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Astronomy - Final End States of Stars

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    Study Guide Astronomy – Final End States of Stars 1. Novae A nova is a powerful but temporary brightening of a star. It happens in a special system where two stars orbit each other , called a binary system . One of these stars is a white dwarf , and the other is a normal star that later grows into a red giant . 1. 1 Mass Transfer Between Stars As the companion star ages, it expands and becomes a red giant . Its outer layers grow so large that the strong gravity of the nearby white dwarf begins to pull in gas from the red giant’s atmosphere. • This gas is mostly hydrogen . • The white dwarf collects this hydrogen on its surface. • Over time, more and more material builds up. This situation is unstable , because the white dwarf cannot quietly store this extra material forever. 1. 2 What Triggers a Nova? When enough hydrogen has piled up, a critical point is reached. At this stage: • Thermonuclear reactions suddenly begin on the surface of the white dwarf. • These reactions rapidly convert hydrogen into helium . • A huge amount of energy is released in a short time. This explosive surface event is what we call a nova . 1. 3 How Does a Nova Look from Earth? Astronomers observe a nova as: • A very rapid increase in brightness , often in less than one day . • The star can shine 10,000 to 100,000 times brighter than the Sun . • After reaching its peak, the brightness slowly fades over the next few months .

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    Study Guide During the explosion: • A small amount of material is thrown into space. • This ejected mass is usually much less than 0.01 times the mass of the Sun . • The material can move at speeds of up to 2,000 km/s . 1. 4 What Stays the Same? Even though the explosion is dramatic: • The white dwarf itself is not destroyed . • Its internal structure remains unchanged . • The orbit of the two stars is not affected. This makes a nova very different from a supernova. 1. 5 Repea Ɵ ng Novae After the explosion: • The white dwarf starts collecting hydrogen again . • Over time, the gas builds up once more. • Another nova can occur after about 50 to 100 years . Stars that produce novae again and again are known as recurrent novae . Key Idea to Remember A nova is a repeating surface explosion on a white dwarf caused by the buildup of hydrogen from a nearby companion star. It is bright, energetic, and temporary — but the star survives and the process can repeat .

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    Study Guide 2. Type I Supernovae A Type I supernova happens at the very end of a white dwarf’s life in a binary star system. Let’s through the process step by step. 2. 1 Mass Builds Up Over Time A white dwarf can slowly pull gas from its companion star. This process is called mass accretion . Sometimes, the buildup of gas causes small explosions called novae , 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 the Chandrasekhar limit , named after Subrahmanyan Chandrasekhar. 2. 2 Gravity Finally Wins When the white dwarf reaches this limit, the electron pressure inside the star is no longer strong enough to balance gravity. As a result, the star begins to collapse suddenly and violently . 2. 3 Runaway Nuclear Reac Ɵ ons Begin During the collapse, the star’s carbon and oxygen are squeezed tightly together. This extreme compression triggers rapid thermonuclear reactions , such as: • Carbon nuclei combining to form magnesium • Oxygen nuclei combining to form sulfur • Carbon and oxygen combining to form silicon These reactions continue, producing even heavier elements, including iron and nickel . 2. 4 A Total Stellar Explosion The energy released by these reactions is enormous. It is so powerful that the white dwarf cannot hold itself together. The star is completely blown apart in a massive explosion known as a Type I supernova .

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    Study Guide 2. 5 Enriching the Universe All the material from the exploded star — now rich in heavy elements — is spread out into interstellar space . This material later becomes part of new stars, planets, and other cosmic objects. 2. 6 Where Type I Supernovae Are Found In other galaxies, Type I supernovae are usually seen in regions filled with old stars . This matches our understanding that they come from old, evolved white dwarfs , not from young, massive stars. Key Idea to Remember A Type I supernova is the dramatic end of a white dwarf that gains too much mass. When it crosses a critical limit, runaway nuclear reactions destroy the star completely and help spread heavy elements throughout the universe .

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    Study Guide 3. Type II Supernovae 3. 1 Life of a Massive Star Before Explosion Stars that are more than 8 times the mass of the Sun live very fast and dramatic lives. Because they are so massive, they shine very brightly and burn their fuel quickly. Unlike smaller stars, these massive stars cannot lose enough mass to quietly end their lives as white dwarfs. Instead, they continue evolving through many nuclear stages. As the star ages: • Hydrogen in the core turns into helium • Helium fuses to make carbon and oxygen • The core keeps getting hotter and denser Each stage happens faster than the previous one because gravity is stronger in massive stars.

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    Study Guide 3. 2 Forma Ɵ on of the Onion - Shell Structure Eventually, the star develops an onion - like layered structure , where different elements are burning in different shells. From the center outward , the layers look like this: • Iron core (at the center) • Shell where silicon, sulfur, and magnesium form iron • Shell where carbon and oxygen form heavier elements • Shell where helium fuses into carbon and oxygen • Outer shell where hydrogen fuses into helium Important idea: Nuclear fusion only happens in the hottest and densest regions , which are near the center. 3. 3 Why an Iron Core Is a Problem Iron 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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