A cosmic ruler
Ken Tapping, July 16th, 2014
When you hear that the great spiral galaxy in the constellation of Andromeda lies at a distance of 2.5 million light years, have you ever asked yourself how we managed to find that out? Actually we have Henrietta Leavitt to thank for that. She was an astronomer at Harvard observatory in the late 19th and Early 20th Century.
When she graduated from Radcliff College in 1893 she was taken on by astronomer Edward Pickering at the Harvard Observatory as a "computer". In those days there were no digital devices to assist with data analysis and number crunching; the work was done by people, usually women. This work required meticulous attention. Leavitt was given the work of analyzing photographic plates, the glass sheets on which images were recorded. Her particular task was extracting data about variable stars – stars that vary in brightness over time.
A star's apparent brightness might vary in the sky for two reasons: either the star itself is changing brightness or something is moving in the way, like another star orbiting it. These days we can detect the minute changes in brightness when one of that star's planets pass between it and us. Sometimes an aging star will spit off a shell of material, which will partially block the star's light. The other variability arises through instability in the star itself.
Stars are balls of hot gas and magnetic fields with a nuclear fusion reactor in the middle. In most cases the complex combination of processes taking place inside it are self-regulating, so the majority of stars live their lives without oscillating in brightness or developing other instabilities. Fortunately for us the Sun is like that. However, some stars, particularly giant stars, might pulsate in size and brightness by enough for observers on Earth to notice this and to measure it.
Leavitt's work consisted of identifying thousands of pulsating variable stars in the Magellanic Clouds. These are two small galaxies visible from the Southern Hemisphere. They look like bits of the Milky Way that have wandered off. She carefully estimated the brightness of these stars and how long they took between consecutive pulsations. What Leavitt did next revolutionized astronomy.
She assumed that these galaxies were far enough away for her to treat all the variable stars in those galaxies as being more or less the same distance from us, rather like the passengers in a car 10 km away. Then she plotted them on a graph of average apparent brightness against the period of the pulsations, and found they were related. That means we can observe one of these variable stars in a distant star cluster or galaxy measure how bright it looks and from the period of pulsation estimate how bright the star is – its luminosity. It is a simple calculation to find out how far away the star is. This is of critical importance to us. Before that we only had triangulation, a standard surveying technique. That method only works for stars in our neighbourhood.
These days, if we can spot one or more of these stars in a distant object, Leavitt has given us a tape measure. These particular pulsating stars are called cepheids, because they are all similar to the star Delta Cephei, a faint star in the constellation of Cepheus. This constellation lies high in our northern skies and Delta Cephei is a favourite of amateur astronomers studying variable stars. Leavitt was considered for the Nobel Prize, but died of cancer before she could receive the award. However she lived to see how big a contribution she gave us…a ruler to measure the cosmos.
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