حل لغز الثقب الأسود والنجوم

[وسيم أحمد الفلو]

ألقى علماء فلك الضوء على كيفية تشكل النجوم حول الثقب الأسود الكبير على عكس ما كان يعتقد في السابق.

وكان العلماء يستبعدون حدوث ذلك في الماضي في مثل هذه الظروف لأنه يتناقض مع المنطق العلمي.

فالسحب الجزيئية التي تتكون منها النجوم تتفتت في حالة تعرضها للجاذبية حسبما ذكر فريق من الباحثين في دورية :ساينس" العلمية.

إلا أن الباحثين قاوا إن النجوم ينما أن تتكون من اسطوانات ناتجة عن تفتت سحب غازية عملاقة عند اقترابها من الثقب الأسود.

وقد توصلوا إلى هذا الاكتشاف بعد اجراء تجارب على أجهزة الكومبيوتر

من خلال خلق أشكال تشبه السحل العملاقة التي تبتلع عند اقترابها من الثقب الأسود مثل الماء عندما ينساب من البالوعة.

ويتفق ما توصل اليه العلماء مع الرصد الميداني من خلال محطة ميلكي واي الفضائية التي توصلت إلى وجود ثقب أسود ضخم محاط بنجوم عملاقة تدور في مدارات بيضاوية.

وقد تمت عملية المشابهة باستخدام جهاز كومبيوتر عملاق، واستغرقت التجربة عاما كاملا، أعقبها تشكيل سحابتين عملاقتين من الغازات أكبر 100 ألف مرة من حجم الشمس، وهما تختفيان داخل الثقب الاسود.

http://www.stfc.ac.uk/resources/image/SolarMass.jpg

[وسيم أحمد الفلو]

The mystery of how young stars can form within the deep gravity of black holes has been solved by a team of astrophysicists at the Universities of St Andrews and Edinburgh.


Solar mass molecular cloud falling towards a black holeThe team, partly funded by the Science and Technology Facilities Council (STFC), made the discovery after developing computer simulations of giant clouds of gas being sucked into black holes. The new research may help scientists gain better understanding of the origin of stars and supermassive black holes in our Galaxy and the Universe. The new discovery is published in the journal Science this week (22nd August 2008).

Until now, scientists have puzzled over how stars could form around a black hole, since molecular clouds - the normal birth places of stars - would be ripped apart by the black hole's immense gravitational pull.

However, the new study by Professor Ian Bonnell (St Andrews) and Dr Ken Rice (Edinburgh) found that stars appear to form from an elliptical-shaped disc, the remnant of a giant gas cloud torn apart as it encounters a black hole.

The discovery of hundreds of young stars, of high masses and making oval-shaped orbits around a black hole three million times more massive than the sun, and at the centre of our Galaxy, is described as one of the most exciting recent discoveries in astrophysics.

Prof Bonnell comments "These simulations show that young stars can form in the neighbourhood of supermassive black holes as long as there is a reasonable supply of massive clouds of gas from further out in the Galaxy.

The simulations, performed on the Scottish Universities Physics Alliance (SUPA) SGI Altix supercomputer - taking over a year of computer time - followed the evolution of two separate giant gas clouds up to 100,000 times the mass of the sun, as they fell towards the supermassive black hole.

The simulations show how the clouds are pulled apart by the immense gravitational pull of the black hole. The disrupted clouds form into spiral patterns as they orbit the black hole; the spiral patterns remove motion energy from gas that passes close to the black hole and transfers it to gas that passes further out. This allows part of the cloud to be captured by the black hole while the rest escapes. In these conditions, only high mass stars are able to form and these stars inherit the eccentric orbits from the disc. These results match the two primary properties of the young stars in the centre of our Galaxy: their high mass and their eccentric orbits around the supermassive black hole.

[Dr Rice comments " The crucial element was the modelling of the heating and cooling of the gas as this tells us how much mass is needed for part of the gas to have enough gravity to overcome its own gas pressure, and thus form a star. The heating is caused by the extreme compression of the cloud as it is squashed and pulled apart by the black hole.

This is balanced by the cooling which requires detailed knowledge of how quickly the radiation can escape the cloud."

Professor Bonnell concluded, “That the stars currently present around the Galaxy's supermassive black hole have relatively short lifetimes of ~10 million years, suggests that this process is likely to be repetitive. Such a steady supply of stars into the vicinity of the black hole, and a diet of gas directly accreted by the black hole, may help us understand the origin of supermassive black holes in our and other galaxies in the Universe."

[وسيم أحمد الفلو]

By Phil Berardelli
ScienceNOW Daily News
22 August 2008

Black holes might have a nurturing side. Scottish researchers have created a computer simulation that explains how supermassive black holes, such as the one at the center of the Milky Way, could promote the birth of nearby stars. The findings expand the possible scenarios for star formation and could help astronomers determine how stars emerged in the very young universe.
The popular conception of black holes is that they obliterate anything in their path, including time, space, and matter. Stars can escape annihilation if their orbits keep them far enough away. But some stars not only orbit perilously close to a supermassive black hole but also appear to have formed in its vicinity. Earlier in this decade, for example, astronomers spotted a population of very young--under 10 million years old--and very massive stars locked in elliptical orbits around the Milky Way's central black hole (ScienceNOW, 13 October 2005).

Could the stars have migrated there? Not likely. They're too young, and there are no nearby star hatcheries that could have produced them. The other possibility is that the stars formed in place. But astronomers also considered that idea unlikely, because the supermassive black hole would have shredded any cloud of gas--from which all stars condense--pulled into its influence.

Now the homegrown scenario seems more realistic, thanks to a computer model developed by astrophysicists Ian Bonnell of the University of St. Andrews in Fife, U.K., and Kenneth Rice of the University of Edinburgh, U.K. The simulation, which required more than a year of supercomputer time, tracked two hypothetical clouds of molecular hydrogen--the basic stellar building material--moving within a light-year or so of a supermassive black hole, much like the one anchoring the Milky Way. The researchers report today in Science that as the clouds fell toward the black hole, its gravity disrupted but did not destroy their clumpy structure. Eventually, the clouds flattened and merged into a disk that followed an elliptical orbit. During the flattening, nearly 200 new stars ignited, within a few hundred thousand years. Nearly all the resulting stars were very massive, meaning that they will live short and violent lives ending in supernovae.

The findings raise the question of where the star-forming clouds in the Milky Way would have come from. Bonnell and Rice speculate that they drifted freely within the galaxy until interaction with some other object or objects, such as larger clouds or other black holes, sent them hurtling toward the supermassive central black hole. But the answer remains unclear.

The simulation is a "breakthrough," says astronomer Mark Voit of Michigan State University in East Lansing, because it helps explain why those massive young stars around the Milky Way's center follow such elongated orbits. It "addresses one of the big open questions in astrophysics," adds Volker Bromm, an astrophysicist at the University of Texas, Austin. Thanks to this work, he says, "one wonders what the next-generation telescopes will find in the far-away universe just a few years from now."

http://sciencenow.sciencemag.org/content/vol2008/issue822/images/200882221.jpg



Riding the whirlwind. Computer simulation shows the birth of massive stars (bright dots) occurring in a gas cloud being pulled in by a giant black hole.
Credit: I. A. Bonnell and W. K. M. Rice, Science