ESO finds 'impossible' star in Leo

(source: digitaljournal.com)

The European Space Agency announced a team using the Very Large Telescope (VLT) recently zoomed in on a faint Milky Way star in the constellation Leo that has so few elements it falls into the "forbidden zone" of a widely accepted star formation theory.

Composed mostly of hydrogen and helium, with only the tiniest traces of heavier elements, the star SDSS J102915+172927 is probably more than 13 billion years old, yet would have been thought impossible by many scientists, according to the ESO astronomers, whose findings appeared September 1 in the journal Nature. The researchers found this visually unremarkable galactic halo star, glowing faintly in the Milky Way's Lion, chemically bizarre, because it contains the lowest amounts of metals of any star studied to date. The astronomers used the VLT's X-shooter and UVES instruments to analyze the unusual chemical composition of SDSS J102915+172927, and found the proportion of metals in the star was at least 20,000 times lower than the Sun's.

ESO/Digitized Sky Survey 2

This pie-chart shows the strange, metal-poor composition of the ancient star SDSS J102915+172927, discovered in the constellation Leo (The Lion) by European Space Agency astronomers using the Very Large Telescope (VLT); it is almost entirely made from hydrogen and helium, with only tiny traces of heavier elements.

According to the researchers, most cosmologists believe the lightest elements, hydrogen and helium, formed immediately after the Big Bang along with some lithium, while most of the heavier elements were generated by stars later, then dispersed by supernova explosions, so that newer stars formed within the element-enriched interstellar medium since then have higher proportions of metals. ESO astronomer Lorenzo Monaco explained, “The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found.” The star's extremely low lithium content also surprised the team, because scientists expect the oldest stars to be composed mostly of hydrogen, helium and lithium, plus a few metals, matching the widely theorized composition of the earliest Universe. “It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star, " project supervisor Piercarlo Bonifacio remarked. But the team concluded there may be many "freakish" stars like this out there. Lead author Elisabetta Caffau stated, “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case.” 

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Star Smile mad

Star eaten by a black hole: still blasting away

(source: blogs.discovermagazine.com)

In late March of 2010, an extraordinary event occurred: a black hole in a distant galaxy tore apart and ate a whole star.
Now, there’s more info: the black hole, lying at the center of a galaxy nearly 4 billion light years away, has about 8 million times the mass of the Sun. When it tore the star apart, about half the mass of the star swirled around the black hole, forming twin beams of matter and energy that blasted outward at a large fraction of the speed of light. The folks at NASA’s Goddard Space Flight Center made a great animation to show this:



The star was ripped apart by tides. The thing about black holes is, they’re small: this one was probably about 15 million kilometers across. A typical star is about a million km across (the Sun is 1.4 million kilometers in diameter, for comparison). This means the star could get really close to the black hole, and that’s why it was doomed. The force of gravity drops with distance, so as the star approached, the side of it facing the black hole felt a far greater force than the size facing away. That stretched the star, and the stretching increased as the star got closer. At some point, the force was so great it exceeded the star’s own gravity, and it could no longer hold on to its material. The black hole won — as they usually do.


The material from the shredded star formed a disk around the black hole, and near the center heated up to millions of degrees as it swirled around at near the speed of light. For reasons not entirely understood, this forms the beams of matter that jet away from the hole, and as it happens one of these beams was aimed pretty much right at us (not to worry, though, since at that vast distance the light was so diminished it took Hubble to see it in visible light at all). That’s what alerted us to the event in the first place; it was detected by the Swift satellite, which was designed to look for high-energy blasts from space.
Normally, things like this fade pretty quickly, but in this case, amazingly, it’s still pouring out energy and will probably be detectable even into 2012. That is partly due to relativity: because we’re looking straight down the beam of material, we see its clock ticking more slowly. This effect works better when the material is moving at high speed, and radio observations show that the blast is still expanding away from the black hole at half the speed of light! And that’s after it slowed down by ramming into material floating in between the stars in that galaxy. It started out moving at more than 90% the speed of light.
The energy it takes to do this is mind-numbing: we’re talking about roughly an octillion tons of matter screaming outward at well over one hundred thousand kilometers per second!
It literally makes the hair on the back of my neck stand up. I’m glad this happened billions of light years away.
Astronomers will continue to observe this event to learn more about it. It probably happens all the time in the Universe, but this is the first time we’ve had the equipment to really get a good look (even if we have to crane our necks a bit from 4 billion light years away). We’re not really sure how often something like this happens, or how it affects the galactic environment. I’ll note I’m not terribly worried about it happening in our Milky Way (we have a 4 million solar mass black hole in the center of our galaxy), since, after all, we’re here. If this happened often enough to be dangerous, we wouldn’t be here to talk about it!



Star Smile mad