Johannes Hirn

Stars 10 times as massive as the Sun, or more, should not exist: as they grow, they tend to push away the gas they feed on, starving their own growth.

Scientists have been struggling to figure out how some stars overcome this hurdle.

Now, a group of researchers led by two astronomers at the University of Toronto suggests that baby stars may grow to great mass if they happen to be born within a corral of older stars –with these surrounding stars favorably arranged to confine and feed gas to the younger ones in their midst.

The astronomers have seen hints of this collective feeding, known as “convergent constructive feedback,” in a giant cloud of gas and dust called Westerhout 3 (W3), located 6,500 light years from us.

Their results are published in April in The Astrophysical Journal.

“This observation may lift the veil on the formation of the most massive stars which remains, so far, poorly understood,” says Alana Rivera-Ingraham, who led the study while she was a graduate student in the Department of Astronomy and Astrophysics at U of T. Rivera-Ingraham is now a postdoctoral researcher at the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France.

To study the formation of high-mass stars, Rivera-Ingraham and collaborators used high-quality and high-resolution far-infrared images from a space telescope launched by the European Space Agency in 2009 —the Herschel Space Observatory. This telescope’s two cameras recorded light that is not visible to the naked eye, spanning a range from infrared radiation partway to the microwave region. Exploiting these cameras, scientists including Peter Martin, professor in the Canadian Institute for Theoretical Astrophysics at the University of Toronto, created something called the HOBYS Key Programme to study the birth of very massive stars in nearby giant clouds of gas and dust in our own Galaxy, including W3. Research on HOBYS at the University of Toronto is supported in part by the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada.

Scientists track the regions of the gas cloud where stars are about to form by mapping the density of dust and its temperature, looking for the most dense regions where the dust is shielded and cold.

“We can now see where stars are about to be born before it even happens, because we can detect the cold dust condensations,” says Martin. “Until Herschel, we could only dream of doing that.”

Stars are born in the denser parts of gas clouds, where the gas gets compressed enough by gravity to trigger nuclear fusion. The more massive the newborn star, the more visible and ultraviolet light it emits, heating up its surroundings —including the dust studied by Herschel.

“The radiation during the birth of high-mass stars is so intense that it tends to destroy and push away the material from which they need to feed for further growth,” says Rivera-Ingraham.

Scientists have modeled this process and found that stars about eight times the mass of our Sun would stop growing because they run out of gas.

But astronomers do see stars that are more massive than this theoretical limit. And by looking at W3, Rivera-Ingraham and her collaborators have found clues to how this might be possible.

The researchers noticed that the densest region of the cloud, in the upper left of the image, was surrounded by a congregation of old high-mass stars. It is as if previous generations of large stars enabled the next ones to grow also massive, and close to each other. The scientists suggest that this is no coincidence: each generation of stars might have created the right conditions for another generation to grow comparably or even more massive in its midst, ultimately leading to the formation of a rare, massive cluster of high-mass stars.

Like young high-mass stars, older stars also radiate and push gas away. If such older stars happen to be arranged favorably around a major reservoir of gas, they can compress it enough to ignite new stars. The process is similar to the way a group of street cleaners armed with leaf blowers can stack leaves in a pile—by pushing from all sides at the same time. This corralling of dense gas can give birth to new, high-mass stars.

A large newborn star will push its food source away, but if it is surrounded by enough large stars, these can keep nudging gas back at it. With such collective feeding at play, the young star could grow very massive indeed.

Next on the to-do list of the astronomers is to test their idea by simulating the situation with computer modeling, by measuring gas motions, and by comparing their results with data from other giant clouds studied by HOBYS. Only then will they be able to discern the mechanism—collective feeding or not—that gives rise to high-mass stars in these giant clouds.

This image does not show any stars because the Herschel Space Observatory’s cameras record far-infrared light instead of visible light. Gas is not visible either, even though it makes up most of the 400,000 solar masses of matter in this cloud: dust amounts to only about one per cent of the mass.

The side of this image spans about two degrees, or four times the diameter of the moon. At the cloud’s distance of 6,500 light years, this corresponds to about 230 light years. North is up and East is to the left.

This three-colour image of W3 uses visible colours to depict far-infrared radiation of different wavelengths, combining the Herschel bands at 70 μm (depicted in blue), 160 μm (depicted in green) and 250 μm (depicted in red). Hotter structures emit more strongly at shorter wavelengths. Red represents very cold shielded dust structures with equilibrium temperature about 10 degrees Kelvin (10 degrees above absolute zero, or -263 degrees Celsius), while blue is somewhat warmer at around 30 degrees Kelvin, with yellow in between.

Compared to the right side of the cloud which is cold and red, the left side glows yellow, because it is heated by the radiation coming from high-mass hot stars off to the left, just outside the field of view of the image.

