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Modeling Cosmic Evolution at AMNH

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Curator Mac Low's research uses computing to follow the evolution of supernova explosions. This photo shows supernova remnant Cassiopeia A. © NASA/CXC/GSFC/U.Hwang et al.

The fifth floor of the Rose Center for Earth and Space is home to the Museum’s Department of Astrophysics, which includes a research group of two dozen graduate students, research scientists, and postdocs. Mordecai-Mark Mac Low is one of three curators in the department. Below is the second in a series of features on the curators’ areas of research.

Curator Mac Low’s office is bright, and most of the floor space is claimed by book-lined shelves and neat stacks of papers. Just outside the door, the hall window—marked with equations scribbled in red and blue marker—looks out onto the gray top of the Hayden Sphere as sunlight pours in from 81st Street.

Mac Low also studies the evolution of stars, but his more theoretical approach to astrophysics requires months of computing time and routine digital conference calls with an international network of collaborators and students.

“I’m a storyteller,” says Mac Low. “I’m verbally oriented, not primarily a mathematician…but getting a computer to do what you want it to do is something that I’m comfortable with and amused by, by and large.”

One story that Mac Low likes to tell is how stars form. Until he began working on this problem a decade ago, most astrophysicists’ models of star formation were based on the assumption of an idealized geometric distribution of gas. But when simulations include more realistic turbulent gas flow, it becomes clear that chaotic motions provide stability against gravitational collapse to the gas between the stars, determining the speed with which stars form.

“The simulations we run are, in their essence, the same as programs used for weather prediction,” says Mac Low. “They follow the motions of a gas as it is heated and cooled and pushed around. Over millions of tiny steps, we follow the gross evolution of a supernova explosion.”

Another story involves a conundrum that has stumped astrophysicists for decades: how do planetesimals—asteroids and dwarf planets—evolve from rocks and boulders in a young solar system whose star is still surrounded by its natal gaseous disk?

As the rocks collide and grow larger, they orbit faster than the gas and feel a headwind that drags them into the star. Mac Low and colleagues found the answer in a phenomenon well known to cyclists: drafting behind the leader. If there are more rocks in one orbit, further rocks falling into that orbit are protected from the headwind and can accumulate there. So much material can accumulate that gravity can collapse it together, forming large asteroids and even dwarf planets in only ten or so orbits.

This story originally appeared in the Fall issue of Rotunda, the Member magazine. For the first article in the Rose Center for Earth and Space series, click here.