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WC13 - Recap Extra­terrestrial Life(II Recap...

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Unformatted text preview: Recap Extra­terrestrial Life (II) Recap Extra­terrestrial Life (II) Describe the current research on extra­solar planets Describe the issues in the communication with extra­terrestrial intelligence. What is terraformation ? What are our chances for the colonization of our Galaxy ? What are our chances for the colonization of our Universe ? The Life of Stars The Life of Stars Seven Ages for our Sun The Life of a Very Large Star Neutron Stars (pulsars) Close Binary Systems The Life of Our Sun Our Sun is a mature star in the main sequence of the Hertzsprung­Russell (H­R) diagram . To predict its past or future astronomers have to examine other stars, which are at the moment of observation in different stages of evolution. This research requires models developed with the thermonuclear data obtained by nuclear physicists. The 1st Age The 1 1st age corresponds to the gravitational collapse which leads to the formation of the protostar. The energy generated by the collapse is pumped out making the young Sun about 500 times more luminous than today. The star itself had a diameter about 50 times larger than today and its rotation was much faster than today. Most stars seen in this phase of development are hidden within cocoons of dust and can be seen only in infrared. The evolution of the protostar involved an increase in its density and a change in the radiation elimination from a direct to a convection process. After about 8 million years of gravitational collapse the Sun becomes of about the same size as today. The 2nd Age The 2 As the temperature at its center reached about 10 million degrees, the nuclear fusion of hydrogen started and an equilibrium was reached between the force of the nuclear phenomena and the pressure of the external layers. The 2nd age corresponds to a Sun which produces energy through nuclear fusion reactions (primarily the conversion of hydrogen to helium). To ignite them the Sun used the gravitational energy produced by the collapse. Today we are after 4.5 billion years of the 2nd age and every second the Sun “burns” about 600 millions of tons of hydrogen. Models show that this process will continue for another 1.5 billion years (nothing to worry about !) The 3rd Age The 3 The 3rd age starts with the moment when the center of the Sun becomes mainly helium and nuclear fusion of hydrogen continues with higher intensity in the external layers. During this age, before becoming 10 billion years old, the Sun will grow 3 times in diameter and 4 times in luminosity, most likely destroying life on Earth (if other events will not do it earlier). The growth in size makes the temperature at the surface to decrease. The 4th Age The 4 The 4th age is an accelerated continuation of the 3rd age, when in only 600 million years the Sun will become about 50 times bigger and about 1500 times brighter than today. The key phenomenon in the 4th age is the nuclear fusion of helium (the triple­alpha reactions) in the star’s core, which requires temperatures around 100 million degrees. This makes the core to expand and implicitly to cool down, making the star’s luminosity to decrease. During the 4th age the Sun will become a “red giant” similar to today’s Antares in the Scorpius constellation. By then Mercury, Venus and maybe even Earth would be “swallowed” by our Sun. The density of the Sun, which today is about 1/5 of the Earth density, would become only 1/10 of the today’s Earth atmosphere. The 5th Age The 5 5th age has the helium in the center of the Sun converted into carbon and oxygen, while the helium fusion continues in the outer layers producing increased amounts of nuclear energy and a further expansion. After 30 million years of 5th age expansion the red giant would be about 400 times bigger than today and would swallow even Mars. Because the triple alpha reactions are very sensitive to temperature changes, in this stage our Sun will become unstable and will eject matter and radiation in the outerspace. Up to 40% of its mass can be lost through this “puffing” phenomenon. Radiation makes the gas visible in a variety of colours. Planetary nebulae Planetary nebulae Dying Stars: various types of matter emissions The 6th Age The 6 The 6th age starts when the nuclear fuel is finished. For a star of Sun’s size the compression in the center is not sufficient to heat up the carbon at a temperature where it could fuse and therefore the the nuclear energy generation stops. During about 50,000 years the Sun will collapse gravitationally through a number of short explosions. Its collapse will stop because of the Pauli principle, which puts a limit on the squeezing of an atom. Our Sun will become a white