Close-up on pulsating star

The Palomar Testbed Interferometer in California

By BBC News Online science editor Dr David Whitehouse

Astronomers have directly measured the change in size of a particular class of pulsating star for the very first time.

The star, a so-called Cepheid variable, which lies in the constellation of Gemini, expands and contracts every 10 days but, like most stars, is far too small to be seen in conventional telescopes.

To make their observation, astronomers combined the light received from two small telescopes situated 110 metres (359 feet) apart to mimic the performance of a very much larger device - a technique called interferometry.

The achievement of Dr Shri Kulkarni and colleagues, at the Palomar Observatory of the California Institute of Technology, Pasadena, will help scientists calibrate the scale of the Universe and allow them to measure the distances to galaxies more accurately.

Expanding Universe

The star, Zeta Geminorum, is situated just over a thousand light-years away. It is one of the brightest stars in a class of stellar objects known as the Cepheid variables, which includes Polaris.

A Cepheid variable star observed by the Hubble Space Telescope...

The importance of Cepheids was first recognised in 1912 by the remarkable American astronomer Henrietta Leavitt, who saw them in a companion galaxy to ours. She noticed that their period of pulsation was related to their true brightness.

This meant that by observing their period of pulsation their true brightness could be calculated and compared to their apparent brightness. The difference between the two, with some corrections, yielded their distance.

It was Edwin Hubble who discovered Cepheids in the nearby Andromeda galaxy in the 1920s. He used the stars to measure the galaxy's distance and in so doing found out that Andromeda was receding from us - the Universe was expanding.

And very recently, the Hubble Space Telescope used Cepheids to refine Hubble's work and produce the most accurate estimate yet for the age of the Universe.

Old and new

But the ability to detect the actual changes in the diameter of one of these pulsating stars has been hampered by the limitations of conventional telescopes.

...and Cepheids observed by Edwin Hubble in 1927.

Now, using the Palomar Testbed Interferometer, Dr Shri Kulkarni's team have watched the disc of Zeta Gem enlarging and contracting over a 10-day period.

It is a remarkable observational feat and the forerunner of more detailed observations to come by this and other optical interferometers. The apparent size of a star on the sky is about the same as the apparent size of an astronaut on the Moon as seen from the Earth.

The new data enables the star's distance to be calculated independently of other methods, allowing our understanding of Cepheids to be checked. The agreement between new and old estimated distances is pleasing to astronomers.

Henrietta Leavitt died in 1921. She would no doubt have been amazed that her successors have seen in close-up the stars that for her were just tiny points of light, barely visible in photographs of a nearby galactic companion.

The Zeta Gem research is reported in the journal Nature.