The Stellar Origin of Copper

By Ken Croswell

April 6, 2007

Image of Orion by Bill and Sally Fletcher. Used by permission.

Most of the copper in pennies and pipes arose in supergiant stars like Rigel and Betelgeuse, say astronomers in Italy. The stars then exploded, casting the copper into space. The new finding means that gold, silver, and copper all owe their existence to massive stars.

Scientists know the origins of most of the chemical elements from hydrogen to uranium. However, copper (atomic number 29) is an exception. Some theories say the element originated in big stars, while other theories point to stars smaller than the Earth: exploding white dwarfs, which astronomers call type Ia supernovae.

Now Donatella Romano at the Astronomical Observatory of Bologna and Francesca Matteucci at the University of Trieste have come down on the side of big stars. Their conclusion is noteworthy in part because Matteucci once held the opposite view, that copper arose instead in type Ia supernovae.

When stars die, they eject some of the elements they have created into space, where subsequent generations of stars inherit them. These younger stars thus preserve a record of the deceased stars' nucleosynthesis.

Romano and Matteucci compared copper and iron abundances that observers have measured in stars of different ages. Some of these stars were in the general Milky Way. Others were in Omega Centauri, the Milky Way's most luminous globular star cluster, which may be the nucleus of a small galaxy that smashed into the Milky Way. Omega Centauri has an unusually low copper-to-iron ratio.

As Romano and Matteucci report in a forthcoming issue of Monthly Notices of the Royal Astronomical Society, their models of galactic chemical evolution can explain the data in both locations only if nearly all copper on Earth arose in massive stars--those born with more than eight times the mass of the Sun.

The stars create copper after leaving the main sequence. While on the main sequence, a massive star generates energy by converting hydrogen into helium at its core. It does so through the CNO cycle, in which carbon, nitrogen, and oxygen catalyze the hydrogen-to-helium reaction. The CNO cycle also changes carbon and oxygen into nitrogen-14.

After the star's core runs out of hydrogen, the star expands into a supergiant, and its core begins converting helium into carbon and oxygen. Helium nuclei also hit the nitrogen-14, transforming some of it into neon-22. When neon-22 hits helium, it becomes magnesium-25 and liberates a neutron. These neutrons then change some of the iron (atomic number 26) that the star acquired at birth into copper. The flux of neutrons is slow, so astronomers refer to this type of nucleosynthesis as the s-process.

At the end of its life, the star explodes as a type Ib, Ic, or II supernova. The explosion hurls the copper into space--and ultimately into future generations of stars.

"I think this is probably the right explanation," comments Andrew McWilliam at the Carnegie Observatories in Pasadena, California. "The origin of copper has been debated for a while, and every single conceivable stellar site seems to have been proposed at one time or another." To confirm copper's origin, McWilliam says astronomers should study copper in additional galaxies, such as those which orbit the Milky Way.

Gold, silver, and platinum also arise chiefly in massive stars--but via the r-process rather than the s-process. The r-process occurs when massive stars explode as supernovae, releasing a rapid flux of neutrons that bombard iron nuclei and convert them into heavier elements. Thus, whereas copper is forged during the lives of massive stars, gold, silver, and platinum originate in these stars' deaths.


Atomic NumberElementProduced Mostly ByEjected Into Galaxy By
29Coppers-process in high-mass starsType Ib, Ic, and II supernovae
47Silverr-process in type Ib, Ic, and II supernovaeType Ib, Ic, and II supernovae
78Platinumr-process in type Ib, Ic, and II supernovaeType Ib, Ic, and II supernovae
79Goldr-process in type Ib, Ic, and II supernovaeType Ib, Ic, and II supernovae

Early in the Milky Way's life, however, the s-process could not make much copper. That's because the first stars had little carbon, nitrogen, oxygen, or iron. Without carbon, nitrogen, and oxygen, a star can't burn its hydrogen via the CNO cycle, which produces the nitrogen-14 that produces the neon-22 that produces the neutrons that convert iron into copper. Furthermore, without iron, the neutrons can't make copper.

Instead, Romano and Matteucci say, the Galaxy's first copper originated when its first stars exploded. Although such explosively synthesized copper makes up just a few percent of the copper on Earth, it accounts for most of the copper in the Galaxy's oldest stars--those with iron abundances below 1.6 percent of the Sun's. These iron-poor stars reside in the Galactic halo.

Ken Croswell earned his doctorate in astronomy from Harvard University for his study of the Galactic halo, and his book The Alchemy of the Heavens describes the origin of elements in the Milky Way; page 166 gives the origins of the ten most abundant elements in the universe. He is also the author of Magnificent Universe.

"An engaging account of the continuing discovery of our Galaxy."--New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.