While different colours correspond to different temperatures, brightness records the quantity of dust. Before Herschel’s two cutting-edge cameras, such a precise determination of the temperature and quantity of dust has been impossible. This capability, combined with the high resolution of Herschel, is exploited by the HOBYS Key Programme to study the early stages of formation of stars more than ten times as massive as the Sun.

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Toaster-sized space telescope launches from India

sgupta — Mon, 25 Feb 2013 13:11:01 +0000

Toaster-sized space telescope launches from India sgupta Mon, 02/25/2013 - 08:11

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Cordell Grant working on the nanosatellite at U of T's Space Flight Laboratory

Johannes Hirn

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Johannes Hirn

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Astronomy

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Space Flight Laboratory builds world's smallest astronomical satellite

The smallest astronomical satellite ever built launched Feb. 25, 2013 as part of a mission to prove that even a very small telescope can push the boundaries of astronomy.

The satellite was designed and assembled at the Space Flight Laboratory of the University of Toronto Institute for Aerospace Studies (UTIAS). It was launched from the Satish Dhawan Space Centre in Sriharikota, India, along with its twin, also designed in Canada, but assembled in Austria.

"The launch appeared to go flawlessly," said Cordell Grant, manager of satellite systems for the UTIAS lab. "It was nerve-wracking watching the launch because something can always go wrong. I felt great when it launched successfully and I felt even better a few hours later when we actually 'talked' to the satellites and ensured they were both working."

Each nano-satellite in the mission, which is known as BRITE, is a cube 20 centimetres per side that weighs less than 7 kilograms. The BRITE satellites are part of the new wave of nano-satellites that can be designed, assembled and deployed fast and relatively cheaply.

“The Space Flight Laboratory has demonstrated that nano-satellites can be developed quickly, by a small team and at a cost that is within reach of many universities, small companies and other organizations,” Grant said. "A nano-satellite can take anywhere from six months to a few years to develop and test, but we typically aim for two years or less."

Up to now, such nano-satellites had been used only to monitor the earth and experiment with new technologies.

“Researchers, scientists and companies worldwide, who have great ideas for space-borne experiments, but do not have the means to fund a large spacecraft, can now see their ideas realized,” said Grant. “BRITE has the potential to open an entirely new market for low-cost high-performance satellites.”

BRITE is the first nano-satellite mission intended for astronomy, and the first-ever astronomy constellation —more than one satellite working toward a common objective— of any size. The previous world-record holder for small astronomy satellites was also designed and assembled in part by U of T. Known as MOST, it launched in 2003 and was the first entirely Canadian satellite for astronomy, weighing in at 53 kilograms. Compared to the 11 metric tons of the Hubble Space Telescope, MOST was aptly called a micro-satellite. It is still operating today.

“BRITE is expected to demonstrate that nano-satellites are now capable of performance that was once thought impossible for such small spacecraft,” said Grant.

BRITE is not intended to take pictures, Grant said, but will simply observe stars and record changes in their brightness over time. Such changes could be caused by spots on the star, a planet or other star orbiting the star, or by oscillations and reverberations within the star itself —like earthquakes on Earth.

To perform precise measurements of the brightness of stars, the telescopes need to be above the atmosphere. Otherwise, scintillation —the atmospheric effect that causes stars to twinkle— overwhelms the relatively small brightness variations of the stars themselves. By avoiding this, a very small telescope in space can produce more accurate data than a much larger telescope on the ground.

Also, unlike telescopes on Earth which are useless during the day, in bad weather or when the stars set below the horizon, telescopes in space can potentially observe stars all the time.

"As their name suggests, the BRITE satellites will focus on the brightest stars in the sky including those that make up prominent constellations like Orion the Hunter," said Grant. "These stars are the same ones visible to the naked eye, even from city centres."

Because very large telescopes mostly observe very faint objects, the brightest stars are also some of the most poorly studied stars, he said. But the brightest stars are also the largest.

"Big, bright stars lead short and violent lives and deaths (supernovas) and in the process seed the universe with heavy elements without which life on Earth would be impossible," Grant said. "To better understand these stars is to better understand how life arose on our planet."

Because big objects oscillate and quake slower than smaller ones, the BRITE satellites do not have to keep their eyes constantly on any given star, but can observe from time to time to see if anything has changed.

"The BRITE satellites can monitor their target stars whatever orbit they are placed on, and don't require a dedicated rocket to place them in a specific orbit," Grant said. "By piggy-backing on any available rocket, the BRITE satellites can thus be launched for relatively little money.

To gather more observations and to increase the lifetime of the mission, scientists will be launching three such pairs of satellites —one Austrian pair, one Polish pair and one Canadian pair supported by the Canadian Space Agency— so that within a few years BRITE will become a constellation of six satellites. Each twin in a pair watches the sky in a different colour (red or blue), providing another exciting layer of data to the scientists.

