Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Astronomical spectroscopy wikipedia , lookup
Astrophysical X-ray source wikipedia , lookup
First observation of gravitational waves wikipedia , lookup
Planetary nebula wikipedia , lookup
Hayashi track wikipedia , lookup
Standard solar model wikipedia , lookup
White dwarf wikipedia , lookup
Nucleosynthesis wikipedia , lookup
Main sequence wikipedia , lookup
Julia M. Riedl, 1211962 Artikel zum Präsentationsthema Wissenschaftliches Arbeiten und Präsentationstechnik Death of Stars with the Mass of 0.3-8 Sun Masses J.M. Riedl A star gets its energy by fusing hydrogen to helium. The fusion creates a radiation pressure that antagonizes the gravitation which causes the star to be stable. However, once the hydrogen in the core runs out, the star begins to fuse helium to carbon and oxygen. Because of the high pressure, the star expands and, if it has the right mass, becomes a red giant. If the star is light enough it collapses after the “helium burning,” leaving a planetary nebula behind and forming a white dwarf. If the red giant has a mass of 1.4 suns or more, it starts to fuse heavier elements until it gets to iron. However, iron has the strongest binding energy in the atomic nucleus of all elements; therefore it is not possible to get energy by fusing it. The star collapses, the inner part of the core is compressed into neutrons causing infalling material to bounce off and form an outward-propagating shock front. While the surrounding material is blasted away in a supernova, the leftover material forms a small star, and depending on the mass of the remaining material, a neutron star or a black hole is formed. A supernova can also evolve from a white dwarf that gains additional mass from the outside. When the dwarf exceeds a critical mass, it collapses and starts a sudden fusion and explodes. In that case, no compact object is left behind. Being the source of all elements in the universe that are heavier than helium i, stars belong to the most exciting topics of astrophysics. They are born from star forming regions – giant nebulas – that collapse in consequence of the gravitation. Stars live from thermonuclear fusion until they die in an even more spectacular way. One such worth seeing star death – the thermonuclear supernova SN 2014J – could be seen in the beginning of this year in the Cigar Galaxy M82. The supernova happened about 12 million light-years away and got spotted by astronomy students. A fainter supernova called SN 2014L has appeared in the spiral galaxy M99. This galaxy is four times further away than the M99, making the occurrence more difficult to see.ii Nevertheless, stars not only die within a gigantic explosion but also end silently, such as with white dwarves. In this article I am going to explain the main parts of the process a star of a certain mass passes through before its death, and what it leaves behind. When a star, which gets its energy by burning hydrogen to helium and has a mass of 0.3-8 sun masses, runs out of hydrogen in the core, the core region collapses because there is no radiation pressure that counteracts the gravitation. Due to this sudden compression, the temperature rises and the fusion of hydrogen in an outer layer begins, while the helium burning in the core is initiated explosively. From then on the helium fuses to carbon and oxygen. It generates a high pressure and the outer layers of the star expand greatly. On account of the larger surface area, the surface temperature drops and the star seems to be red. The star has thus become a red giant. When there is no helium left in the core, it collapses again causing the temperature to rise Julia M. Riedl, 1211962 Artikel zum Präsentationsthema further. Therefore the helium and hydrogen in outer layers are able to begin fusing. If the star has enough mass, this alternating growing and collapsing continues until it is hot enough for heavier elements to fuse. If the red giant has only a mass of less than about 1.4 sun masses, the fusion stops after the helium in the core has been burned up. Due to gravitational pressure, the star gets compressed and becomes a white dwarf. The outer layers are pushed away, ionized by the UV-rays of the star and begin to glow as a new planetary nebula. The density of a white dwarf is extremely high – one cubic centimeter weighs about one ton. A white dwarf only shines because of the energy in its core. After millions of years it will cool down until it will slowly becomes a black dwarf.iii If the red