Grad Student's Work Leads to 'Basic Milestone' By Emil Venere Laura Ferrarese's patient sifting of Hubble Space Telescope data has paid off. Ferrarese, an Italian-born graduate student working on her doctorate in the Department of Physics and Astronomy, has provided measurements essential for a scientific breakthrough. Astronomers announced the historic findings last week. An international team of researchers using the space telescope measured the distance of a remote galaxy, M100, located 56 million light-years away in the constellation Virgo. Such precise measurements will eventually be the key to calculating the age and size of the universe, two of the most fundamental goals in cosmology. But in order to measure M100's distance, astronomers first needed to identify stars called Cepheid variables. Ferrarese used a complicated software program to compare 12 sets of images of the distant galaxy taken at different times. By carefully analyzing the images and comparing all the data, she was able to find 62 of the rare Cepheid variables. "It's a basic milestone; I'm not exaggerating," said the 28-year-old astrophysicist, who arrived at Hopkins four years ago. Astronomers use Cepheids as a yardstick to measure distances to objects in space. The stars pulsate on a regular basis, growing brighter and dimmer. By studying the pulsations, scientists can determine the stars' actual brightness. From there, it is a relatively straightforward calculation to determine how far away the stars are, since the brightness of a star as seen from Earth drops off at a geometrically predictable rate with distance. Detecting Cepheids is one of the space telescope's major strengths, said Hopkins astronomer Holland Ford, who has a dual appointment at the Space Telescope Science Institute, which operates the telescope. "Hubble is a Cepheid machine," Dr. Ford said. "It just really does Cepheids better than anything we've ever had on the ground." Astronomers are now realizing the goal envisioned by scientists who planned and designed the telescope. "I'm very proud of having done this work because it's so interesting," Ferrarese said. "It's something that has never been done before, something that, only a few years ago, if you asked another astronomer, 'Wouldn't it be nice to find 60 Cepheids in Virgo?' they'd say you must be crazy." Her contribution to the research can't be overlooked, since she virtually volunteered to analyze the space telescope data in the interests of scientific accuracy, Dr. Ford said. "She is a watchdog for data quality," said Dr. Ford, who also is involved in the research. "She is the data-quality assurance person." Abhijit Saha, an associate astronomer at the Space Telescope Science Institute, also played a key role in the findings. He helped write the computer program used to find the Cepheids. Without the software, Ferrarese would not have been able to distinguish the difference between Cepheids, other objects and signals produced by the static of cosmic interference. "You have these 12 images and then you have to sort of find the needle in the haystack," Dr. Saha said. "These are very crowded fields, and the light from one star runs into the light of the next one. They are also very, very faint." The field contains about 100,000 stars. Because the images are taken at different times, an astronomer can identify which of the stars are pulsating by comparing the images, he said. Ferrarese's findings were compared with a separate analysis produced by another team of scientists. Since both sets of data showed comparable results, the astronomers were able to say with confidence that they had indeed identified Cepheid variables. Though Cepheids are rare, once found, they provide a reliable measurement for estimating intergalactic distances, and might be used to construct a precise calculation of the expansion rate of the universe, called the Hubble Constant. For decades astronomers have been trying to determine the value of the Hubble Constant, first developed by American astronomer Edwin Hubble in 1929. Hubble found that the farther a galaxy is, the faster it is receding. This uniform expansion effect is strong evidence that the universe began in an event called the Big Bang and that it has been expanding ever since. The M100 findings represent a critical step in finding the true value of the Hubble Constant. But the work is by no means definitive. Over the next two years astronomers will use the space telescope to find and study Cepheids in about 20 additional galaxies, located in different regions of space, Dr. Ford said. Those findings will be used to provide further information that should allow astronomers to find the Hubble Constant's value. Scientists are aiming to calculate the value to an accuracy of 10 percent, he said. Astronomers may encounter some surprises once they reach that plateau. Researchers are already puzzled over implications of the M100 findings. Data from the space telescope observations imply that the universe may range in age from 8 billion to 12 billion years. But those age estimates don't make sense, since some stars are believed to be 18 billion years old. Furthermore, the estimates pose problems for current theories about the formation and development of the universe. The research may force cosmologists to rewrite theories that have been accepted for decades, namely, the paradigm that the universe is expanding uniformly in all directions. "We may learn that our cosmological paradigm is not correct," Dr. Ford said. "Or we may learn that there is more about stellar evolution than we had thought. It would be very exciting if we threw out a paradigm. "The size and the age of the universe are fundamental," he added. "That's what we're after."
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