How is spectroscopy used to determine the composition of stars

In practice, analyzing stellar spectra is a demanding, sometimes frustrating task that requires both training and skill. Studies of stellar spectra have shown that hydrogen makes up about three-quarters of the mass of most stars.

Generally, but not invariably, the elements of lower atomic weight are more abundant than those of higher atomic weight. Take a careful look at the list of elements in the preceding paragraph. Two of the most abundant are hydrogen and oxygen which make up water ; add carbon and nitrogen and you are starting to write the prescription for the chemistry of an astronomy student.

We are made of elements that are common in the universe—just mixed together in a far more sophisticated form and a much cooler environment than in a star. The metallicity of the Sun, for example, is 0. The Chemical Elements lists how common each element is in the universe compared to hydrogen ; these estimates are based primarily on investigation of the Sun, which is a typical star.

Some very rare elements, however, have not been detected in the Sun. Estimates of the amounts of these elements in the universe are based on laboratory measurements of their abundance in primitive meteorites, which are considered representative of unaltered material condensed from the solar nebula see the Cosmic Samples and the Origin of the Solar System chapter.

When we measure the spectrum of a star, we determine the wavelength of each of its lines. If the star is not moving with respect to the Sun, then the wavelength corresponding to each element will be the same as those we measure in a laboratory here on Earth.

But if stars are moving toward or away from us, we must consider the Doppler effect see The Doppler Effect. We should see all the spectral lines of moving stars shifted toward the red end of the spectrum if the star is moving away from us, or toward the blue violet end if it is moving toward us Figure 2. The greater the shift, the faster the star is moving. Such motion, along the line of sight between the star and the observer, is called radial velocity and is usually measured in kilometers per second.

Figure 2: Doppler-Shifted Stars. When the spectral lines of a moving star shift toward the red end of the spectrum, we know that the star is moving away from us. If they shift toward the blue end, the star is moving toward us.

William Huggins , pioneering yet again, in made the first radial velocity determination of a star. He observed the Doppler shift in one of the hydrogen lines in the spectrum of Sirius and found that this star is moving toward the solar system. Today, radial velocity can be measured for any star bright enough for its spectrum to be observed. As we will see in The Stars: A Celestial Census , radial velocity measurements of double stars are crucial in deriving stellar masses.

There is another type of motion stars can have that cannot be detected with stellar spectra. Unlike radial motion, which is along our line of sight i. We see it as a change in the relative positions of the stars on the celestial sphere Figure 3. These changes are very slow.

Even the star with the largest proper motion takes years to change its position in the sky by an amount equal to the width of the full Moon, and the motions of other stars are smaller yet. Figure 3: Large Proper Motion. For this reason, with our naked eyes, we do not notice any change in the positions of the bright stars during the course of a human lifetime.

If we could live long enough, however, the changes would become obvious. White light what we call visible or optical light can be split up into its constituent colors easily and with a familiar result: the rainbow.

All we have to do is use a slit to focus a narrow beam of the light at a prism. This setup is actually a basic spectrometer. The resultant rainbow is really a continuous spectrum that shows us the different energies of light from red to blue present in visible light.

But the electromagnetic spectrum encompasses more than just optical light. It covers all energies of light, extending from low-energy radio waves, to microwaves, to infrared, to optical light, to ultraviolet, to very high-energy X-rays and gamma rays. Three types of spectra: continuous, emission line and absorption. Each element in the periodic table can appear in gaseous form and will produce a series of bright lines unique to that element.

Hydrogen will not look like helium which will not look like carbon which will not look like iron Thus, astronomers can identify what kinds of stuff are in stars from the lines they find in the star's spectrum.

This type of study is called spectroscopy. This is called a red-shift , because the lines are moving toward the red end of the spectrum. The velocity away from Earth can then be calculated from the amount of red-shift.

When the star or galaxy is moving toward the observer, the spectral lines shift toward the blue end. This is called a blue-shift.

Again, the velocity can be calculated from the amount of shift. A spectrometer uses a prism or diffraction rating to spread out a incoming beam of light into its spectrum of different colors or wavelengths. The tool allows astronomers to determine the chemical composition of planets and stars, as well as to indicate the speed and direction of a star or galaxy.

Instead of gratings and prisms, various combinations of filters and detectors are used to measure portions of the X-ray and gamma-ray spectra. At the other extreme - that is, very long wavelengths - radio spectra of stars and other radio sources are measured by "tuning" a radio telescope to different frequencies. A radio telescope - the largest is more than m 1, ft across - is like a giant optical reflector with a radio amplifier at the focus.

Radio spectra are much more accurate than optical spectra. Multiple radio telescopes, placed thousands of kilometers apart, can determine the position of a radio-emitting star as accurately as an optical telescope can, to better than 0.

Create a List. List Name Save. Rename this List. Rename this list. List Name Delete from selected List.