- Issue 1 |
- August 2009
Exploring the Cosmic Origin of the Elements
“Where do all the elements that make up our bodies and our world come from?”
Fig. 1 Artist’s rendering of a
white dwarf star accreting
hydrogen from a nearby star.
This leads to novae or, rarely,
type Ia supernovae.
For most of recorded history, the answer to this question has been the stuff of speculation, if not myth. Today DOE scientists, in concert with their colleagues around the world, synergistically combine cutting edge measurements in nuclear accelerator labs with computer simulations and satellite observations to probe the mysteries of our Galaxy and the Universe.
As a consequence of their work, we now know some answers to the “elements puzzle.” Some of the elements, for example, were formed in the Big Bang, when the universe was created. Others were cooked up in the seething maelstrom of stars. Still others, we think, are created in cataclysmic stellar explosions, such as supernovae or novae (see Figure 1). But how much material is synthesized in exploding stars is still a mystery.
Fig. 2 Satellite observatories like INTEGRAL can see
our Galaxy in “26Al light.” The “hot spots” indicate
recent (106 years) nucleosynthesis events.
To answer this, we launch satellites with special “eyes” to capture traces of these cosmic detonations, and try to devise explosion simulations that match their snapshots. A magnificent example of this took place ninety-five thousand miles above Earth, where NASA’s Compton Gamma Ray Observatory spent about 10 years gathering data not in visible
light but in “Aluminum-26 light” which, when coalesced into a detailed map of our galaxy (Figure 2) revealed numerous hot spots. The lumpy distribution, representing a hundred trillion trillion kg of radioactive Aluminum-26, is inconsistent with some models of how the elements are created. Whatever created this Aluminum-26, it must have happened recently—within the last million years or so—because this exotic aluminum decays into Magnesium in that time, emitting energy in the form of gamma rays—the source of the hot spots.
Fig. 3 The Oak Ridge Rutgers University Barrel
Array, a high-resolution instrument with specialized
particle detectors arranged in a barrel shape around
a target. The instrument is housed at the Holifield
Radioactive Ion Beam Facility at Oak Ridge
To make sense of these and related discoveries, an international effort has been launched t o make laboratory measurements of the nuclear reactions that create, and subsequently destroy, this unusual Aluminum in exploding stars. In 2009 at DOE’s Holifield Radioactive Ion Beam Facility at ORNL, a beam of unstable Aluminum-26 bombarded a target of hydrogen to determine how fast this exotic aluminum is burned up before giving off its special light. Figure 3 shows some of the sophisticated detectors used for this study, which was a search for “sweet spots” in the nuclear reaction that would destroy more Aluminum than previously thought. When combined with complementary measurements at other facilities, we will get a better handle on exactly what this map is telling us about exploding stars.
For all that we have discovered so far, there is still much to learn. For example, we know very little about how elements heavier than iron came into being. DOE’s Facility for Rare Isotope Beams (aka FRIB), planned for nearly a decade, will give us the ability to study this right here on Earth, to find additional pieces in the elements puzzle.
This research was sponsored by the US Department of Energy, Office of Science’s Office of Nuclear Physics.