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Stellar Classification and Evolution: The origin, life, and death of stars

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What is a star? A cloud of gas and plasma, mainly hydrogen and helium The core is so hot and dense that nuclear fusion can occur. The fusion converts light elements into heavier ones

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Every star is different Luminosity: Tells us how much energy is being produced in the core Can be calculated using apparent magnitude and distance Color: Tells us the surface temperature of the star Determined by analyzing the spectrum of starlight Mass: Determines the life cycle of a star and how long it will last Given relative to our suns mass (ex: 0.8 solar masses)

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Units of luminosity We measure the luminosity of every day objects in Watts. How bright is a light bulb? By comparison, the Sun outputs: 380,000,000,000,000,000,000,000,000 Watts This is easier to write as 3.8 x 10 26 Watts To make things easier we measure the brightness of stars relative to the Sun.

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Units of temperature Temperature is measured in Kelvin The Kelvin temperature scale is the same as the Celsius scale, but starts from -273 o. This temperature is known as absolute zero -273 o C-173 o C0 o C100 o C 0 K100 K273 K373 K 1000 o C 1273 K Kelvin = Celsius + 273

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Measuring the temperature The temperature of a star is indicated by its color Blue stars are hot, and red stars are cooler Red star 3,000 K Yellow star 5,000 K Blue star 10,000 K

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Colors of Stars Stars appear different colors depending on the peak wavelength of light they emit. The sun, whose data is depicted in this graph, appears yellow-orange to our eyes.

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Spectral Class ( Oh Boy, A Failing Grade Kills Me) Determined by analyzing a stars spectra O stars are the hottest and most massive M stars are the coolest and least massive Our Sun is a G star

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Spectral Classes

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The Hertzsprung Russell Diagram We can also compare stars by showing a graph of their temperature and luminosity

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Hertzprung-Russell Diagram What information is plotted on the H-R Diagram? Temperature and luminosity What are the main stages of stars? Main sequence, giant, supergiant, dwarf, Do stars always stay in the same stage? No, they change throughout their lifetimes

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Birth of Stars Stars (and their solar systems) are created in giant molecular clouds of cosmic dust and gas When gravity causes intense heat and pressure in the core of the proto-star, it triggers fusion and a star is born The planets and other solar system objects are formed from the left-over materials in the proto-planetary disk surrounding this new star

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The Carina Nebula (HST photo)

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Artist rendering

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Mass and Stellar Evolution The life cycle of a star is determined by its mass More massive stars have greater gravity, and this speeds up the rate of fusion O and B stars can consume all of their core hydrogen in a few million years, while very low mass stars can take hundreds of billions of years.

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Brown Dwarf a Failed Star If a proto-star does not have enough mass, gravity will not be strong enough to compress and heat its core to the temperatures that trigger fusion If the mass is less than 0.08 x solar mass, it will form a Brown Dwarf Brown Dwarfs are not true stars, but they do give off small amounts of light as they cool

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The Main Sequence Longest life stage of a star Energy radiating away from star balances gravitational pull inward (hydrostatic equilibrium) Main-sequence stars fuse hydrogen into helium at a constant rate Star maintains a stable size as long as there is ample supply of hydrogen atoms The Sun will spend a total of ~10 billion years on the main sequence

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OUR SUN A Main Sequence Star SDO image

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When hydrogen in the core starts to run low In stars with masses more than 0.4 x solar mass, fusion slows down Outer layers of the star begin to swell and surface temperatures fall The shell surrounding the core begins to fuse hydrogen Stars move out of the Main Sequence

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Giants and Supergiants Old stars Helium produced through shell fusion becomes part of the core Stars core temperature increases as the more massive core contracts The increased core temperature causes the helium left to fuse into carbon atoms (triple-alpha process)

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Size: Giants : 10X bigger than our sun Supergiants 100 X bigger than our sun

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A red supergiant nearing the end of it's life Betelgeuse

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Death of Stars Depends on MASS Low mass stars are less than 8 solar masses High mass stars are greater than 8 solar masses

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Low-Mass Giants/Supergiants In low mass stars (0.4 8.0 x solar mass) strong solar winds and energy bursts from helium fusion shed much of their mass The ejected material expands and cools, becoming a planetary nebula (which actually has nothing to do with planets, but we didnt know that in the 18 th century when Herschel coined the term) The core collapses to form a White Dwarf

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Helix Nebula (HST photo)

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White Dwarf Stars The burned-out core of a star less than 8 x solar mass becomes a white dwarf The carbon-oxygen core that remains is about the size of earth, but much more dense Theoretically, after all of the stored energy radiates out into space, these stars will become giant crystals of carbon and oxygen (Black Dwarfs)

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Astronomers overexposed the image of Sirius A so that the dim Sirius B could be seen. HST photo White Dwarf Stars White dwarf Sirius B main sequence star Sirius A

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White Dwarfs in binary systems can explode Occasionally we observe a White Dwarf star that suddenly becomes dramatically brighter and then fades to its original luminosity over a period of months or years. It may repeat this process, if the companion star is stable This is a Nova Artists rendering

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Nova (artists rendering)

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Massive stars continue fusion Massive stars (> 8 x solar mass) have more gravity than low-mass stars When helium fusion ends, gravitational compression collapses the core and the temperature rises beyond 600 million K Fusion of the atoms from heavier elements begins, and the star becomes a luminous supergiant These stars produce neon, magnesium, oxygen, sulfur, silicon, phosphorous, and iron

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Supernova explosions The iron-rich core signals the impending violent death of the massive star The core collapses in seconds, and the resulting temp. exceeds 5 billion K Intense heat breaks apart the atomic nuclei in the core, causing a shock wave After a few hours, the shockwave reaches the stars surface, blasting away the outer layers in a supernova

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Artists rendering of a Supernova

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Crab Nebula (HST image) Remnants of a Supernova recorded in 1064 11 ly across Supernova remnants are strong sources of X- rays and radio waves

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Supernova 1987A This HST picture shows three rings of glowing gas encircling the site of supernova in February 1987. The supernova is 169,000 ly away in the dwarf galaxy called the Large Magellanic Cloud

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Neutron Stars The cores left over after Supernovae can become Neutron Stars-- very small, dense balls of NEUTRONS 1 teaspoon of this would be approximately 1 billion tons on Earth Due to the great density it rotates very rapidly, and some become PULSARS

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Pulsars Rapidly-spinning neutron stars with very strong magnetic fields. Jets of charged particles are ejected from the magnetic poles of the star. This material is accelerated, producing beams of light in all wavelengths from the magnetic poles. We can see this lighthouse effect many times per second Computer model

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Pulsar Chandra X-Ray Observatory image shows a pulsar at the center of the Crab Nebula http://earthsky.org/space/ binary-pulsar-gives-up- secrets-then- disappears?utm_source=E arthSky+News&utm_camp aign=63c2b12cb9- EarthSky_News&utm_med ium=email&utm_term=0_ c643945d79-63c2b12cb9- 394144225

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Black Holes Supermassive stars (>25 x solar mass) collapse into neutron stars too massive to be stable They collapse in on themselves, forming a region of infinite density and zero volume a SINGULARITY at the center of a Black Hole Space curves inward and traps all matter and electromagnetic radiation

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