High-Mass Stars
High-mass stars are celestial bodies that possess significantly greater mass than low-mass stars, often reaching up to eight times the mass of the Sun. These stars undergo nuclear fusion in their cores, primarily through the carbon-nitrogen-oxygen (CNO) cycle, allowing them to convert hydrogen into helium relatively quickly. This rapid conversion leads to shorter lifespans compared to their low-mass counterparts, which use the proton-proton chain reaction for fusion. As high-mass stars exhaust their nuclear fuel, they develop a helium core surrounded by hydrogen, eventually reaching temperatures high enough to fuse helium into heavier elements.
The evolutionary process of high-mass stars culminates in a dramatic supernova explosion, which occurs when the core's fusion processes cease. The core collapses under its own gravity, leading to the formation of a neutron star, a dense remnant composed primarily of neutrons. If the original star is sufficiently massive, this process can result in a black hole, a region of space with an intense gravitational pull that prevents anything, including light, from escaping. High-mass stars play a crucial role in the cosmic cycle, as their supernovae distribute elements throughout the universe, contributing to the formation of new stars and planetary systems.
High-Mass Stars
FIELDS OF STUDY: Astronomy; Cosmology; Stellar Astronomy
ABSTRACT: High-mass stars have hydrogen fusion cores and masses much greater than the sun. They burn brightly but have relatively short lives that end in powerful explosions called supernovas. Supernova explosions cast into the galaxy the elements created within the core of the stars. Scientists have determined that everything on Earth, including humans, contains elements that came from long-ago supernovas produced by high-mass stars.
High-Mass versus Low-Mass Stars
Scientists use a number of techniques to classify stars in order to study them. One technique is to determine whether stars are high or low mass, which can be discovered by examining the amount of matter they contain.
High-mass stars contain significantly more matter than low-mass stars. Their mass may be up to eight times greater than that of the sun. Both high- and low-mass stars have cores in which nuclear fusion is constantly taking place, converting hydrogen to helium to create energy. However, the conversion process happens differently in high-mass and low-mass stars, although both processes take thousands of years.
High-mass stars convert hydrogen to helium mainly via the carbon-nitrogen-oxygen (CNO) cycle, so called because carbon, nitrogen, and oxygen isotopes act as catalysts, starting with carbon-12. They move quickly through the six-step process, giving them a comparatively shorter life span. Low-mass stars use the proton-proton chain reaction and take a longer time to expend their energy.
Both high- and low-mass stars eventually exhaust their energies and undergo change. The hydrogen in the core is used up, leaving a helium core surrounded by a hydrogen shell. When the helium core reaches a high enough temperature, the star begins fusing helium into carbon instead. This produces a carbon core surrounded by helium and hydrogen shells. What happens to the star next depends on whether it is high or low mass.
How High-Mass Stars Become Supernovas
The extra matter in high-mass stars means that they are more affected by gravity than low-mass stars. The carbon core contracts and heats up. The immense heat allows the carbon to fuse into oxygen, which then produces neon, then magnesium, then silicon, and finally iron. Iron does not generate enough energy for fusion, so the fusion process ends.
Once nuclear fusion has stopped, the star can no longer counter the force of gravity, and the core begins to collapse. As it does, the electrons in the iron atoms are forced ever closer to their nuclei, until they combine with protons to form neutrons. In an instant, the star shrinks from an object larger than the sun to one just a few kilometers in diameter. It then rebounds, causing a massive explosion known as a supernova that blasts the outer layers of the star into the universe.
A Neutron Star Is Born
Although the outer layers of a high-mass star are blown away in a supernova, the core remains. It is incredibly dense due to the combination of protons and electrons to form neutrons. This dense core forms a neutron star. The neutron core does not support fusion, so no energy is created. Without an outward flow of energy to counteract the pull of gravity, the star continues to shrink until it is only about 20 kilometers (12.4 miles) in diameter. Yet despite its smaller size, a neutron star contains more matter than the sun. Less than a teaspoon of this matter would weigh more than one hundred million tons on Earth. Thus, neutron stars have great gravitational pull despite their relatively small size.
When a high-mass star at least ten times the size of the sun becomes a supernova, a large neutron star forms with no energy output to counteract its immense gravity. This creates a black hole, a region of space with such incredible gravitational pull that it draws in all energy and matter that comes near. High-mass stars and their resulting supernovas are key ways that matter is distributed throughout the universe.
PRINCIPAL TERMS
- black hole: a region of space with a gravitational pull so strong that nothing, not even light, can escape it.
- carbon-nitrogen-oxygen (CNO) cycle: a six-step process that converts hydrogen into helium within the cores of high-mass stars.
- neutron star: a dense, rapidly spinning star made of the material that remains after a star becomes a supernova. It is formed from the neutrons created by the reaction of the star’s remaining protons and electrons.
- supernova: a massive explosion that results when a dying star exhausts its fuel. The star’s core collapses and explodes, releasing large amounts of energy and matter into the universe.
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