Astrophile: Lucky strike turns a dark cloud into a star

[bystander]

Lucky strike turns a dark cloud into a star
New Scientist | Astrophile | Ken Croswell | 2012 Sep 07

Object: Barnard 68
Mass: Around the same as 1 to 3 suns
Distance: 300 to 500 light-years

The contenders are lined up, each aspiring to greatness. Suddenly, a bullet ploughs into one of these shadowy figures, which then collapses. Success! A star is born. Star-making proceeds quite differently in the heavens than in Hollywood.

How to Become a Star (Credit: ESO)

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New observations back up this tale of violent transformation for Barnard 68 – a cold cloud of gas and dust, known as a Bok globule, which hovers in the constellation Ophiuchus and may give us one of the most detailed glimpses of the first stage of star birth.

Until recently, the inky blob had an uncertain future – some astronomers thought it was a sun in the making, while others thought it's a washout.

Right now, the gas and dust in Barnard 68 is more frigid than Pluto and so dark that if someone were sitting inside they wouldn't be able to see a single star in the sky. It also has an extension of cloud and gas to its lower left.

In 2009, Andreas Burkert of the University of Munich in Germany, who wasn't involved in the new work, and João Alves, now of the University of Vienna in Austria, proposed that the cloud's extension is really a separate cloud that is striking Barnard 68 like a slow-motion bullet.

The bullet cloud's impact should cause the larger globule to collapse until it becomes so dense and hot that it shines like a sun-like star.

New observations by Markus Nielbock of the Max Planck Institute for Astronomy in Heidelberg, Germany, and his colleagues now support this idea.

They used ESA's Herschel Space Observatory to measure the temperature of dust in the cloud, from its centre to its edge. Their results show that it is definitely cold enough for star formation.

They also made ground-based radio observations, which reveal that the bullet's velocity differs slightly from that of the main cloud. This suggests they are indeed two separate clumps of material.

Twirling cloud

Nielbock thinks Barnard 68's future is still unclear, but he cautiously predicts that it is on the verge of collapsing.

Burkert is bolder, saying that in a few hundred thousand years the collapse will heat the cloud's centre so much that it will shine as a star – albeit one powered by gravity. Our sun spent millions of years in this gravity-powered phase before switching to nuclear energy.

The bullet is probably striking off-centre, Burkert adds, which gives the main cloud a twirl so that a disk of leftover material will arise around the newborn star. That disk might one day form planets like Earth, perhaps complete with its own Hollywood stars.

"I don't know of any other object that is so close and well-studied where we can understand how a sun forms," Burkert says.

The earliest phases of star formation observed with Herschel (EPoS):
The dust temperature and density distributions of B68 - M. Nielbock et al

• arXiv.org > astro-ph > arXiv:1208.4512 > 22 Aug 2012 (v1), 06 Sep 2012 (v2)

APOD: Molecular Cloud Barnard 68 (2012 Jan 29)

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[Ann]

Very interesting!

A picture of this molecular cloud was the Astronomy Picture of the Day on January 29, 2012. In the discussion thread, I expressed severe doubt that this small-looking cloud was up to making new stars. The cloud is apparently quite light-weight, containing only about two solar masses, and only a small part of a collapsing cloud will go into making new stars. Chris corrected me, pointing out that Barnard 68 has, for the most part, sharp rather than ragged edges, suggesting that it's collapsing.

I find it very interesting that professional astronomers have shared my doubts about the star-making potential of Barnard 68. I may certainly be wrong here, but I can't help thinking that they, like me, have considered Barnard 68 too puny to do the job.

Or not. Maybe it's all about finding a trigger to make the cloud, more or less any cloud, collapse. The idea that two colliding clouds may cause the kind of collapse that leads to star formation seems convincing. Maybe you don't even need two separate clouds collapsing, but just some disturbance within a cloud. That would certainly explain why colliding galaxies often have such high rates of star formation.

Ann

[bystander]

[i][url=http://apod.nasa.gov/apod/ap050521.html]APOD: Snake in the Dark (2005 May 21)[/url] Credit & Copyright: [url=http://www.home.earthlink.net/~gstevens913/]Gary Stevens[/url][/i]

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Barnard 68 (B68) is part of a much larger complex of dark nebula known as the Great Dark Horse Nebula in Ophiuchus. The rear leg and hind quarter of the dark horse are probably better known as the Pipe Nebula. If you zoom into the top center of that APOD, you can find another familiar image, the Snake Nebula (B72). In that APOD, to the right of B72 are the smaller Bok globules, from the top, B68, B69, B71, and B70. An annotated image of the Dark Horse Nebula can be found in Tom J. Martin's Photo Blog.

