Secondary method of measuring distance to galaxies stronger than previously thought

September 29th, 2008 - 2:00 pm ICT by ANI  

Canberra, September 29 (ANI): Australian and US scientists have found that a secondary method of measuring the distance to nearby galaxies is more robust than previously thought.

According to a report by ABC News, astrophysicists Professor Jeremy Mould from the University of Melbourne and Dr Shoko Sakai from the University of California, Los Angeles, tested the reliability of using stars from the tip of the red giant branch (TRGB) to calculate distance.

They found these stars to be just as good, if not better than Cepheids.

Astronomers measure distance in space using various methods depending on how far away the stars are.

For nearby distances, Cepheid variables - bright stars that expand and contract by 10 percent - have long been the primary standard candles used.

This is because the relationship between how long a Cepheid takes to shrink and grow, and its brightness is well known.

By comparing the actual observed brightness with its intrinsic brightness, astronomers can determine the stars distance from earth.

But, some astronomers have recently cast suspicion on the capacity of Cepheids as standard candles, according to Mould.

The question has been whether the relationship is more complicated than we think. Things like the composition of the star may make a difference, said Mould.

The researchers trawled through archival Hubble Space Telescope images, examining 14 galaxies that contain TRGBs.

Towards the end of their life, red giants produce a flash as they exhaust their outer hydrogen shell and begin to burn helium from their core.

The luminosity of the flash is well understood and therefore the stars make good standard candles.

But unlike the Cepheid variables, the red giant flash isnt influenced by the stars age or composition, the researchers said.

It looks like this alternative standard candle is immune to things like differences in the chemical composition of stars, said Mould. It confirms the Cepheid variable scale using a completely independent technique, he added.

According to Dr Scott Croom of the University of Sydney, the TRGB technique, which is a well understood idea, is potentially more robust that the Cepheid variable technique.

Its an independent method which confirms that weve done a pretty good job of getting the distance scale right, he said. (ANI)

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