Measuring colors is only one way of analyzing starlight. Another way is to use a spectrograph to spread out the light into a spectrum see the Radiation and Spectra and the Astronomical Instruments chapters. In , the German physicist Joseph Fraunhofer observed that the spectrum of the Sun shows dark lines crossing a continuous band of colors.
In the s, English astronomers Sir William Huggins and Lady Margaret Huggins Figure 1 succeeded in identifying some of the lines in stellar spectra as those of known elements on Earth, showing that the same chemical elements found in the Sun and planets exist in the stars.
Since then, astronomers have worked hard to perfect experimental techniques for obtaining and measuring spectra, and they have developed a theoretical understanding of what can be learned from spectra.
Today, spectroscopic analysis is one of the cornerstones of astronomical research. Figure 1: William Huggins — and Margaret Huggins — William and Margaret Huggins were the first to identify the lines in the spectrum of a star other than the Sun; they also took the first spectrogram, or photograph of a stellar spectrum. When the spectra of different stars were first observed, astronomers found that they were not all identical. Since the dark lines are produced by the chemical elements present in the stars, astronomers first thought that the spectra differ from one another because stars are not all made of the same chemical elements.
This hypothesis turned out to be wrong. The primary reason that stellar spectra look different is because the stars have different temperatures. Most stars have nearly the same composition as the Sun, with only a few exceptions.
Hydrogen, for example, is by far the most abundant element in most stars. However, lines of hydrogen are not seen in the spectra of the hottest and the coolest stars. In the atmospheres of the hottest stars, hydrogen atoms are completely ionized. Because the electron and the proton are separated, ionized hydrogen cannot produce absorption lines.
Recall from the Formation of Spectral Lines section, the lines are the result of electrons in orbit around a nucleus changing energy levels. In the atmospheres of the coolest stars, hydrogen atoms have their electrons attached and can switch energy levels to produce lines. However, practically all of the hydrogen atoms are in the lowest energy state unexcited in these stars and thus can absorb only those photons able to lift an electron from that first energy level to a higher level.
Photons with enough energy to do this lie in the ultraviolet part of the electromagnetic spectrum, and there are very few ultraviolet photons in the radiation from a cool star. 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.
This image compares the spectra of the different spectral classes. The spectral class assigned to each of these stellar spectra is listed at the left of the picture. The strongest four lines seen at spectral type A1 one in the red, one in the blue-green, and two in the blue are Balmer lines of hydrogen. Note how these lines weaken at both higher and lower temperatures, as Figure 2 also indicates.
The strong pair of closely spaced lines in the yellow in the cool stars is due to neutral sodium one of the neutral metals in Figure 2. Both colors and spectral classes can be used to estimate the temperature of a star. Spectra are harder to measure because the light has to be bright enough to be spread out into all colors of the rainbow, and detectors must be sensitive enough to respond to individual wavelengths.
In order to measure colors, the detectors need only respond to the many wavelengths that pass simultaneously through the colored filters that have been chosen—that is, to all the blue light or all the yellow-green light. Annie Jump Cannon was born in Delaware in In , she went to Wellesley College, one of the new breed of US colleges opening up to educate young women.
Wellesley, only 5 years old at the time, had the second student physics lab in the country and provided excellent training in basic science. After college, Cannon spent a decade with her parents but was very dissatisfied, longing to do scientific work. On average 1 in 30 stars are F-type stars. Procyon , the brightest star in Canis Major is of spectral type F5. G -type stars have surface temperatures between 5, and 6, K. On average 1 in 12 stars are G-type stars.
Our Sun is of spectral type G2. K -type stars have surface temperatures between 3, and 5, K. On average 1 in 8 stars are K-type stars. Pollux , the lower of the two bright stars in Gemini is of spectral type K0. M -type stars have surface temperatures between 2, and 3, K.
They are the reddest and coolest of the common stellar spectral types. On average 3 in every 4 stars are M-type stars. Betelgeuse , the red star in Orion is of spectral type M1. Find out more about the technique of spectroscopy.
Read more about Wien's Law and how it relates to the electromagnetic spectrum. You are here Home » Spectral Types. Latest Gallery Images.
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