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Astronomy: Different types of Stars
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Stars lead very different lives. Very high mass stars, with up to 100 times the mass of our Sun, burn up their energy reserves at a high rate and live for about 10 million years. High mass stars live longer, approximately 100 million years. And stars like our Sun - the intermediate stars - can live for 10 billion years and sustain a long liveable region for orbiting planets. Intermediate starsA star like our Sun can be expected to live more than 10 billion years with only a small decrease in surface temperature and a small increase in size. The usage of its energy reserves is relatively slow. Slowly from the center to the surface layers, the hydrogen is transformed into helium by nuclear reactions.This long term stable environment makes the development of life on orbiting planets more likely than on bigger stars. For instance, the next billion years our Sun's luminosity will increase by only 1%. But in 5 billion years the Sun's luminosity has increased by 50% and in 8 billion years its energy output has increased 10 times. Then, the Sun has transformed into a red giant, roasting our planet. In case of the Sun, the Sun will just engulf Earth or just run short when swelling into a red giant. The swelling is caused by the helium that stars burn in the end of the star's life, initiated by the star's growing gravitation and temperature. This energy causes the star to balloon to a giant, glowing red because the surface temperature drops, while the total energy output increases dramatically. Planetary nebula and white dwarfsThe helium burns with flashes that push away the red giant's outer layers. At the core's surface, the remaining helium is burning and around the star is the ejected gas, also called a planetary nebula.The name planetary nebula has nothing to do with planet formation, but as seen from a distance with a telescope it looks the same as the gas disks from which planets are formed. The star of a planetary nebula is soon short of fuel and becomes a white dwarf. The white dwarfs have diameters compared to Earth, and a mass compared to the Sun. Massive and high massive starsHigh mass stars burn their hydrogen in a set of reactions using carbon, nitrogen ans oxygen as catalysts. This process only runs at high core temperatures. A star with the size of 15 solar masses (spectral type B0) has a central temperature of 35 million oC. It pours out nearly 40.000 times the energy output of our Sun.The most massive stars (type O) drive an outflow of gas from their surfaces called a stellar wind, a more exterme form of our solar wind. A star of type O5 with a mass of 60 Suns and a surface temperature of 42.000 oC will blow away more than a quarter of its mass during its 3,5 million years lifetime. |
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A supernovaWhen the hydrogen energy reserves of a massive/ high massive star are depleting, the star starts to burn helium and it grows into a red giant, just like an intermediate star would. But the bigger mass causes the star to grow into a red supergiant.Searching for new energy the star starts to turn carbon into neon and magnesium, then burn neon to make oxygen and more magnesium. This magnesium is transformed into silicon, and oxygen into sulphur, and silicon into iron. Ultimately, the star is left with a heart of iron and in a doomed attempt to extract yet more energy the iron core tries to shrink even further. When gravity is bigger than the star's fusion process the infall starts at an incredible speed, protons and electrons merge to become neutrinos. When a ball of a few kilometers of neutrinos is formed, the infall stops and a huge shock starts from the center. The neutrino shock wave blasts the outer layers into space at a speed of 20.000 km/s, a supernova has occured. The outer layers are expelled in the supernova and only a neutron star or a black hole remains. A neutron star or a black holeA neutron star is comparable to a white dwarf star, but with even a bigger mass and a smaller diameter (approximately 20 km).And a black hole has such strong gravity that even light is pulled back, so we can't observe black holes directly. Yet indirectly, we can establish the existence of a black hole by observing the gravity pull on other bodies in the proximity. Related subjects>> Our Sun>> A brown star >> Betelgeuse, a red supergiant star >> Supernova SN1987A >> First observation of birth of a black hole >> A suspected black hole in the Milky Way Galaxy |
 M101 contains two expanding shells, the remnant of 2 hypernovae - an even bigger version of a supernova and linked to massive gamma ray bursts. These 2 remnants are the biggest ever observed.  Although a super- or hypernova has a devestating effect on its nearby environment, binary stars have been observed which seemed to have survived the collapse and explosion of a twin star. This picture shows the light echo of SN1993J, which is one of the biggest supernovae of modern times, together with SN1987A. |
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