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Iron in the Fire: The Little-Star Supernovae That Could

Exploding white dwarf stars and an unexpected abundance of iron explain the chemistry of a unique star cluster
supernovae, dwarf stars, hubble telescope, Large Magellanic Cloud



NASA, ESA, and the Hubble Heritage Team (STScI/AURA); acknowledgment: J. Hughes (Rutgers U.)

Although stars other than our sun are light-years away, their gifts are all around us: the oxygen we breathe, the calcium in our bones, the iron in our blood. High-mass stars have been especially generous. During their short lives, they cook up such elements as oxygen and magnesium and eject them into space via supernova explosions; the blasts themselves forge still heavier elements, such as gold and platinum. About 80 percent of the Milky Way's supernovae arise from large stars, so their by-products are everywhere—from the magnesium in chocolate to the gold at Fort Knox.

In contrast, supernovae from small stars are element-creating underdogs. Named type Ia, these supernovae occur when a white dwarf—a dense star smaller in diameter than Earth—accretes too much material from a companion star and gains a mass of at least 1.4 suns. The resulting explosion spews heavy elements into space. These "little-star" supernovae do excel at producing one vital element, iron; they have created roughly two thirds of the iron in Earth's core and in your blood. Despite that important accomplishment, though, small-star supernovae have left a lesser mark on the world. The same pattern appears in the heavens, where elements generated by large-star explosions overwhelm those from their smaller and rarer brethren.

But now a surprising exception has turned up: in NGC 1718, a two-billion-year-old cluster of about 100,000 stars located in the Large Magellanic Cloud—the Milky Way's brightest satellite galaxy, around 160,000 light-years from Earth. "NGC 1718 has very unusual chemical abundances," says Takuji Tsujimoto, an astronomer at the National Astronomical Observatory of Japan. "It has chemical abundances equivalent to type Ia supernovae."

The key clues are magnesium and iron. Magnesium nuclei originate in high-mass stars and normally outnumber iron nuclei, the latter a product of both types of supernovae. But new observations by astronomer Janet Colucci of the University of California, Santa Cruz, and her colleagues reveal that the odd star cluster has far more iron than magnesium. Its iron-to-magnesium ratio is eight times the solar value.

In the past, astronomers have found a few individual stars with high iron-to-magnesium ratios; each star probably formed from a pocket of gas that happened to lie near one exploding white dwarf. But until now, no one has seen these abnormal abundances among such a profusion of stars.

What happened in NGC 1718? The odd chemistry "means that the formation history of this cluster should be quite different from other clusters," says Kenji Bekki, an astronomer at the University of Western Australia. He and Tsujimoto propose in The Astrophysical Journal Letters that it takes a virtual village of exploding white dwarfs to endow an entire cluster of 100,000 stars with such high iron-to-magnesium ratios.

It also takes another galaxy: The Large Magellanic Cloud has its own satellite—the Small Magellanic Cloud—which dances around its mate on an elliptical orbit, sometimes coming close, other times veering away.

To explain NGC 1718, Tsujimoto and Bekki propose that the drama actually started with another star cluster, whose high-mass stars exploded and catapulted gas out of part of the Large Magellanic Cloud. Then the Small Magellanic Cloud swung by, dumping its gas into the hole left by the exploding stars. This gas was magnesium- and iron-poor, because the smaller of the two galaxies has fewer stars to create these elements. Then white dwarfs in the same cluster that had blasted the hole began to detonate, enriching the infalling gas with their iron but little magnesium.

Recent observations suggest that type Ia explosions begin 100 million years after stars form. The time lag occurs because white dwarfs evolve from stars that live longer than high-mass stars do. Tsujimoto and Bekki calculate that 17 to 20 exploding white dwarfs cast their iron-rich debris into the gas, which then spawned NGC 1718, whose many new stars inherited the high iron-to-magnesium ratio.

"I agree with the authors," says Rosemary Wyse, an astronomer at Johns Hopkins University. "In all likelihood, the cluster formed from material that was extremely rich in the ejecta from type Ia supernovae." Two decades ago, she and Gerard Gilmore of the University of Cambridge invoked exploding white dwarfs to explain milder abundance anomalies in the Magellanic Clouds. She says the cluster demonstrates how a galaxy's chemical evolution doesn't always proceed smoothly. Instead, irregularities can arise, with chemical abundances that deviate from the norm, allowing underdogs like type Ia supernovae to be occasional champions.

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