A quest for far-out mileposts in space
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.”
Astronomer Henrietta Swan Leavitt discovered in 1912 that the brighter a Cepheid glowed, the more infrequently it pulsed. Scientists realized that if they could figure out the precise distance to one Cepheid, they could nail down the relationship between the stars’ intrinsic brightness and period of pulses.
Then they could calculate the distance to any visible Cepheid by measuring its pulses and the dimming of its starlight as its passed through space.
Benedict envisioned them as mileposts of the universe.
New technology crucial
Nailing down the distance to a Cepheid is no easy task, given that the closest is more than 800 light-years away.
It would require technology that was barely imaginable when Benedict was growing up in California and the Middle East, where the family moved after his father took a job as a chemical engineer for an oil company.
They lived for a time in Saudi Arabia. Desert life in an American compound of 1,400 could be solitary, but there was one saving grace. He was occasionally allowed to use the town supervisor’s 10-inch telescope, at that time the largest in the country.
Benedict took his first astrometry course at the University of Michigan, taught by a tough former World War II Navy captain. The exacting nature of the work appealed to Benedict’s sense of order.
He got a doctorate from Northwestern University in Illinois and then found a temporary job as a research associate at the University of Texas at Austin.
His Cepheid quest began in the summer of 1977. He was vacationing in Ohio when he got a call from a professor at the university telling him NASA was preparing to put a telescope in orbit, soon to be named Hubble.
There were thousands of ideas for the device, but a small group realized that it would be the perfect instrument to determine the distance to a Cepheid.
Would he be interested in joining the quest?
The original members of the Space Telescope Astrometry Team were a who’s who of the field, led by Yale’s Van Altena.
Benedict, the novice of the group, was chosen because he knew how to select guide stars -- fixed points in space used to hold a telescope in position.
Hubble’s centerpiece was a massive 94-inch primary mirror that allowed the 42-foot-long telescope to bring the universe’s exotic creatures into sharp focus.
Thousands of scientists were clamoring to use Hubble, but the demand wasn’t enough to prevent marathon delays in getting it into space.
The launch, originally set for 1983, was pushed back when there were problems in grinding the telescope’s mirror to a precise curvature.
Launch had been rescheduled for late 1986, but the shuttle fleet was grounded for almost three years after the Challenger exploded in January of that year.
Hubble was finally launched in 1990, but then fate struck again. After just a few weeks in space, scientists discovered that Hubble had blurry vision because its main mirror had indeed been ground to the wrong shape. Engineers fixed it by creating a set of “glasses” for the telescope. Those were installed in 1993.
As the years passed, the original members of the astrometry team gradually peeled away. Some members retired; others took work that they found more rewarding than waiting for observation time on Hubble.
For the next decade, the team worked on perfecting techniques and tackling smaller astrometric projects as they waited to be given the thousands of hours of Hubble time needed for the Cepheid project. Through attrition, Benedict became the leader of the team.
In 2003 -- 26 years after the astrometry team was assembled -- Benedict received an e-mail from NASA.
The Cepheid project was on.
And the winners are . . .
Benedict chose 10 Cepheids, all within the Milky Way.
The task of analyzing the data from Hubble fell to a former molecular biologist who spent five years studying RNA and DNA sequences in mice for cancer research. Barbara McArthur joined the astrometric group after answering an ad for a computer analyst; she was hired thanks to her tirelessly obsessive mind for detail.
The distance measurements relied on Hubble’s Fine Guidance Sensors, which were designed to lock onto stars so that the telescope’s camera would hold still.
Benedict selected a group of nearby “reference stars” in the same region of the sky as each Cepheid. By keeping these stars in a constant relative position, the astrometrists could track the apparent motion of the Cepheid as Earth traversed its orbit.
Using Hubble’s fine eyesight, McArthur, with the help of graduate student Jacob Bean, could calculate a parallax for each Cepheid down to a level of a few milliarcseconds -- the equivalent of measuring a few inches from 1,500 miles away.
It took two years of observations to collect the necessary data. For every measurement, there were dozens of variables to correct for, such as the heating and cooling of Hubble and the microscopic jiggles of the telescope as it hurtled through space at roughly 17,500 mph.
Over time, each Cepheid seemed to develop its own personality. “Some are really ornery,” McArthur said.
When the parallax measurements were complete, Benedict tackled the problem of determining each Cepheid’s true brightness using Hubble’s spectrometer.
Starlight dims as it passes through dust and gas in space. To determine true brightness, the astrometrists had to figure out how much dimming occurred over that particular stretch of space.
Benedict already knew the true brightness of the reference stars, which were chosen because they were common types with known attributes. He calculated the amount of dimming to those stars and then adjusted the apparent brightness of each Cepheid.
Benedict plotted the results on a chart that compared the luminosity and frequency of each Cepheid’s pulses. If they were true standard candles, the line on a logarithmic graph should shoot like an arrow.
“To my intense delight, it was a straight line,” he said.
Their work was complete.
He went the distance
Nearly 30 years had passed since Benedict joined the Cepheid project. By now, he was the last of the original team members and, perhaps, the only one in the cosmos who could truly appreciate the end point of so much work and waiting.
The results were published in April in the Astrophysical Journal.
The new calculations narrow the error margin on Cepheid distances from as much as 40% to less than 10%. For example, the distance to the Cepheid known as L Carina was honed from between 1,000 to 2,000 light-years away, to an estimated 1,400 to 1,600 light-years.
Wendy L. Freedman, director of the Carnegie Observatories in Pasadena, Calif., said the Cepheid distances also would help refine the Hubble Constant, one of the key values in cosmology that describes the rate at which the universe is expanding. The constant’s 10% error margin could be cut in half, Freedman said, bringing science closer to answering such fundamental questions as how much dark energy and matter exists in the universe.
Still, even with the improvements, the yardstick of the universe remains an imperfect tool whose minuscule error margins can amount to unimaginable distances.
Benedict, whose hair turned gray, then nearly white as he waited to measure the Cepheids, is unperturbed that so much work remains.
“Science is in the details,” he said.
He has already begun thinking about calculating the Cepheid distances to the microarcsecond, which is like measuring the thickness of a coin from the moon.
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