Read Online For the Love of Physics by Walter Lewin - Free Book Online

relationship for all Cepheids (even those outside the SMC). I won’t elaborate here on this method, as it gets quite technical; the important thing to appreciate is that working out the luminosity-period relation was a milestone in measurements of distances. Once you know a star’s luminosity and its brightness, you can calculate its distance.
    The range in luminosity, by the way, is substantial. A Cepheid with a period of three days has about a thousand times the Sun’s luminosity. When its period is thirty days, its luminosity is about thirteen thousand times greater than the Sun’s.
    In 1923, the great astronomer Edwin Hubble found Cepheids in the Andromeda Galaxy (also known as M31), from which he calculated its distance at about 1 million light-years, a genuinely shocking result to many astronomers. Many, including Shapley, had argued that our own Milky Way contained the entire universe, including M31, and Hubble demonstrated that in fact it was almost unimaginably distant from us. But wait—if you google the distance to the Andromeda Galaxy, you’ll find that it’s 2.5 million light-years.
    This was a case of unknown unknowns. For all his genius, Hubble had made a systematic error. He had based his calculations on the known luminosity of what later came to be known as Type II Cepheids, when in fact he was observing a kind of Cepheid variable about four times more luminous than what he thought he was seeing (these were later named Type I Cepheids). Astronomers only discovered the difference in the 1950s, and overnight they realized that their distance measurements for the previous thirty years were off by a factor of two—a large systematic error that doubled the size of the known universe.
    In 2004, still using the Cepheid variable method, astronomers measured the distance to the Andromeda Galaxy at 2.51 ± 0.13 million lightyears.In 2005 another group measured it by using the eclipsing binary stars method, to get a result of 2.52 ± 0.14 million light-years, about 15 million trillion miles. These two measurements are in excellent agreement with each other. Yet the uncertainty is about 140,000 light-years (about 8 × 10 17 miles). And this galaxy is by astronomical standards our next-door neighbor. Imagine the uncertainty we have about the distances of so many other galaxies.
    You can see why astronomers are always on the hunt for what are called standard candles—objects with known luminosities. They allow us to estimate distances using a range of ingenious ways of establishing reliable tape measures to the cosmos. And they have been vital in establishing what we call the cosmic distance ladder.
    We use parallax to measure distances on the first rung on that ladder. Thanks to Hipparcos’s fantastically accurate parallax measurements, we can measure the distances of objects up to several thousand light-years with great precision this way. We take the next step with Cepheids, which allow us to obtain good estimates of the distances of objects up to a hundred million light-years away. For the next rungs astronomers use a number of exotic and complicated methods too technical to go into here, many of which depend on standard candles.
    The distance measurements become more and more tricky the farther out we want to measure. This is partly due to the remarkable discovery in 1925 by Edwin Hubble that all galaxies in the universe are moving away from one another. Hubble’s discovery, one of the most shocking and significant in all of astronomy, perhaps in all of science in the past century, may only be rivaled by Darwin’s discovery of evolution through natural selection.
    Hubble saw that the light emitted by galaxies showed a distinct shift toward the less energetic end of the spectrum, the “red” end where wavelengths are longer. This is called redshift. The larger the redshift, the faster a galaxy is moving away from us. We know this effect on Earth with sound as the Doppler effect; it explains why we can tell