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Posted on 08/18/2010 10:06:11 AM PDT by decimon
Using ESO’s Very Large Telescope, European astronomers have for the first time demonstrated that a magnetar — an unusual type of neutron star — was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar. This now raises a fundamental question: just how massive does a star really have to be to become a black hole?
To reach their conclusions, the astronomers looked in detail at the extraordinary star cluster Westerlund 1 [1], located 16 000 light-years away in the southern constellation of Ara (the Altar). From previous studies (eso0510), the astronomers knew that Westerlund 1 was the closest super star cluster known, containing hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two thousand times the diameter of the Sun (as large as the orbit of Saturn).
“If the Sun were located at the heart of this remarkable cluster, our night sky would be full of hundreds of stars as bright as the full Moon,” says Ben Ritchie, lead author of the paper reporting these results.
Westerlund 1 is a fantastic stellar zoo, with a diverse and exotic population of stars. The stars in the cluster share one thing: they all have the same age, estimated at between 3.5 and 5 million years, as the cluster was formed in a single star-formation event.
A magnetar (eso0831) is a type of neutron star with an incredibly strong magnetic field — a million billion times stronger than that of the Earth, which is formed when certain stars undergo supernova explosions. The Westerlund 1 cluster hosts one of the few magnetars known in the Milky Way. Thanks to its home in the cluster, the astronomers were able to make the remarkable deduction that this magnetar must have formed from a star at least 40 times as massive as the Sun.
As all the stars in Westerlund 1 have the same age, the star that exploded and left a magnetar remnant must have had a shorter life than the surviving stars in the cluster. “Because the lifespan of a star is directly linked to its mass — the heavier a star, the shorter its life — if we can measure the mass of any one surviving star, we know for sure that the shorter-lived star that became the magnetar must have been even more massive,” says co-author and team leader Simon Clark. “This is of great significance since there is no accepted theory for how such extremely magnetic objects are formed.”
The astronomers therefore studied the stars that belong to the eclipsing double system W13 in Westerlund 1 using the fact that, in such a system, masses can be directly determined from the motions of the stars.
By comparison with these stars, they found that the star that became the magnetar must have been at least 40 times the mass of the Sun. This proves for the first time that magnetars can evolve from stars so massive we would normally expect them to form black holes. The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars and those above 25 solar masses would produce black holes.
“These stars must get rid of more than nine tenths of their mass before exploding as a supernova, or they would otherwise have created a black hole instead,” says co-author Ignacio Negueruela. “Such huge mass losses before the explosion present great challenges to current theories of stellar evolution.”
“This therefore raises the thorny question of just how massive a star has to be to collapse to form a black hole if stars over 40 times as heavy as our Sun cannot manage this feat,” concludes co-author Norbert Langer.
The formation mechanism preferred by the astronomers postulates that the star that became the magnetar — the progenitor — was born with a stellar companion. As both stars evolved they would begin to interact, with energy derived from their orbital motion expended in ejecting the requisite huge quantities of mass from the progenitor star. While no such companion is currently visible at the site of the magnetar, this could be because the supernova that formed the magnetar caused the binary to break apart, ejecting both stars at high velocity from the cluster.
“If this is the case it suggests that binary systems may play a key role in stellar evolution by driving mass loss — the ultimate cosmic ‘diet plan’ for heavyweight stars, which shifts over 95% of their initial mass,” concludes Clark. Notes
[1] The open cluster Westerlund 1 was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970–74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100 000, and this is why it has taken so long to uncover the true nature of this particular cluster.
Westerlund 1 is a unique natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our Milky Way live and die. From their observations, the astronomers conclude that this extreme cluster most probably contains no less than 100 000 times the mass of the Sun, and all of its stars are located within a region less than 6 light-years across. Westerlund 1 thus appears to be the most massive compact young cluster yet identified in the Milky Way galaxy.
