October 21, 1999
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Starstruck: Alyssa Goodman studies the gestation period of stars

By Ken Gewertz
Gazette Staff


Alyssa Goodman of the Astronomy Department: "Ultimately, what I'm trying to figure out is how these big clouds of dust and gas get ready to form stars." Photo by Kris Snibbe.

As a child, Alyssa Goodman dreamed of being Jacques Cousteau and exploring the mysteries of the ocean depths.

She has not so much given up that dream as traded it for one even grander – exploring the vast distances and mysterious physical processes of interstellar space.

Specifically, Goodman studies molecular clouds. These are gigantic assemblages of dust and gas hundreds of light years across that float in the cold dark spaces between the stars.

On average, they contain 100 atoms of matter per cubic centimeter. On earth, this would be considered a vacuum, but in deep space it is dense enough to cause the atoms to get together to form molecules. The molecules form on slightly larger particles called "dust grains," which themselves are only as large as – or a bit larger than – the wavelength of light. The dust in molecular clouds causes the clouds to block starlight and appear as areas of darkness.

Astronomers study these clouds because they are the breeding ground where new stars form. Goodman investigates not the actual birth of stars but the processes leading up to it.

"Ultimately, what I’m trying to figure out is how these big clouds of dust and gas get ready to form stars," she said.

Scientist, Teacher, Role Model

Goodman was recently promoted to a tenured position in the Astronomy Department of the Faculty of Arts and Sciences (FAS). With the exception of three years as a postdoctoral fellow at the University of California-Berkeley (1989-92), she has been associated with the Astronomy Department and with the Harvard-Smithsonian Center for Astrophysics (CfA) since she came to Harvard as a graduate student in 1984. She received her Ph.D. from Harvard in 1989.

Philip Myers, lecturer in astronomy and associate director for radio astronomy at the CfA, was Goodman’s thesis adviser and had this to say about her: "She was an outstanding graduate student and took on a very hard thesis topic, measuring the Zeeman effect in interstellar magnetic fields. In addition to being a very effective scientist, she’s a terrific organizer and presenter of data, a fantastic speaker, a great teacher, and she serves as a role model, especially for women students."

Goodman traces her interest in molecular clouds to a project she worked on in 1983 as an M.I.T. undergraduate. At a summer program of the NASA Goddard Institute for Space Studies her assignment was to make a polarization map of the constellation Orion. This involved organizing large amounts of data to show how dust particles in interstellar space were polarized or aligned by magnetic fields. The project not only gave her a senior thesis topic but also helped her decide on a career in astronomy.

"A lot of that summer was grunt work, looking up information in books, but I could see what it was leading to, and that made it interesting to me," she said.

Goodman continued to study interstellar magnetic fields as a graduate student. These fields are analogous to magnetic fields generated between the positive and negative poles of a magnet and often demonstrated in elementary science classes by sprinkling iron filings on a sheet of paper. The difference between the demonstration and the real thing (in addition to the enormous difference in size) is that the origin of these deep space fields has never been satisfactorily explained.

The ‘Zeeman Effect’

Goodman has made several important discoveries about these fields. For her Ph.D. research she measured the Zeeman effect in molecular clouds. The Zeeman effect (discovered in 1896 by Dutch physicist Pieter Zeeman) refers to the splitting of a spectral line into a group of closely spaced lines under the influence of a magnetic field. The effect was later explained through quantum theory.

Goodman compares the energy levels of magnetic fields to a series of steps. The Zeeman effect measures increments so tiny that they can be compared to a layer of dust covering the top of a step. Goodman showed that constructing a Zeeman map of an interstellar cloud, in conjunction with polarization measurements, would give a detailed picture of how a cloud is influenced by its magnetic field.

The best way to map the structure and direction of interstellar magnetic fields involves measuring the polarization of far-infrared radiation from dust particles. The only drawback to mapping interstellar clouds by this method is that the necessary readings can only be made from space. Currently Goodman, along with other scientists, has proposed a satellite-based radio-style telescope that would produce a three-dimensional map of magnetic fields throughout the galaxy.

Called the Milky Way Magnetic Field Mapping Mission, the project is estimated to cost approximately $65 million. The group is applying for a grant from NASA.

"Personally, I think it would be well worth the money to put up a satellite and collect this data," Goodman said.

Star Formation

Goodman’s observations of magnetic fields have led her to think about their role in star formation. She expects that when young stars are just beginning to form in the dense cores of interstellar clouds, they emit flows of matter which shake up the magnetic field and ultimately slow down the core’s collapse. As a result, molecular clouds in the galaxy as a whole will form stars at a rate that is in line with the stellar death rate – a result that is believed to be desirable.

Much of Goodman’s work has resulted in the production of great quantities of data on the structure of the interstellar medium. Another of her contributions has been a tool to better analyze and compare this mass of data.

The Spectral Correlation Function (SCF) is a tool that she developed to analyze extremely complicated interstellar maps and compare them with theoretical models. Before more sensitive observational instruments coupled with supercomputers resulted in the proliferation of such data, astronomers used to make such comparisons impressionistically. Goodman’s SCF has allowed astronomers to benefit from the enormous quantities of data now at their disposal.

Goodman said that she is extremely happy to be at Harvard because, as a research associate of the Smithsonian Astrophysical Observatory and a professor in the Astronomy Department, she is in contact with the best minds in the field.

"The people I interact with are not just the few dozen members of the Astronomy Department, but the 300 or so Ph.D. scientists in the CfA. It’s the best you can do – the center of the astronomy universe."

 


Copyright 1999 President and Fellows of Harvard College