22 November 2005

It is lousy weather again so there is no observing. But that doesn't mean no astronomy. Tonight I went to an astronomy lecture at Columbia University.

The lecture was held in the Astronomy and Astrophysics departmental library in Pupin Hall of the university's Morningside campus on the Upper West Side. The library is more like a morgue for old journals. I only saw one shelf with copies of the notices from the 2000s. It is on the top floor of the building under a hip roof section. It is lighted by hanging fixtures with white globes of varying sizes that are attached to the ceiling at irregular locations and makes it look like a star field with different brightness of stars.

Before the talk there was a family astronomy event for the kids. It was run by the undergraduate students from one of the labs. It was all female students. Two other astronomy lab classes arrived for the lecture and they were all female students as well.

The talk tonight was by George Johnson to hype his recently released book Miss Leavitt's Stars.

Henrietta Leavitt wasn't an astronomer but was hired by Harvard University back in the early 1900s as a minimum wage clerical worker assigned to visually scan photographic plates and catalog the stars on them. Her job title was "computer." For this she was paid the grand sum of $0.25/hour, $0.10 more than a cotton mill worker of the time, or about $5.00/hour in 2005 dollars. During the course of her employment she discovered the periodicity-luminosity relationship of Cepheid variables that became one of the "standard candles" used in computing the size of the universe.

Cepheid variables are stars that expand and contract at a very regular rate. Along with this change in physical size there is a change in brightness. The change in brightness, rather than the star's actual size, is registered on photographic media by a change in the size of the dot recorded: brighter star = bigger dot. Knowing the exposure factor for the individual plate you can mathematically calculate the brightness of the stars on it based on their size.

At the time Harvard wasn't particularly interested in doing science, but was just trying to catalog all the stars in both hemispheres. Southern sky pictures were made on glass plates at their telescope in the mountains of South America and taken by burro to a ship where they were shipped to Boston. Leavitt was assigned to look for Cepheid variables in the Magellanic Clouds. They expected that she might fine a dozen or so. She found 1,777.

To complete her task two plates showing the same part of the sky, but taken at different times, were compared. One plate was kept as the original negative and the other was duplicated as a positive. When they were stacked on top of each other the stars would cancel each other out and disappear if they were the same size. If the star's brightness had changed a gray circle would show around the star. The star would be logged by hand in a ledger book, and the ledger book would be given to the men scientists for interpretation.

Leavitt noticed that brighter stars always pulsed at a slower rate than dimmer stars and realized that this information could be used to calculate the distance of objects in the universe.

• All stars in the Magellanic Cloud are the same distance away.
• The brighter the star the slower the blink rate.
• Given the location of the star and its blink rate you can calculate its brightness
  or
  given the location and its brightness you could predict its blink rate.
• If you know how far away a star is and its blink rate and the blink rate of a similar star in another location you can tell how bright that star is and deduce its distance.

In the time honored tradition, Leavitt had her observations published by the university in a paper authored by its director. The relationship she proposed went on, after her death, to be used by Edwin Hubbell in calculating the size of the universe and is at the root of all methods for determining astronomical distances.

A similar talk Johnson aired yesterday over WNYC radio can be heard here.

There had been talk early on that the roof/dome might be open after the lecture for stargazing but that had disappeared from the second announcement email and it is just as well given the weather. It was mostly cloudy and very windy. The current temperature is 38°F but the wind chill temperature is only 29°F.