stars – Australian Science

Explosions visible from across the Universe

Markus — Mon, 01 Jul 2013 00:06:58 +0000

Gamma ray bursts are the most violent and energetic events in the entire universe. Powerful blasts of high energy gamma radiation, bright enough to be seen from literally the very edge of the visible Universe. And yet, we know surprisingly little about them. Of course, we have theories, but every now and again astronomers spot something which those theories don’t fully cover. Such as a couple of years ago, when an international team of astronomers caught a glimpse of the longest lasting gamma ray burst ever seen.

December 9th, 2011. NASA’s Swift telescope detected a sudden spike of gamma rays from somewhere within the constellation of Phoenix, visible in the Southern hemisphere. Gamma ray bursts (GRBs for short) like these are normally subdivided into two types, short bursts and long bursts, which have different causes. Short bursts last for just a couple of seconds, while long bursts can last for several minutes. But this particular burst, dubbed GRB 111209A (pictured above), was very different. One of a unique, rare breed of extra long lived GRBs, and by far the longest ever observed – lasting an unprecedented 7 hours!

Ever since then, astronomers have been picking at the data which were recorded, trying to work out exactly what kind of unusual circumstances would cause such a goliath GRB. One team, a collaboration between Bruce Gendre now at the French National Center for Scientific Research, and David Coward and Eric Howell at the University of Western Australia, managed to get to what may be the bottom of the puzzle. In short, these extra long GRBs are caused by the deaths of the Universe’s most massive stars.

It’s a sobering fact that all stars die. Some burn slowly for billions of years. The very smallest may burn for trillions. But the most massive stars burn out rapidly before dying in titanic explosions which we know as supernovae. It seems that the most massive of the massive, exceptionally rare, die in a unique type of GRB.

So far, only three of these ultra-long GRBs have been found, appearing so bright that some astronomers initially believed that they came from inside our own galaxy. As it happens, this wasn’t the case. Analysing GRB 111209A, Gendre and his colleagues hypothesised the source of it to be an extremely hot, massive star.

A furiously burning blue supergiant (possibly a hypergiant), hundreds of times the diameter of the Sun. When a star like this dies, its core collapses into a black hole, which then proceeds to start devouring the star from the inside out. As that black hole hungrily eats, its mammoth gravitational forces brutally tearing matter apart at the subatomic level, it generates an immense amount of energy which causes two jets of material to shoot out from its north and south poles. These are blasted outwards out with such ferocity that they actually punch two holes in the doomed star.

Read that again. A star which is hundred of times as massive as the sun, and these jets are powerful enough to blast two holes in it! The amount of energy required to accomplish that is truly difficult to comprehend.

The stars which die in these titanic events still require a specific set of circumstances. These stars are apparently enriched in elements heavier than helium (astronomers refer to these as “metals”, even though chemists would disagree on a few counts). More of these heavier elements cause a star’s stellar wind to increase, causing its rotation speed to slow as it loses material. But faster spinning stars seem to be more likely to cause these long bursts, which led some to wonder if the massive star behind GRB 111209A was also cannibalising another nearby star – a process which could possibly cause its rotation to speed up.

The exact cause is still a matter of puzzles and hypotheses. We won’t know more until we observe more of these ultra-long GRBs, and seeing as we’ve only ever spotted 3 of them, it may remain a puzzle for a while yet. But a tantalising glimpse like this of one of the Universe’s most extreme events gives us a good reason to keep watching and waiting. When we do finally understand, it will almost certainly have been worth waiting for.

Image credits:
Top – NASA/Swift/B. Gendre (ASDC/INAF-OAR/ARTEMIS)
Bottom – CNRS/ARTEMIS – Céline Lavalade

Cite this article:
Hammonds M (2013-07-01 00:06:58). Explosions visible from across the Universe. Australian Science. Retrieved: May 05, 2024, from http://australianscience.com.au/space/explosions-visible-from-across-the-universe/

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It’s a small world after all

Markus — Mon, 11 Mar 2013 00:20:42 +0000

What we know of exoplanets has developed at the same time as the technology which we use to discover them. This is, in my opinion, the most exciting thing about the entire field of study. For instance, when we first started spotting planets around alien suns, we found huge gas giants. Hot jupiters, extremely massive and close to their parent stars. For a while, some conjectured that this type of planet may be quite common in the Universe. But since then, we’ve developed more powerful methods of searching the sky and, as it turns out, smaller planets are much more common than huge superjovian worlds. The latest piece in the puzzle comes courtesy of NASA’s Kepler space teescope. Near the end of last month, NASA announced the discovery of the smallest exoplanet ever found around a sun-like star!

