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Life Cycle of a Star The changes that a star goes through is determined by how much mass the star has. Two Types of Life Cycles: Average Star- a star with relatively low mass Massive Star- a star with relatively high mass Stellar Nebula All stars begin in a cloud of gas and dust called a stellar NEBULA. Eagle Nebula Lagoon Nebula Orion Nebula (Hubble Space Telescope) It begins in a gas cloud called a nebula Star formation is ongoing; star-forming regions are seen in our galaxy as well as others: •Gravity causes the nebula to contract. •The nebula separates into smaller ‘chunks’. The gas and dust in these ‘chunks’ condense and becomes hotter. •At 15 million ºC fusion begins and the star emits light!! •At this point, we call the new star a “protostar” The release of energy that causes the star to shine, also stops the star from contracting. It counteracts gravitational force. We call these ‘Main Sequence’ stars. Stars with one solar mass remain in the main sequence for about 10 billion years, until all of the H has fused to He. FUSION • http://youtu.be/oIe1EDExxyg FUSION Life Cycle of Stars – sketch this http://www.seasky.org/cosmic/sky7a01.html The Life of an Average Star (one solar mass) An Average Star (low mass star) is condensed in a nebula and begins a nuclear reaction that causes hydrogen to form helium, releasing energy in the form of heat and light. A low mass star will stay in this MAIN SEQUENCE phase for a about 10 billion years, until it begins to use up all of its hydrogen. The Life of an Average Star Towards the end of it’s MAIN SEQUENCE phase, a star begins to burn all of its hydrogen. The He in the core is hot enough to fuse into carbon. Gasses start to expand. The outer layers begin to cool, causing a red color. We call this a ‘red giant’. The Life of an Average Star The star begins to quickly blow off its layers of gas, forming a cloud around the star called a planetary nebula. The core of the star that remains in the center of the nebula is very hot but not very bright. Life of an Average Star When a star has burned all its fuel it will collapse under the pressure of gravity. The remaining core is very small and dense. It is known as a ‘white dwarf’. - When it stops shining, it is known as a black dwarf. Life of a Massive Star http://www.seasky.org/cosmic/sky7a01.html Stellar Nebula All stars, regardless of size, begin in a stellar nebula. Life of a Massive Star Stars with more mass than the sun (high mass stars) burn their hydrogen faster than low mass stars, so their MAIN SEQUENCE phase is much shorter. These stars burn hotter and brighter than low mass stars. Life of a Massive Star When the high mass star burns off it’s hydrogen its outer layers begin to expand and contract repeatedly and rapidly Temperatures at the core are much higher than a red giant. Nuclear fusion causes elements to combine into an iron core at amazing speeds. Life of a Massive Star There is no longer an outward force to counter gravity’s inward force. In less than a second, the core collapses on itself under the intense gravity. A massive explosion occurs called a ‘supernova’. A supernova is so energetic that it can outshine its galaxy in terms of luminosity. A supernova produces more energy than the sun will over the course of its entire existence. Life of a Massive Star If the core survives the supernova explosion, and the surviving core is less than 3 solar masses, it becomes a neutron star. Neutron stars are very tiny (10 km), very dense (1 trillion times more dense than a white dwarf) and made up of neutrons. Life of a Massive Star If the surface of the neutron star is hotter than about a million K, the surface would be liquid form, while if it's cooler than that, it would be solid. Below that is a solid crust, about a kilometer thick. This crust is very hard and very smooth. Gravity would probably prevent any irregularities larger than half a centimeter. Life of a Massive Star If the mass is too dense (over 3 solar masses) it will continue to collapse in on itself, forming a black hole. The gravitational pull of a black hole is so great, light can not escape… more about black holes later…let’s stick to stars for now… Some interstellar clouds are too small for fusion ever to begin. They gradually cool off and become dark. Jupiter is a good example. A protostar must have 0.08 the mass of the Sun (which is 80 times the mass of Jupiter) in order to become dense and hot enough that fusion can begin. Shock Waves and Star Formation Shock waves from nearby star formation can be the trigger needed to start the accretion process in an interstellar cloud: Shock Waves: Waves of matter driven outward from high temperatures and pressures Shell of gas rushing out Shock Waves and Star Formation Other triggers to start the collapse process in an interstellar cloud: • Death of a nearby average star • Supernova •Galaxy collisions Shock Waves and Star Formation This region may very well be several generations of star formation: Characterizing Stars Temperature and Color (review) Hottest = blue color Medium = orange/yellow color Coolest = red color MAGNITUDE: Brightness Increases from bottom to top The Hertzsprung-Russell (HR) Diagram Are these stars brighter or dimmer than the sun 1 L is equal to the brightness of the sun REMEMBER: Temperature Increases from right to left Sketch this HR diagram (quickly). Mark on the diagram where you would find the following: Hot & Bright Cool & Bright Sun Hot and Dim Cool and Dim HR Diagram Basics Characteristics of Stars The Hertzsprung-Russell (HR) Diagram Temperature & Color – The color of a star indicates the temperature of the star – Stars are classified by temperature Decreasing O, termperature (bright to dim) B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ] Spectral Type Classification O B A F G 50,000 K K M 3,000 K temperature How bright a star appears to us on earth What affects apparent magnitude? The Inverse-Square Law The farther a star is from Earth, the dimmer it looks to us. Doubling the distance makes the star look onefourth as bright. Tripling the distance decreases the star’s brightness by a factor of 9. (Luminosity) This is the real brightness of a star as though all the stars were At the same distance- Standard comparison is 10 parsecs Absolute Magnitude – the actual brightness of a star • Absolute magnitude tells how bright a star really is, no matter how far from Earth it is. • Are the car lights actually dimmer as the car moves away?