann Law

The second aspect of Kirchoff's first law, that the hotter an object is, the brighter it appears is summed up in the Stefan Boltzmann Law.

This law will be important in figuring out the nature of stars.

Spectral Classification Schemes for Stars

Stars can be classified on the basis of their spectra. Astronomers still use a system introduced in the late 19th century. The significance is gone but the labels hang on. Stars are grouped in the following categories: O(hottest),B,A (Vega and Sirius), F, G(good old Sun), K(Arcturus), M (coolest). These labels are a standard part of astronomical vocabulary.

Motivation of the Hertzsprung-Russell Diagram

We now know a lot about stars. Let's imagine taking data on a lot of stars and plotting it up. For the x axis (the abscissa) we can choose: temperature, or spectral class, or color. For the y axis (the ordinate) we can choose: luminosity relative to the Sun, or absolute magnitude.

What do we get? Look at Figure 16.20 for a pleasing-to-the-eye representation.

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