1. Explain the concept of a black body and why stars behave like black bodies to first approximation.

2. Explain the types of information that can be obtained by using Wien’s Law, the Stefan-Boltzmann Law, and the Planck Law.

3. Discuss how the color of a star’s spectrum can be related to the star’s surface temperature.

4. Discuss how the knowledge of a star’s surface temperature can be used to determine how much energy is coming out of a unit area on the star’s surface.

5. Describe the various stellar spectral types and their properties.

¶ color

¶ spectrum

¶ surface temperature

¶ black body

¶ Wien’s Law

¶ Stefan-Boltzmann Law

¶ Planck Law

¶ continuum

¶ emission line

¶ absorption line

¶ energy state

¶ stellar classification

¶ OBAFGKM

¶ The color of a star and other properties of its spectrum tell us about a star’s surface temperature.

¶ Stars are similar to objects which physicists call black bodies. Black bodies are perfect absorbers and emitters of electromagnetic radiation.

¶ According to Wien’s Law, as a black body (or star) gets hotter, more radiation will start to come out at shorter wavelengths (e.g., the color may shift from red to blue as it gets hotter).

¶ According to the Stefan-Boltzmann Law, if a black body (or star) gets even a little hotter, a lot more energy will come out of it at all wavelengths. The amount of energy emitted per second per cm² (E) is proportional to temperature raised to the fourth power (E=sT⁴ where s is a constant).

¶ The actual energy emitted by a black body as a function of wavelength is given by Planck’s Law.

¶ According to Planck’s Law a black body emits a continuous spectrum as a function of wavelength. Continuum is the term for the continuous spectrum that we would measure from a body if no spectral lines were present.

¶ There are two types of spectral lines:

¶ According to the laws of Quantum Mechanics, there are only a specific number of energy states in which a given atom, ion, or molecule can exist.

[Quantum Mechanics is a branch of physics which is able to predict the consequences of electromagnetic radiation having both wave-like and particle-like properties. Note that a bundle of energy at one specific wavelength, i.e., a quantum, is also called a photon.]

¶ Spectral lines arise from changes in the amounts of energy present in the atoms, ions, or molecules that cause them.

¶ The allowed energy states of an atom, ion, or molecule are discrete (i.e., separate and distinct); they are quantized.

¶ When atoms, ions, or molecules lose (emit) photons of the same discrete energy, an emission line is produced in the background continuum source.

¶ When atoms, ions, or molecules gain (absorb) photons of the same discrete energy (provided by a background continuum source), an absorption line is produced.

¶ The neutral hydrogen atom has one proton in its nucleus and an electron associated with it.

¶ If the neutral hydrogen atom absorbed enough energy, the electron would be knocked out of the hydrogen atom and we would have an ionized hydrogen atom (which is simply a proton) and a free electron. The neutral hydrogen atom would have to absorb a very energetic UV photon for this to happen.

¶ There are specific wavelengths (not all wavelengths) of less energetic photons that the neutral hydrogen atom can also absorb. However, absorption of these energies would not cause the electron to be knocked free of the proton. It would simply cause the electron to move to a higher (‘more excited’) energy state.

¶ When an electron is free, it has a natural tendency to recombine with the proton and return to its lowest energy state (called the ground state). An electron in an excited energy state also has a natural tendency to return to its ground state.

¶ The absorption and emission caused by electronic transitions between energy states in the hydrogen atom are termed the Lyman Series, the Balmer Series, the Paschen Series, the Brackett Series, etc.

¶ Each atom, ion, and molecule (by definition) represents a unique combination of electrons, protons, and neutrons. Thus, each atom, ion, and molecule has its own distinct set of spectral lines that it is capable of giving rise to—even more distinct than a finger print.

¶ Both from the color of a star’s continuum light and the spectral lines that it exhibits we can determine a star’s surface temperature. [The properties of the spectral lines are also sensitive to gas pressure and density.]

¶ The stellar classification scheme, from hottest to coolest, is as follows: O B A F G K M. Some people memorize this by saying: ‘Oh, Be a Fine Girl/Guy, Kiss Me.’

¶ Note, for example, that the material in the atmospheres of O stars is highly ionized (50,000 degrees Kelvin). Neutral hydrogen can be seen in the atmospheres of A stars (10,000 degrees Kelvin). Molecules can be seen in the atmospheres of M stars (3,500 degrees Kelvin). The Sun is a G type star (6,000 degrees Kelvin).