Science

  • Issue 44 / October - December 2003



    Violent Deaths of Massive Stars and the Story of Black Holes

    Emin Kilic

    Everything started with an explosion. About 14 billion years ago, when the universe was only 10 millionths of a second old, it consisted of high energy photons with a temperature of above 1 trillion degrees. The protons, electrons, and neutrons of which our bodies are made were produced during the first 4 seconds of the Big Bang. Technically, we are about 14 billion years old! By the time the universe was 2 minutes old, protons and neutrons combined to make heavy hydrogen (deuterium), and further reactions started to convert deuterium into helium. But heavier atoms could not be built because there were no stable nuclei with atomic weights of 5 or 8. If we use the analogy of a stairway to represent cosmic element building, then we can see the lack of stable nuclei with atomic numbers of 5 and 8 as gaps in the stairway, thus the step-by-step reactions could not jump over these gaps to climb the stairs (or to form heavier atoms). So how did we get the heavy atoms on Earth that are essential for life, if they were not produced during the Big Bang?
    Every soul shall have a taste of death (Quran 3:185). Like everything else, stars live and die. Would it make any difference if you knew that the iron in your blood and the calcium in your bones had been assembled inside stars? Atoms heavier than iron are formed by rapid nuclear reactions that can only occur when a massive star explodes. Gold, which is not crucial for our lives, and iodine, which is important for our health, are available, thanks to the violent deaths of massive stars.

    The death of a star leads to one of three final states. Most stars, including our Sun, will become white dwarfs, stars about the size of the Earth, with no usable fuels. But the most massive stars explode and leave extraordinary objects behind; either a neutron star or a black hole.

    The Sun resists its own gravity by generating energy through nuclear fusion. Under extreme conditions, four hydrogen atoms are combined to form a helium atom, and the mass difference between these atoms are converted to energy which can be calculated by Einsteins famous equation, E=mc2, where m is the amount of mass converted to energy and c is the speed of light. In 4.5 billion years, the Sun will exhaust the fuel, hydrogen and helium stored in its core. This will start the chain of events that will result in its death. Since it will not be able to generate any energy to balance the huge weight of its outer layers, it will collapse. This will result in an increase in the temperature around its core. This temperature increase in the shell around the core will start new reactions which will produce excess amounts of energy. This extra energy will cause the Sun to expand and become a red giant. Its size will increase to such an extent that it will swallow Mercury and Venus, and maybe even our planet, Earth. As a giant star, it will have a strong solar wind that carries gas into space. Eventually, it will lose its outer layers, and produce a beautiful planetary nebula.1 Soon the remains of the Sun will collapse and form a very compact object; a white dwarf. Imagine squeezing the Sun into a planet the size of the Earth. Gravity on a white dwarf is 10 million times greater than it is on Earth. Thus, a person weighing 150 pounds will weigh 1.5 billion pounds on a white dwarf. The white dwarf will burn 100 times fainter than our Sun; if the Earth survives the red giant phase, it will fall into a deadly deep freeze, and would not be a pleasant place to live.2

