- 3225 - STARS - lifetime of stars? Stars last a long time, but eventually they will die. The energy that makes up stars comes from the interaction of individual atoms. So, to understand the largest and most powerful objects in the universe, we must understand the most basic atomic structures. The smallest controls the biggest?
------------------ 3225 - STARS - lifetime of stars?
- As the star's life ends these same basic principles once again come into play to describe what will happen to the star. Various aspects of stars determine how old they are as well as understanding the life and death processes they experience.
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- The stars took a long time to form, as gas drifting in the universe was drawn together by the force of gravity. This gas is mostly hydrogen, because it's the most basic and abundant element in the universe. Hydrogen is a single proton with a single electron. Some of the gas might consist of some other lighter elements including isotopes of hydrogen. Enough of this gas begins gathering together under gravity as each atom is pulling on all of the other atoms.
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- This gravitational pull is enough to force the atoms to collide with each other, which in turn generates heat. As the atoms are colliding with each other, they are vibrating and moving more quickly which is the definition of what “heat energy” really is: “atomic motion“.
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- Eventually, the collisions get so hot that the individual atoms have so much kinetic energy, that when they collide with another atom they don't just bounce off each other. With enough energy, the two atoms collide and the nucleus of these atoms “fuse” together.
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- This is mostly hydrogen, which means that each atom contains a nucleus with only one proton. When these nuclei fuse together, nuclear fusion, the resulting nucleus has two protons, which means that the new atom created is helium.
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- Stars may also fuse heavier atoms, heavier than helium, together to make even larger atomic nuclei, called nucleosynthesis, is believed to be how many of the elements in our universe were formed.
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- The atoms inside the star collide together, going through a process of “nuclear fusion“, which generates heat, electromagnetic radiation (including visible light), and energy in other forms, such as high-energy particles.
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- This period of “atomic burning” is what most of us think of as the life of a star, and it's in this phase that we see most stars up in the heavens.
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- This heat generates a pressure which pushes the atoms apart. But at the same time gravity is trying to pull them together. Eventually, the star reaches an equilibrium where the attraction of gravity and the repulsive pressure are balanced out, and during this period the star burns in a relatively stable way, like our Sun. Until it runs out of fuel, that is.
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- As the hydrogen fuel in a star gets converted to helium, and to some heavier elements, it takes more and more heat to cause this nuclear fusion. The mass of a star and the consequential plays a role in how long it takes to "burn" through the fuel.
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- More massive stars use their fuel faster because it takes more energy to counteract the larger gravitational force. The larger gravitational force causes the atoms to collide together more rapidly. While our Sun will probably last for about 5,000 million years, more massive stars may last as little as 100 million years before using up their fuel.
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- As the star's fuel begins to run out, the star begins to generate less heat. Without the heat to counteract the gravitational pull, the star begins to contract. These atoms made up of protons, neutrons, and electrons, are called “fermions“.
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- One of the rules governing fermions is called the “Pauli Exclusion Principle“, which states that no two fermions can occupy the same "state," which is a way of saying that there can't be more than one identical one in the same place doing the same thing.
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- The result of this is that the Pauli Exclusion Principle creates yet another slight repulsive force between electrons, which can help counteract the collapse of a star, turning it into a “white dwarf“. This principle was discovered by the Indian physicist Subrahmanyan Chandrasekhar in 1928.
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- Another type of star, the “neutron star“, come into being when a star collapses and the neutron-to-neutron repulsion counteracts the gravitational collapse. However, not all stars become white dwarf stars or even neutron stars. Chandrasekhar realized that some stars would have very several different fates.
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- Chandrasekhar determined any star more massive than about 1.4 times our sun (a mass called the Chandrasekhar limit) wouldn't be able to support itself against its own gravity and would collapse into a “white dwarf“ star. Stars ranging up to about 3 times our sun would become neutron stars.
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- Beyond that there's just too much mass for the star to counteract the gravitational pull through the exclusion principle. It's possible that when the star is dying it might go through a supernova explosion, expelling enough mass out into the universe that it drops below these limits and becomes one of these types of stars ... but if not, then what happens? The mass continues to collapse under gravitational forces until a blackhole is formed.
