LECTURE

distance to your nger using basic trigonometry. The same principle applies to stars, for as the Earth orbits the Sun, the closest stars should appear to move against the background stars. The Greeks understood these principles, but even the nearest stars are so remote that detecting their movements requires very delicate observation. Not until 1838 were the rst accurate measurements made.

In the rst decade of the 20th century, American astronomer Henrietta Leavitt (1868–1921) found that variations in the light from stars called Cepheid variables could be used to calculate their true brightness. Comparing this with their apparent brightness on Earth made it possible to estimate their true distance even if they were well beyond the range of parallax measurements. In 1924, Hubble showed that at least some Cepheids existed outside our galaxy, the Milky Way—proving for the rst time that the Universe consisted of many galaxies, not just one.

Second, astronomers tried to determine the motions of the stars. In 1814, German glassmaker Joseph von Fraunhofer (1787–1826) invented the spectroscope, a device like a prism, which splits light into its component wavelengths. Fraunhofer identi ed dark “absorption lines” in the spectra of starlight. These correspond to particular elements in the stars themselves, because each element absorbs light energy at different frequencies.

In the late 19th century, Vesto Slipher, at the Lowell Observatory in Flagstaff, Arizona, showed that some stellar absorption lines were shifted away from their expected frequencies. Slipher interpreted these shifts as the results of a Doppler effect, an apparent change in wavelengths caused by the relative movements of the two bodies. (We experience the Doppler effect when the pitch of an ambulance siren appears to change as it passes us.) As Slipher showed, this meant that changes in absorption lines could tell us whether distant objects were moving toward us or away from us, and at what speed.

Using these ndings, Hubble showed that all remote objects are shifted to the red end of the spectrum, which meant they were moving away from us. Furthermore, the more remote they were, the greater was the red shift, or the rate at which they were moving away from the Earth.