star formation Archives

January 14, 2010December 24, 2015 by Nancy Atkinson

Latest from Hubble: Star Formation Fizzling Out in Nearby Galaxy

NGC 2976.. NASA, ESA, and J. Dalcanton and B. Williams (University of Washington, Seattle)

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Most galaxies are throughout the universe are happenin’ places, with all sorts of raucous star formation going on. But for a nearby, small spiral galaxy, the star-making party is almost over. In this latest Hubble release, astronomers were surprised to find that star-formation activities in the outer regions of NGC 2976 are fizzling out, and any celebrating is confined to a few die-hard partygoers huddled in the galaxy’s inner region.

The reason? Well, the star birth began when another party-crashing galaxy interacted with NGC 2976. But that happened long ago, and now star formation in the galaxy is fizzling out in the outer parts as some of the gas was stripped away and the rest collapsed toward the center. With no gas left to fuel the party, more and more regions of the galaxy are going to sleep.

“Astronomers thought that grazing encounters between galaxies can cause the funneling of gas into a galaxy’s core, but these Hubble observations provide the clearest view of this phenomenon,” explains astronomer Benjamin Williams of the University of Washington in Seattle, who directed the Hubble study, which is part of the ACS Nearby Galaxy Survey Treasury (ANGST) program. “We are catching this galaxy at a very interesting time. Another 500 million years and the party will be over.”

NGC 2976 does not look like a typical spiral galaxy. It has a star-forming disk, but no obvious spiral pattern. Its gas is centrally concentrated, but it does not have a central bulge of stars. The galaxy resides on the fringe of the M81 group of galaxies, located about 12 million light-years away in the constellation Ursa Major.

“The galaxy looks weird because an interaction with the M81 group about a billion years ago stripped some gas from the outer parts of the galaxy, forcing the rest of the gas to rush toward the galaxy’s center, where it is has little organized spiral structure,” Williams says.

The galaxy’s relatively close distance to Earth allowed Hubble’s Advanced Camera for Surveys (ACS) to resolve hundreds of thousands of individual stars. What look like grains of sand in the image are actually individual stars. Studying the individual stars allowed astronomers to determine their color and brightness, which provided information about when they formed.

The image was taken over a period in late 2006 and early 2007.

“This type of observation is unique to Hubble,” Williams says. “If we had not been able to pick out individual stars, we would have known that the galaxy is weird, but we would not have dug up evidence for a significant gas rearrangement in the galaxy, which caused the stellar birth zone to shrink toward the galaxy’s center.”

Simulations predict that the same “gas-funneling” mechanism may trigger starbursts in the central regions of other dwarf galaxies that interact with larger neighbors. The trick to studying the effects of this process in detail, Williams says, is being able to resolve many individual stars in galaxies to create an accurate picture of their evolution.

Williams’ results will appear in the January 20, 2010 issue of The Astrophysical Journal.

Source: HubbleSite

January 11, 2010December 24, 2015 by Nancy Atkinson

Time-Lapse Movie Shows Massive Stars Form Similarly to Smaller Stars

It has been difficult for astronomers to see how massive stars form, since these stars are rare, form quickly and tend to be enshrouded in dense, dusty material which obscures them from view. But astronomers using the Very Long Baseline Array (VLBA) radio telescope were able to take images of the wavelengths of light emitted by a massive young star located 1,350 light years away in the Orion constellation. The created a ‘movie’ from the data, which they say shows the first evidence that young massive stars form from an accretion disk, just as smaller stars form.

“It is the first really ironclad confirmation that massive young stars are surrounded by orbiting accretion disks, and the first strong suggestion that these disks launch magnetically driven winds,” said Mark Krumholz, from the University of California at Santa Cruz.

The astronomers, led by Lynn D. Matthews from the Haystack Observatory at MIT, were able to see a disk of gas swirling close to the young massive star, known as Source I (said like “Source Eye”) in the high-resolution time-lapse movie they created.

