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A quest for far-out mileposts in space

A Texas astronomer spends 30 years finding distances to stars called Cepheids, give or take a few hundred light-years.

November 26, 2007|John Johnson Jr. | Times Staff Writer

AUSTIN, TEXAS — In the vastness of space, how far is far?

That question has simmered in G. Fritz Benedict's mind since he was 8, when a family friend took him into the backyard of his home and pointed to the constellation Orion.

"Something in my brain went 'snap,' " said Benedict, an astronomer at the University of Texas at Austin.

The experience set him on a lifelong quest to answer one of the most arcane questions in astronomy: How exactly do you measure the universe?

Astronomers have wrestled with the issue for millenniums, and for most of that time they haven't even come close.

Their calculations have been painstakingly constructed into a framework known as the cosmic distance ladder.

Each rung is made up of a stellar object whose distance is roughly known and can be used to measure the distance of neighboring objects.

The problem is that over cosmic distances, minuscule inaccuracies can compound into huge miscalculations.

"People say, 'What's the big deal?' " said Benedict, a youthful man of 62 with a quick smile and an inexhaustibly detailed mind. "I tell them, 'What if I handed you a yardstick and told you, 'I don't know if it's 32 or 42 inches long?' "

Caltech astronomer Shri Kulkarni is more blunt. "Astrometry is the fundamental basis of astronomy," he said. "It's the way you know such things as the size of the universe. Other than that, you know nothing."

Benedict's quest has taken him on a lonely 30-year journey filled with bureaucratic roadblocks and technical delays that made him doubt whether he would ever get the chance to make his measurements.

While most of his peers went to work on the big questions of the universe -- uniting gravity and quantum mechanics, searching for extraterrestrial life or figuring out how the universe will end -- Benedict has obsessed over one set of measurements: the distance to a type of star known as a Cepheid.

There's been little glory. And even as the tools of cosmic surveying are reaching once-unimaginable accuracy, American astrometry is fading as students move into sexier topics.

"There's a rather gloomy future," lamented retired Yale University astronomer William F. van Altena, the most illustrious astrometrist of his era. "This is really a sad state of affairs."

Early efforts stumbled

Surveying the universe is mostly an exercise in being wildly wrong.

In the 3rd century B.C., the mathematician Aristarchus of Samos made one of the earliest attempts to calculate the distance to the sun -- the first rung in the cosmic distance ladder.

Using the apparent similarity in size between the sun and moon, he came up with 4 million miles.

That was off by 89 million.

It took more than 2,000 years for English astronomer Edmond Halley to devise a strategy for a more precise calculation.

Astronomers knew that observers in different parts of the world saw the sun from slightly different angles. That angular difference, or parallax, could allow scientists to calculate a distance to the sun.

Halley's plan was to send scientists around the world to observe the Transit of Venus -- a rare event in which the planet passes across the face of the sun.

Venus would appear to traverse a different part of the sun depending on the observer's location. Its journey would take a different amount of time depending on whether the planet passed over a wide section of the sun or a narrow one. The elapsed time of the transit, which could vary by several minutes, would indicate the observer's viewing angle.

The undertaking proved so difficult that it took until the late 1800s -- almost 150 years after Halley's death -- before adequate observations allowed analysts to calculate the distance of about 93 million miles.

Modern astronomers used the parallax method to measure the distance to our nearest starry neighbors, the triple star system Alpha Centauri about four light-years away, or nearly 26 trillion miles. Instead of observing the stars from two different spots on Earth, they took measurements at two extremes of Earth's orbit around the sun.

The parallax technique, however, fails as the angular difference grows too small. In the 1990s, the European satellite Hipparcos couldn't measure distances beyond about 500 light-years. To survey deeper parts of the universe, astronomers rely on "standard candles" -- objects that shine with a known luminosity, such as the rare exploding stars known as Type 1a supernovas.

Because their brightness is the same, their distance can be calculated by measuring how much the light has dimmed after passing through light-years of dust and gas.

The problem is that there aren't enough Type 1a supernovas. Benedict was drawn to an unusual type of pulsating objects that, unlike supernovas, are abundantly scattered through the universe.

"Cepheids are the flashing 'Eat at Joe's' sign in a galaxy," Benedict said. "They're easy to spot and very satisfying."

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