Transcript Stars
Stars What is a Star? • Stars are large balls of hot gas. • They are the objects that heat and light the planets in a system. • A star is a ball of plasma held together by its own gravity – Nuclear reactions occur in stars (H He) – Energy from the nuclear reactions is released as electromagnetic radiation • They look small because they are a long way away, but in fact many are bigger and brighter than our Sun. • The heat of the star is made in the center by nuclear fusion reactions. • There are lots of different colours and sizes of stars. Characteristics of Stars • DISTANCE – Measured in light-years • The distance which a ray of light would travel in one year • About 6,000,000,000,000 (6 trillion) miles • 186,000 miles per second Characteristics of Stars • Magnitude (brightness) – A measure of brightness of celestial objects • Smaller values represent brighter objects than larger values – Apparent magnitude • How bright a star appears to be from Earth – Absolute magnitude (luminosity) • How bright a star actually is Characteristics of Stars • Temperature & Color – The color of a star indicates the temperature of the star – Stars are classified by temperature • Decreasing temperature (bright to dim) • O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ] http://www.seasky.org/cosmic/sky7a01.html Hertzsprung-Russell Diagram http://www.dustbunny.com/afk/stars/lifecycle/hrdiagram.gif Main Sequence Stars • A major grouping of stars that forms a narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram Types of Stars Classification Class Temperature Color O 20,000- 60,000 K Blue B 10,000 – 30,000 K Blue-white A 7,500 – 10,000 K White F 6,000 – 7,500 K Yellow-white G 5,000 – 6,000 K Yellow K 3,500 – 5,000 K Orange M 2,000 – 3,500 K Red http://www.answers.com/topic/stellar-classification Life Cycle of Stars http://hea-www.cfa.harvard.edu/CHAMP/EDUCATION/PUBLIC/ICONS/life_cycles.jpg Life Cycle of Stars How are stars made? • Begin their lives as giant clouds of dust and gas called nebulae • Gravity may cause the nebula to contract • Matter in the gas cloud will begin to condense into a dense region called a protostar • The protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins. • Begins the main sequence phase of the star • Most of its life is in this phase Life Cycle of Stars Life span of a star depends on its size. – Very large, massive stars burn their fuel much faster than smaller stars – Their main sequence may last only a few hundred thousand years – Smaller stars will live on for billions of years because they burn their fuel much more slowly • Eventually, the star's fuel will begin to run out. Life Cycle of Stars • It will expand into what is known as a red giant. • Massive stars will become red super giants. • This phase will last until the star exhausts its remaining fuel. • At this point the star will collapse. Life Cycle of Stars • Most average stars will blow away their outer atmospheres to form a planetary nebula • Their cores will remain behind and burn as a white dwarf until they cool down • What will be left is a dark ball of matter known as a black dwarf Life Cycle of Stars • If the star is massive enough, the collapse will trigger a violent explosion known as a supernova • If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star Life Cycle of Stars • The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole Life Cycle of Stars http://www.seasky.org/cosmic/sky7a01.html