Spectra Q & A

This article by Jim Shields of San Francisco, California appeared in Rainbow Optics Newsletter #3.

Questions & Answers about Spectra

by Jim Shields

My first few times out with a spectroscope filled my mind with unanswered questions. I was impressed with the prominence of the hydrogen lines in Vega, and amazed at the complexity of the spectra of Betelgeuse, but I wanted to understand what was going on. One of the best places I've found for answers is Stars and Their Spectra by James Kaler, published by Cambridge University Press.

Here are some of my questions and a brief summary of the answers I've found, hopefully suitable for use at star parties and the like. I figure I won't really understand the subject myself until I can explain it to you.

How are spectral lines formed?

By electrons jumping between different energy levels in the atoms in the star's outer layers. Bound electrons can absorb and emit energy only in certain discrete amounts. When an electron absorbs a photon of light with just the right amount of energy, it jumps to a higher energy level. When the electron spontaneously jumps back to a lower energy level, a photon is emitted. Enough electrons jumping between any two given energy levels of a given element will result in a spectral emission or absorption line at a characteristic wavelength.

For example, the strongest spectral line in a hot main-sequence star like Vega lies in the blue-green part of the spectrum. It is a dark or absorption line resulting from electron jumps from the second to the fourth energy level of the neutral hydrogen atom, and is known as hydrogen beta (in the Balmer series).

When I look at a star, why do I see dark absorption lines rather than bright emission lines?

Gas under high pressure produces a continuous spectrum, a rainbow of colors. Continuous radiation viewed through a low density gas results in an absorption-line spectrum. What's happening here is that radiation emitted by gas under high pressure deep within the star is being absorbed by low density gas in the star's outer layers.

What kinds of deep sky objects have emission-line spectra?

A low density gas shows an emission-line spectrum, when not observed against a background of continuous radiation. Thus emission lines are found in the spectra of planetary and diffuse nebulae, and in some stars. In the latter case the lines often arise from gas clouds ejected from the star by strong stellar winds.

Vega and Deneb are both A-type stars, yet the prominent hydrogen lines of Vega are missing in the spectra of Deneb. Why?

All spectral lines have a certain width, caused by atomic collisions perturbing the energy levels, which permits the atoms to absorb light slightly away from the line center. Generally, the greater the pressure and density of the gas, the broader and more prominent the lines will be.

Vega is a main-sequence or dwarf star, Deneb a supergiant. At the relatively low pressure and density typical of the outer layers of supergiants, atomic collisions are relatively rare and the spectral lines are consequently much narrower (and less prominent) than in dwarfs of this spectral type.

Why are the hydrogen lines weaker in cooler stars?

At cooler temperatures most electrons are in the ground state, at the lowest energy level, because of the relatively few atomic collisions. Jumps to and from the ground state of neutral hydrogen produce spectral lines in the Lyman series, the strongest of which fall in the ultraviolet part of the spectrum. In cooler stars the relative scarcity of electrons at the second (and higher) energy levels results in weaker absorption lines in the Balmer series, visible to the human eye.

Why do spectral lines from elements heavier than hydrogen and helium ("metals") become more prominent with decreasing temperature?

At cooler temperatures fewer elements are ionized, i.e., have lost one or more electrons through atomic collisions. Spectral lines from neutral (and many singly-ionized) elements tend to fall into the visual part of the spectrum, whereas lines from more highly-ionized atoms are shifted into the ultraviolet.

What are the thick bands in the spectra of Betelgeuse? How are they formed?

These bands arise from molecules and are formed in a manner similar to but much more complex than atomic spectra. In addition to the atomic energy levels, in a molecule there are also vibrational and rotational states to be accounted for. The collection of jumps between the various vibrational and rotational states of a given electronic state, in all their combinations, results in a series of lines which converge toward a characteristic wavelength known as the band head.

Why are these bands absent in hotter stars?

Stable molecules cannot form in the conditions of high pressure and density typical of the outer layers of hotter stars because of the frequent atomic collisions.