Johns Hopkins Gazette: October 31, 1994

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
    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
    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
    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
    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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