Peering into the Cosmic Cradle

(An expanded version of an Eyepiece article based on a Hayden Planetarium lecture by Dr. Grace Wolf-Chase)

Hubble Space Telescope photo of a stellar
nursery in the Trifid Nebula
Photo: NASA and Jeff Hester (Arizona State University)

New stars are being born all the time all the time in our galaxy, but the process by which they come into being was cloaked in mystery until just a few decades ago. Swathed in clouds of gas and dust that hinder their detection in visible light, nascent stars known as protostars are hidden to our eyes, yet today’s astronomers—armed with powerful new telescopes to probe the far infrared and radio spectrum in which protostars emit most of their radiation—are unveiling the secrets of star birth, yielding surprises unaccounted for by theory. Grace Wolf-Chase, an astrophysicist at the University of Chicago and the Adler Planetarium and an expert on star formation, talked about “The Pangs of Star Birth” at the Hayden Planetarium on May 6.

Dr. Wolf-Chase is a native New Yorker who grew up in Bergen County, New Jersey, did her undergraduate work at Cornell, and got her Ph.D. at the University of Arizona. “Not only were my childhood memories of the old Hayden Planetarium a major factor in my wanting to become an astronomer,” she says, “but my wonderful experiences in informal learning I had here so many years ago so inspired me that not only did I want to do astronomy research, but I also wanted to work in informal education, which is one of the reasons why I joined the Adler Planetarium.”

The study of star formation is a very new science, but theories of the origin of our sun and solar system go back centuries. In 1755 Immanuel Kant proposed the nebular hypothesis in which the solar system formed from a huge spinning cloud of gas and dust that collapsed inward under its own gravity and flattened out, with slow-moving material falling into the center and faster particles spreading out along the edge, the material eventually coalescing to form the Sun and planets. A similar theory was created around the same time by Pierre Laplace. In the past few decades the nebular hypothesis has been borne out by observation of solar systems in the making.

HST photo of a gaseous pillar
in the Eagle Nebula
Photo: Jeff Hester, P. Scowen
(Arizona State University), NASA

In 1970, after Keith Jefferts, Robert Wilson, and Arno Penzias (the latter two, Nobel laureates for their discovery in the 1960s of the cosmic background radiation) detected millimeter-wave emissions from carbon monoxide in the Orion Nebula, it was realized that stars must form in enormous, very cold molecular clouds, about 100 light-years across and only 10-12 degrees above absolute zero. The regions are dense enough so that gravity can cause the gas and dust to accrete into stars.

A surprising discovery was of bipolar jets of material spewing up to 10 light-years from the protostar, so well collimated that they are the equivalent of a garden hose shooting a tight stream of water 20 miles into the sky. These jets travel faster than 1 million km/hour, fast enough that the Hubble Space Telescope can track their motion. They can launch more than 1 trillion metric tons of gas into space a day. It appears that most young stars go through an outflow phase. Star formation also connected with strong molecular fields, which pull material off of the protostellar disk; as the protostar collapses down by a factor of 1 million, it spins ever quicker and must shed angular momentum, which is perhaps the function of outflow.

Proplyds (protoplanetary disks,
the tan ovals) in the Orion Nebula.

Several questions that Dr. Wolf-Chase and other specialists are addressing, and with the help of the next generation of space telescopes hope to answer, include: What determines the mass of a star? Does outflow blow away a lot of the material in some stars? What is the relationship between the gas that’s being spit out of the star and the gas that has to be falling in onto the star? And how do high-mass stars form? To answer these and other questions, a new generation high-resolution telescopes such as ALMA (the Atacama Large Millimeter Array) are being built. ALMA—the Spanish word for soul—will be at 16,000 feet in the Atacama Desert in Chile and consist of 64 12-meter antennas that can be arranged in different configurations. It will have better resolution than either the Hubble or Very Large Array; it is the largest, most sensitive instrument at these wavelengths in the works.

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