By assembling 19 individual images of Source I taken by the VLBA at monthly intervals between March 2001 and December 2002, the high-resolution movie reveals thousands of masers, radio emitting gas clouds that can be thought of as naturally occurring lasers, located close to the massive star. According to Matthews, only three massive stars in the entire galaxy are known to have silicon monoxide masers. Because the silicon monoxide masers emit beams of intense radiation that can pierce the dusty material surrounding Source I, the scientists could probe the material close to the star and measure the motions of individual gas clumps.

Click here to see the time-lapse movie.

For almost 20 years, astronomers have known that low-mass stars form as a result of disk-mediated accretion, or from material formed from a structure rotating around a central body and driven by magnetic winds. But it had been impossible to confirm whether this was true for massive stars, which are eight to 100 times larger than low-mass stars. Without any hard data, theorists proposed many models for how massive stars might form, such as via collisions of smaller stars.

“This work should rule out many of them,” Krumholz said.

Because massive stars are believed to be responsible for creating most of the chemical elements in the universe that are critical for the formation of Earth-like planets and life, understanding how they form may help unravel mysteries about the origins of life.

The VLBA consists of a network of 10 radio telescope dishes located across North America, and can be thought of as a virtual telescope 5,000 miles in diameter. Used as a zoom lens to penetrate the dusty cloud surrounding the massive star, the VLBA captured images up to 1,000 times sharper than those previously obtained by other telescopes, including NASA’s Hubble Space Telescope.

The team’s paper was published in the Jan. 1 issue of the Astrophysical Journal.

Lead image caption: Artist’s conception of the rotating disk of hot, ionized gas surrounding Orion Source I, blocking the star from our view. A cool wind of gas is driven from the upper and lower surfaces of the disk and is sculpted into an hourglass shape by tangled magnetic field lines. Image: Bill Saxton, National Radio Astronomy Observatory/Associated Universities, Incorporated/National Science Foundation

Source: MIT

January 4, 2010December 24, 2015 by Jon Voisey

New Studies on the Vela Star Forming Region

A false-color infrared image of the star forming complex in Vela. Two new studies have measured for the first time the dust emission at very long infrared wavelengths, and found a set of young stars that are accreting material and flaring. Credit: NASA and the Spitzer Space Telescope

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This week at the AAS meeting scientists revealed two new studies on a star forming region in Vela. The first used the Balloon-borned Large Aperture Submillimeter Telescope (BLAST, a proptotype detector for the one on the new Herschel Space Telescope) to classify the young stars and begin mapping the warm dust in the region. The second searched the nebula for flaring young stars. Both studies are to appear in an upcoming publication of the Astrophysical Journal.

Although star formation has been well modeled and understood theoretically, observational astronomy is often made more difficult due to the fact that it occurs shrouded in dusty nebulae. Visible light absorbed by the nebula and reemitted as lower energy infrared light. Most of the wavelengths in this region cannot permeate Earth’s atmosphere.

In order to study regions like this, astronomers are forced to use balloon based and space observatories. Astronomers Massimo Marengo, Giovanni Fazio, and Howard Smith, together with an international team of scientists used BLAST to study just such a star forming region in Vela. The first of their studies searched the nebula for newly formed stars. To do this, they searched for behaviors shown to be indicative of star formation, “such as proto-stellar jets and molecular outflows.” Additionally, to truly classify as a proto-star, the object was required to show up at more than one wavelength. In searching for these candidates, they confirmed 13 cores originally reported by a previous team, but discounted one because it did not have the proper spectral characteristics (although they may still later collapse to form stars).

By analyzing the mass of the forming regions, the team was also able to show that the Core Mass Function (CMF, a function that describes the frequencies of proto-star cores of various masses) is very similar to the Initial Mass Function (IMF, which is the same thing but for already formed stars). Although this is unsurprising, it is a necessary observation to confirm our understanding of how stars form and to show that stars do indeed come from such nebulae.

