1/17/2024 0 Comments Hydrogen absorption spectrum![]() ![]() If the cloud is too hot, the electrons in hydrogen have absorbed so much energy that they can break free from the atom. The strength of the Balmer lines (that is, how much absorption they cause) depends on the temperature of the cloud. The absorption lines from hydrogen observed in the visible part of the spectrum are called the Balmer series, and they arise when the electron in a hydrogen atom jumps from level 2 to level 3, level 2 to level 4, level 2 to level 5, and so on. Recall from Lesson 3 that the electrons in a gas are the cause of absorption lines-all the photons with the correct amount of energy to cause an electron to jump from one energy level to a higher energy level get absorbed as they pass through the gas. That is, spectral type A had the strongest lines, B slightly weaker than A, C slightly weaker than B, and so on.įor more information on her life and work, visit the homepage for Annie Jump Cannon at Wellesley College. ![]() The original classification scheme used the strength of the lines of hydrogen to order the spectral types. However, some classes were eventually merged with others, and not all letters were used. Originally, she started out using the letters of the alphabet to designate different classes of stars (A, B, C…). In the early 1900s, an astronomer named Annie Jump Cannon took photographic spectra of hundreds of thousands of stars and began to classify them based on their spectral lines. The absorption lines visible in the spectra of different stars are different, and we can classify stars into different groups based on the appearance of their spectral lines. Recall from Lesson 3 that the spectrum of a star is not a true blackbody spectrum because of the presence of absorption lines. Once again, we know from the colors of these stars that the blue star is hotter than the yellow star, because its apparent color indicates that the peak of its emission is in the blue, while the other star’s peak is in the yellow part of the spectrum.Īt Hubblesite, they have an extended tutorial on the " Meaning of Color in Hubble Images." which includes a discussion of the filters used by astronomers to determine the color of astronomical objects. They adopted the double star system Albireo as the “Cal Star,” because the two stars (one blue and one yellow) match the school’s colors. These red stars have the coolest temperatures among the stars in the cluster.Īnother good example is this color image of Albireo taken by students at the University of California, Berkeley. You can tell that many of the stars are similar in color however some stand out as being much redder than the others. Astronomers took images through different colored filters (in this case, near-infrared, I, visual, V, and ultraviolet, U), and added the three images together to produce a close approximation of the colors we would see of these stars with our own eyes. In addition, look at this image: Hubble Space Telescope image of star cluster 47 Tucanae. Note how for the three different objects with three different temperatures, that for the coolest object: its B intensity is smaller than its V intensity, for the warmer object: they are roughly the same, and for the hottest object: its B intensity is larger than its V intensity.Ĭredit: PhET and the Penn State Astronomy & Astrophysics When you click the play button, you see an animated curve representing blackbodies of different temperatures, and it marks the B and V measurements through these two filters for the different blackbodies. Intensity, There is a yellow band showing the frequency range that corresponds to the V filter, and a blue band that illustrates the frequency range for the B filter. The animation below shows a plot of Frequency vs. The difference between these two, B-V, is the star’s color. The B filter measures the star’s brightness in blue light, and the V filter measures the star’s brightness in yellow light. They then re-observe the same star with a visual (V), or yellow, filter. To do this, they put a blue filter (B) on the telescope and observe the star. Measuring a star’s spectrum is not always easy, but astronomers can often measure a star’s color reasonably easily. For example, a 4500 K blackbody peaks in the red part of the spectrum, a 6000 K blackbody in the green part of the spectrum, and a 7500 K blackbody in the blue part of the spectrum. If you study this plot, or one of the interactive blackbody radiation demonstrators we used in the last lesson, you can prove to yourself that the color of a star provides a fairly accurate measurement of its surface temperature.
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