Giant stars have a larger radius and luminosity than main-sequence stars due to their helium or heavier element cores. Stars with more than 0.4 solar masses will become giants, and stars with more than 8 solar masses will eventually collapse into a supernova, creating a neutron star.
Giant stars are massive stars with a radius and luminosity much larger than a main-sequence star with a similar surface temperature. Main sequence stars have a mixed core, composed of hydrogen and helium. Giant stars have a core made of helium or even heavier elements like carbon. That’s because the giant stars have begun to use up substantial portions of their hydrogen fuel.
The giant phase is inevitable for any star with more than 0.4 solar masses. Stars with a solar mass between 0.4 and 0.5 accumulate helium in their core as they age, and eventually a core of pure helium accumulates, but they don’t have the pressure and temperature to fuse helium. The hydrogen at the periphery of the core forms a shell of rapid fusion activity, because the enormous gravity of the core is compressing the hydrogen onto it. The size of the star expands and becomes much more diffuse. When the Sun becomes a red giant in five billion years, its surface will reach Earth’s current orbit.
Stars with more than 0.5 solar masses can fuse helium nuclei into oxygen and carbon through the triple alpha process. Although the core must reach a temperature of 108 K before ignition, when this happens, it produces excess energy, which increases the size of the core, decreasing the pressure in the hydrogen-producing shell. This slows down fusion reactions and counterintuitively decreases the size and temperature of the star. Thus, a more massive star ends up being less luminous than a less massive one. Such stars are part of the so-called horizontal branch, because on a graph of luminosity against spectral type they form a horizontal line.
If less than 8 solar masses, but greater than 0.5, the star will accumulate carbon in its core and start fusing helium on a shell outside the core. It becomes an “asymptotic giant branch” or AGB star as helium fusion accelerates and inflates its host star. These can create supergiant and hypergiant stars.
For stars larger than 8 solar masses, the cores fuse down to iron. When such a star builds an iron core larger than 1.44 solar masses, core collapse begins. The mutually repulsive electron shells around the iron nuclei fail to repel each other under the great pressure and temperature, and they start to fuse into another state of matter called neutronium, consisting of neutrons tightly packed into a giant nucleus. city-sized atomic bomb.
When fusion reactions in the core cease, the star cannot produce enough energy to counteract its gravity and collapses. As light elements fall inward, they bounce off the nearly incompressible neutronium nucleus. The bounce is enough to blast the star’s mantle into space at thousands of kilometers per hour. This event is called a supernova and is how elements heavier than iron are created.
The remnant is what is called a star remnant, or a neutron star. A teaspoon of its matter weighs two million tons.
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