Astronomers use light echoes to measure the distance to a star

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Munich, Feb 10 : A team of astronomers have used an ESO (European Southern Observatory) telescope to measure the distance to a star, using the help of light echoes.

The star, known as a Cepheid, belongs to a class of variable stars that constitute one of the first steps in the distance scale of the Universe.

"Our measurements with ESO's New Technology Telescope at La Silla allow us to obtain the most accurate distance to a Cepheid," said Pierre Kervella, lead-author of the paper reporting the result.

Cepheids are actually pulsating stars that have been used as distance indicators since almost a hundred years.

For the research, the team of astronomers studied RS Pup, a bright Cepheid star located towards the constellation of Puppis ('the Stern') and easily visible with binoculars.

RS Pup varies in brightness by almost a factor of five every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15 000 times more luminous. It is the only Cepheid to be embedded in a large nebula, which is made of very fine dust that reflects some of the light emitted by the star.

Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star.

According to Kervella, "The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope."

"As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time," he explained.

This delay is called a 'light echo'.

By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star. It is simply the measured delay in time, multiplied by the velocity of light (300,000 km/s).

Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup.

From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, plus or minus 90 light years.


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