When a great star dies, it will by all means create a black hole. But how can you possible find an invisible object which may be located hundreds of light years away? With more stars in the sky than sand grains in all the world’s beaches, how is it possible to spot a small black star that swallows light instead of shining like a lighthouse at night? Astronomers do not properly look for black holes, but for the effects they provoke in the surrounding space. Astronomers look for a visible star which may have remained trapped in a black hole’s orbit but this too is not easy to recognize. It is like looking for a needle in a haystack, with the difference that if the needle is not felt there is no hope of finding it. In the 80s the Japanese made a big step ahead in this field by launching the x-ray satellite Ginga, provided with a device able to spot any source of x rays in the universe. In practice, it is x-rays which inform us of the presence of a black hole. In 1989, the satellite Ginga recorded a sudden increase of x-rays in an area not distant from our galaxy. The source of radiation was an invisible object 3,000 light years away around which there seemed to rotate a scarcely brilliant star. Such star had been classified as Cygnus v404. It was exactly the star the astronomers were looking for. The hypothesis is that the invisible object is a black hole originated from a star, the most common type. According to the theory a black hole like this should have a mass equal to ten of our sun, but with a diameter equal to the city of London. A second type of black holes is less common, being these far bigger than the previous ones, located in the middle of galaxies. A matching analogy of what occurs to Cygnus v404 is with a well built man and a very thin woman. While they rotate, the man hardly shifts whereas the woman counterbalances their weights and moves much faster. Calculations demonstrate that Cygnus v404 completes a total orbit around its partner once every 6.5 days. In order to gain such a high speed, it has to rotate around a body with a remarkable gravitational mass. It has been calculated that the mass of Cygnus v404’s mysterious partner equals four times that of the sun. This means that it is noticeably heavier, as related to theoretical calculations, than a neutron star. Almost surely it is a black hole. This black hole is driving v404 into a fatal ballet, subtracting gas from the unlucky star to feed a big disc of matter. While advancing spirally towards the black hole, these particles heat up and send a last x-ray signal to the external world. By the same token the astronomers have spotted about ten similar objects. One of the first was cygnus x1. On this object K. S. Thorne and Stephen William Hawking made a bet: is cygnus x1 a black hole? William Hawking had bet it wasn’t. As he points out – this does not mean I did not believe in black holes. It’s just I needed some sort of insurance policy. I had worked a lot on black holes and it would have only been a waste of time had we found out they did not exist. I would at least have had the consolation to win the bet. When, back in 1974, Hawking and Thorne made the bet, there were no positive proofs but as time went by proofs became more convincing and Hawking had to pay the bet. As Thorne himself recounts, in 1990 Hawking was in Los Angeles and came to my office to make some copies of what I had won, the yearly subscription to Penthouse, with his wife’s great disappointment –as Hawking says. This hand-written bet on a paper sheet, became the symbol of the first acknowledgement of the existence of black holes by the scientific community. An article published on Nature on October 20th reveals that a black hole had been discovered, in the Triangle Galaxy, (also called M33, about 3 million light years far from earth) which orbited around a star with a 3.5 days’ period. The peculiarity of this body (catalogued as M33 X-7) is that it is extremely massive: it is thought to have a mass 15.65 times as much as the sun’s, being so the most massive as yet observed black hole, and also its partner has a quite high mass value (about seventy times as much as our Sun’s). Jeffrey McClintock of Harvard-Smithsonian Center for Astrophysics of Cambridge, has explained that it is an enormous star that has an enormous black hole as partner. At the end the partner will become a supernova: so there will be a couple of black holes. The data obtained through observations, however, is in contrast with today’s theories concerning black holes, and so Jerome Orosz, of San Diego State University, one of the article’s authors, has maintained that it is this discovery that arouses all sorts of questions about the possibile origins of such a black hole. In fact, a black hole is born from the collapse of a massive star but, in the case of a binary star system, the star with a bigger mass collapses first and turns into a black hole. This did not happen with M33 X-7, because the star which gave origin to the black hole had a smaller mass than its partner. Furthermore, the generating star should also have had a much bigger radius, even superior to the actual distance between the two celestial bodies, such that the two stars would have had to share part of each other’s atmosphere. On the basis of our present knowledge, this status is not likely to give life to such a big-massed black hole, owing to the loss of gas matter. The study of the newly discovered black hole might bring new information about the origin of black holes and their evolution or to reviewing our current theories. Maybe no answers will be given, but at least we shall know where to find them

