Astronomers Find New Way to Measure Cosmic Distances

Cepheid stars in galaxies such as M81, shown here. The stars could offer a new way to measure distances to objects in the universe. Image courtesy of Ohio State University.” width=”580″ height=”535″ class=”size-medium wp-image-32187″ />
hio State University astronomers are using the Large Binocular Telescope to look for ultra long period cepheid stars in galaxies such as M81, shown here. The stars could offer a new way to measure distances to objects in the universe. Image courtesy of Ohio State University.

Using a rare type of giant Cepheid variable stars as cosmic milemarkers, astronomers have found a way to measure distances to objects three times farther away in space than previously possible. Classical Cepheids are stars that pulse in brightness and have long been used as reference points for measuring distances in the nearby Universe. But astronomers have found a way to use “ultra long period” (ULP) Cepheid variables as beacons to measure distances up to 300 million light years and beyond.

Classical cepheids are bright, but beyond 100 million light years from Earth, their signal gets lost among other bright stars, said Jonathan Bird, doctoral student in astronomy at Ohio State, who discussed his findings at the American Astronomical Society conference on Monday.

But ULPs are a rare and extra-bright class of Cepheid, which pulse very slowly.

Astronomers have also long sthought that ULP cepheids don’t evolve the same way as other cepheids. In this study, however, astronomers found the first evidence of a ULP cepheid evolving the same way as a classical Cepheid..

There are several methods for calculating the distance to stars, and astronomers often have to combine methods to indirectly measure a distance. The usual analogy is a ladder, with each new method a higher rung above another. At each new rung of the cosmic distance ladder, the errors add up, reducing the precision of the overall measurement. So any single method that can skip the rungs of the ladder is a prized tool for probing the universe.

Krzysztof Stanek, professor of astronomy at Ohio State, applied a direct measurement technique in 2006, when he used the light emerging from a binary star system in the galaxy M33 to measure the distance to that galaxy for the first time. M33 is 3 million light years from Earth.

This new technique using ULP cepheids is different. It’s an indirect method, but this initial study suggests that the method would work for galaxies that are much farther away than M33.

“We found ultra long period cepheids to be a potentially powerful distance indicator. We believe they could provide the first direct stellar distance measurements to galaxies in the range of 50-100 megaparsecs (150 million – 326 million light years) and well beyond that,” Stanek said.

Because researchers generally don’t take note of ultra long period cepheids, there are few of them in the astronomical record. For this study, Stanek, Bird and Ohio State doctoral student Jose Prieto uncovered 18 ULP cepheids from the literature.

Each was located in a nearby galaxy, such as the Small Magellanic Cloud. The distances to these nearby galaxies are well known, so the astronomers used that knowledge to calibrate the distance to the ULP cepheids.

They found that they could use ULP cepheids to determine distance with a 10-20 percent error — a rate typical of other methods that make up the cosmic distance ladder.

“We hope to reduce that error as more people take note of ULP cepheids in their stellar surveys,” Bird said. “What we’ve shown so far is that the method works in principle, and the results are encouraging.”

Bird explained why astronomers have ignored ULP cepheids in the past.

Short period cepheids, those that brighten and dim every few days, make good distance markers in space because their period is directly related to their brightness — and astronomers can use that brightness information to calculate the distance. Polaris, the North Star, is a well known and classical cepheid.

But astronomers have always thought that ULP cepheids, which brighten and dim over the course of a few months or longer, don’t obey this relation. They are larger and brighter than the typical cepheid. In fact, they are larger and brighter than most stars; in this study, for example, the 18 ULP cepheids ranged in size from 12-20 times the mass of our sun.

The brightness makes them good distance markers, Stanek said. Typical cepheids are harder to spot in distant galaxies, as their light blends in with other stars. ULP cepheids are bright enough to stand out.

Astronomers have also long suspected that ULP cepheids don’t evolve the same way as other cepheids. In this study, however, the Ohio State team found the first evidence of a ULP cepheid evolving as a more classical cepheid does.

A classical cepheid will grow hotter and cooler many times over its lifetime. In-between, the outer layers of the star become unstable, which causes the changes in brightness. ULP cepheids are thought to go through this period of instability only once, and going in only one direction — from hotter to cooler.

But as the astronomers pieced together data from different parts of the literature for this study, they discovered that one of the ULP cepheids — a star in the Small Magellanic Cloud dubbed HV829 — is clearly moving in the opposite direction.

Forty years ago, HV829 pulsed every 87.6 days. Now it pulses every 84.4 days. Two other measurements found in the literature confirm that the period has been shrinking steadily in the decades in between, which indicates that the star itself is shrinking, and getting hotter.

The astronomers concluded that ULP cepheids may help astronomers not only measure the universe, but also learn more about how very massive stars evolve.

Some of these results were reported in the Astrophysical Journal in April 2009. Since that paper was written, the Ohio State astronomers have started using the Large Binocular Telescope in Tucson, Arizona to look for more ULP cepheids. Stanek says that they’ve found a few good candidates in the galaxy M81, but those results have yet to be confirmed.

Sources: AAS, The Ohio State University

5 Replies to “Astronomers Find New Way to Measure Cosmic Distances”

  1. One a related distance-in-space subject, I’ve often wondered why we don’t lob two probes that note where stars are out to stellar north and south with one of those ion engines on each to just keep them accelerating.

    We’d be able to make more and more accurate measurements on the stellar distances between in our local neighborhood of stars each year to to the increasing parallax, plus we wouldn’t have to wait six months between measurements.

  2. There are number of things such a probe could do by pumbing interstellar distances. This might include a measurement of dark energy. If a lone black hole should be found to be passing near the solar system that too would be worth giving a visit to.

    The propulsion system of choice would be a VASiMR VAriable Specific Impluse Magnetodynrodynamic Rocket. A VASIMR proulsion unit powered by a nuclear reactor could reach a few percent the speed of light relative to the sun.

  3. “VASiMR VAriable Specific Impluse Magnetodynrodynamic Rocket”

    Wow! Star Trek’s got nothing on this thechnobable 🙂

    I looked up in google and it says Magnetoplasma instead of Magnetodynrodynamic. I’m split on which one sounds cooler though.

    “VASIMR proulsion unit powered by a nuclear reactor could reach a few percent the speed of light relative to the sun.”
    This sounds like a mission to do for it’s own sake. I would love to see something like this. If my calculation is right, at 1% of c, the probe could cross 30AU in about 7 days. from the sun to pluto’s closest approach. Of course there will be the time it takes to build up the speed.

  4. I was curious as to why no ULP Cepheids have been discovered in our own galaxy? Surely some must have been studied (unknowingly) over the years, thus making it amenable to other tests of its period-luminosity-function and possibly distance determination by means of triangulation. I agree with the authors, though, that this method, properly verified and calibrated could greatly improve estimates to galaxies within range. A preprint of the short paper can be found here: 🙂

  5. Err… Nancy, typo in the fourth paragraph, first line: “sthought” ?

    BTW, great article — as usual.

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