In our previous stop on our tour of celestial objects of cosmological importance, we looked at a handful of galaxies measured by the early 20th-century astronomer Vesto Slipher. The former Indiana farm boy wrestled with a modestly endowed telescope and a 450-pound spectrometer to make an astonishing discovery. He found the ‘spiral nebulae’ like Andromeda (M31) and the Sombrero (M104) were moving away from us at astonishing speeds, up to 1000 km/s and far faster than any nearby stars. The speeds of these spiral assemblies strongly suggested they lay outside our own group of stars, and were perhaps separate galaxies in their own right far outside our own.
But in science, a strong suggestion is not proof.
In the first years of the 20th century, astronomers had no way of knowing for sure the distance to these spiral assemblies. Indeed, a hundred years ago, they only could estimate the distances to a handful of nearby stars. The true scale of even our own galaxy was a complete mystery. No one knew whether the Milky Way was all there was to the universe, and whether it was a hundred light years across, or a thousand, or a trillion. Never mind the distances to the mysterious ‘spiral nebulae’, which may simply have been nearby star systems in the process of formation.
The key to the distance to the spiral nebulae, which we now know to be separate galaxies, and to the universe itself, lay unexpectedly in a class of unassuming stars, many of which you can see from your backyard with a pair of binoculars or without any optics at all.
These stars are called Cepheid variables, named after the relatively bright star δ (delta) Cephei in the constellation Cepheus, which is one of the first of these types of stars discovered. Cepheids vary in brightness noticeably over the course of a few days to a week. Hundreds are now known in our galaxy, and you can see a few of the brighter Cepheid variables yourself. Bright stars such as η (eta) Aquilae and Polaris, the North Star, are Cepheid variables. These stars, along with δ Cephei, are easy targets for even the most casual stargazer. If you’re keen, you can track for yourself the change in brightness of some of these stars. Eta Aquilae, for example, in the wings of the constellation Aquila, the Eagle, varies from magnitude 3.5 to 4.3 over a period of 7.18 days.
In 1912 an obscure and underpaid astronomer named Henrietta Leavitt discovered dozens of Cepheids in the Large Magellanic Cloud (LMC), a large irregular congregation of stars visible from the deep southern hemisphere. While examining photographic plates of the LMC, she discovered an amazing property of Cepheid variables. She noticed the brighter Cepheids had longer periods of variability. Since these stars were all at roughly the same (unknown) distance, she concluded their true brightness was related to their period of variability. Which meant, if she could figure out the true distance to some nearby Cepheids, and assuming Cepheids everywhere had the same properties and behavior, she could calibrate these stars to be a “standard candle” to measure distances. Astronomers would simply have to measure the period of variability of a Cepheid variable star to find its true brightness. And by measuring its apparent brightness– an easy measurement– they could find out the distance to the star.
Leavitt made an enormous and ultimately successful effort to make this calibration using photographic measurements and other data, and once she did so, astronomers could use the pulsation period of Cepheid variables to estimate their distance and the distance to any star clusters or galaxies to which these stars belonged.
This was a revolutionary discovery. Edwin Hubble used Leavitt’s work to detect and measure a few of these variable stars in the Andromeda “nebula” and determined it was not a nebula at all, but a galaxy in its own right lying more than 2 million light years away, some 20 times the span of our Milky Way. This discovery exploded the size of the known universe, and was one of the most stunning and famous scientific discoveries in history. The image at the top of the page shows a modern image of the first Cepheid variable star detected by Hubble.
Hubble rightly became famous, of course, and remains so to this day. For her effort, Henrietta Leavitt was paid just $10.50 a week. She died in obscurity in 1921 at the age of 53, nearly forgotten, one of dozens of women employed as human “calculators” in the late 19th and early 20th centuries to help astronomers make many key discoveries. To his credit, Hubble later said Leavitt deserved the Nobel Prize for her work.
There are more than 700 known Cepheid variables in our galaxy, and thousands more visible in big telescopes in galaxies out to a distance of 100 million light years.
By the mid 20th century, as astronomers learned more about how stars work, they learned that Cepheids were a type of pulsating variable star. The basic mechanism of pulsation was first proposed by Arthur Eddington nearly a century ago, well before astronomers understood how stars worked. In the outer layers of these stars, suggested Eddington, ionized helium atoms act as a valve. When the star is faintest and the outer layers are compressed and hot, helium atoms in the star’s outer atmosphere become doubly ionized, that is, they lose both electrons, and they become more opaque to the light inside the star. This causes heat from inside the star to build up and eventually push out these outer layers, causing them to cool slightly. This allows the helium atoms to pick up an electron and grow more transparent. When the star’s light passes through the transparent outer layer, the layers cool, collapse, and become opaque again, and the process repeats itself. It does this for millions of years until the star’s innards evolve and blow off the outer atmosphere for good. So Cepheid variables are relatively short-lived phases of a large star’s overall lifespan.
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