Finding Our Place in the Universe by Hélène Courtois

Author:Hélène Courtois
Language: eng
Format: epub
Tags: Cosmography; Astronomy; Milly Way; Laniakea
Publisher: The MIT Press

Figure 3.4    Broadening of the hydrogen emission line from a spiral galaxy. (Also plate 5.)

To get a rough idea of the process at work, one can make a “spiral galaxy” at home with a small plastic pinwheel. Looking from the side as it spins, we can see certain blades (say, those on the right) moving away while those on the opposite side (the left, in this case) draw closer. In the same way, when it’s possible to observe a spiral galaxy from the side—and not from the front—the hydrogen emission line appears larger because of the Doppler effect: at one edge of the galaxy, the hydrogen clouds are moving away from us (toward longer wavelengths), and at the other edge, the clouds are moving closer (toward shorter wavelengths). The rotation of the galaxy on itself gives the line an easily identifiable and measurable M shape.

This broadening of the spectral line depends directly on the galaxy’s rotational velocity. But just as planets orbit the sun because of its gravitational attraction, the bigger a spiral galaxy is, the faster its stars and gases revolve around its center. Accordingly, when we know the rotational velocity of a galaxy, we can estimate its mass, which allows us to deduce its total absolute luminosity, a value that stands around 1037 watts. By comparing this value to the apparent brightness measured with optical telescopes, we can then determine the galaxy’s distance. The Tully-Fisher method is illustrated in figure 3.5. In 1977, these two North American astronomers worked out the relation using just a dozen spiral galaxies. At the time, the distances were calculated with about a 30% margin of error. Since then, much larger samples have been tested, and the degree of accuracy is constantly improving. In the best cases, we can now measure the distance of spiral galaxies with an 8% margin of error.

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