HE 1523-0901

Bill Keel, Ray White III, Chris Conselice,
Cerro Tololo Inter-American Observatory



Larger image.




As a halo field star whose age
has been measured using
radioactive thorium and uranium
and other neutron-capture
element dating, HE 1523-0901
appears to be even older than
the ancient stars found in the
galaxy's globular clusters such
as 47 Tucana, at left.

A halo field star, HE 1523-0901 appears to be even older than the ancient stars found in globular clusters. Extremely scarce in elements heavier than hydrogen and helium ("metals"), HE 1523-0901 was found as part of a search for metal-poor halo stars in the Hamburg/ESO Survey, which gave it its "HE" designation in combination with its position (see: press releases from ESO and the McDonald Observatory; Frebel et al, 2007; and Karen Sanderson, Nature, May 11, 2007). (More discussion on the chemical compositions of halo field stars and globular cluster stars.)

When massive and fast evolving Population III stars matured into supergiant stars after a few million years of life or so, however, they blew off heavier elements in strong stellar winds, and many probably exploded as "pair-instability" supernovae, contaminating the universe with gas and dust rich with heavier elements than hydrogen and helium. As a result, the second generation of stars (Population II) were born with the first doses of heavier elements (such as carbon, oxygen, silicon, sulfur, neon, magnesium, and iron), although most have an heavy-element abundance that is only 1/10th to 1/1000th of Sol's metallicity. Therefore, the first Population II stars (such as HE 0107-5240) should have been contaminated the least. By comparison, Sol was born of later generations of Population I stars which formed from gas and dust that has been contaminated over billions of years of stellar evolution.

Edward L. Wright, COBE, DIRBE, NASA -- larger infrared image
(Like many other Population II stars, HE 0107-5240 lies outside the galactic bulge and disk -- more.)

Most Population II stars are found outside the spiral disk of the Milky Way. Some are found in globular clusters, but most move in a huge cloud around the disk called the galactic halo, which has a luminous inner component defined by globular star clusters and other easily observable stars (with coronae of hot gas possibly expelled by supernovae and of high-velocity neutron stars) and an outer dark-matter component inferred from its gravitational impact on the Milky Way's spiral disk. As these "halo stars" were born when the Milky Way was young, their motions through and outside of the spiral disk still carry the imprint of the process by which the galaxy formed, when gravity brought gas together to create the first stars.

© ESO


Larger illustration.



HE 1523-0901's age
was deduced by
measuring very
precisely the
abundance of the
radioactive elements
thorium and uranium
after 13.2 billion
years of decay into
lighter elements (more).

HE 1523-0901 may be only about half a billion years younger than the universe itself, where the Big Bang created the universe about 13.7 billion years ago (more discussion on the age of the universe). Many now believe that most, if not all, of the first generation of stars that formed from the gas and dust created by the Big Bang were massive, fast-burning, short-lived, and composed only of the four lightest elements, hydrogen and helium with traces of lithium and beryllium. Hence, HE 1523-0901 is a very rare star to be both extremely old and yet contain very heavy elements such as uranium in its spectrum. Some astronomers argue that the observation of uranium in the optical absorption lines of spectra from ultra–metal-poor stars (such as CS 31082-001 and HE 1523-0901) imply that these ancient stars were contaminated from the supernova explosion of a binary companion star (Qian and Wasserburg, 2002), which may yet be present as a neutron star or black hole with accretion processes that generate detectable x-rays (Eric M. Schegel, 2002).

© ESO


Larger illustration.



The presence of uranium
(U II line) in the star's
atmosphere was observed
and modelled against a
synthetic spectrum without
any uranium (more).

The best direct method for dating the age of an ancient star currenty involves studying the ratios of different isotopes of certain elements in stellar atmospheres. Called nucleo-chronometry, the dating procedure uses radioactive decay and a radioactive element’s half-life, which is the amount of time it takes for half of the original amount of the element to decay into another. Uranium and thorium have half-lives of 4.7 billion and 14 billion years, respectively, but uranium is a better radioactive clock for dating stars than thorium, because thorium’s half-life is actually a little longer than the known age of the universe -- currently estimated at 13.7 billion years. Dating involves deriving the ratios for all the elements that the star formed with and then measures what ratios of those elements are actually observed in the star's atmosphere today (more discussion is available from astronomer Pamela L. Gray).

Each radioactive element, moreover, must be "anchored" to another element within the star. Fortunately, in the case of HE 1523-0901, three anchor elements (europium, osmium, and iridium) were detected so that an usually accurate age could be derived. The combination of two radioactive elements with three anchor elements provided six so-called “cosmic clocks” which were used to date HE 1523-0901's age with unusual precision (McDonald Observatory news release).