giant has more than 1.4 sun masses and the helium in the inner layers is already burned, the core collapses, provoking the temperature to rise until the carbon can fuse to neon and magnesium. Later, sulfur and silicium will be formed, and the fusion of silicium produces iron. But iron has the strongest binding energy of the atomic nucleus of all elements. The fusion chain stops here because it requires more energy to fuse iron than the process can produce. The radiation pressure sinks and the core of the star collapses on account of the huge gravitational pull. This collapse frees a gigantic amount of energy – about 1046 Joule. Only neutrons that are produced by merging electrons and protons of the inner core together can stop that implosion. The rest of the material bounces off and forms a huge explosion – a supernova. The light of a supernova explosion is extremely high, so that it is possible to see it as a new object in the starry sky (stella nova = new star). The remaining material of the star forms a pulsar – a neutron star - or a black hole, depending on its mass.iv If the leftover material of a supernova has less than about 2.5 sun masses, the neutron core Note: American English (trillion=1012) Wissenschaftliches Arbeiten und Präsentationstechnik does not further collapse and forms a fast spinning neutron star called a pulsar. In a neutron star, the neutrons are packed very tightly, causing the density of the star to be a million times higher than the density of a white dwarf. Neutron stars only have an average diameter of 20 kilometers, but the weight of a normal star.v Not only their volume but also their magnetic field is compressed, so that a field forces are reached that are a trillion times higher than the magnetic field of the earth. Here is the spinning axis of the pulsar not the same as the axis of the magnetic field. Therefore, the electromagnetic waves discharging from the magnetic poles always point in another direction. If a pole points towards the earth during its rotation, the radio waves can be measured frequently.vi If the mass of the leftover material after a supernova is higher than 2.5 sun masses, not even neutrons can resist the strong gravitational force, and the star is unable to regain stability. The core collapses again until it has no volume but infinite density and has become a black hole. This condition is called singularity. A black hole can be imagined as hole in space-time from where not even electromagnetic waves can escape. This means that black holes can only be observed indirectly.vii Sometimes a light star that ended up as a white dwarf can also become a supernova. If the white dwarf gains additional mass from outside – for example from a companion star in a binary star system – it can reach a critical mass and collapse due to gravitation. The collapse leads to a higher temperature which causes the sudden start of carbon fusion. Through the abrupt admission of energy, the star explodes in a thermonuclear supernova, shooting all of the material into the space and leaving no compact object behind. The companion star becomes a “runaway”-star that flies with the former orbital velocity away.viii i Borgeest, Ulf: Supernovae.Sternentod mit Knalleffekt.Online im Internet: URL: http://www.geo.de/GEO/natur/supernovae-sternentod-mit-knalleffekt-3028.html [Stand 2014-05-07]. ii MacRobert, Alan: Supernova in M82 Passes Its Peak.Online im Internet: URL: http://www.skyandtelescope.com/astronomy-news/observing-news/supernova-in-m82-passes-its-peak/ [Stand 2014-06-05]. iii Rote Riesen und weiße Zwerge.Das Ende eines Sterns.Online im Internet: URL: http://www.scinexx.de/dossier-detail-52-9.h0030062284tml [Stand 2014-05-20]. iv Feuerwerk im Weltall.Schwere Sterne explodieren als Supernovae.Online im Internet: URL: http://www.scinexx.de/dossier-detail-52-10.html [Stand 2014-05-20]. v Neutronenstern.Online im Internet: URL: http://www.astronews.com/glossar/eintraege/neutronenstern.html [Stand 2014-05-20]. vi Superdichte Klumpen und Leuchttürme im All.Was sind Neutronensterne und Pulsare?.Online im Internet: URL: http://www.scinexx.de/dossier-detail-52-11.html [Stand 2014-05-20]. vii Kosmische Staubsauger.Vom Neutronenstern zum schwarzen Loch.Online im Internet: URL: http://www.scinexx.de/dossier-detail-52-12.html [Stand 2014-05-20]. viii Kayser, Rainer: Supernovae.Normale Sterne füttern Weiße Zwerge.Online im Internet: URL: http://www.weltderphysik.de/gebiet/astro/news/2011/supernovae-normale-sterne-fuettern-weisse-zwerge/ [Stand 2014-05-24].