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Stars What is a star? • Objects that heat and light the planets in a system • A ball of plasma held together by its own gravity – Nuclear reactions (fusion) occur in stars (H He) – Energy from the nuclear reactions is released as electromagnetic radiation • Stars are born with a specific mass • Mass is the main factor in determining the star’s brightness, temperature, expected lifetime, type of death, and spectra. • Classified according to their spectra Astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral features. • Spectral lines are caused by different elements present in a star • Overall spectrum is strongly related to the surface temperature of the star • Stars are categorized into classes by: 1. Strength of the spectral lines 2. Shape • The 7 classes of stars are: O B A F G K M • From O M: 1. Stars decrease in temperature 2. Stars generally become less massive (smaller) O B A F G K M Oh Be A Fine Guy/Girl, Kiss Me! One Bug Ate Five Green Killer Moths • The Hertsprung-Russell or H-R diagram reveals that about 90% of all stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right. • Not all stars fall on the main sequence. Stars below the main sequence are called white dwarfs and those above it are called giants. Since hotter stars are bluer, and cooler stars are redder, a White Dwarf is hotter than a Red Giant. • Color: blue • Temperature: 28,00050,000 K • Size: very large and massive • Spectra: ionized helium, ultraviolet continuum • Examples: 10 Lacertra, Zeta Puppis, and Iota Orionis A The rock stars of the universe-they live fast and die young! • Color: blue • Temperature: 10,00028,000 K • Size: large and massive • Spectra: neutral helium lines,some hydrogen • Examples: Beta Centauri, Rigal, Spica B stars are relatively rare, comprising only 0.1% of main sequence stars. • Color: white • Temperature: 75,00010,000 K • Size: moderate sized, very luminous • Spectra: strong hydrogen lines, ionized metals • Examples: Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega) A stars are amongst the most common naked eye stars. • Color: white-yellow • Temperature: 6,00075,000 K • Size: 1.2 to 1.6 times bigger than the Sun • Spectra: weak hydrogen lines, strong Calcium and other ionized metals • Examples: Canopus, Procyon Often used as targets for extrasolar planet searches and SETI programs. • Color: yellow • Temperature: 5,0006,000 K • Size: 0.8 to 1.1 times the mass of the Sun • Spectra: weak hydrogen lines, neutral and ionized metals • Examples: Alpha Centauri A, Capella The best known example of a G star is our SUN! • Color: orange • Temperature: 3,5005,000 K • Size: smaller and cooler than the Sun • Spectra: faint hydrogen lines, strong neutral metallic lines • Examples: Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran) Also used as targets for extrasolar planet searches like project Ozma in 1960. • Color: red • Temperature: 2,5003,500 K • Size: range from 0.5-25 (main sequencesupergiants) times the mass of the Sun • Spectra: strong metallic lines and wide titanium oxide bands • Examples: Antares and Betelgeuse Most common class by number of stars, since 90% of all stars are red dwarfs. 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 1. Begin their lives as clouds of dust and gas called nebulae 2. Gravity may cause the nebula to contract 3. Matter in the gas cloud will begin to condense into a dense region called a protostar 4. Protostar continues to condense, it heats up – Eventually reaches a critical mass and nuclear fusion begins. 5. 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 – 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 • Will expand into what is known as a red giant • Massive stars will become red supergiants • 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 (ionized gas emission) • 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 White Dwarf Planetary Nebula Nebulas Life Cycle of Stars • If the star is massive enough, the collapse will trigger a violent explosion known as a supernova • Supernova = stellar explosion that briefly outshines an entire galaxy, gives off as much energy as the Sun over its entire life span, before fading from view over several weeks or months Cassiopeia Supernova Remnant Life Cycle of Stars • 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 • Neutron star = star remnant that results from collapse of a massive star after a supernova – Densest and smallest stars known – Can have a mass of about two times that of the Sun Life Cycle of Stars http://www.seasky.org/cosmic/sky7a01.html