[bystander]

Dark Clouds, Young Stars, and a Dash of Hollywood
Max Planck Institute for Astronomy | 2012 Oct 30

New results from space telescope's explorations of stellar birthplaces

An astronomical project led by researchers from the Max Planck Institute for Astronomy (MPIA) has examined the earliest stages of star formation in unprecedented depth: Using the European Space Agency's Herschel Space Telescope and techniques more commonly encountered in Hollywood blockbuster computer graphics than in astronomy, the researchers produced a three-dimensional map of the molecular cloud B68, a possible future birthplace for a low-mass star. Turning their attention to much more massive molecular clouds, the researchers also managed to identify a previously unobserved class of object that is likely the earliest known precursor of the birth of massive stars.

[i]False-colour image of the dark cloud Barnard 68, prepared using data from the Herschel Space Telescope at different far-infrared wavelengths. The way in which the cloud appears to change shape depending on wavelength is a sign of uneven external illumination. In the bottom left corner, there are traces of an isolated object. This could be a cloud fragment in collision with Barnard 68. The wavelengths for the images are 100, 160, 250 and 350 micrometers, respectively. The image colors show the intensity of the radiation received at that wavelength, from purple and blue at low intensity to high intensities in red and white. [b]Image credit: MPIA / Markus Nielbock[/b][/i]

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Stars are born in hiding, when dense regions within clouds of gas and dust collapse under their own gravity. But the clouds not only provide the raw material for star formation, they also absorb most of the light from their interior, hiding from view the crucial details of stellar birth – one of the key astronomical processes if we want to understand our own origins!

Now, two groups in the EPoS ("Earliest Phases of Star formation") project led by MPIA's Oliver Krause, using ESA's Herschel Space Telescope, report new results in understanding the earliest stages of star formation.

On the trail of the origin of low-mass stars (with less than about twice the mass of our Sun), a team led by Markus Nielbock (MPIA) has completed a detailed investigation of one of the best-known potential stellar birthplaces: the dark cloud (or "globule") Barnard 68 in the constellation Ophiuchus. Combining the Herschel Space Telescope's unrivaled sharpness and sensitivity in the far-infrared range with a method more often encountered in visual effects companies working on Hollywood blockbusters than in astronomy, the researchers were able to construct the most realistic 3D model of the cloud to date.

The method, adapted for this particular use by MPIA's Ralf Launhardt, uses what is known as raytracing: For each minute portion of the object that we can see, the line of sight is traced back into the object itself. The contribution by each portion of the light's path – is light being absorbed at this particular point? is it being emitted? if yes, at which wavelengths? – are added up. Raytracing is routinely used to produce realistic-looking computer-generated creatures, objects or whole scenes. Here, it helped to match light emitted within Barnard 68 at different wavelengths with simplified models of the cloud's three-dimensional shape, density and temperature distribution.

The results have shaken up some of what astronomers thought they knew about this cloud. The emerging picture is one of Barnard 68 condensing from a drawn-out filament, heated by unevenly distributed external radiation from the direction of the central plane of our home galaxy. The astronomers also found some signs pointing to a cloud fragment in collision with Barnard 68, which might lead to the cloud's collapse, and the formation of one or more low-mass stars, within the next hundreds of thousands of years, and whose existence had been predicted by a previous study (Burkert & Alves 2009).

As cosmic clouds go, Barnard 68 is rather small. Clouds of this size will give birth to a few low-mass stars at most. To find out how massive stars are born (mass greater than about twice the mass of the Sun), a team led by MPIA's Sarah Ragan turned Herschel's PACS camera to 45 significantly more massive dark clouds. The clouds contain numerous stars about to be born, so-called "protostars". While previous missions, such as NASA's Spitzer Space Telescope, have also searched for protostars, Herschel enables astronomers to probe deeper into the clouds than ever before. Younger protostars are hidden much more effectively within their clouds than older ones. Herschel managed to find the youngest and most primitive protostars known.

The new observations swelled the ranks of known protostars from 330 to nearly 500 and, most excitingly, led to the discovery of a new type of not-quite-a-star: dense regions at a mere 15 degrees above absolute zero (-258 degrees Celsius) with no sign of a protostar. These regions are likely to be in an early precursor stage of star formation. In astronomy, where timescales of hundreds of millions or of billions of years are the norm, the fact that this precursor stage is expected to last less than 1000 years makes it extremely short-lived. Studying these elusive, pristine objects lays a necessary foundation for all subsequent studies of star formation.

The Earliest Phases of Star formation (EPoS) observed with Herschel:
The dust temperature and density distributions of B68 - M. Nielbock et al

• Astronomy & Astrophysics 547 A11 (Nov 2012) DOI: 10.1051/0004-6361/201219139
  arXiv.org > astro-ph > arXiv:1208.4512 > 22 Aug 2012 (v1), 08 Sep 2012 (v3)

The Earliest Phases of Star Formation (EPoS): A Herschel Key Program -
The precursors to high-mass stars and clusters - Sarah Ragan et al

• Astronomy & Astrophysics 547 A49 (Nov 2012) DOI: 10.1051/0004-6361/201219232
  arXiv.org > astro-ph > arXiv:1207.6518 > 27 Jul 2012 (v1), 06 Sep 2012 (v2)