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In the Dragonfish's Mouth

sgupta — Thu, 01 Dec 2011 15:41:16 +0000

In the Dragonfish's Mouth sgupta Thu, 12/01/2011 - 10:41

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The dragonfish star cluster is a batch of newborn stars found by three ��οastronomers. (Photo by Peter Shearer)

Johannes Hirn

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Johannes Hirn

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Faculty of Arts & Science

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��οastronomers discover the next generation of superstars to stir up the galaxy

Three astronomers at the University of Toronto have found the most numerous batch of young, supermassive stars yet observed in our galaxy: hundreds of thousands of stars, including several hundreds of the most massive kind --blue stars dozens of times heavier than our Sun. The light these newborn stars emit is so intense, it has pushed out and heated the gas that gave them birth, carving out a glowing hollow shell about a hundred light-years across.

These findings will be published in the Dec. 20 issue of Astrophysical Journal Letters. For the researchers, the next step is already clear:

"By studying these supermassive stars and the shell surrounding them, we hope to learn more about how energy is transmitted in such extreme environments," said Mubdi Rahman, a PhD candidate in the Department of Astronomy & Astrophysics at the University of Toronto, who led the work with his supervisors, Professors Dae-Sik Moon and Christopher Matzner.

Such large nurseries of massive stars have been noticed in other galaxies, but were so far away that all stars are often blurred together on images taken by telescopes.

"This time, the massive stars are right here in our galaxy, and we can even count them individually," Rahman said.

Studying the individual stars will require intricate measurements. The cluster of bright stars is located nearly halfway across our galaxy, 30,000 light-years away, and the line of sight is blocked by dust.

Before the light from these stars can reach us, most of it is absorbed by the intervening dust in our galaxy. This makes the brightest stars in the cluster appear as dim as smaller, nearby stars. The fainter stars in the cluster appear so dim that they are not seen.

"All this dust made it difficult for us to figure out what type of stars they are," Rahman said."These stars are incredibly bright, yet, they're very hard to see."

The researchers used the New Technology Telescope at the European Southern Observatory in Chile to collect whatever light they could from a few dozen stars. They measured in details how much light the stars emit in each colour, and were finally able to confirm that at least a dozen stars in the cluster were of the most massive kind, some possibly a hundred times more massive than our Sun.

In fact, before turning a ground telescope toward the stars themselves, Rahman first noticed the glow from the large shell of heated gas using the WMAP satellite, which is sensitive to microwaves (between radio waves and visible light). To make an image of the gas shell being blown away and heated up, the researchers used the Spitzer satellite, which works with infrared light (between microwave and visible light).

Rahman suggested the name “Dragonfish” after comparing the infrared image of the celestial gas shell with Peter Shearer’s illustration of the deep-sea creature with the same name. The astronomical image resembles a dark gaping mouth-like shape with teeth, two eyes, and a bright fin to the right. The “mouth” is the volume from which the gas has been cleared by the light of the stars, pushed outward to form a shell that is particularly bright in spots corresponding to the eyes and the fin of the animal.

“We were able to see the effect of the stars on their surroundings before seeing the stars directly”, Rahman says. This would be like seeing lit faces and red cheeks from the heat of a campfire, without being able to see the logs and flames themselves.

In the same way that red embers are cooler than the blue flame of a welding torch, the gas is cooler than what is heating it, and thus glows redder than the blue stars. Compared to the colours of a rainbow ranging from red to blue, most of the light emitted by the heated gas is in fact redder than red, and thus infrared --less affected by gas or dust, and invisible to our naked eyes, but not to appropriate telescope instruments. At the other end of the rainbow, the giant stars in the cluster are bluer than blue, and emit mostly in the ultraviolet, which is blocked by dust and thus not visible on the image.

“But we had to make sure what was at the heart of the shell,” Rahman said.

Now that the astronomers have identified several stars there as very massive, they know that these stars will burn their nuclear fuel relatively quickly in astronomical terms: within a few million years (thousands of times faster than for our Sun) even though the giant blue stars contain dozens of times more fuel than our Sun.

"Still, if you thought the inside of the shell was empty, think again," Rahman said.

For each of the few hundred superstars the researchers may have spotted, there are thousands of average stars more akin to our Sun. When the superstars have burned through their fuel, they will explode and release metals and other heavy atoms that may help form rocky planets around smaller, quieter stars -- perhaps providing the building blocks for life.

"There may be newer stars already forming in the eyes of the Dragonfish," Rahman said.

Some areas in the shell glow particularly bright, and the researchers think the gas there may have been compressed enough to ignite even more stars.

The gas now in the shell is the remainder of the very gas that gave birth to the stars, and there is a lot of it: the mother shell is more massive than the cluster of its babies. But with no mother anymore to keep them reined in via its mass and gravity, all the young stars may start wandering off in all directions.

"We’ve found a rebel in the group, a runaway star escaping from the group at high speed," Rahman said. "We think the group is no longer tied together by gravity: however, how the association will fly apart is something we still don't understand well."

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