All stars so far analysed in Westerlund 1 have masses at least 30–40 times that of the Sun. Because such stars have a rather short life — astronomically speaking — Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly a “newborn” cluster in our galaxy. More information
The research presented in this ESO Press Release will soon appear in the research journal Astronomy and Astrophysics (“A VLT/FLAMES survey for massive binaries in Westerlund 1: II. Dynamical constraints on magnetar progenitor masses from the eclipsing binary W13”, by B. Ritchie et al.). The same team published a first study of this object in 2006 (“A Neutron Star with a Massive Progenitor in Westerlund 1”, by M.P. Muno et al., Astrophysical Journal, 636, L41).
The team is composed of Ben Ritchie and Simon Clark (The Open University, UK), Ignacio Negueruela (Universidad de Alicante, Spain), and Norbert Langer (Universität Bonn, Germany, and Universiteit Utrecht, the Netherlands).
The astronomers used the FLAMES instrument on ESO’s Very Large Telescope at Paranal, Chile to study the stars in the Westerlund 1 cluster.
ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”. Links
* Research paper * More information: Black Hole Press Kit
Contacts
Simon Clark The Open University UK Tel: +44 207 679 4372 Email: jsc@star.ucl.ac.uk
Ignacio Negueruela Universidad de Alicante Alicante, Spain Tel: +34 965 903400 ext 1152 Email: ignacio.negueruela@ua.es
Richard Hook ESO, La Silla, Paranal and E-ELT Press Officer Garching bei München, Germany Tel: +49 89 3200 6655 Email: rhook@eso.org
‘I thought it burned off it’s fuel (mass). Where did all the mass come from?’
It’s a process called ‘nucleosynthesis’. The energy comes from fusing hydrogen and hydrogen together to form helium.
In stars larger then the sun, they continue to fuse helium to produce Carbon, Nitrogen and Oxygen.
Thanks for posting your patient answers. Don’t worry if the questioner gets it or not.
There are probably others besides myself who enjoy reading the background details that you are providing (stuff that the main article assumes the reader already knows).
Thanks again. Very interesting.
Why is it that iron is the end-point of stellar fusion? Why can’t iron fuse to become heavier elements as part of the normal star-burning progression? I’ve always wondered about this.
I know that the heavier elements are all created during a supernova but I’ve never understood why they can’t be created in the same fashion as the lighter ones are, by normal stellar fusion.
European astronomers have for the first time demonstrated that a magnetar — an unusual type of neutron star — was formed from a star with at least 40 times as much mass as the Sun. ... Westerlund 1, located 16,000 light-years away in the southern constellation of Ara..
(cough) Not to point out the obvious, but what they are observing from this 'magnatar happened 16,000 years ago. Ergo, for all we know Westerlund 1 could have went from magnetar to Super Nova 15,000 years ago and we won't know it for another thousand years. 
Gotta run. I have to hook up those 480V lines to the Magnets on my Cold Fusion Reactor I'm making in my garage. If that doesn't cause those darn Hydrogen Atoms to Fuse, I may have to rethink a step or two. Like replacing one with an Anti-Matter Hydrogen atom.
(its safe, really, honest -- I read a book on it)
HUH? Are you 'series'?
No offense, and I see you have your hands full already, but Einstein mathematically proved they exist, Hawking confirmed they exist, and the first one was 'seen'(1) by Cosmologists in 1971 (iirc).
Black Holes are a Fact of Science. Not even a 'theroy' now.
(1) the Black Hole itself isn't 'seen' (after all it's Black) but its existence confirmed by tracking orbits of stars around them. If a vertical Black Line on the axis is jagged (offset) at the star's orbit, there's a Black Hole in the middle.
Nuclear reactions are similar to chemical reactions in that some of the reactions are exothermic, and some are endothermic. It has to do with the binding energy between the products and the binding energy of the reactants.
In nuclear reactions you are looking at the binding energy within the nucleus.
Once Iron is fused, the next step would be to fuse Zinc. Zinc, unfortunately has more binding energy than you would get from fusing Iron.
Zinc, unfortunately has more binding energy than you would get from fusing Iron.And that extra needed energy is there during the time of a supernova?
Yep, that’s how we get elements heavier than Iron. That’s also why elements heavier than iron are so much rarer.
Star ping!
thanks
Hmmm... There seems to be a link between magnetars and pissing contests...
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