Astronomy - Final End States of Stars - Quick Guides | Astronomy

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Study GuideAstronomy–Final End States of Stars1. NovaeAnovais a powerful but temporary brightening of a star. It happens in a special system wheretwostars orbit each other, called abinary system. One of these stars is awhite dwarf, and the other isa normal star that later grows into ared giant.1.1Mass Transfer Between StarsAs the companion star ages, it expands and becomes ared giant. Its outer layers grow so large thatthe strong gravity of the nearby white dwarf begins topull in gasfrom the red giant’s atmosphere.•This gas is mostlyhydrogen.•The white dwarf collects this hydrogen on its surface.•Over time, more and more material builds up.This situation isunstable, because the white dwarf cannot quietly store this extra material forever.1.2What Triggers a Nova?When enough hydrogen has piled up, acritical pointis reached. At this stage:•Thermonuclear reactionssuddenly begin on the surface of the white dwarf.•These reactions rapidly converthydrogen into helium.•A huge amount of energy is released in a short time.This explosive surface event is what we call anova.1.3How Does a Nova Look from Earth?Astronomers observe a nova as:•Avery rapid increase in brightness, often inless than one day.•The star can shine10,000 to 100,000 times brighter than the Sun.•After reaching its peak, the brightness slowly fades over the nextfew months.

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Study GuideDuring the explosion:•A small amount of material is thrown into space.•This ejected mass is usuallymuch less than 0.01 times the mass of the Sun.•The material can move at speeds of up to2,000 km/s.1.4What Stays the Same?Even though the explosion is dramatic:•Thewhite dwarf itself is not destroyed.•Itsinternal structure remains unchanged.•Theorbit of the two starsis not affected.This makes a nova very different from a supernova.1.5RepeaƟng NovaeAfter the explosion:•The white dwarf startscollecting hydrogen again.•Over time, the gas builds up once more.•Another nova can occur after about50 to 100 years.Stars that produce novaeagain and againare known asrecurrent novae.Key Idea to RememberAnovais a repeating surface explosion on a white dwarf caused by the buildup of hydrogen from anearby companion star.It is bright, energetic, and temporary—butthe star survives and the process can repeat.

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Study Guide2. Type I SupernovaeAType I supernovahappens at the very end of a white dwarf’s life in a binary star system. Let’sthrough the process step by step.2.1 Mass Builds Up Over TimeA white dwarf can slowly pull gas from its companion star. This process is calledmass accretion.Sometimes, the buildup of gas causes small explosions callednovae, but even after these events,the white dwarf can continue gaining mass.Over a long time, its mass may approach a critical value known as theChandrasekhar limit, namedafter Subrahmanyan Chandrasekhar.2.2 Gravity Finally WinsWhen the white dwarf reaches this limit, theelectron pressureinside the star is no longer strongenough to balance gravity.As a result, the star begins tocollapse suddenly and violently.2.3 Runaway Nuclear ReacƟons BeginDuring the collapse, the star’scarbon and oxygenare squeezed tightly together. This extremecompression triggers rapidthermonuclear reactions, such as:•Carbon nuclei combining to form magnesium•Oxygen nuclei combining to form sulfur•Carbon and oxygen combining to form siliconThese reactions continue, producing even heavier elements, includingiron and nickel.2.4 A Total Stellar ExplosionThe energy released by these reactions is enormous. It is so powerful that the white dwarf cannothold itself together.The star iscompletely blown apartin a massive explosion known as aType I supernova.

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Study Guide2.5 Enriching the UniverseAll the material from the exploded star—now rich in heavy elements—is spread out intointerstellarspace.This material later becomes part of new stars, planets, and other cosmic objects.2.6 Where Type I Supernovae Are FoundIn other galaxies, Type I supernovae are usually seen in regions filled withold stars.This matches our understanding that they come fromold, evolved white dwarfs, not from young,massive stars.Key Idea to RememberA Type I supernova is the dramatic end of a white dwarf that gains too much mass. When it crosses acritical limit, runaway nuclear reactions destroy the star completely and help spread heavy elementsthroughout the universe.

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Study Guide3. Type II Supernovae3.1 Life of a Massive Star Before ExplosionStars that aremore than 8 times the mass of the Sunlive very fast and dramatic lives.Because they are so massive, they shine very brightly and burn their fuel quickly.Unlike smaller stars, these massive starscannot lose enough massto quietly end their lives aswhite dwarfs. Instead, they continue evolving through many nuclear stages.As the star ages:•Hydrogen in the core turns intohelium•Helium fuses to makecarbon and oxygen•The core keeps gettinghotter and denserEach stage happens faster than the previous one because gravity is stronger in massive stars.

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Study Guide3.2 FormaƟon of the Onion-Shell StructureEventually, the star develops anonion-like layered structure, where different elements are burningin different shells.From thecenter outward, the layers look like this:•Iron core(at the center)•Shell wheresilicon, sulfur, and magnesiumform iron•Shell wherecarbon and oxygenform heavier elements•Shell whereheliumfuses into carbon and oxygen•Outer shell wherehydrogenfuses into heliumImportant idea:Nuclear fusion only happens in thehottest and densest regions, which are near the center.3.3Why an Iron Core Is a ProblemIron is a special element.Unlike lighter elements,iron does NOT release energy when it fuses.This creates a crisis:•The star can no longer produce energy in its core•Energy continues to leak outward•Gravity starts to win

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