Astronomer Advances Novel Theory On Star Formation

This is the final installment in a six-part series on astronomy research at Harvard.

Astronomers have recently discovered that stars like the sun are born rotating very slowly, spin faster and then slower as they age. The unexpected result provides new insights into the ways stars form and evolve.

Lee Hartman of the Harvard/Smithsonian Center for Astrophysics, one of the new theory's pioneers, and his colleagues, including-John Stauffer of the Dominion Astrophysical Observatory in Canada, have shown that stars spin at various speeds during their lifetimes. These scientists are currently devoting much of their energy and time to decipering this curious behavior.

Hartman explains that if a forming star does not rid itself of its spin efficiently, its rotation increases too rapidly as it contracts. The acceleration produced by the rotation can cancel the force of gravity and prevent the final collapse into a compact normal star.

Creation

Two theories of stellar formation dominate astrophysics. One holds that as gravity pulls together a massive cloud of interstellar material, the cloud is spun down sufficiently by magnetic coupling with the surrounding gas to form a star.

The second postulates that a large rapidly spinning disk of material forms first. Most of the material in this disk slowly spirals in, forming the star, while the remaining gas can be used to form planets.

While it is still impossible to determine which, if either, is correct, Hartman says that recent data shows that young stars spin slowly. Moreover, this suggests that the disk theory is incorrect, as it says that stars should be rapidly rotating when formed.

What was even more surprising was the finding that after formation, stars like the Sun spin up rapidly. With rotational periods of less than a day, stars spin down as they age. The Sun's present rotational period is 25 days.

Most astronomers expected that stellar rotation slows uniformly with age, says Douglas K. Dunean of the Mt. Wilson and Las Campanas Observatories, but "nobody had expected a period when stars spin up."

"This is important enough that the past theories have to be revised," adds Sallie L. Baliunas of the Center for Astrophysics. "People only predicted that stars slowed down, but not that they could speed up that much."

Currently, the team is in the midst of acquiring new data to determine empirically exactly how the process works.

Rotation causes different parts of the stellar surface to have different speeds relative to the earth. The faster the star rotates, the greater the spread in speeds.

As stars age, they eject matter, which causes them to spin down as angular momentum is carried away, explains Ross D. Cohen of the University of California at San Diego. If stars contract, they may spin up by this effect alone. The spin up of stars is apparently possible during periods when star contraction is much more rapid than matter ejection.

Despite the billions of stars in the galaxy, most are too faint to measure accurately, Large telescopes, such as the Multiple Mirror Telescope of the Smithsonian Institution and the University of Arizona near Tuscon, are employed to collect the necessary data.

Even using this telescope, "all of the stars we've looked at are a few hundred light years from the Sun," Hartman says.

"We cannot go back into the past to study the sun's history, but we can look at stars of similar size to develop a picture of the Sun's development," Bartman says

Rapid rotation of star causes a tremendous in crease in the ultraviolet light. X-rays, and gamma radiation it the Sun rotated rapidly in its south, this excess radiation may have had important effects on the youthful Earth's atmosphere.

In last, this is one of the reasons Hartman, as a student, decided to explore the properties of young stars.

"I was studing other stars to try and learn something about the sun's evolution, "he says." I found it fascinating that a few young stars were reported to he spinning so much faster than sun, so I decided to look more closely into these stars." he add