Without the explosions of supernovae, the heavy elements so essential to life itself would be unavailable, and stars would lack the raw materials to form planets. Thus Carl Sagan’s famous “We are star-stuff” quotation, an idea validated by our extrasolar studies, which allow us to correlate the presence of planets with the existence of heavy elements in their stars. Much remains to be done here, but stars with higher metallicity and more heavy elements do appear more likely to have planets.
Volker Bromm (now at the University of Texas) puts it this way:
“We’re now just beginning to investigate the metallicity threshold for planet formation, so it’s hard to say when exactly the window for life opened. But clearly, we’re fortunate that the metallicity of the matter that birthed our solar system was high enough for the Earth to form. We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars. As the universe evolved, it progressively seeded itself with all the heavy elements necessary for planets and life to form. Thus, the evolution of the universe was a step-by-step process that resulted in a stable G-2 star capable of sustaining life. A star we call the Sun.”
While at the Harvard-Smithsonian Center for Astrophysics, Bromm worked with Lars Hernquist and Naoki Yoshida (now at Nagoya University) on simulations of the first supernova explosions, studies designed to plot their evolution and the subsequent birth of stars like the Sun. The latter two, working with Kazuyuki Omukai (National Astronomical Observatory of Japan) have now released simulations offering a still more precise picture of the earliest stars, work that incorporates dark matter in the mix. The result: A protostar with one percent of the Sun’s mass would evolve into a massive star a hundred times as massive as Sol, one that would burn for no more than a million years and synthesize heavy elements.
All of this gets us into the spread of heavy elements not simply in later generations of stars but relatively soon after the Big Bang. Indeed, Bromm’s earlier work with Avi Loeb had determined that a first-generation supernova could produce the heavy elements needed to allow the first Sun-like stars to form. The upshot is that many second-generation stars would have had the size, mass and temperature of the Sun, but with such low abundances of metals that they would have been unable to form rocky planets. For that, we need subsequent generations of stars and a more metal-rich interstellar medium. But over what time frame?
Image: The first primordial stars began as tiny seeds that grew rapidly into stars one hundred times the mass of our own Sun. Seen here in this artist impression, swirling clouds of hydrogen and helium gasses are illuminated by the first starlight to shine in the Universe. In the lower portion of the artwork, a supernova explodes ejecting heavier elements that will someday be incorporated into new stars and planets. Credit: David A. Aguilar, CfA
From an astrobiological perspective, it would be fascinating to learn how quickly stars that could support planets and life might have formed. Here Fermi again raises his head — If the universe might have supported life billions of years ago (Charles Lineweaver has written fascinatingly on this), then shouldn’t there be civilizations billions of years older than our own? It’s an elegant supposition, but what Yoshida and team have accomplished thus far is but to simulate a protostar’s birth, one whose further growth will require more intensive computational resources as the simulation progresses toward the supernova stage.
Even so, says Lars Hernquist, “This general picture of star formation, and the ability to compare how stellar objects form in different time periods and regions of the universe, will eventually allow investigation in the origins of life and planets.” And that’s something anyone with a yen to understand life’s place in the universe will want to keep an eye on. The paper is Yoshida, Omukai and Hernquist, “Protostar Formation in the Early Universe,” Science Vol. 321, No. 5889 (August 1, 2008), pp. 669-671 (abstract).