What can astronomers learn about a star from its spectral lines?

Answer 1

They can learn how to open their textbooks to find the answers to their questions.

But since you asked:
surface temperature
composition
rotation rate (maybe)
speed away or towards earth
age (indirectly)
radius (indirectly)
luminosity class
spectral class
how many planets it has

If you list all those in your answer, your teacher/professor will know you cheated.

Answer 2

kris knows that you can't tell how many planets a star has by looking at the star's spectrum.

The composition is fairly obvious. Each ionization state of each element has its own set of absorption or emission lines.

The rotation rate is the result of doppler broadening of the spectral lines. The faster the rotation, the more the broadening.

The radial velocity comes from the doppler shift in the observed frequency of the lines.

Actually, you'd probably already have the star's color temperature from the blackbody curve, but the Saha equation will give you a check from the distribution of power among the various ionization states.

Answer 3

From The National Enquirer and from YahooAnswers.

Answer 4

Physical Conditions in Stars

Not only do different elements have different spectral signatures, but the signatures of atoms depend on whether or not tha atoms are ionized, and on how many of their electrons have been removed. Such information gives the astronomer valuable insight into the temperatures and pressures in stars. Also, spectral lines are modified by electrical or magnetic fields. After 130 years, astronomers are still finding new ways to get information out of the spectra of stars.

After helium was discovered, astronomers began looking for other new elements in the cosmos, and a number were announced and named. All later turned out to be ordinary elements like oxygen under extreme and unfamiliar physical conditions, for example with many of their electrons stripped away.

The Doppler Effect

Besides allowing the astronomer to determine the chemical compositions of stars, spectroscopy provides the astronomer with another powerful technique: the Doppler Effect.

Most people have had the experience of hearing the pitch of a train whistle or ambulance siren drop as the source moved past. As the sound source moves toward the observer, the sound waves are compressed, making the pitch of the sound higher. As the sound source moves away from the observer, the sound waves are stretched out, making the pitch of the sound lower. In a similar way, light from an approaching star has its wavelengths shortened, or blue shifted, and light from a receding star has its wavelengths lengthened, or red-shifted.

Light that is red- or blue-shifted merely changes color. There is no way to tell from color alone that a star is moving. Contrary to many popular illustrations, red-shifted stars do not look red; as the visible spectrum of a star is shifted to longer wavelengths, its ultraviolet spectrum is shifted into the visible range. The key to the Doppler effect is that spectral lines change position. The change in position is easily measured on a photographic plate. The Doppler Effect allows astronomers to determine three important facts:

* Astronomers can determine if stars or galaxies are moving toward or away from Earth. Almost all galaxies are receding from us, and the farther away they are, the faster they are receding.
* Many stars consist of two or more stars orbiting a common center of gravity. Even if such a pair of stars is too close to see the individual stars with the most powerful telescope, it is possible to detect their orbital motion. The spectrum of the star will consist of two sets of lines that shift as the stars move toward and away from Earth. Such star systems are called Spectroscopic Binaries.
* As stars rotate, one side of the star moves toward us and the other side moves away. The spectral lines of the approaching side will be slightly blue-shifted, while the receding side will be red-shifted. The spectral line will be broader than normal. By measuring the broadening of spectral lines, it is possible to determine how fast the star is rotating.

As an indication of the subtle details spectroscopy can reveal, it is possible to detect vertical movements of material on the Sun from its Doppler shift. Just as earthquakes on the Earth send waves through the Earth's interior, disturbances on the Sun send waves through its interior, and these waves can be detected from their effects on the surface of the Sun. Study of these disturbances, called helioseismology, is revealing details of the Sun's interior in astonishing detail.
Motions Of The Stars

The stars are not fixed, but move measurably over the years. Astronomers refer to the motion of stars as proper motion, and measure the apparent motion of the stars on photographs taken years apart. Most stars would take centuries to move the apparent width of the Moon, but they do move.

If we know the distance to a star, we can calculate its velocity across our line of sight. The Doppler Effect enables us to determine the radial velocity of the star, or its speed along our line of sight. If we know the two velocities, we can determine the true velocity and direction of the star's movement in space.
The Sun's Motion

The stars appear to move both because they are in motion and because we ourselves are in motion. Just as a driver in a snowstorm sees the snowflakes appear to radiate away from his direction of motion, astronomers also observe that most of the stars in the sky are moving away from a certain region of the sky. This direction, in the constellation Hercules (straight overhead for U.S. observers in the early evening in summer) is the direction the Solar System is moving, at about xx miles (xx km) per second.
Spectral Classification

It is obvious even to the unaided eye that stars differ. Some are reddish, others yellow, still others bluish-white. When we determine distances to the stars still more differences appear: some very nearby stars are extremely faint, while very distant stars are sometimes very bright.

Spectroscopy provides a means of making sense out of the variety of the stars. The very hottest stars are blue-white and show only the spectral lines of helium. Somewhat cooler stars are white and show lines for hydrogen as well. Still cooler stars (yellow and orange) show the signatures of elements heavier than helium (what astronomers call metals), and the very coolest stars (red) are cool enough for a few very sturdy molecules to form in their atmospheres.

Astronomers divide stars into spectral classes, labelled with letters. Originally the classes were designated A, B, C ..., but some classes were dropped and others rearranged in order. The major classes of stars, from hottest to coolest, are now designated O, B, A, F, G, K and M. There is a handy phrase for remembering the letter sequence: "Oh, be a fine girl (guy), kiss me". Four other classes are used for peculiar stars: W for very rare, hot stars, N, R, and S for chemically peculiar cool red stars. For stars that fall between the main classes, subdivisions are used: an F5 star is halfway between classes F and G, for example. Our own Sun is classed as G2.