Another unsurprising confirmation of stellar formation models is that forming cores in the nebula are notably warmer when they’ve reached the density sufficient to create fusion in the core and have an embedded protostar. These results, “can thus provide guidelines
for understanding the physical conditions where the transition between pre- and proto-stellar cores takes place.”

The second of their studies analyzed known young stars to search for large flares thought to be caused by material being accreted onto the young star. The region was imaged once and then a second time six months later. Over this period, 47 of some 170,000 observed stars had increases in brightness consistent with what was expected for flaring. Closer inspection of these stars 19 had the further characteristics (mass, age, environment) expected of such flares. Eight showed evidence of being extremely young (on the order of a hundred thousand years or less) and were still enshrouded in gravitationally bound disks of dust.

Although this cannot confirm the prediction of such youthful flares being due to infalling material (as opposed to magnetic fields or interactions with a companion) it does show that BLAST and its successor, Herschel, will be a powerful tool for further study.

December 15, 2009December 24, 2015 by Nancy Atkinson

Incredible New Hubble Image is Full of Stars!

This is a Hubble image of the star cluster R136 at the heart of the Tarantula Nebula. It's a starburst region that's home to several extremely massive stars, including R136a1, which is almost 200 times more massive than the Sun. Image Credit: By NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O'Connell (University of Virginia, Charlottesville), and the Wide Field Camera 3 Science Oversight Committee

A brand new Hubble image from Wide Field Camera 3 shows the most detailed view of the largest stellar nursery in our local galactic neighborhood. The massive, young stellar grouping, called R136, is only a few million years old and resides in the 30 Doradus Nebula, a turbulent star-birth region in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. There is no known star-forming region in our galaxy as large or as prolific as 30 Doradus. Many of the diamond-like icy blue stars are among the most massive stars known. Several of them are over 100 times more massive than our Sun. In a few million years, this region should provide an incredible show: that’s when these hefty stars are destined to pop off like a string of firecrackers, as supernovas.

The image, taken in ultraviolet, visible, and red light by Hubble’s Wide Field Camera 3, spans about 100 light-years. The nebula is close enough to Earth that Hubble can resolve individual stars, giving astronomers important information about the birth and evolution of stars in the universe. The Hubble observations were taken Oct. 20-27, 2009. The blue color is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen.

Ground-based version of the Doradus Constellation. Credit: A. Fujii

The LMC is located 170,000 light-years away and is a member of the Local Group of Galaxies, which also includes the Milky Way.

Click here for larger (and eye-popping!) versions of this image.

You can also “zoom” in and out of this image here on the “Starry Critters” website.
Source: HubbleSite

September 9, 2009December 24, 2015 by Brian Ventrudo

Magnetic Fields Have Key Influence on Star Formation

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When a giant cloud of interstellar gas and dust collapses to form a new cluster of stars, only a small fraction of the cloud’s mass ends up in stars. Scientists have never been sure why. But a new study provides insights into the role magnetic fields might play in star formation, and suggests more than the influence of gravity should be taken into account in computer models of stellar birth.

Gravity favors star formation by drawing material together, so if most material does not coalesce into stars, some additional force must hinder the process. Magnetic fields and turbulence are the two leading candidates. Magnetic fields channel flowing gas, making it hard to draw gas from all directions, while turbulence stirs the gas and induces an outward pressure that counteracts gravity.

“The relative importance of magnetic fields versus turbulence is a matter of much debate,” said astronomer Hua-bai Li of the Harvard-Smithsonian Center for Astrophysics. “Our findings serve as the first observational constraint on this issue.”

Li and his team studied 25 dense patches, or cloud cores, each one about a light-year in size. The cores, which act as seeds from which stars form, were located within molecular clouds as much as 6,500 light-years from Earth.

The degree of polarization of light from the clouds is influenced by the direction and strength of the local magnetic fields, so the researchers measured polarization to determine magnetic field strength. The fields within each cloud core were compared to the fields in the surrounding, tenuous nebula.

