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Objects in the Telescope May Be Closer Than They Appear
12 January 2011 5:55 pm
SEATTLE, WASHINGTON—To measure distances accurately, you need a precise ruler. And to calibrate your ruler, you need a good standard. Astronomers have discovered that some of their standards and rulers are less trustworthy than they thought. In particular, the prototype of a class of stars used to determine cosmic distances turned up a few surprises, according to scientists presenting new observations here at the 217th meeting of the American Astronomical Society.
Delta Cephei, a star in the northern sky that's visible with the naked eye, gave its name to a class of stars now known as cepheids. These giant stars swell and shrink periodically, causing them to brighten and dim regularly. In the early 20th century, Harvard University astronomer Henrietta Leavitt discovered that the brighter the star, the slower its brightness varies. This Leavitt Law enables astronomers to use cepheids as a cosmic ruler or "standard candle": Just measure how fast they vary, find their average true luminosity from the Leavitt Law, compare this to the apparent brightness in the sky, and out pops the distance, since stars appear fainter when they're farther away.
Checking out cepheids in far-off galaxies made it possible to determine distances to these galaxies, too. But new observations by NASA's Spitzer Space Telescope now indicate that these distance estimates may be influenced by unexpected properties of cepheids. As a cosmic ruler, they are a bit less precise than had been thought, says astrophysicist Massimo Marengo of Iowa State University in Ames. In the future, he adds, "We can and should take this into account."
Using the sensitive infrared cameras of Spitzer, Marengo and his colleagues discovered a hitherto unknown nebula of gas and dust surrounding Delta Cephei, sporting a bow shock in the direction of the star's motion through the surrounding thin interstellar gas, similar to the bow wave in front of a ship moving through water. From the known speed of the star and the size of this bow shock, they could calculate that Delta Cephei must be losing mass at a prodigious rate of about 2000 tons per second—a million times faster than the sun. Spitzer found that about 25% of all other cepheids are also surrounded by an infrared nebula.
"I like these results a lot," says astronomer Scott Engle of Villanova University in Pennsylvania, who also studies unexpected behavior of cepheids. "There has been a long-standing problem with the masses of cepheids—they usually turn out to be less massive than theoretical models of stellar evolution had predicted. This discrepancy might be explained if all cepheids experience substantial mass loss."
But it's not all good news. Marengo explains that the nebulae may absorb light, making cepheids appear slightly dimmer than they really are. In principle, this means that astronomers should correct for this effect before applying the Leavitt Law.
Cepheids are not the only cosmic rulers that may need some extra scrutiny. Last week, a team led by astrophysicist Marco Tavani of the University of Rome Tor Vergata announced the detection of energetic gamma-ray flashes from the Crab Nebula, while other scientists have also found the x-rays of this glowing remnant of an exploded star to be not as steady as they thought.
X-ray and gamma-ray astronomers use the Crab Nebula as a standard, says astrophysicist Colleen Wilson-Hodge of NASA's Marshall Space Flight Center in Huntsville, Alabama, so "we need to rethink how we calibrate our instruments." But, adds Tavani, "the Crab Nebula is still the best calibrator we have."
Likewise, cepheids will remain extremely useful for determining cosmic distances, says Marengo. In fact, taking future infrared observations of new instruments like the James Webb Space Telescope into account, distance estimates may even become much more precise.