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Life Cycle of Stars Pumbaa: Timon? Timon: Yeah? Pumbaa: Ever wonder what those sparkly dots are up there? Timon: Pumbaa. I don't wonder; I know. Pumbaa: Oh. What are they? Timon: They're fireflies. Fireflies that uh... got stuck up on that big... bluish-black... thing. Pumbaa: Oh. Gee. I always thought that they were balls of gas burning billions of miles away. Timon: Pumbaa, wit' you, everything's gas. A Star is Born • Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces. • As the cloud collapses the temperature increases and a protostar is formed. • These young stars undergo further collapse, becoming hot enough to fuse hydrogen into helium as main sequence stars. Looking at the Birthplace of Stars • Horsehead Nebula • Rosette Nebula Everything Depends on Mass • The more mass a star starts out with, the brighter and hotter it will be. • The color of the star depends on the surface temperature of the star. • Its temperature depends on its mass, or how much gas and dust were accumulated during formation. The Constant Battle Against Gravity • Gravity is always present, compressing stars toward their center of mass. • Nuclear Fusion is the force that acts against gravity, releasing energy outwards Adolescent Stars • The force of gravity is pulling in on the star while the radiation from fusion is pushing out. • The star is burning its most abundant fuel, HYDROGEN! Nuclear Fusion • • • • • • • Conservation of Mass and Energy E = mc2 Nucleus 1 + Nucleus 2 Nucleus 3 + Energy Proton - Proton Chain Reaction 1H + 1H 2D + Energy 2D + 1H 3He + Energy 3He + 3He 4He + 1H + 1H + Energy After a while all the Hydrogen gets turned into He and lots of energy has been released. Red Giants • After millions to billions of years, depending on their initial masses, stars run out of their main fuel - hydrogen. • Without the outward pressure generated from these reactions to counteract the force of gravity, 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 increase. • 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. Planetary Nebula • After expanding and reaching the enormous red giant phase, the outer layers of the star continue to expand. • The core contracts; the helium atoms in the core fuse together, forming carbon atoms and releasing energy. • The core is now stable since the carbon atoms are not further compressible. • Now the outer layers of the star start to drift off into space, forming a planetary nebula (a planetary nebula has nothing to do with planets). • The star loses most of its mass to the nebula. Egg Nebula formed only a few hundred years ago. It takes light about 3000 years to reach us from the Egg Nebula. White Dwarf • The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of years. • When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps full of diamonds, highly compressed carbon). A white dwarf in the M4 Globular Cluster SUPERNOVA: A star that dramatically increases in brightness because of an explosion on its surface. • Means “new” in Latin but is actually the death of a star. • Here’s a before and after picture of a nova in 1992. A Closer Look H-R Diagram Our Star the Sun. • Diameter 1.4 x 106 km • Mass 2.0 x 1030 kg (300,000 times bigger than the Earth). • On the H-R Diagram our sun is a yellow star in the main sequence. • In about 5 billion years our Sun will start to become a Red Giant Chemistry and the Sun Element Hydrogen Helium Oxygen Carbon Nitrogen Silicon Magnesium Neon Iron Sulfur Abundance (% of total # of atoms) 91.2 8.7 0.078 0.043 0.0088 0.0045 0.0038 0.0035 0.0030 0.0015 Abundance (% of total mass) 71.0 27.1 0.97 0.40 0.096 0.099 0.076 0.058 0.140 0.040