Pulsating star mystery solved

Artist's impression of OGLE-LMC-CEP0227

Artist's impression of the binary star OGLE-LMC-CEP0227. The smaller of the two stars is a pulsating Cepheid variable and the orientation of the system is such that the stars eclipse each other during their orbits.

A lucky alignment of orbits has enabled astronomers measure the mass of a specific kind of star, and in the process solve a decades-old mystery.

The star in question is of a type known as a ‘Cepheid variable’, or just Cepheid. Cepheids are unstable stars that are larger and much brighter than the Sun. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle.

The time taken to brighten and fade again is longer for stars that are more luminous and shorter for the dimmer ones. This is known as the period-luminosity relation.

Astronomers can use this to their advantage when estimating the distances to galaxies. By timing the rise and fall in light of a Cepheid in a distant galaxy, they can work out how intrinsically bright the star must be.

By comparing that inherent brightness with the actual brightness measured, they can work out how far away the star must be.

The Cepheid period-luminosity relation, discovered by Henrietta Leavitt in 1908, was used by Edwin Hubble to make the first estimates of the distance to what we now know to be galaxies.

More recently Cepheids have been studied with the Hubble Space Telescope and with ground-based telescopes to make highly accurate distance estimates to many nearby galaxies.

The period-luminosity relation makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe.

Large Magellanic Cloud

The Large Magellanic Cloud galaxy, in which the Cepheid star was discovered. Further Cepheid discoveries in this galaxy could help astronomers pin down its distance to better than 1% precision.

Competing theories

Unfortunately, despite their importance, Cepheids are not fully understood. In particular, predictions of their masses derived from the theory of pulsating stars are 20% less than predictions from the theory of the evolution of stars. This embarrassing discrepancy has been known since the 1960s.

To resolve this mystery, astronomers needed to find a binary star containing a Cepheid where the orbit happened to be seen edge-on from Earth. In these cases, known as eclipsing binaries, the brightness of the two stars dims as one passes in front of the other, and again when it passes behind the other star.

In pairs such as these, astronomers can determine the masses of the stars to high accuracy using well-known physical laws.

Unfortunately, neither Cepheids nor eclipsing binaries are common, so the chance of finding such an unusual pair seemed very low. None are known in the Milky Way.

“Very recently we actually found the double star system we had hoped for among the stars of the Large Magellanic Cloud,” says Wolfgang Gieren, a member of the team. The Large Magellanic Cloud is a small galaxy that orbits our Milky Way galaxy at a distance of around 160,000 light-years.

The system “contains a Cepheid variable star pulsating every 3.8 days,” adds Gieren. “The other star is slightly bigger and cooler, and the two stars orbit each other in 310 days.”

The observers carefully measured the brightness variations of this rare object, known as OGLE-LMC-CEP0227, as the two stars orbited and passed in front of one another. They also used spectrographs to measure the motions of the stars towards and away from the Earth—both the orbital motion of both stars and the in-and-out motion of the surface of the Cepheid as it swelled and contracted.

This very complete and detailed data allowed the observers to determine the orbital motion, sizes and masses of the two stars with very high accuracy—far surpassing what had been done before for a Cepheid.

The mass of the Cepheid is now known to about 1% and agrees exactly with predictions from the theory of stellar pulsation. The larger mass predicted by stellar evolution theory was shown to be significantly in error.

The team hopes to find other examples of these remarkably useful pairs of stars to exploit the method further. They think that from such binary systems they will eventually be able to pin down the distance to the Large Magellanic Cloud to 1%, which would mean an extremely important improvement of the cosmic distance scale.

Adapted from information issued by ESO / L. Calçada.

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