How old is universe? Hubble team inquires

October 27, 1994|By Frank D. Roylance | Frank D. Roylance,Sun Staff Writer

GREENBELT -- An international team of scientists using the Hubble Space Telescope says it has reached the first major milepost in its quest for answers to some of the biggest questions in science -- the size and age of the universe.

The astronomers announced yesterday at the Goddard Space Flight Center and in the journal Nature that they have made the first accurate measurement of the distance to a beautiful spiral galaxy in the springtime constellation Virgo. They say the galaxy, called M100, is about 56 million light years away.

With that data, and measurements to 19 more galaxies over the next three years, they hope to nail down a precise figure for the elusive "Holy Grail" of astronomy, called the Hubble Constant, which is the rate of the expansion of the universe. When they have that, they can answer the big questions.

"It's really a grand quest," said Dr. Holland Ford, professor of physics and astronomy at the Johns Hopkins University and a member of the project team.

So far, their work suggests that the universe has been expanding for 8 billion to 12 billion years, since the origin of all time, matter and space in a cataclysmic event called the Big Bang.

Even 12 billion years is young by current estimates, and it poses a potential problem for astrophysicists, who have found stars they believe to be 15 billion years old. If the younger numbers hold up, it means either that our understanding of stellar evolution is wrong or that current beliefs about the origins of the universe are wrong.

"It will mean we really have a dilemma, and that's exciting. That's what science is really all about," said Dr. Alexei V. Filippenko, a University of California astronomer who is not part of the Hubble telescope team.

The apparent paradox has not discouraged the investigators.

"It's a first result and we're very excited about it, because it's a portent of things to come," said Abhijit Saha, 38, an associate astronomer at the Space Telescope Science Institute in Baltimore and a member of the research team. "It's only the beginning of a very exciting two or three years."

The search for the Hubble Constant is one of the space telescope's "Key Projects," and one that scientists had in mind when they named and designed the orbiting observatory more than 20 years ago.

The modern quest for the age of the universe really began in 1929, when American astronomer Edwin Hubble discovered that everything he saw in the night sky was moving away from Earth, suggesting that the entire universe is expanding. And the farther away we look (and, therefore, the farther back in time), the faster everything is receding.

The rate of the expansion for objects at a given distance is called the Hubble Constant, but no one has ever calculated it accurately. If the rate could be figured accurately, scien

tists could figure out how long the expansion has been going on, when it began, and maybe how it would end.

To figure it out, astronomers need two things: the speed of the object, which is relatively easy to determine by spectrography, and its distance, which is not so easy.

The most reliable yardstick has been a rare type of pulsating star called a Cepheid variable. Astronomers know that the rate at which Cepheids vary in their apparent brightness is a direct indicator of their actual brightness. Knowing exactly how bright a given Cepheid star is, and how much any light source fades with distance, it is relatively easy to figure out how far away individual Cepheids are.

For the most reliable estimate of the Hubble Constant, however, astronomers need to look at the most distant objects they can measure. And ground-based telescopes can't easily see individual Cepheids as far away as M100.

An Indiana University team using a Hawaiian telescope announced last month that it had calculated the Hubble Constant based on observations of just three Cepheid stars in another galaxy in Virgo.

The Hubble researchers dismissed that claim yesterday as premature, based on too small a sample of stars to be reliable.

Using the space telescope's new Wide Field/Planetary Camera, however, the Hubble astronomers last May and June were able to snap 12 photographs of M100 and identified as many as 60 Cepheids blinking among a field of 100,000 stars. The Cepheids were found five to six times farther away than they can be routinely seen from the ground.

Laura Ferrarese, 28, a native of Padua, Italy, and a doctoral candidate in physics at Hopkins, led one of the computer-assisted searches.

With Hubble, she said, "you see [Cepheids] immediately, and it's so striking." Plotted on a graph, each star's pulsating light "looks like it is out of a [text] book."

The 20 "best" Cepheids Ms. Ferrarese found were confirmed by a parallel search led by Dr. Wendy L. Freedman, at the Carnegie Observatories in California, and used to figure M100's distance.

"It's just so amazing," Ms. Ferrarese said. "None of these was known from the ground."

Knowing the distance to M100, scientists can now grab other yardsticks needed to extend their measurements far beyond the limits at which Cepheids can be seen.

One of those yardstick is based on Type II supernovae, brilliant remnants of massive stars that ran out of fuel and exploded. Now that scientists know how far away the Type II supernovae in Virgo really are, they can calculate the distance to others much farther away.

"Type 2s are really bright, so they can take you easily several hundred megaparsecs" into space, said Dr. Saha. The Virgo Cluster is only about 17 megaparsecs away. (A megaparsec is 3.26 million light years. A light year is 5.9 trillion miles.)

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