Chapter 25: Space Physics 1. The Sun - - It is a medium sized star. Mainly composed of helium and hydrogen. Emits infrared radiation, visible light, ultraviolet radiation. Sources of energy in the Sun: Energy from nuclear reactions in the core of the sun. Hydrogen undergoes nuclear fusion to form helium, thus releasing energy. Energy from the core heats the outer shell, causing them to glow and emit radiation. 2. The Stars - Nuclear Reactions in Stars - Stars like the Sun are powered by nuclear fusion. - The core is hot and dense enough for hydrogen to fuse into helium. Fusion in the core releases energy, maintaining high core temperatures. - Some core energy moves to outer layers, which emit electromagnetic radiation. Distance light travels in a vacuum in one year. 1 light-year = 9.5×10¹²km = 9.5×10¹⁵mm - 3. The Galaxies - Large collection of stars, gas and dust. - Our Milkyway galaxy is elliptical. 4. Origin and Life Cycle of Stars Formation: - Interstellar clouds are clouds of gas and dust that occupy the space between stars. Molecular clouds are interstellar clouds which contain hydrogen, and they are cold and dense enough for star formation. Protostar: A very young star that is still gathering mass from its parent cloud. The collapse of a clump of molecular cloud due to gravitation attraction starts a series of energy transfers. Gravity pulls the hydrogen molecules closer together, transferring their gravitational potential energy to kinetic energy. As the molecules collide their kinetic energy is transferred to thermal energy. The clump contracts into a spinning sphere of super-hot gas ( protostar ) It continues to gain mass by pulling more material from the cloud. It's final mass if less than 8 solar masses, it is a medium sized star. If the final mass is greater than 8 solar masses, it is a massive star. Life Cycle of Stars: [Medium Stars] Stable Stage (Main Sequence Star) - Hot bodies radiate heat which radiates radiation pressure. Radiation pressure acts outwards to make the star expand. It is balanced by gravity which acts inwards to make the star contract. When they are balanced, the star is stable. Red Giant - Hydrogen begins to get used up in the nuclear reactions. The temperature of the core decreases. Radiation pressure decreases. The star is unstable. It begins to contract. The core collapses. - This causes some gravitational potential energy to transfer to thermal energy. The temperature of outer shell increases and temperature of core increases. - The outer shell expands and cools (turning red) which turns into a red giant that is bigger and cooler. - The hotter core allows for helium fusion which forms carbon, which needs a higher temperature due to greater charge. End Stage - The core collapses into a white dwarf star after all the helium is used up. It shrinks. Since the white dwarf is not hot enough inside to fuse heavier elements, so it will cool to become a black dwarf. On the other hand, the outer shells, will blow away and be expelled due to the high radiation pressure, and will create a planetary nebula. Life Cycle of Stars: [Massive Stars] Stable Stage (Main Sequence Star) - Hot bodies radiate heat which radiates radiation pressure. Radiation pressure acts outwards to make the star expand. It is balanced by gravity which acts inwards to make the star contract. When they are balanced, the star is stable. It is burning hydrogen in main sequence star. Red Supergiant - Hydrogen begins to get used up in the nuclear reactions. The temperature of the core decreases. Radiation pressure decreases. The star is unstable. It begins to contract. After helium fusion, the core collapses into a red super giant. - This causes some gravitational potential energy to transfer to thermal energy. The temperature of outer shell increases and temperature of core increases. - The very hot core allows for fusion of heavier elements which forms elements up to iron, which need a higher temperature. Lighter elements continue to fuse in outer shells further from the core. - The outer shell expands into a red supergiant. Elements heavier than iron cannot be formed by nuclear fusion. End Stage - The core collapses for the last time and explodes into a supernova after all the fuel is used up. This now provides the energy needed to create elements heavier than iron. The elements will be pushed into space as a nebula with lighter elements (including hydrogen). The mass of the supernova decreases. What happens to supernova, depends on the final mass of supernova. - If it is lesser than three solar masses, a neutron star forms. The force is so strong that electrons and protons are forced together to create neutrons - If it is greater than three solar masses, it will continue collapsing until it becomes so dense that not even light can escape, and the star becomes a black hole.