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- And that is what you call the death of a star.
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- Stars are of the fundamental building blocks of the universe. They not only make up galaxies, but many also harbor planetary systems. Our Sun gives us a first-class example to study, right here in our own solar system. It's only eight light-minutes away, so we don't have to wait long to see features on its surface.
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- Astronomers have a number of satellites studying the Sun, and they've known for a long time about the basics of its life. For one thing, it's middle-aged, and right in the middle of the period of its life called the "main sequence". During that time, it fuses hydrogen in its core to make helium.
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- Throughout its history, the Sun has looked pretty much the same to us. It has always been this glowing, yellowish-white object in the sky. It doesn't seem to change. However, it does change, but in a very slow way compared to the rapidity in which we live our short, fast lives.
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- If we look at a star's life on the scale of the universe's age (about 13.7 billion years) then the Sun and other stars all live pretty normal lives. That is, they are born, live, evolve, and then die over tens of millions or billions of years.
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- Astronomers sort stars in a series of "bins" using these characteristics: temperature, mass, chemical composition. Based on its temperature, brightness (luminosity), mass, and chemistry, the Sun is classified as a middle-aged star that is in a period of its life called the "main sequence".
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- The density in the core reaches a point where the temperature is at least 8 to 10 million degrees Celsius. The outer layers of the protostar are pressing in on the core. This combination of temperature and pressure starts a process called nuclear fusion. That's the point when a star is born.
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- The star stabilizes and reaches a state called "hydrostatic equilibrium", which is when the outward radiation pressure from the core is balanced by the immense gravitational forces of the star trying to collapse in on itself. When all these conditions are satisfied, the star is "on the main sequence" and it goes about its life busily making hydrogen into helium in its core.
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- The star’s mass plays an important role in determining the physical characteristics of a given star. It also gives clues to how long the star will live and how it will die. The greater than the mass of the star, the greater the gravitational pressure that tries to collapse the star. In order to fight this greater pressure, the star needs a high rate of fusion.
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- The greater the mass of the star, the greater the pressure in the core, the higher the temperature and therefore the greater the rate of fusion. That determines how fast a star will use up its fuel.
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- A massive star will fuse its hydrogen reserves more quickly. This takes it off the main sequence more quickly than a lower-mass star, which uses its fuel more slowly.
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- When stars run out of hydrogen, they begin to fuse helium in their cores. This is when they leave the main sequence. High-mass stars become “red super giants“, and then evolve to become “blue super giants“.
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- It is fusing helium into carbon and oxygen. Then, it begins to fuse those into neon and so on. Basically, the star becomes a chemical creation factory, with fusion occurring not just in the core, but in layers surrounding the core.
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- Eventually, a very high-mass star tries to fuse iron. This is the kiss of death for that star. Why? Because fusing iron takes more energy than the star has available. It stops the fusion factory dead in its tracks.
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- When that happens, the outer layers of the star collapse in on the core. This happens pretty quickly. The outer edges of the core fall in first, at the amazing speed of about 70,000 meters per second. When that hits the iron core, it all starts to bounce back out, and that creates a shock wave that rips through the star in a few hours.
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- In the process, new, heavier elements are created as the shock front passes through the material of the star. This is what's called a "core-collapse" supernova. Eventually, the outer layers blast out to space, and what's left is the collapsed core, which becomes a neutron star or blackhole.
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- Stars with masses between a half a “solar mass” and about eight solar masses will fuse hydrogen into helium until the fuel is consumed. At that point, the star becomes a “red giant“ star.. The star begins to fuse helium into carbon, and the outer layers expand to turn the star into a “pulsating yellow giant“ star.
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- When most of the helium is fused, the star becomes a red giant again, even larger than before. The outer layers of the star expand out to space, creating a “planetary nebula“. The core of carbon and oxygen will be left behind in the form of a “white dwarf“.
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- Stars smaller than 0.5 solar masses will also form “white dwarfs“, but they won't be able to fuse helium due to the lack of pressure in the core from their small size. Therefore these stars are known as helium white dwarfs. Like neutron stars, blackholes, and supergiants, these no longer belong on the main sequence of stars.
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- July 18, 2021 STARS - Lifetime of stars? 3225
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