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The Heavyweights of the Universe

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The Heavyweights of the Universe _ ASTRONOMY The heavyweights of the Universe Alternative mechanism may explain the formation of neutron stars that are more massive than normal Salvador Nogueira PUBLISHED IN AUGUST 2011 The Crab nebula is bathed in particles emitted…. 034-037_fisica_ING.indd 34 9/21/12 5:37 PM … by a neutron star (right) at its central region magine compacting the Sun until it is the page 60). The transformation of two hydrogen same size as a city. Is that radical thinking? nuclei, each with one proton, into a helium nu- It might be, but nature performs the same cleus, which has two protons, is accompanied by experiment when it creates the so-called a subtle reduction in the total mass. Part of the neutron stars, which are among the small- mass is converted into energy and escapes from Iest and densest objects in the Universe. Astrono- the star. This escaped energy is the source of a mers know basically how this process happens, star’s power, which can bathe an entire planetary but there are few who admit that a great deal of system in radiation. This energy generated inside work remains before science can explain what the star osets its gravitational force, which acts is out there. One of the mysteries is how neutron in the opposite direction. Because of this equilib- stars appear that have a greater mass than that rium, the star remains approximately the same forecast by the theory of stellar formation and size throughout most of its life. evolution. A group of researchers in Brazil is at- Over millions of years, however, the fuel avail- tempting to illuminate the subject by reviving a able for nuclear fusion gradually runs out. With- controversial hypothesis. In general terms, they out hydrogen, heavier elements, such as heli- are suggesting that there must be more than one um, carbon, and oxygen, are used until a limit is way of creating neutron stars. reached: iron. This is the final frontier: the fu- Neutron stars appear because of the death of sion of iron nuclei consumes more energy than stars that have very large mass, at least eight times the energy released at the end of the process. At greater than the Sun’s mass. To understand what this stage, the production of energy in the central happens, it is necessary to first consider what region is interrupted, and gravity begins working astronomers know about how stars live and die. unimpeded without any force to oset it. Made from concentrated gas (mostly hydrogen) and dust, stars begin to shine when the concen- COSMIC BOMB tration of material makes the atoms in the central At this point, the star collapses, triggering a com- region of these heavenly bodies join together, plex sequence of events. The result is the explo- NASA / JPL a process known as nuclear fusion (see text on sion of the outermost layers of the star, when 90% PESQUISA FAPESP 35 034-037_fisica_ING.indd 35 9/21/12 5:37 PM of its mass is launched into space. What remains after this violent episode, known as a supernova, is a very compact stellar Life and death of stars core. If the mass of the core is relatively small, this compression produces what Stars with a mass eight is normally called a neutron star. If the times greater than mass is larger and the compression con- that of the Sun shine SUPERGIANT tinues, a black hole is formed, an object for millions of years so dense that nothing escapes its attrac- until they exhaust tion, not even light. their nuclear fuel and MAIN explode in the form of SEQUENCE ccording to the currently accept- a supernova. The outer ed theory, neutron stars (so called layers are launched Abecause they have high propor- into space, and the tions of uncharged particles (neutrons) central core generates inside them) should all have the same a neutron star or a dimensions: a mass almost 40% greater black hole than that of the Sun compressed into a sphere less than 20 km in diameter. “But no one knows exactly what mass PROTOSTAR a star needs to have during its life to die and leave a neutron star or a black hole,” says astronomer Jorge Horvath from the Institute of Astronomy, Geophysics RECYCLING and Atmospheric Sciences (IAG) at the University of São Paulo, coordinator of a group that is investigating the charac- teristics of neutron stars. “Until recently, it was believed that all neutron stars had this pattern,” says MOLECULAR João Steiner, another astronomer from CLOUD the IAG. “But last year, a case was dis- covered that is clearly bigger.” The name of the object is PSR J1614- a larger mass that is approximately 1.73 223, a neutron star located 