Life Cycle of a Small Star

Life Cycle of a Small Star
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Stars truly are born from stardust, and because stars are the factories that produce all the heavy elements, our world and everything in it also comes from stardust.

Clouds of it, consisting mostly of hydrogen gas molecules, float around in the unimaginable coldness of space until gravity forces them to collapse in on themselves and form stars.

All stars are created equal, but like people, they come in many variations. The primary determinant of a star's characteristics is the amount of stardust involved in its formation.

Some stars are very big, and they have short, spectacular lives, while others are so small that they barely had enough mass to become a star in the first place, and these have extremely long lives. The life cycle of a star, as NASA and other space authorities explain, is highly dependent on mass.

Stars approximately the size of our sun are considered small stars, but they aren't as small as red dwarves, which have a mass about half that of the sun and are as close to being eternal as a star can get.

The life cycle of a low-mass star like the sun, which is classified as a G-type, main sequence star (or a yellow dwarf), lasts about 10 billion years. Although stars of this size do not become supernovae, they do end their lives in dramatic fashion.

The Formation of a Protostar

Gravity, that mysterious force that keeps our feet glued to the ground and the planets spinning in their orbits, is responsible for star formation. Within the clouds of interstellar gas and dust that float around the universe, gravity coalesces molecules into small clumps, which break free of their parent clouds to become protostars. Sometimes the collapse is precipitated by a cosmic event, such as a supernova.

By virtue of their increased mass, protostars are able to attract more stardust. Conservation of momentum causes the collapsing matter to form a rotating disk, and the temperature increases because of increasing pressure and the kinetic energy released by gas molecules attracted to the center.

Several protostars are believed to exist in the Orion Nebula, among other places. Very young ones are too diffuse to be visible, but they eventually become opaque as they coalesce. As this happens, the accumulation of matter traps infrared radiation in the core, which further increases the temperature and pressure, eventually preventing more matter from falling into the core.

The envelope of the star continues to attract matter and grow, however, until something incredible occurs.

The Thermonuclear Spark of Life

It's hard to believe that gravity, which is a comparatively weak force, could precipitate chain of events that leads to a thermonuclear reaction, but that's what happens. As the protostar continues to accrete matter, the pressure at the core becomes so intense that hydrogen begins fusing into helium, and the protostar becomes a star.

The advent of thermonuclear activity creates a intense wind that pulses from the star along the axis of rotation. Material circulating around the perimeter of the star is ejected by this wind. This is the T-Tauri phase of the star's formation, which is characterized by vigorous surface activity, including flares and eruptions. The star can lose up to 50 percent of its mass during this phase, which for a star the size of the sun, lasts for a few million years.

Eventually, the material around the star's perimeter begins to dissipate, and what's left coalesces into planets. The solar wind subsides, and the star settles into a period of stability on the main sequence. During this period, the outward force generated by the fusion reaction of hydrogen to helium occurring at the core balances the inward pull of gravity, and the star neither loses nor gains matter.

Small Star Life Cycle: Main Sequence

Most of the stars in the night sky are main sequence stars, because this period is the longest by far in the life span of any star. While on the main sequence, a star fuses hydrogen into helium, and it continues to do so until its hydrogen fuel runs out.

The fusion reaction happens more quickly in massive stars than it does in smaller ones, so massive stars burn hotter, with a white or blue light, and they burn for a shorter time. Whereas a star the size of the sun will last for 10 billion years, a super massive blue giant might only last for 20 million.

In general, two types of thermonuclear reactions occur in main-sequence stars, but in smaller stars, such as the sun, only one type occurs: the proton-proton chain.

Protons are hydrogen nuclei, and in a star's core, they are traveling fast enough to overcome electrostatic repulsion and collide to form helium-2 nuclei, releasing a v-neutrino and a positron in the process. When another proton collides with a newly formed helium-2 nucleus, they fuse into helium-3 and release a gamma photon. Finally, two helium-3 nuclei collide to create one helium-4 nucleus and two more protons, which go on to continue the chain reaction, so, all in all, the proton-proton reaction consumes four protons.

One sub-chain that occurs within the main reaction produces beryllium-7 and lithium-7, but these are transition elements that combine, after collision with a positron, to create two helium-4 nuclei. Another sub-chain produces beryllium-8, which is unstable and spontaneously splits into two helium-4 nuclei. These sub processes account for about 15 percent of the total energy production.

