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In The Stars: Omega Centauri Blues

In order to go supernova, a star must be at least 10 times more massive than the sun, forming a creature called a red supergiant.
by Phil Berardelli
Science & Technology Editor
Washington (UPI) Mar 28, 2005
The late astronomer Carl Sagan was fond of saying "we are starstuff," referring to the fact that the atoms in our bodies were once cooked up inside an ancient giant star that exploded.

It is rather beguiling, the thought of being literally born of the cosmos - and it is true: the carbon, oxygen, nitrogen and heavier elements within each of us were produced by a supernova - but the process also is a lot more complex than it sounds, and a lot more puzzling.

For one thing, not all stars eventually explode - in fact, most do not. Many, like our own sun, burn more or less steadily for 10 billion years or so, then heave a massive sigh, temporarily growing to hundreds of times their normal size, then shrinking radically and cooling more or less permanently into a white dwarf.

In order to go supernova, a star must be at least 10 times more massive than the sun, forming a creature called a red supergiant.

These stars live fast and die young - well under 100 million years - when a change in their nuclear reaction triggers a near-instantaneous collapse.

As a result, internal heat and density spike so high that the doomed stars fuse their constituent atoms of hydrogen and helium into the heavier elements.

Then, like a billion-billion nuclear bombs exploding simultaneously, the supernova occurs, ripping the star apart and propelling its matter outward at great speed with awesome force.

The star's remnant settles down into a pulsar or maybe even a black hole, but the supernova continues outward, shredding anything in its path for hundreds of light-years.

That latter detail becomes somewhat worrisome when one considers that just such a red supergiant resides about 425 light-years away from Earth.

Betelgeuse, which occupies the right shoulder in the constellation Orion, is about 15 times more massive than the sun, about 630 times its diameter and maybe 60,000 times brighter.

Its fuel consumption is so intense Betelgeuse could go supernova in a few million years, creating havoc for the residents of surrounding star systems - possibly including ours.

The question is why do some stars form as relatively small and sedate as the sun, while others grow as massive and fierce as Betelgeuse? Astronomers still are unsure, as they are about why stars emerging out of clouds of dust and gas can contain widely differing concentrations of helium.

The puzzle is likely to continue with the word from a team working at the European Southern Observatory's Very Large Telescope in Paranal, Chile, that it has collected some perplexing information about the stars in Omega Centauri, a very distant globular cluster in the southern hemisphere.

Globular clusters are galaxy wannabes. They are groups of up to a million stars - compared to as many as 400 billion in the Milky Way - all of which are thought to have emerged together from the same interstellar cloud at approximately the same time.

Omega Centauri seems to be an exception to this rule, however. Definitely a globular cluster, it harbors two distinct populations of stars, about 75 percent burning in the red color range and 25 percent in the blue.

Using a sensitive multi-object spectrograph called FLAMES, the astronomers compiled more than 200 hours of exposures and found that the blue stars are the most helium-rich stars known.

Piecing the saga together from other evidence, the team said those helium-rich blue stars represent the late-comers to the cluster. Their births followed the emergence of the red majority.

As expected, the Omega Centauri reds converted hydrogen into helium by nuclear fusion. Also expected, some of the stars, with masses of 10 times or more of the sun's, eventually went supernova, spreading their helium among the interstellar medium and providing seed matter for the second-generation blues.

So far, so good, but the problem is helium-rich stars are supposed to be red, not blue. In other words, not only are the Omega Centauri blues super-abundant in helium, but they also are burning in the wrong color range.

"The only way we can explain this discrepancy is by assuming that the two populations of stars have a different abundance of helium," said Giampaolo Piotto, of the University of Padova, Italy, and leader of the team.

"We find that while the red stars have a normal helium abundance, the bluer stars must be enriched in helium by more than 50 percent with respect to the other population."

Under the standard model of star formation, this makes no sense. The Milky Way galaxy, for example, needed about 8 billion years to increase its helium content from the primordial 24 percent created by the Big Bang to the present 28 percent in the local helium-rich population.

The Omega Centauri blues, in contrast, contain at least 39 percent helium.

Another team member, Luigi Bedin, said the explanation might lie partly in the reds that went supernova and partly in the size - and gravity - of the cluster.

"The scenario we presently favor is one in which the high-helium content originates from material ejected during the supernovae explosions of massive stars," said Bedin, an astronomer at the ESO.

"It is possible that the total mass of Omega Centauri was just right to allow the material expelled by high-mass supernovae to escape, while the matter from explosions of stars with about 10-12 times the mass of the sun was retained."

When the supergiants among the Omega Centauri reds exploded - within a few tens of millions of years - they ejected helium-enriched matter that "polluted" the globular cluster, Bedin said. The blues then formed from this helium-rich gas.

Perhaps, but the team acknowledged certain questions remain about this process. The biggest of which concerns Omega Centauri itself: Why is it the only one among all known globular clusters that was able to produce super-helium-rich stars?

In the Stars is a weekly series examining new discoveries about the cosmos. E-mail: sciencemail@upi.com

All rights reserved. 2005 United Press International. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by United Press International. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of United Press International.

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A Tale Of Two Populations
Paranal, Chile (SPX) Mar 16, 2005
On the basis of stellar spectra totalling more than 200 hours of effective exposure time with the 8.2-m VLT Kueyen telescope at Paranal (Chile), a team of astronomers has made a surprising discovery about the stars in the giant southern globular cluster Omega Centauri.



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