Bibliography

Hawking W. Stephen, Dal big bang ai buchi neri, Rizzoli, Milano, 1988;

Hawking W. Stephen, Inizio del tempo e fine della fisica, Mondadori, Milano, 1992;

Hawking W. Stephen, Buchi neri e universi neonati, Rizzoli, Milano, 1995;

Hawking W. Stephen & Roger Penrose, La natura dello spazio e del tempo, Sansoni, Milano, 1996;

Kaufmann J. Williams, Le nuove frontiere dell’astronomia, Sansoni, Firenze, 1980;

Shipmen B. Harry, Buchi neri, Quasar e universo, Zanichelli, Bologna, 1982;

www.pd.astro.it/MOSTRA/NEW/A3003EVO.HTM, INAF istituto nazionale di astrofisica, Renato Falomo, Daniela Fantinel;

www.esa.int/SPECIALS/Missione_Possibile/SEMOEJD3M5E_0.html, ESA European space agency, Fulvio Drigani;

www-groups.dcs.st-and.ac.uk/~history/Biographies/Newton.html, School of Mathematical and Computational Sciences;University of St Andrews, John J O'Connor e Edmund F Robertson;

www.jpl.nasa.gov/news/news.cfm?release=2007-122, NASA, Brian Dunbar;

www.pd.astro.it/MOSTRA/NEW/A5022RAD.HTM, INAF istituto nazionale di astrofisica, Renato Falomo, Daniela Fantinel;

www.ecplanet.com/print.php?id=3205&madre=9, ECplanet, Edoardo Capuano;

www.pd.astro.it/MOSTRA/NEW/EVOL.HTM, INAF istituto nazionale di astrofisica, Renato Falomo, Daniela Fantinel.

Iconography

www.scienzaonline.com/astronomia/img/nascita-buco-nero3g.jpg, scienza online, Guido Donati

www.gsfc.nasa.gov/gsfc/spacesci/pictures/blackhole/BH1m.jpg, NASA, Brian Dunbar;

www.pd.astro.it/MOSTRA/NEW/IMAGES/BHOLE1.JPG, osservatorio astronomico di Padova, Renato Falomo;

www.astrosurf.com/cosmoweb/documenti/buchineri.html,astro surf;

www.lastronomia.it/News2006/07-2006.htm, L’astronomia, M. Ferrara, F. Oldani, R. Serpilli;

www.ivreastrofili.it/Astrofisica/Testi/Relativit%E0%20generale.htm,gruppo astrofili Eporediesi

www.nasa.gov/images/content/163830main_Hawking_Griffin_Rees.jpg, NASA, Brian Dunbar;

http://diamante.uniroma3.it/hipparcos/BlackHole.jpg, Università degli studi di Roma, Joram Marino;

www.scienzagiovane.unibo.it/mortestelle/neri.html, Università di Bologna, R. Giacomelli. B. Poli;

http://chandra.harvard.edu/photo/2007/m33x7/m33x7.jpg, osservatorio astronomico di Roma, Marco Castellani;

www.pd.astro.it/MOSTRA/IMAGES/120702.JPG, osservatorio astronomico di Padova, Renato Falomo

http://astrolink.mclink.it/ids/lib/nanabi.htm, Astro-link, Stefano Iacus;

www.astrosurf.com/cosmoweb/stelle/evoluzione.html, astro surf;

www.castfvg.it/zzz/ids/steneutr.html, circolo astrofili talmassons, Lucio Furlanetto;

www.pd.astro.it/MOSTRA/NEW/EVOL.HTM#stelle6, osservatorio astronomico di Padova, Renato Falomo;

www.bo.astro.it/sait/spigolature/spigo101base.html, osservatorio astrofisica di Bologna, Marco Lolli;

www.torinoscienza.it/dossier/apri?obj_id=8864, Torino scienza, Patrizia Picchi,

http://diva.mporzio.astro.it/webdiva/News/news_universo_nov.htm, osservatorio astronomico di Roma, Francesco D’Alessio.