Show me related lesson plans. A rainbow rises over a misty forest. Credit: U. Fish and Wildlife Service. A spectrum is simply a chart or a graph that shows the intensity of light being emitted over a range of energies.
Have you ever seen a spectrum before? Nature makes beautiful ones we call rainbows. Sunlight sent through raindrops is spread out to display its various colors the different colors are just the way our eyes perceive radiation with slightly different energies. Spectroscopy can be very useful in helping scientists understand how an object like a black hole, neutron star, or active galaxy produces light, how fast it is moving, and what elements it is composed of.
Spectra can be produced for any energy of light, from low-energy radio waves to very high-energy gamma rays. Each spectrum holds a wide variety of information. The precise origin of these 'Fraunhofer lines' as we call them today remained in doubt for many years, until Gustav Kirchhoff, in , announced that the same substance can either produce emission lines when a hot gas is emitting its own light or absorption lines when a light from a brighter, and usually hotter, source is shone through it.
With that discovery, scientists had the means to determine the chemical composition of stars through spectroscopy. Stars aren't the only objects for which we can identify chemical elements. Any spectrum from any object allows us to look for the signatures of elements. This includes nebula, supernova remnants and galaxies. Once we have identified specific elements in a spectrum, we can also look to see if the emission lines from those elements has been shifted from where we might expect to find them.
While we usually talk about emission spectra as though the wavelengths of the lines are fixed, that is only true when the source emitting the lines and the detector "seeing" the lines are not moving relative to one another. When they are moving relative to each other, the lines will appear shifted. The image below shows a typical SDSS spectrum with some labels to point out several features. Study the image; the text below it describes some of its features.
The spectrum of a star is composed mainly of thermal radiation that produces a continuous spectrum. The star emits light over the entire electromagnetic spectrum, from the gamma rays to radio waves. What this means is that if you observe the spectrum of a very hot or very cool star with a typical telescope on the surface of Earth, the most common element in that star, hydrogen, will show very weak spectral lines or none at all.
The hydrogen lines in the visible part of the spectrum called Balmer lines are strongest in stars with intermediate temperatures—not too hot and not too cold. Calculations show that the optimum temperature for producing visible hydrogen lines is about 10, K. At this temperature, an appreciable number of hydrogen atoms are excited to the second energy level. They can then absorb additional photons, rise to still-higher levels of excitation, and produce a dark absorption line.
Similarly, every other chemical element, in each of its possible stages of ionization, has a characteristic temperature at which it is most effective in producing absorption lines in any particular part of the spectrum. Astronomers use the patterns of lines observed in stellar spectra to sort stars into a spectral class.
There are seven standard spectral classes. Recently, astronomers have added three additional classes for even cooler objects—L, T, and Y. In the s, Williamina Fleming devised a system to classify stars based on the strength of hydrogen absorption lines. But we saw above that hydrogen lines alone are not a good indicator for classifying stars, since their lines disappear from the visible light spectrum when the stars get too hot or too cold. Instead of starting over, Cannon also rearranged the existing classes—in order of decreasing temperature—into the sequence we have learned: O, B, A, F, G, K, M.
As you can read in the feature on Annie Cannon: Classifier of the Stars in this chapter, she classified around , stars over her lifetime, classifying up to three stars per minute by looking at the stellar spectra. Each of these spectral classes, except possibly for the Y class which is still being defined, is further subdivided into 10 subclasses designated by the numbers 0 through 9.
A B0 star is the hottest type of B star; a B9 star is the coolest type of B star and is only slightly hotter than an A0 star. And just one more item of vocabulary: for historical reasons, astronomers call all the elements heavier than helium metals , even though most of them do not show metallic properties. If you are getting annoyed at the peculiar jargon that astronomers use, just bear in mind that every field of human activity tends to develop its own specialized vocabulary.
Just try reading a credit card or social media agreement form these days without training in law! It is these details that allowed Annie Cannon to identify the spectral types of stars as quickly as three per minute! As Figure 2 shows, in the hottest O stars those with temperatures over 28, K , only lines of ionized helium and highly ionized atoms of other elements are conspicuous. Hydrogen lines are strongest in A stars with atmospheric temperatures of about 10, K.
Ionized metals provide the most conspicuous lines in stars with temperatures from to K spectral type F. In the coolest M stars below K , absorption bands of titanium oxide and other molecules are very strong. By the way, the spectral class assigned to the Sun is G2. The sequence of spectral classes is summarized in Table 1. This graph shows the strengths of absorption lines of different chemical species atoms, ions, molecules as we move from hot left to cool right stars. The sequence of spectral types is also shown.
Suppose you have a spectrum in which the hydrogen lines are about half as strong as those seen in an A star.
Looking at the lines in our figure, you see that the star could be either a B star or a G star. But if the spectrum also contains helium lines, then it is a B star, whereas if it contains lines of ionized iron and other metals, it must be a G star.
If you look at Figure 3, you can see that you, too, could assign a spectral class to a star whose type was not already known. All you have to do is match the pattern of spectral lines to a standard star like the ones shown in the figure whose type has already been determined.
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