dwarf, with a diameter about 100 times smaller than today (about the size of Earth) and a huge density (a matchbox of its material would weigh tons !). The gravitational collapse will produce energy, the temperature at the surface will reach 30,000 degrees and the brightness of the Sun will be about 50 times larger than today. Many white dwarfs were observed since the first one was The 7 Age The 7 th The 7th age corresponds to a cooling dwarf. It becomes first red and then black. Our Sun as a black dwarf will be a kind of huge diamond, still orbited by Jupiter, Saturn, Uranus and Pluto. Although with a mass only 60% of today’s, the Sun will still be substantially heavier than these planets. After a short period of time when the temperature conditions around these planets was quite amenable, they will become again the cold worlds that they are today, because the black dwarf produces no radiation and no heat. Seven Ages of the Sun in the H­ Seven Ages of the Sun in the H­ R diagram 6 luminosity white dwarf 5 4 main sequence 7 2 3 1 red giant 30,000oC temperature 3,000oC The Life of Other Stars The Life of Other Stars The main sequence in the H­R diagram contains a variety of stars with masses between 0.1 and 20 solar masses. The higher the mass the higher the luminosity and the surface temperature (luminosity increases with the 3rd or 4th power of its mass) Larger stars live shorter than small stars (the largest can live just millions of years, while the smallest more than a trillion years) Small Stars Small Stars Protostars with masses smaller than our Sun take millions of years to form and then follow the same 7 ages but at a slower speed. They can reach the stage of a red giant after hundreds of billions of years. Protostars with masses between 0.08­0.5 solar masses appear as red dwarfs. Their surface temperature is less than 4000oC. Recent studies showed that these stars are the most abundant in our galaxy (about 70% of all stars). Protostar with less than 0.08 solar masses (about 80 times the mass of Jupiter), are brown dwarfs, dense gaseous bodies with a surface temperature well below 3000oC and with luminosities about 10,000 times smaller than the Sun. Large Stars Large Stars Stars with significantly higher masses than our Sun will have a much faster evolution. For instance a star with 5 solar masses will stay in the 2nd age for only about 100 million years (while for the Sun it is about 6 billion years). When it becomes a red giant (4th age), this star will become a pulsating variable (e.g. a Cepheid variable). In the 5th age it will become a supergiant, which can loose about 80% of its mass to become a white dwarf. If the mass of the star is at least 4 solar masses the core temperature can reach about 600 million degrees, which is sufficient for the fusion of carbon. Very Large Stars Very Large Stars We shall call “very large” a star with more than 8 solar masses. Their main characteristic is that at the end of their life they will not form white dwarfs. Instead they will explode – the supernova process – producing remnants. We shall discuss later the two possible remnants of very large stars. Very Large Stars (II) Very Large Stars (II) There are many stars much larger than our Sun. For instance, in 1968 the Canadian astronomer Alan Batten discovered a star 60 times heavier then our Sun ! Larger stars burn stronger and quicker. A star twice heavier than the Sun would burn hydrogen only 1/5 of the time for the Sun (2 billion years), while Batten’s star would burn its fuel in a few million years. As these stars are more massive, the compression of carbon in the center is huge and fusion reactions involving carbon continue for about 600 years. As heavier elements are being fused in the center the energy produced increases continuously. The heavier elements burn quicker than carbon: neon burns for about 1 year at 1 billion degrees and silicon for 1 day at 3 billion degrees. Supernovas (I) Supernovas (I) A huge explosion is produced when near the center of the red supergiant the fuses iron, which has the highest binding energy per nucleon. The fusing of iron will use energy and will cool a volume just above the stellar core. That cooling in the middle a star heated at billions of degrees will break the star. The explosion just above the core pushes into the core material. When the mass of the core is larger than 1.4 solar masses the collapse overwhelms the stability of atoms. Electrons combine with protons producing neutrons and a huge number of fast neutrinos, which interact through the weak force with the external layers. In less than a second the core becomes a body of neutrons only 20 km in diameter. Nuclear reactions in the external layers cause them to break into fragments which are ejected at speeds of about 40,000 km/s. A supernova is created, which will light