Star: From Birth till Death
Lecturer in Physics Twinkle, Twinkle little star How I wonder, what you are Up above the warld so high Like a diamond in the sky Birth of a Star Imagine an enormous cloud of gas and dust many light- years across. Gravity, as it always does, tries to pull the materials together. A few grauns of dust collect a few more, then a few more, then more still. Eventually, enough gas and dust is collected into a giant ball that, at the center of the ball, the temperature (from all the gas and dust bumping into each other under the great pressure of the surrounding material) reaches 15 million degrees or so. A wondrous event occurs, nuclear fusion begins and the ball of gas and dust starts to glow. A new star has begun its life in our Universe. This magical thing is called “nuclear fusion Why does it start happening inside the ball of gas and dust? It happens like this As the contraction of the gas and dust progresses and the temperature reaches 15 million degrees or so, the pressure at the center of the ball becomes enormous. The electrons are stripped off of their parent atoms, creating plasma. The contraction continues and the nuclei in the plasma start moving faster and faster. Eventually, they approach each other so fast that they overcome the electrical repulsion that exists between their protons. The nuclei crash into each other so hard that they stick together, or fuse, in doing so, they give off a great deal of energy This energy from fusion pours out from the core, setting up an outward pressure in the gas around it that balances the inward pull of gravity When the releascd energy reaches the outer layers of the ball of gas and dust, it moves om into space in the form of electromagnetic radiation. The ball, now a star, begins to shine. New come in a variety of sizes and range from blue to from than half the size of our Sun over 20 times the Sun’s size. It all depends on how much gas and dust is collected during the star’s formation The of the star depends on the surface temperature of the star its temperature de again, on how much gas and dust were accumulated during formation. The more ma a star starts out with, the brighter and hotter it will be For a star everything depends on its mass Beginning of the End Throughout their lives, stars fight the inward pull of the force of gravity It is only the outward pressure created by the nuclear reactions pushing away from the stars core that keep the star “intact” But these nuclear reactions require fuel, in particular, hydrogen. Eventually the supply of hydrogen runs out and the star begins its demise The Red Giant After millions to billions of years, depending on their initial masses, stars run out of their main fuel hydrogen Once the ready supply of hydrogen in the core is gone, nuclear processes occurring there cease. Without the outward pressure generated from these reactions to counteract the force of graviry, the outer layers of the star begin to collapse inward toward the core. Just as during formation, when the material contracts, the temperature and pressure uncrease This newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward. The star expands to larger than it ever was during its lifetime a few to about a hundred times bigger, The star has become a “Red Giant”. The Death of a Star What happens next in the life of a star depends on its initial mass, Whether it was a “massive” star (some 5 or more times the mass of our Sun) or whether it was a “low or medium mass” star (about 0.4 to 3,4 times the mass of our Sun), the nex steps after the red giant phase are very, very different The Supernovae The Fate of Massive Stars one of the most energetic explosions in nature, making them like a 10″ Megaton bomb (ie, a few octillion nuclear warheads). For stars which are some 5 or more times as massive as our Sun. After the outer layers of the star have swollen into a red super giant e a very big red giant), the core begins to yield to gravity and starts to shrink As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core phase of its In than a second, the star begins the final gravitational collapse The core temperature rises to over 100 billion degrees as the iron atoms are crushed together, and core recoils out from the heart of the star in an explosive shock wave In one of the most spectacular events in the Universe, the shock propels the matenal away from the star in a tremendous explosion called a supernova The Neutron Star The result is a catastrophic collapse of the core, which cannot be halted until the core has shrunk to a size of about 10km and a density of the order of 200 million tons/cm’ In fact, the whole core of the star becomes nothing but a dense ball of neutrons. It is possible that this core will remain intact after the supernova, and be called a neutron star The net result is the formation of a neutron star with the liberation of about 10 ‘ergs of energy in neutrinos and 10’lergs of kinetic and luminous energy. (The luminous energy release is about the amount of energy that the sun will release in its 10 billion year lifetime). Neutron stars are typically about ten miles in diameter, have about l 4 times the mass of our Sun, and spin very rapidly (one revolution takes mere seconds!). Neutron stars are fascinating because they are the densest objects known. Due to its small size and high density, a neutron star possesses a surface gravitational field about 300,000 times that of Earth. Neutron stars also have very untense magnetic fields about 1,000,000,000,000 times stronger than Earth’s. Neutron stars which emit pulses are called Pulsans The White Dwarf The Fate of Low or Medium Mass Stars A white dwarf is what stars like our Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains. This core becomes a very hot (T 100,000K) young white dwarf, which cools down over the course of the next billion years or so. Eventually, only about 20% of the stars initial mass remains and the star spends the rest of its days cooling and shrinking until it is only a few thousand miles in diameter. It has become a white dwarf. The Black Dwarf White dwarfs have no way to keep themselves hot (unless they accrete matter from other close by stars therefore, they cool down over the course of many billions of years. Eventually, such stars cool completely and become black dwarfs. Black dwarfs do not radiate at all. The Black Hole End of a Star However, if the original star was very massive (say 15 or more times the mass of our Sun), even the neutrons will not be able to survive, the core collapses and a black hole will form. Black holes are objects so dense that not even light can their gravity and, since nothing can travel faster than light nothing can escape from inside a black hole. Nevertheless, there is now a great deal of observational evidence for the existence of two types of black holes: those with masses of a typical star (4-15 times the mass of our Sun), and those with masses of a ical galaxy (Galaxy mass black holes are thought to have the mass of about 10 to 100 billion Suns!), This evidence comes not from seeing the black holes directly, but by observing the behavior of and other material near them!