HE 0107-5240

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



Larger image.




As a halo field star with the
lowest metallicity known for
any Milky Way star in late
October 2002, HE 0107-5420
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 0107-5240 appears to be even older than the ancient stars found in globular clusters. Extremely scarce in elements heavier than hydrogen and helium ("metals"), HE 0107-5240 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 University of Michigan; and Christlieb et al, 2002). The star is only 1/200,000th as enriched in metals as Sol and is 20 times less enriched than the previously known, lowest-metal star CD-38 245 that was found in 1977. (More discussion on the chemical compositions of halo field stars and globular cluster stars.)

ESO


Larger image.



HE 0107-5240 has the
lowest, known abundance
of elements heavier than
hydrogen and helium of
stars in the Milky Way
Galaxy that have thus
far been measured for
"metallicity" (more).

Given its extremely low metallicity, mass, and stage of evolution, HE 0107-5240 may be only about about a billion years younger than the universe itself. Hence, if the Big Bang created the universe about 13 to 16 billion years ago, HE 0107-5240 may be 12 to 15 billion years old (more discussion on the age of the universe). Many now believe that most, if not all, of the first generation of stars (Population III) 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. When 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 supernovae, contaminating the universe with richer gas and dust. 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.

The discovery of HE 0107-5240 demonstrates that stars that are less massive than Sol can form from very metal-poor gas. This finding was unexpected, as most current theoretical calculations indicate that it should have been very difficult to form low-mass stars shortly after the Big Bang because heavier elements are needed to efficiently cool gas clouds as they contract into stars (more discussion on the expected mass of Population III stars from Bernard Carr, 1994 versus Richard B. Larson, 1999). However, the existence of HE 0107-5240 suggests that there must be other ways of achieving the necessary cooling. Moreover, the star's discovery suggest that even relatively low-mass Population III stars could have formed and survived until today, still shining faintly below easy detectability as main sequence dwarf stars in distant reaches of the galactic halo.

According to astronomer Catherine Pilachowski (who wrote an analysis of HE 0107-5240's discovery for Nature), HE 0107-5240 contains a "striking excess" of nitrogen and carbon although these elements not usually abundant in stars of the cool, giant family to which the star belongs. Hence, it's possible that HE 0107-5240 once orbited a more massive and short-lived, binary companion which contaminated the lower mass star with those elements. If so, then it's possible that most, if not all, of the galaxy's extremely metal-poor stars surviving to the present day originally formed as part of a binary pair, a byproduct of the formation of a massive star.