The magnetic fields tended to line up in the same direction, even though the relative size scales (1 light-year-sized cores versus 1000 light-year-sized nebulas) and densities were different by orders of magnitude. Since turbulence would tend to churn the nebula and mix up magnetic field directions, their findings show that magnetic fields dominate turbulence in influencing star birth.

“Our result shows that molecular cloud cores located near each other are connected not only by gravity but also by magnetic fields,” said Li. “This shows that computer simulations modeling star formation must take strong magnetic fields into account.”

In the broader picture, this discovery aids understanding of how stars and planets form and, therefore, how the universe has come to look the way it is today.

Source: Harvard-Smithsonian Center for Astrophysics

August 27, 2009April 26, 2016 by Brian Ventrudo

Star-Birth Myth Shattered

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An international team of astronomers has debunked a long-held belief about how stars are formed.

Since the 1950’s, astronomers believed groups of new-born stars obeyed the same rules of star formation, which meant the ratio of massive stars to lighter stars was pretty much the same from galaxy to galaxy. For every star 20 times more massive than the Sun or larger, for example, there’d be 500 stars equal to or less than the mass of the Sun.

“This was a really useful idea. Unfortunately it seems not to be true,” said team research leader Dr. Gerhardt Meurer of Johns Hopkins University in Baltimore.

This mass distribution of newly-born stars is called the ‘initial mass function’, or IMF. Most of the light we see from galaxies comes from the highest mass stars, while the total mass in stars is dominated by the lower mass stars which can’t be seen, so the IMF has implications in accurately determining the mass of galaxies. By measuring the amount of light from a population of stars, and making some corrections for the stars’ ages, astronomers can use the IMF to estimate the total mass of that population of stars.

Results for different galaxies can be compared only if the IMF is the same everywhere, but Dr. Meurer’s team has shown this ratio of high-mass to low-mass newborn stars differs between galaxies. Small ‘dwarf’ galaxies, for instance, form many more low-mass stars than expected.

To arrive at this finding, Dr. Meurer’s team used galaxies in the HIPASS Survey (HI Parkes All Sky Survey) done with the Parkes radio telescope near Sydney, Australia. A radio survey was used because galaxies contain substantial amounts of neutral hydrogen gas, the raw material for forming stars, and the neutral hydrogen emits radio waves.

The team measured two tracers of star formation, ultraviolet and H-alpha emissions, in 103 of the survey galaxies using NASA’s GALEX satellite and the 1.5-m CTIO optical telescope in Chile.

Selecting galaxies on the basis of their neutral hydrogen gave a sample of galaxies of many different shapes and sizes, unbiased by their star formation history.

H-alpha emission traces the presence of very massive stars called O stars, the birth of a star with a mass more than 20 times that of the Sun.

The UV emission, traces both O stars and the less massive B stars — overall, stars more than three times the mass of the Sun.

Meurer’s team found the ratio of H-alpha to UV emission varied from galaxy to galaxy, implying the IMF also did, at least at its upper end.

“This is complicated work, and we’ve necessarily had to take into account many factors that affect the ratio of H-alpha to UV emission, such as the fact that B stars live much longer than O stars,” Dr. Meurer said.

Dr. Meurer’s team suggests the IMF seems to be sensitive to the physical conditions of the star-forming region, particularly gas pressure. For instance, massive stars are most likely to form in high-pressure environments such as tightly bound star clusters.

The team’s results allow a better understanding of other recently observed phenomena that have been puzzling astronomers, such as variation of the ratio of H-alpha to ultraviolet light as a function of radius within some galaxies. This now makes sense as the stellar mix varies as the pressure drops with radius, just like the pressure varies with altitude on the Earth.

The work confirms tentative suggestions made first by Veronique Buat and collaborators in France in 1987, and then a more substantial study last year by Eric Hoversteen and Karl Glazebrook working out of Johns Hopkins and Swinburne Universities that suggested the same result.

Source: CSIRO