Sunday, May 17, 2009

More on Hubble

Edwin’s Hubble’s famous discoveries about our universe were made possible by the remarkable work of Henrietta Leavitt. Overcoming her deafness and male chauvinism, Leavitt became the first famous American woman astronomer by solving one of astronomy’s greatest challenges.

After graduating from Radcliffe, Leavitt joined the Harvard Observatory in 1893 as a low-level technical assistant – women were not allowed to operate telescopes at that time. Her job was to count stars on photographic plates and measure their brightness.

Leavitt saw beyond her mundane task and discovered a special relationship for stars of a certain type — Cepheid variables. These are stars whose brightness waxes and wanes in a regular periodic fashion. Several types of stars vary in brightness, but Leavitt relaized that among Cepheid variables, the longer it took the star to go through its brightness cycle the brighter the star was at its peak. In fact, Leavitt demonstrated that from the length of a Cepheids’ cycle, its “period”, the star’s true, or “intrinsic”, brightness can be determined with great precision. Then by measuring how bright that star appears to us on Earth, its distance from Earth can be calculated. Leavitt discovered how Cepheids could become one of astronomy’s most precise “standard candles.”

As the notion of standard candles is of great importance in astronomy, it merits a bit more explanation. Imagine looking down a residential street at night and seeing a row of porch lights. If we knew that each house had a 100 watt blub on its porch, and we knew the distance to the nearest house, we could figure out the distance to every one of the other houses. We would measure how bright each porch light looks from our house and use the fact that the intensity of light drops with the square of the distance it travels – the light from a house twice as far away will appear one-quarter as bright. Astronomers can identify Cepheids, measure their periods, and, using Leavitt discovery, compute their distance.

This was huge! Measuring the distance to very remote objects has always been the most challenging task in astronomy. Thanks to Henrietta Leavitt, astronomers were finally able to measure vast distances – distances on a galactic scale and beyond.

With Leavitt’s discovery, Hubble forever changed our view of the universe. He recognized the importance of Leaviit's contribution and recommended that she be awarded the Nobel Prize in Physics. Unfortunately, Leavitt died in 1921, before the completion of Hubble’s work, and Nobel Prizes are not awarded posthumously.

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