the entire galaxy for many days. Supernovas (II) Supernovas (II) During the first day of the supernova, the absolute magnitude of that star can increase to about 600 million solar luminosities, which is brighter than a small galaxy. Supernovae can be very dangerous, but they are quite rare. On average a galaxy has a supernova every 500 years. Most of the energy released in the supernova (99%) is under the form of fast neutrinos. Nuclear reactions produce in the clouds of debris elements much heavier than iron. Cosmic rays studies discovered cosmic nuclei heavier than uranium, which could be produced only in the extreme heat of supernovas. Types of Supernovas Types of Supernovas Type II supernovas have hydrogen lines prominent in their spectrum, because the outer layers of the supergiant contain a lot of unconsumed hydrogen. The spectra of type I supernovas do not have hydrogen lines (in types Ib and Ic because hydrogen was eliminated or burned before the explosion). Type Ia supernova occurs in a binary system involving a white dwarf (containing carbon and oxygen) and a red supergiant. Material from the low density supergiant is sucked by the white dwarf. As the mass of the white dwarf increases past 1.4 solar masses, it collapses and almost the whole star material is blasted into space. Type Ia supernovas are more than 10 times brighter than the type II supernovas. Supernova Remnants Supernova Remnants The expanding cloud of debris creates a growing hole containing in the middle the small neutron core. Fast electrons moving in the expanding shell emit radiation from radio to X. In 1054 Chinese astronomers observed a supernova which was only 6000 light­years away being visible during the day. Its remnant is visible today with modest telescopes. It is called the Crab nebula because of its complex, crablike filamentary structure. The gas of this nebula is moving at speeds over 1000 km/sec. The Crab Nebula The Crab Nebula The 1987 Supernova The 1987 Supernova The most recent naked­eye supernova flared up in 1987 in the Large Magellanic Cloud, the nearest galaxy to our Milky Way. B31, the star that exploded was a blue supergiant with a mass equal to about 20 solar masses. Neutrino the explosion several hours before the optical observations. The above detectors noted image shows glowing gas rings excited by the radiation from the 1987 supernova. By 2005 more the debris plowing into these rings made the picture of this remnant more complex. Cosmic Jazz Cosmic Jazz 1968 is the year when the first pulsars were discovered. As the name suggests, a pulsar would produce radio signals with a constant frequency. The first pulsar was discovered at Cambridge, UK. It always appeared at the same time during the day and produced signals every 1.3 seconds for about 4 minutes. A graduate student (Jocelyn Bell) had the courage to say that this was not human signal but of extra­terrestrial origin. Soon after this announcement, a team at Green Bank (US) identified another pulsar (named NP0532) inside the Crab Nebula. The initial explanation was related to spinning white dwarfs. The discovery at Jodrell Bank (UK) of a pulsar with a frequency of 1/30 sec. Proved that the source of these radio signals must be spinning objects smaller than a white dwarf. Pulsars are Neutron Stars Pulsars are Neutron Stars In 1931 S.Chandrasekhar proved that after a supernova a central remnant equal to 1.4 solar masses would not end up as a white dwarf. Its gravitational collapse would not be stopped by the Pauli exclusion principle and the atomic electrons would collapse into the nucleus and would combine with protons to produce neutrons. A “neutron star” would spin with a high frequency engaging in this movement the surrounding gas/plasma. This rotation of plasma in extremely high magnetic fields (1012 times higher than around Earth). The pulses of radio and visible radiation are due to the fact that the radiation can escape only through the magnetic poles. A typical pulsar has a diameter of 20­30 km but with a mass similar to that of our Sun. The density of the neutron star was not uniform: a solid crust was covering a dense liquid interior and probably a solid center. Close Binary Systems Close Binary Systems White dwarfs with red giants – type 1a supernovas Pulsars with red giants – bursts of X rays Binary pulsars: At least 20 known systems. The first, PRS1913+16, was discovered in 1974. It consists of two neutron stars each with 1.4 solar masses rotating around each other in 7.75 hours. Their pulses are with a period of 0.059 seconds. In almost 30 years of observations it was possible to notice that the two pulsars move towards each other because of their loss of energy through gravitational waves. Other systems: Binary white dwarfs White dwarf with pulsar ...
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