‘Standard candles’ Cepheids not quite as standard as once thought

WASHINGTON - Astronomers have now gathered proof that Cepheids, which were thought of “standard candles” used to illuminate the size of the universe, are not quite as standard.

Standard candles are astronomical objects that make up the rungs of the so-called cosmic distance ladder, a tool for measuring the distances to farther and farther galaxies.

The first rung consists of pulsating stars called Cepheid variables, or Cepheids for short. Measurements of the distances to these stars from Earth are critical in making precise measurements of even more distant objects.

Now, NASA’s Spitzer Space Telescope has given the first direct evidence that these stars can lose mass-or essentially shrink.

“We have shown that these particular standard candles are slowly consumed by their wind,” said Massimo Marengo of Iowa State University, Ames, Iowa.

“When using Cepheids as standard candles, we must be extra careful because, much like actual candles, they are consumed as they burn.”

The star in the study is Delta Cephei, which is the namesake for the entire class of Cepheids and was discovered in 1784. Intermediate-mass stars can become Cepheids when they are middle-aged, pulsing with a regular beat that is related to how bright they are - a trait that allows astronomers to take the pulse of a Cepheid and figure out how bright it is.

One question has been whether or not they lose mass. Winds from a Cepheid star could blow off significant amounts of gas and dust, forming a dusty cocoon around the star that would affect how bright it appears. This, in turn, would affect calculations of its distance.

Marengo and his colleague used Spitzer’s infrared vision to study the dust around Delta Cephei. A nearby companion star happens to be lighting the area, making the bow shock easier to see.

By studying the size and structure of the shock, the team was able to show that a strong, massive wind from the star is pushing against the interstellar gas and dust.

Follow-up observations of other Cepheids conducted by the same team using Spitzer have shown that other Cepheids, up to 25 percent observed, are also losing mass.

“Everything crumbles in cosmology studies if you don’t start up with the most precise measurements of Cepheids possible,” said Pauline Barmby of the University of Western Ontario.

“This discovery will allow us to better understand these stars, and use them as ever more precise distance indicators.”

The study appeared online Jan. 6 in the Astronomical Journal. (ANI)