First Detection of Thorium in Another Galaxy

By Ken Croswell

May 8, 2007

The discovery of thorium in the Ursa Minor Galaxy offers the first chance to measure another galaxy's age via radioactive dating, say astronomers in Japan. Furthermore, the abundance of thorium and other heavy elements hints that this galaxy evolved differently than the Milky Way.

Thorium is a heavy, radioactive element with an atomic number of 90. Thus, each thorium atom has ninety protons in its nucleus. Until now, no one had seen such a heavy element beyond our Galaxy.

Wako Aoki, Satoshi Honda, and Nobuo Arimoto of the National Astronomical Observatory in Tokyo and Kozo Sadakane of Osaka Kyoiku University observed COS 82, a seventeenth-magnitude orange giant star in the Ursa Minor dwarf spheroidal galaxy. Only 220,000 light-years from Earth, the Ursa Minor dwarf is a satellite of the Milky Way, orbiting our Galaxy just as the Moon orbits the Earth. Unlike the Milky Way, Ursa Minor has no gas from which to create new stars, and its stars are old.

Those stars are spread out from one another, giving the Ursa Minor Galaxy a diffuse appearance. As a result, despite its proximity, astronomers discovered the galaxy only in 1954. All its stars put together emit less light than the single most luminous star in the Milky Way.

Aoki and colleagues chose to search for thorium in COS 82 because other astronomers have discovered high levels of r-process elements there. The r-process is thought to occur when a massive star explodes as a supernova, bombarding iron nuclei with a rapid flux of neutrons. The r-process is responsible for the creation of most elements heavier than iron (atomic number 26) and all elements heavier than bismuth (atomic number 83), including thorium.

To astronomers, the best-known r-process element is europium (atomic number 63). COS 82 has a very high europium-to-iron ratio. It's twenty times the Sun's.

Using the 8.2-meter Subaru telescope atop Mauna Kea in Hawaii, Aoki and colleagues succeeded in detecting a weak thorium line in the star's spectrum. This spectral line appears at an orange wavelength of 5,989 Angstroms.

Timothy Beers of Michigan State University calls the discovery exciting. "This star formed out of gas which presumably was uniquely associated with that dwarf galaxy," he says. "So it's not just a general field star, which we don't know exactly where it came from."

Only twelve stars are known in the entire Milky Way with europium-to-iron ratios exceeding ten times the solar ratio. All these r-process-enhanced stars reside in the Milky Way's halo, the population of ancient stars that surrounds the Galaxy's spiral disk. Furthermore, all twelve stars have nearly the same abundance of iron.

Astronomers usually express iron abundances logarithmically, using the symbol [Fe/H]. An [Fe/H] of 0 means a solar iron abundance; an [Fe/H] of -1, an iron abundance that is one-tenth solar; an [Fe/H] of -2, an iron abundance that is one-hundredth solar; an [Fe/H] of -3, an iron abundance that is one-thousandth solar; and so on.

All known r-process-enhanced stars in the Milky Way have an [Fe/H] of around -3. That means they have just 0.1 percent of the Sun's iron abundance. Therefore, these stars formed early in the Galaxy's life, before supernova explosions had the chance to create much iron.

In striking contrast, the thorium star in the Ursa Minor dwarf galaxy has an [Fe/H] of -1.4, which corresponds to an iron abundance that is 4 percent of the Sun's. Thus, this star has 40 times more iron than the typical r-process-enhanced star in the Milky Way.

The difference suggests to Aoki's team that the Ursa Minor dwarf galaxy evolved differently than the Milky Way's halo. They cite work by two other Japanese astronomers, Takuji Tsujimoto and Toshikazu Shigeyama, who argue that r-process enhancements in stars of higher iron abundance could have occurred in a dwarf galaxy. When the Ursa Minor Galaxy was young and still had iron-free gas, these astronomers say the galaxy had roughly 25 times more mass than it does today. As a result, the galaxy's gravity caused the gas to move fairly fast with respect to its stars. Supernova explosions injected iron-rich debris into this iron-free gas. But because the iron-free gas was moving fast, the iron-rich supernova debris did not sweep up much of this pristine gas and thus did not get much diluted. Thus, r-process-enhanced stars such as COS 82 could form from this supernova debris having higher iron abundances than did r-process-enhanced stars in the Galactic halo.

But Beers favors another explanation. He thinks the r-process enhancements are local rather than global. In particular, he thinks the thorium star in Ursa Minor once had a massive companion star. This companion exploded, showering the surviving star with high levels of thorium and other r-process elements.

To distinguish between the two ideas, astronomers must study other stars in the Ursa Minor dwarf. If several other stars have r-process enhancements, then these are probably due to the galaxy's evolution. On the other hand, if COS 82 remains unique, then it probably owes its thorium abundance to a former companion star that exploded.

Because thorium is radioactive, scientists use it to date stars. Thorium's half-life is 14.05 billion years, so half of it decays over this period of time. When Aoki and colleagues compare the star's thorium level with that of stable r-process elements, such as europium, they find a lower thorium-to-europium ratio than in the Sun. Thus, more of the star's original thorium has decayed, so the star must be older than the Sun.

Aoki's team derives an age for COS 82 which matches that of an r-process-enhanced star in the Milky Way's halo named CS 22892-052, which is 12 or 13 billion years old. This, in turn, suggests the Ursa Minor dwarf is as old as the Galactic halo.

However, the uncertainty in COS 82's age is large--6 billion years--due to measurement uncertainties. Also, thorium and europium have quite different atomic numbers. Although both elements arose in the r-process, different supernovae probably created different proportions of the two elements. Thus, no one knows exactly what COS 82's original thorium-to-europium ratio was.

To derive a better age, Aoki and colleagues recommend a search for uranium. Unlike europium, uranium (atomic number 92) has just two more protons than thorium, so astronomers think that all supernovae create the two elements in the same proportion. Thus, a star's original thorium-to-uranium ratio is known. By observing both thorium and uranium levels in COS 82, astronomers should be able to obtain an age for the star good to just 2 billion years--which should allow them to say whether the Ursa Minor Galaxy is as ancient as the Milky Way's halo.

However, detecting uranium will be a challenge. The element has been seen in a few stars within the Milky Way, but never beyond.

Aoki and colleagues will publish their discovery of extragalactic thorium in Publications of the Astronomical Society of Japan.

Ken Croswell earned his Ph.D. in astronomy for studying the Galactic halo and is the author of a book about the Milky Way, The Alchemy of the Heavens, as well as Magnificent Universe and Ten Worlds.

"An engaging account of the continuing discovery of our Galaxy...wonderful."--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.

"On the basis of its striking design and photographs, this handsome, large-format volume is well worthy of praise. And astronomer Croswell's concise yet conversational, information-packed text wins it sky-high accolades in the narrative sphere as well."--Publishers Weekly, starred review. See all reviews of Ten Worlds here.