Thursday, April 28, 2016

Star-chaeology: The Next (Stellar) Generation

“We are looking back in time by simply studying the grandfathers and all our stellar ancestors.” Dr. Anna Frebel is an Assistant Professor of Physics at MIT and the author of Searching for the Oldest Stars, and she looks for and studies stars that are almost as old as the universe itself. “That's why we call this kind of work stellar archaeology.”

Frebel and her colleagues target neighboring stars in the Milky Way and nearby dwarf galaxies, looking for the ones that formed in the second or third generation of stars following the Big Bang. Being on the small side (about 0.6-0.8 times the mass of our own Sun), these stars are extremely efficient at burning hydrogen into helium, allowing them to survive for more than 13 billion years — and providing astronomers with a window into the early universe.

Identifying these stars from among all of the billions of others relies on understanding how chemical abundances — how much of each element in the periodic table — have changed over time, from the Big Bang to today.  Hydrogen and helium, the most abundant elements, were created in that initial event and became the basic building blocks to produce everything else.  Elements up to iron are formed in the cores of stars, as energy is released in fusion reactions that combine lighter atoms together to form heavier ones. Once a star has gone through all of its lighter elements and built up a core of mostly iron, however, everything changes.  Fusion reactions involving tightly-bound iron atoms don’t produce energy; instead, they use it up.  With its energy source depleted, the star can no longer sustain itself and a brilliant supernova is a likely outcome.  One of the most energetic events in the universe, supernovae release a drove of neutrons, which bombard nearby atoms and build them up into heavier, unstable ones.  These short-lived elements then decay radioactively to produce all of the naturally-occurring elements heavier than iron.

Each generation of stars since the Big Bang has added more and more heavy elements to the mix, and that provides a way to sort stars by age. As Frebel explains, “Because all the elements have been created successively, piece by piece throughout cosmic time, we just look for the stars with the least amounts of heavy elements in them. That tells us that they have formed very early in the universe when there simply haven't been enough stars and stellar generations to produce all that material.”

Studying these old stars can tell us how the early universe was changed by the influx of new elements created by the very first generation of stars, and that story is still evolving.  Just last October, Frebel and her graduate student Alexander Ji observed a set of old stars in a nearby dwarf galaxy that contained a surprise:  these early stars all seemed to be unexpectedly enriched in certain heavy elements, suggesting that a single, rare event — and not a supernova — had left its mark on their surroundings before they formed.  One possible explanation involves a neutron star merger, a collision between two orbiting supernovae remnants that can release enough neutrons to drive the production of heavy elements.  The role of neutron star mergers in the early universe in the formation of star stuff had previously been somewhat downplayed, but these observations prompted Frebel to rethink that conclusion.  “Our story is changing, and we now think these heavy elements are probably not primarily produced by supernova in the early universe but by neutron star mergers.”

For Frebel, that kind of surprising development, leading to an unexpected breakthrough, is what the scientific process is all about.  “It can be very hard to change your mind, but If you have exciting new evidence, it's not that hard. Nature really leads you on the right path of understanding what happened back then.”

—Podcast and post by Meg Rosenburg

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