Post-Main Sequence – The Golden Years

The golden years in the life cycle of a human being are those in which energy begins to wane, and the same is true for a star. The golden years for a low mass star occur when the star has consumed all of the hydrogen fuel in its core, and this period is also known as post-main sequence. The fusion reaction in the core ceases, and the outer helium shell collapses, creating thermal energy as potential energy in the collapsing shell is converted to kinetic energy.

The extra heat causes hydrogen in the shell to begin fusing again, but this time, the reaction produces more heat than it did when it occurred only in the core.

Fusion of the hydrogen shell layer pushes the edges of the star outwards, and the outer atmosphere expands and cools, turning the star into a red giant. When this happens to the sun in about 5 billion years, it will expand half the distance to the Earth.

The expansion is accompanied by increased temperatures at the core as more helium gets dumped in by the hydrogen fusion reactions occurring in the shell. It gets so hot that helium fusion begins in the core, producing beryllium, carbon and oxygen, and once this reaction (called the helium flash) starts, it spreads quickly.

After the helium in the shell is exhausted, the core of a small star can't generate enough heat to fuse the heavier elements that have been created, and the shell surrounding the core collapses again. This collapse generates a significant amount of heat – enough to begin helium fusion in the shell – and the new reaction begins a new period of expansion during which the star's radius increases by as much as 100 times its original radius.

When our sun reaches this stage, it will expand beyond the orbit of Mars.

Sun-Sized Stars Expand to Become Planetary Nebulae

Any story of the life cycle of a star for kids should include an explanation of planetary nebulae, because they are some of the most striking phenomena in the universe. The term planetary nebula is a misnomer, because it has nothing to do with planets.

It's the phenomenon responsible for the dramatic images of the Eye of God (the Helix Nebula) and other such images that populate the internet. Far from being planetary in nature, a planetary nebula is the signature of a small star's demise.

As the star expands into its second red giant phase, the core simultaneously collapses into a super-hot white dwarf, which is a dense remnant that has most of the mass of the original star packed into an Earth-sized sphere. The white dwarf emits ultraviolet radiation that ionizes the gas in the expanding shell, producing dramatic colors and shapes.

What's Left Over Is a White Dwarf

Planetary nebulae are not long lasting, dissipating in about 20,000 years. The white dwarf star that remains after a planetary nebula has dissipated, however, is very long lasting. It's basically a lump of carbon and oxygen mixed with electrons that are packed so tightly that they are said to be degenerate. According to the laws of quantum mechanics, they can't be compressed any farther. The star is a million times more dense than water.

No fusion reactions occur inside a white dwarf, but it remains hot by virtue of its small surface area, which limits the amount of energy it radiates. It will eventually cool down to become a black, inert lump of carbon and degenerate electrons, but this will take 10 to 100 billion years. The universe isn't old enough for this to have occurred yet.

Mass Affects Life Cycle

A star the size of the sun will become a white dwarf when it consumes its hydrogen fuel, but one with a mass in its core of 1.4 times the size of the sun experiences a different fate.

Stars with this mass, which is known as the Chandrasekhar limit, continue to collapse, because the force of gravitation is enough to overcome the outward resistance of electron degeneration. Instead of becoming white dwarfs, they become neutron stars.

Since the Chandrasekhar mass limit applies to the core after the star has radiated much of its mass away, and since the lost mass is considerable, the star must have about eight times the mass of the sun before it enters the red giant phase to become a neutron star.

Red dwarf stars are those with a mass of between half to three-quarters of a solar mass. They are the coolest of all the stars and don't accumulate as much helium in their cores. Consequently, they don't expand to become red giants when they have exhausted their nuclear fuel. Instead, they contract directly into white dwarfs without the production of a planetary nebula. Because these stars burn so slowly, though, it will be a long time – perhaps as much as 100 billion years – before one of them undergoes this process.

Stars with a mass of less than 0.5 solar masses are known as brown dwarfs. They aren't really stars at all, because when they formed, they didn't have enough mass to initiate hydrogen fusion. The compressive forces of gravity do generate enough energy for such stars to radiate, but it's with a barely perceptible light on the far red end of the spectrum.

Because there's no fuel consumption, there's nothing to prevent such a star from staying exactly the way it is for as long as the universe lasts. There could be one or many of them in the immediate neighborhood of the solar system, and because they shine so dimly, we'd never know they were there.

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