Kepler-37b really is tiny. In fact, the whole Kepler-37 system is tiny – the entire system discovered so far can fit inside the orbit of Mercury! The innermost little world is under 100th the mass of Earth, it’s expected to have a radius of around 3867 km (assuming the same average density as the planets in our own solar system) making it smaller than Earth’s moon. One can only apprehensively wonder if this will spark yet another debate over how large an object has to be before it’s considered a planet. With such a tiny orbit, it also has a year lasting just 13 Earth days. Even though the star Kepler-37 is slightly smaller and cooler than the Sun, it’s still enough to heat the surface of tiny 37b to a roasting 700 Kelvin (nearly 430°C). Needless to say, while we all like stories which talk about potential alien life, this is unlikely to be a home for any lifeforms we might recognise.

Kepler-37b is very definitely the runt of the litter. Its sibling worlds, denoted by the letters c and d, are respectively slightly smaller than Earth and about twice the size of Earth. Of course, these planets are also very close to their parent star. The interesting thing is that we’re discovering more and more small worlds around other stars. More and more exoplanet astronomers are warming to the idea that small rocky planets are likely to be the most common in our galaxy. Our current technology might have trouble spotting them further than a certain distance from their parent stars, but they’re likely to be out there waiting to be found.

Even detecting Kepler-37b was quite a notable feat. It was only possible, in fact, because of a set of rather special circumstances. The star Kepler-37 is particularly quiet, lacking the noisy sunspots and features which cause brightness variation in most stars, making it a particularly clear target. It’s also relatively bright in Kepler’s field of view.

To learn more about this star, and hence get greater accuracy on the measurement of the planets it carries in tow, NASA astronomers used a technique known as asteroseismology. Not dissimilar to the way geologists measure earthquakes, asteroseismology is the study of vibrations within a star, measured by accurately observing pulsations in the star’s surface. All stars are constantly bubbling and boiling, and this causes the whole star to vibrate at a number of resonant frequencies – soundwaves – in exactly the same way a bell vibrates when it rings. By measuring the precise frequencies of those soundwaves, a lot can be determined about the interior of a star. Incidentally, this same technique can be used to effectively “listen” to the Sun.

Interestingly, because Kepler-37 has such an eerily peaceful surface for a star, it was very easy to measure those vibrations, making Kepler-37 the smallest star ever to be studied this way. Normally, only large stars are observed using asteroseismology because the measurements need to be very precise. Conveniently though, the Kepler telescope was built for breathtaking precision.

A tiny planet discovered orbiting a singing star 215 light years away. How poetic!

Image credits:
Top – NASA/Ames/JPL-Caltech
Middle – Karl Tate/ © space.com
Bottom – NASA/Ames/JPL-Caltech

Cite this article:
Hammonds M (2013-03-11 00:20:42). It's a small world after all. Australian Science. Retrieved: May 05, 2024, from http://australianscience.com.au/space/its-a-small-world-after-all/

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Kepler Astronomers Find First Planetary System Around a Binary Star

Editorial Board — Fri, 31 Aug 2012 06:50:17 +0000

NASA’s Kepler mission has found the first multi-planet solar system orbiting a binary star, characterized in large part by University of Texas at Austin astronomers using two telescopes at the university’s McDonald Observatory in West Texas. The finding, which proves that whole planetary systems can form in a disk around a binary star, is published in the August 28 issue of the journal Science.

“It’s Tatooine, right?” said McDonald Observatory astronomer Michael Endl. “But this was not shown in Star Wars,” he said, referring to the periodic changes in the amount of daylight falling on a planet with two suns. Measurements of the star’s orbits showed that daylight on the planets would vary by a large margin over the 7.4-Earth-day period as the two stars completed their mutual orbits, each moving closer to, then farther from, the planets (which are themselves moving).

The binary star in question is called Kepler-47. The primary star is about the same mass as the Sun, and its companion is an M-dwarf star one-third its size. The inner planet is three times the size of Earth and orbits the binary star every 49.5 days, while the outer planet is 4.6 times the size of Earth with an orbit of 303.2 days.

Artist's concept of the Kepler-47 system.

The outer planet is the first planet found to orbit a binary star within the “habitable zone,” where liquid water could exist and thus create a home for life. However, the planet’s size (about the same as Uranus) means that it is an icy giant, and not an abode for life. It’s a tantalizing taste of discoveries waiting to be made.

The combination of observations from the NASA mission and McDonald Observatory allowed astronomers to understand the characteristics of Kepler-47’s two stars and two planets.

The Kepler mission looks for minute dips in the amount of light coming from a star that might indicate a planet is passing in front of it, an event called a “transit.” The space telescope is also adept at identifying eclipsing binary stars, in which two stars pass in front of each other as they orbit each other. In the case of Kepler-47, they found both stellar eclipses and planet transits in one system.

