WASHINGTON -- Astronomers using the Hubble Space Telescope have peered deep into the centers of spiral galaxies and found the first solid clues to the ages of those galaxies -- and to the mysteries of their evolution.
Some, they say, have large central regions -- like the yolk of a fried egg -- that bulge with ancient stars. All the stars formed early in the history of the universe, and little seems to have changed in the 10 billion years since.
But at the heart of other spiral galaxies -- the ones with smaller hubs of these ancient stars -- Hubble scientists say they can see dense clusters of hot young stars. These galactic cores are still being fattened by torrents of gas, dust and stars streaming in from the galaxies' spiral arms.
To the researchers, it all suggests that spiral galaxies were born during a period of intense star formation early in the history of the universe, but that their development since then has taken very different tracks.
"Our challenge is to explain what makes galaxies look the way they do," said David Leckrone, NASA's Hubble project scientist.
Galaxies are the vast communities of dust, gas and billions of stars that speckle the universe by the hundreds of millions. Our own sun is just one of the billions of stars that circle the pinwheel-shaped spiral galaxy we call the Milky Way.
But not all galaxies look the same. In addition to the familiar spiral galaxies, there are also elliptical galaxies -- huge clouds of mostly ancient stars that can be nearly spherical, or cigar-shaped. There is also a class of galaxies called irregulars because they have no definite structure.
Before Hubble, astronomers could only guess at the origins of the galaxies. Today, sorting out the natural history of galaxies is one of Hubble's top priorities.
By looking at galaxies at increasing distances from Earth, Hubble can see them as they appeared at increasingly distant moments in time. That's because -- just like a bus -- it takes the light more time to get here from greater distances. For astronomers, it's like piecing together evolution by examining fossils from different geological eras.
Also, with Hubble's power, and its ability to see through dust and gas, astronomers can see more detail in the structures at the galactic core, and that can hold clues to the forces that shaped them. It can also more accurately measure the starlight that reveals a star's age.
Astronomers already knew that elliptical galaxies were filled with ancient stars -- all about 10 billion years old -- and little gas or dust to fuel new star formation.
But the spiral galaxies were a puzzle. They are rich in gas and dust, and that made it hard to see into their centers and measure the true age of the stars there.
An international team of astronomers led by Raynier Peletier of England's University of Nottingham turned the space telescope on 20 spiral galaxies with large central bulges.
Using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), Peletier's team was able to see through the dust. They found that the cores of these galaxies were filled with 10-billion-year-old stars much like those in the elliptical galaxies.
"Apparently everywhere in the universe these intermediate-sized galaxies must have started forming early on," Peletier said. One might expect them to have been replenished with new, young stars since then, "but that doesn't seem to be the case."
But other astronomers, led by C. Marcella Carollo, formerly at the Johns Hopkins University but now at Columbia University, looked at spiral galaxies with small central bulges. And they told a far more complex story.
At the core of these galaxies -- where ground-based telescopes saw only a shapeless mass of light -- Hubble found dense clusters of bright stars as young as 1 billion years old, vibrant regions of starbirth and other complicated structures.
Something had destabilized these galaxies since their first stars formed. Dust, gas and stars in the galaxies' spiral arms seemed to be falling in toward the ancient stars in the center, triggering all the new starbirth, heating and enlarging the center.
Our own Milky Way galaxy doesn't seem to fit neatly into either camp, said Donald F. Figer, an expert on the Milky Way at the Space Telescope Science Institute in Baltimore.
The Milky Way, Figer said, "really represents a dividing line between both types of galaxies."
Pub Date: 10/07/99