ASTR 1514 1st Edition Lecture 25Hydrostatic Equilibrium- The mass in a star is held together by gravity- Pressure in each layer of a stable star balances the weight on that layer- As the weight increases from the surface of the star to the center, the pressure also increases.- If the pressure did not balance the weight, the star would be either expanding or contracting. - Hydrostatic Equilibrium = balance between weight (gravity) and pressure. - Larger gravity means larger pressure - Outward pressure = inward force of gravity. Pressure and TemperatureHow can a star support itself?- The star must provide very high pressure in its center to support the overlying layers- Relationship between pressure and temperature: 1. Ideal Gas Law: P = nkT (Pressure = density x constant x Temperature)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- If a star could attain a high temperature at its center, it would be more likely to be able to provide the pressure required to support its immense weight- Density, temperature, and pressure increase toward the centerFusion- Stars do nuclear fusion in their core- Nuclear fusion will release energy that will heat the gas, producing the large pressure needed to support the star- Fusion – Combining the light nuclei to form heavier nuclei (happens in the sun and most stars)- Fission – Breaking heavy nuclei to form lighter nuclei (happens in nuclear reactions on Earth, and some stellar phenomena)- How Fusion produces Energy1. Nuclei are similar to atoms in that they have energy levels excited states, and ground states. (Ex: Can go from excited nuclear state to the ground nuclear state, giving off energy in the form of a photon (gamma ray).) 2. Total energy of 4 separate hydrogen nuclei is higher than the total energy of a single helium nuclei3. Combining the 4 hydrogen nuclei into a single helium nucleus, you could harvest the leftover energy.4. One Helium Nucleus weighs less than 4 hydrogen nuclei (protons) E = Mc25. Some mass is converted into energy. Turning 4H into one He- Electromagnetic Force – Long range (larger than size of atom) force that causes like charges to repel each other, and opposite charges to attract each other. - Strong Force – Short range (smaller than size of atom) attractive nuclear force. Only works on particles in the nucleus (Neutrons and protons), So if two hydrogen nuclei are close enough together, the strong force will take over and fusion will happen. - Protons have a positive electrical charge, neutrons have no charge. Electrical forces push protons apart. - Stronger nuclear force binds protons together- Fusion Requires ramming protons together at high speed ( high temperature)- Proton-Proton Chaino A series of reactions take place that fuse 4 hydrogen nuclei into one helium nuclei.o This process requires high temperatures and high density. (High temps so that the Coulomb repulsion is overcome) (High density so that there are more collisions)o This process is only occurring in the center of a star (while there is fuel in the center) Where it is hot and dense enough.o The Fusion reaction releases a positron and a neutrino when a photon is turned into a neutrono Gamma rays are also released – these carry the energy. The Sun Fusion- Been around for about 4.6 Billion years- Generates a lot of energy over a long time- Source of energy: Nuclear Fusion of hydrogen to helium in the central core- Fusion often called hydrogen burning- Happens for all main – sequence starsRadiation (Energy) Transport- Radiation Transport – Process of moving radiation (from center of star to surface)- Three Methods for Energy Transport1. Conduction: Transfer of energy by atomic motions - Ex: Heating pan on electric stove- Not important in main sequence stars2. Radiation: Transfer of energy by photons- Ex: Black dog in the sun (heats up)- Important in main sequence stars3. Convection: Transfer of energy by motion of the gas- Ex: Boiling water on the stove- Important in Main Sequence Stars- Type of radiation transport depends on the location in the sun (Inner part of the sun – radiation transport occurs through radiation)- Toward the Surface of the sun – Radiation occurs through convection- Outer part of the sun – Surface – Radiation emitted into spaceMoving the Energy to the Surface- It takes 100,000 years for energy to reach the surface from the core- Each photon scatters many times before reaching the surface- Energy is spread out throughout the sunSo while the core is about 15 million degrees K, the surface of the sun is only about