So Kepler astronomers Jerome Orosz (lead author on the study) and William Welsh of San Diego State University flagged the Kepler-47 system as worthy of follow up from the ground. They asked the McDonald Observatory Kepler team to work with them.

Endl studied the binary star with the 9.2-meter Hobby-Eberly Telescope (HET, one of the world’s largest telescopes), as well as the 2.7-meter Harlan J. Smith Telescope at McDonald.

“The challenging thing is that this is a very faint star,” Endl said, “about 6,000 times dimmer than can be seen with the naked eye.”

He was taking spectra of the system — looking for characteristics in its light to indicate the motions of the primary star. (The secondary star is too faint to measure.) The McDonald observations enabled astronomers to calculate the mass of the primary star.

These values, along with the Kepler eclipse and transit timings, were plugged into a model that calculated the relative sizes of all the bodies involved, Endl said.

The Kepler team at McDonald Observatory also includes Bill Cochran (a co-Investigator of the Kepler mission), research scientist Phillip MacQueen, graduate students Paul Robertson and Eric Brugamyer, and recent graduate Caroline Caldwell.

“This is the type of research where McDonald Observatory really excels,” Cochran said. “We have excellent scientific instruments on our telescopes, and the queue-scheduled operation of the HET allows us to obtain spectra at the optimal times when they will give us the best information about the stars.”

Source.

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The smoking guns of dying stars

Markus — Mon, 09 Jul 2012 01:28:21 +0000

VY Canis Majoris, the largest known star in our galaxy, with it's huge smoky clouds of gas and dust being lost into interstellar space. Credit: NASA, ESA, and R. Humphreys (University of Minnesota)

The ancient Greeks once believed that the heavens were immutable. A vast starry vista, eternally unchanging above our heads. But we now know this to be untrue in the slightest. A lot has changed since then, however, and over the past few hundred years, astronomers have unfurled ever increasing knowledge of the lives of stars. Look out across a starry night sky and you can see stars in all stages of their lives, from brightly burning newborn stars, to ageing giants in their final gasping breaths. As stars near the ends of their lives, they begin to shine a rich red colour as they expand and dramatically increase in size.

As red giants, these stars are burning the last of their fuel, and as they do so they begin to sputter and gasp. Just as a candle flame does when it starts to burn out, red giant stars flicker – though for these, the largest stars in the universe, those flickers can take thousands of years each. These final bursts of energy are known as thermal pulses, and they mark the star’s final days. During the last couple of million years of its life, a red giant star will lose incredble amounts of material to interstellar space. Even when not caught mid-pulse, these stars lose several times the mass of our planet every year. And the puzzle of exactly how they do so has been recently been unravelled by a team of astronomers led by researchers at the University of Sydney.

All stars emit a stellar wind – a steady stream of particles being constantly accelerated outwards by the star. Even the Sun emits its own solar wind (with a speed measured at 535 kilometres per second as I write this). The wind from red giant stars is slightly different. As the star loses more mass, stellar material cools and condenses into dust. This dust then catches starlight which is so intense near to the star that it actually pushes that dust outwards. This effect, known as radiation pressure, causes the tiny reflective dust grains to act like minute sails and accelerates them away. Each dust grain is tiny compared to the kind of dust we find on our bookshelves, much more like fine smoke than any dust we’d recognise. Instead of being called stardust, it could easily be called starsmoke!

“The winds that stream from the upper atmosphere of the red giant stars are responsible for removing massive amounts of matter. The grains that we have discovered here will come as a real shock to the accepted wisdom in the field. They are both much larger and much closer to the stellar surface than anyone expected.” said Barnaby Norris, a PhD student at the University of Sydney, and lead author on the research published earlier this year in Nature.

This stardust emitted by these smoky smouldering old stars is transparent, not unlike finely powdered glass. The implications of this stardust being detected to close to an ageing star could help us to better understand the processes that occur in stars as they die, and how they manage to lose such prodigous amounts of mass so rapidly. As Norris said, “Hopefully our findings will help to illuminate a key step in the grand cycle as matter is expelled from stars into the galaxy only to seed new generations of stellar and planetary birth.”

On a final poetic note, all of the chemical elements essential to life are created within stars, before being seeded back into the cosmos when stars die. This means that this condensing stardust contains the fundamental raw materials needed for life to eventually form. As Carl Sagan so often used to muse, we are literally made of stardust.

The Cat's Eye nebula, a star which has now ended its life and is casting its outer layers into the cosmos. The concentric rings show material lost as stellar wind during the star's final thermal pulses.. Credit: ESA, NASA, HEIC and The Hubble Heritage Team (STScI/AURA)

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