A tour of the milky way

In the latter part of the eighteenth century, the astronomer William Herschel made systematic observations to attack the problem of the Galaxy's structure. Herschel had constructed telescopes that dwarfed other instruments of the day.

With these powerful tools he set out to establish the extent of the Galaxy by counting stars in selected regions of the sky. Herschel thought that directions in which he found large numbers of faint stars were places in which the Galaxy extended the farthest. One of William Herschel's large telescopes, used by him to inter the shape of the Galaxy.

This procedure of star counting required a number of assumptions to make it workable. One assumption can be stated as: brightness means nearness (or faintness means farness). In other words, you assume that all stars are essentially the same and have the same luminosities.

If you have looked even casually at the night sky, you have seen that stars do not have the same apparent brightness (flux). If you assume, as Herschel did, that all stars have the same luminosity, then the differences in flux will reflect differences in distances from the earth. The brightest stars will be the closest, the faintest ones the most distant. You sometimes make this kind of distance judgment too. If you view a city at night from a high vantage point, you can estimate distances by the brightness of visible lights. You judge that the brightest lights are the closest ones. (Note that your estimates are inaccurate if smog enshrouds a city. You then misjudge that lights dimmed by smog are farther away than they actually are.)

With this and other assumptions, Herschel devised a program of counting stars in selected regions of the sky. From the number counted at each flux level, he estimated the number at each distance in various directions. He thus mapped out the distribution of stars and the distance to the edge of the Galaxy in each direction.

He could do this in relative terms: how far the stars were from the earth compared with the distance of a standard star. Herschel chose Sirius as his standard. He could calculate a star's distance compared with that of Sirius using the inverse-square law. Because he did not know the distance to Sirius in kilometers, he could not establish the scale of the Galaxy. He could-and did-estimate its relative proportions as far out as he could count stars. (Fig. 23.9)