    Medium mass stars, like the Sun, die relatively quietly as they exhaust their fuel and form white dwarfs. In contrast, massive stars live spectacular lives and destroy themselves in violent explosions. Massive stars have too great a mass to die as white dwarfs. They consume hydrogen and become red giants, but unlike the medium mass stars, their core temperature is high enough, about 1 billion degrees, to ignite carbon fusion. After they fuse carbon, they burn oxygen, neon, and magnesium to make silicon and sulfur, and then the silicon fuses to make iron. Iron is the most tightly bound of all atomic nuclei. Nuclear fusion is able to produce energy by combining less tightly bound nuclei into a more tightly bound nucleus, but iron is the limit. Once the core of the star has been converted to iron, there are no nuclear reactions that can burn iron and release the energy. Thus, the iron core is a dead end. The iron core sucks energy from the rest of the star. Since the star cannot produce any energy, it cannot resist its own gravity. In a fraction of a second, the star collapses in on itself. The collapsing core of the massive star quickly becomes a neutron star or a black hole. This collapse happens so rapidly that our most powerful computers are unable to predict the details. The envelope of the star collapses and bounces back off the dense core, which triggers a violent supernova explosion that expels the outer layers of the star to form an expanding supernova remnant. This explosion enriches the neighboring media with iron and other metals. If you throw a water balloon at your friend, your friend will get wet. Massive stars are not water balloons, but they are iron, silver, and metal balloons. When they explode, they seed the interstellar medium3 with metals. If there had not been a massive star death near our solar system when the Sun and the Earth were forming, our solar system would be iron-poor, and we would not be living today. Massive stars die so that we might live. The Quranic verse We have sent down iron, with its mighty strength and diverse uses for mankind (57:25) sheds light on this fact 14 centuries before it was discovered. Only in the past century, with the utilization of modern telescopes, have we had evidence of this. Nursi explained the above verse as iron is sent down together with the globe of the Earth from the Supreme Treasury, as a tremendous bounty. That is to say, the thing most necessary for the house of the Earth is iron, for when the All-Glorious Creator separated the Earth from the Sun and sent it down for mankind, He sent down iron together with it, and met most of mankinds needs with it. The All-Wise Quran decrees in a miraculous fashion: Use this iron in your works and try to excavate it and take advantage of it.4

    A neutron star, on average, is 1.4 times more massive than the Sun, and is compressed to a radius of about 6 miles. Its density is so high that matter is stable only as a fluid of neutrons. An atom is mostly empty space. The nucleus of an atom is very small compared to the size of the atom. If we represent the nucleus of an atom with a blueberry, then the distance between the nucleus and the electrons would be as great as the height of the Empire State building. If you could eliminate the empty space in atoms, you would be able to squeeze stars larger than the Sun into a radius of about 6 miles (the radius of a neutron star). A neutron star spins several times a second, and has a magnetic field a trillion times stronger than that of the Earth. Observational evidence for neutron stars was first found in 1967 when astronomers found a neutron star (pulsar) rotating around itself in 1.3 seconds and sending radio pulses to Earth. If you have a large enough antenna, you can pick up periodic radio signals from pulsars. On Earth, a teaspoon of the material from a neutron star would weigh 100 million tons.

    Another scenario for the end product of the death of a massive star is a black hole. When the core of a star contains more than 3 times the mass of the Sun no known force can stop it when it collapses. The object will not stop collapsing when it reaches the size of a white dwarf or a neutron star, because the electrons or neutrons cannot support the weight of the star. The object will collapse to zero radius (or almost zero radius) and form a black hole. Objects need high speeds to be able to leave another object, to be able to resist falling back due to the gravitational pull of the other object. For example, a space shuttle must reach a speed of 11.2 km/s to to be able to leave the gravitational pull of the Earth in order to go into space. Gravity is so strong near black holes that the escape speed from a black hole is greater than the speed of light. Thus, even light cannot escape; this is the reason why these phenomena are called black holes.

    As an object collapses, its gravity increases. If it collapses to zero radius, its density and gravity become infinite. Such a point is called a singularity. Clocks slow down near a singularity. If we were able to watch a person falling into a black hole, we would see them moving more slowly as they came closer to the black hole. In fact, the person would never disappear from sight. From where we were standing, this person would fall more and more slowly, until finally they would hardly seem to move at all. Generations later, our grandchildren would be able to look at this friend approaching the black hole, but never crossing the event horizon (the boundary of the black hole). Black holes are not giant vacuum cleaners that will pull in everything in the universe. A black hole has a huge gravity pull, but its force is quite small if you are not near it. If the sun were replaced by a black hole of a similar mass, the orbits of the planets in our solar system would not change at all. The gravity of a black hole becomes extreme only when approached. There are many black holes in the universe, but they do not pose any threat for us as long as we stay away from them. Next time you advise your children to stay away from strangers, remember to tell them to stay away from black holes, as well.

    Footnotes:
    1A planetary nebula is an expanding shell of gas ejected from a star, and it has nothing to do with planets.
    2Seeds, M.A., Horizons: Exploring the Universe, 2002, Brooks/Cole
    3The gas and dust between stars.
    4Nursi, S., Flashes, Sozler Yayinevi, 28th Flash

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