3,000 light times the solar mass. THE PROJECTS years away from Earth that was discov- Why do these two different groups ered by a group from the National Radio- exist? “The results point to more than 1. Hadronic matter and QCD in astrophysics: astronomy Observatory (NRAO) in the one mechanism for neutron star forma- supernovas, grbs and compact stars – nº 2007/03633-3 United States. Presented in an article tion,” says Horvath. published in Nature, this star appears to This idea seems compatible with the 2. Investigation of high energy and high have two solar masses – it is mammoth, distribution of neutron stars in places density astrophysical phenomena - nº 2008/09136-4 in terms of objects of this type. like globular clusters, which are inhab- This fi nding obliged the astronomi- ited principally by very old stars with a TYPE cal community to accept the fact that smaller mass than the mass that would 1. Thematic Project 2. Young Investigators Awards Program there is signifi cant variation in the size be necessary to give rise to neutron stars, of neutron stars. This realization fits according to the star formation theory. COORDINATORS well with the forecasts made recently by Recent observations by astronomers 1. Jorge Horvath – IAG/USP 2. German Lugones - UFABC Horvath’s group, which were published from various countries have shown there in the June issue of the journal Monthly are many more neutron stars in these INVESTMENT Notices of the Royal Astronomical Soci- regions than would be expected if they 1. R$ 154,250.00 (FAPESP) 2. R$ 91,207.65 (FAPESP ) ety. In this work, Horvath, Eraldo Ran- were the exclusive product of the explo- gel and Rodolfo Valentim conducted sion of large-mass stars. a statistical analysis of the mass of 55 Stars that originally have a mass that heavily studied neutron stars and found is less than eight times that of the Sun there are two common patterns: one is do not generate neutron stars when formed by stars with a smaller mass (ap- they collapse. Instead, they generate proximately 1.37 times that of the Sun) another class of object: white dwarves, and with little variation, as expected; with the mass of a sun compressed to the other is a more variable type with a volume such as that of the Earth. 36 SPECIAL ISSUE SEPTEMBER 2012 034-037_fisica_ING.indd 36 9/21/12 5:37 PM At the Federal University of ABC in Santo André, in the Greater São Paulo Metropolitan Region, Germán Lugones’ group has been performing calculations SUPERNOVA and simulations of how di erent internal compositions of these stars would a ect their mass, radius, evolution and other properties. One of the team’s results is that certain phenomena that arise when matter is found in the form of quarks – such as the transition to a superconduc- tor state – naturally explain the existence of stars with masses much greater than the classic 1.4 times the solar mass. This is why the discovery of PSR J1614-223 was an important sign that research- ers may be on the right path. Lugones believes that a more radical version of quark stars – the strange star, or auto- BLACK HOLE linked quark star, in which the entire star is composed of these particles – should be considered a candidate if stars with a mass even greater than that of PSR J1614-223 are observed. “According to theoretical studies car- ried out over the last few years by our group, the density necessary for particles NEUTRON STAR of matter to break down into quarks is SUPERNOVA 5 to10 times greater than the density of REMNANTS the inside of an atomic nucleus,” says Lugones, emphasizing that this density may be reached quite easily in the cen- ter of neutron stars with greater mass. This is how the Sun will likely end its The defi nitive answer is yet to emerge, No one knows if this occurs. There days. In some binary systems, the white but it is almost certain that future re- are gaps in understanding of both the dwarf uses gravity to rob mass from its search will involve reformulations in physics behind these processes and the companion star until it reaches a limit the theories of how neutron stars arise observable properties of neutron stars. that induces a new collapse. This event and behave. Manuel Malheiro, a researcher at the is explosive and produces a specific Aerospace Technological Institute and a type of supernova, called Ia, in which INSIDE AND OUT collaborator with Horvath and Lugones, the entire mass of the star is violently If there is mystery regarding the size and has been at the University of Rome since launched into space. mass of neutron stars, it is no simpler 2010, where he is investigating the com- when the subject is their composition.
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