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Astronomers Prove What Separates True Stars from Wannabes

Astronomers have shown what separates real stars from the wannabes. Not in Hollywood, but out in the universe.
"When we look up and see the stars shining at night, we are seeing only part of the story," said Trent Dupuy of the University of Texas at Austin and a graduate of the Institute for Astronomy at the University of Hawaii at Manoa. "Not everything that could be a star 'makes it,' and figuring out why this process sometimes fails is just as important as understanding when it succeeds"
He and co-author Michael Liu of the University of Hawaii have found that an object must weigh at least 70 times the mass of Jupiter in order to start hydrogen fusion and achieve star-status. If it weighs less, the star does not ignite and becomes a brown dwarf instead.

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Title: Star Formation triggered by cloud-cloud collisions
Author: S. K. Balfour, A. P. Whitworth, D. A. Hubber, S. E. Jaffa

We present the results of SPH simulations in which two clouds, each having mass M_o=500 solar masses and radius R_o=2pc, collide head-on at relative velocities of Delta v_o=2.4,2.8,3.2,3.6and4.0kms-1. There is a clear trend with increasing Delta v_o. At low Delta v_o, star formation starts later, and the shock-compressed layer breaks up into an array of predominantly radial filaments; stars condense out of these filaments and fall, together with residual gas, towards the centre of the layer, to form a single large-N cluster, which then evolves by competitive accretion, producing one or two very massive protostars and a diaspora of ejected (mainly low-mass) protostars; the pattern of filaments is reminiscent of the hub and spokes systems identified recently by observers. At high Delta v_o, star formation occurs sooner and the shock-compressed layer breaks up into a network of filaments; the pattern of filaments here is more like a spider's web, with several small-N clusters forming independently of one another, in cores at the intersections of filaments, and since each core only spawns a small number of protostars, there are fewer ejections of protostars. As the relative velocity is increased, the mean protostellar mass increases, but the {\it maximum} protostellar mass and the width of the mass function both decrease. We use a Minimal Spanning Tree to analyse the spatial distributions of protostars formed at different relative velocities.

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Title: The dangers of being trigger--happy
Author: J. E. Dale, T. J. Haworth, E. Bressert

We examine the evidence offered for triggered star formation against the backdrop provided by recent numerical simulations of feedback from massive stars at or below giant molecular cloud sizescales. We compile a catalogue of sixty--seven observational papers, mostly published over the last decade, and examine the signposts most commonly used to infer the presence of triggered star formation. We then determine how well these signposts perform in a recent suite of hydrodynamic simulations of star formation including feedback from O--type stars performed by Dale et al (2012a, b, 2013a, b, 2014). We find that none of the observational markers improve the chances of correctly identifying a given star as triggered by more than factors of two at most. This limits the fidelity of these techniques in interpreting star formation histories. We therefore urge caution in interpreting observations of star formation near feedback--driven structures in terms of triggering.

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Title: Triggered Star Formation from Bubbles S51, N68, and N131
Authors: Chuan-Peng Zhang, Jun-Jie Wang

We investigated the environment of the infrared dust bubbles S51, N68, and N131 from catalogue of Churchwell et al. (2006), and searched for evidence of triggered star formation.

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Title: The Salpeter Slope of the IMF Explained
Authors: M. S. Oey

If we accept a paradigm that star formation is a self-similar, hierarchical process, then the Salpeter slope of the Initial Mass Function (IMF) for high-mass stars can be simply and elegantly explained as follows. If the instrinsic IMF at the smallest scales follows a simple -2 power-law slope, then the steepening to the -2.35 Salpeter value results when the most massive stars cannot form in the lowest-mass clumps of a cluster. It is stressed that this steepening MUST occur if clusters form hierarchically from clumps, and the lowest-mass clumps can form stars. This model is consistent with a variety of observations as well as theoretical simulations.

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 How nature shapes the birth of stars

Using state of the art computer simulations, a team of astronomers from the University of Bonn in Germany have found the first evidence that the way in which stars form depends on their birth environment. The team, based at the University of Bonn in Germany, publish their results in the journal Monthly Notices of the Royal Astronomical Society.
The group of scientists now have evidence that the mass distribution of stars does indeed depend on the environment in which they form.

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Baby stars born to napping parents

University astronomers believe that a young star's long "napping" could trigger the formation of a second generation of smaller stars and planets orbiting around it.
It has long been suspected that the build up of material onto young stars is not continuous but happens in episodic events, resulting in short outbursts of energy from these stars.
However, this has been largely ignored in models of star formation.
Now, by developing advanced computer models to simulate the behaviour of young stars, University Astrophysicists from the School of Physics and Astronomy Dr Dimitris Stamatellos and Professor Anthony Whitworth, have offered a new insight in star formation.

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A Perth astronomer has discovered how thousands of new stars are formed almost simultaneously and why more stars are born in galaxies further from the Milky Way.
International Centre for Radio Astronomy Research astronomer Kenji Bekki used computer simulations to work out how large clusters of galaxies affect the formation of stars.
Galaxies tend to attract each other and form groups from a few dozen to several thousand.

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Age of the stars
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Scientists from the UPC Barcelona Tech have precisely calculated the age of the stars

A group of scientists led by the UPC has calculated white dwarf stars to be 8 billion years old. The research results will be published in the prestigious scientific journal 'Nature'.
A team of scientists from the Universitat Politècnica de Catalunya Barcelona Tech (UPC Barcelona Tech), the Catalan Institute for Space Studies, the Institute of Space Sciences of the Spanish National Research Council (CSIC), the National University of La Plata  (Argentina), and Liverpool John Moores University (UK), led by researcher Enrique García-Berro of the UPC Barcelona Tech's Department of Applied Physics, has demonstrated that the white dwarf stars in the NGC 6791 star cluster are 8 billion years old, not 6 billion as previously believed. The research opens up new opportunities for extending our knowledge of the origin of the universe. The results will be published in the scientific journal Nature on May 13.

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Magnetic Fields Play Larger Role in Star Formation than Previously Thought
The simple picture of star formation calls for giant clouds of gas and dust to collapse inward due to gravity, growing denser and hotter until igniting nuclear fusion. In reality, forces other than gravity also influence the birth of stars. New research shows that cosmic magnetic fields play a more important role in star formation than previously thought.
A molecular cloud is a cloud of gas that acts as a stellar nursery. When a molecular cloud collapses, only a small fraction of the cloud's material forms stars. Scientists aren't sure why.
Gravity favours star formation by drawing material together, therefore some additional force must hinder the process. Magnetic fields and turbulence are the two leading candidates. (A magnetic field is produced by moving electrical charges. Stars and most planets, including Earth, exhibit magnetic fields.) Magnetic fields channel flowing gas, making it hard to drawn the 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. Our findings serve as the first observational constraint on this issue" - astronomer Hua-bai Li of the Harvard-Smithsonian Centre for Astrophysics.

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