Life of a Star

By: Baylee, Carter, and Hannah

Beginning Formation

Star formation begins in the giant molecular cloud, which is made up of hydrogen and other elements that make up stars. The dense part of the cloud, the cloud core, is where star formation begins as the cloud contracts.. From this point, a protostar is formed from areas of intense radiation within the cloud core. When the star begins to form its own core and can pull other material into it by gravitational force, it becomes a protostar. This is the earliest phase in star formation, and it generally lasts up to 10,000,000 years.  

Giant molecular cloud

Transition into adulthood

The density of the star’s core increases and it begins the transition into ‘adulthood’ when it becomes a zero age main sequence star. This is when a star first begins to burn hydrogen through fusion reactions in its core, but is still unstable because it hasn’t established equilibrium between its gravitational force and the force from the fusion reactions. This is also when the star joins the main sequence of the HR diagram. The diagram shows the luminosity and surface temperature of all stars. Stars that have a greater luminosity fall higher on the diagram and stars with a greater surface temperature are towards the left of the diagram.

Main sequence stars

T-Tauri Star (artist representation)

The youngest visible stars in spectral classes F G K and M on the HR diagram are T-Tauri stars. The  surface temperatures are the same as main sequence stars with the same mass, but are more luminous because their radii are larger. Their central temperature is too low for hydrogen fusion, and they are incredibly unstable. T-Tauri stars are pre-main sequence star showing intense emission lines.

90% of the stars in the universe are main sequence, ranging in mass from one tenth the size of the sun to 200 times its mass. These stars have fusion reactions in their cores, which fuse hydrogen atoms to form helium. At this point, the star has achieved thermo-gravitational equilibrium, so its force of gravity is balanced against the pressure from its fusion reactions, making it stable.

If a star’s mass is too small, it becomes a brown dwarf instead of entering the main sequence. Dimmer and cooler than the dimmest M main sequence stars, its in between the size of a huge planet and a small star. Two spectral classes were created to accommodate them, class L and T. The brown dwarf does not fuse hydrogen.

Brown Dwarf (scaled representation

Main sequence cont.

Red Dwarf (artist representation)

There are several types of stars within the main sequence. Red dwarf stars are one of the most common types of stars, they are of spectral type M and cannot be seen with the naked eye because they are so dim. They have very small masses, smaller than the sun, and low surface temperatures. Because of their small masses, they burn through their supply of hydrogen very slowly, so they have significantly longer life spans than other stars and can live for trillions of years. When they do burn out, they most often become white dwarfs but can turn into red giants if their mass is big enough.

Stars in the main sequence fuse hydrogen to form helium by one of two nuclear processes. Stars the size of the sun or smaller use the proton-proton chain, which fuses hydrogen by a series of reactions, the first of which is the reaction of one proton with another. Stars 1.3 times more massive than the sun and larger use the carbon cycle, which uses carbon, oxygen, and nitrogen to fuse four hydrogen nuclei to form one helium nucleus. This process does not change the number of carbon, nitrogen, or oxygen nuclei within this star.

After this point in the star's life, it could take several paths depending on its mass. This has to do with the Vogt-Russell theorem, which states that the mass and chemical composition of a star will determine the entire course of its evolution.

Red Giant (artist representation)

When a star with a large mass gets to the end of its life, it becomes a red giant. This happens when a main-sequence star stops fusing hydrogen in its core, and gravity takes over, compressing the star. As it compresses, its temperature increases to the point where it can fuse helium to form carbon.The energy produced through this fusion causes the star to grow to many times its original size. At the end of the red giant phase, a shell of ionized gas is released from the star and surrounds it. The luminous shell expands into space and forms a ring around the star, the planetary nebula.

During the red giant phase, the star begins to use the s-process (slow neutron capture) to build up massive nuclei through neutron capture. This increases the atomic weight of the nucleus by one. This process is very slow, especially compared to the rate at which it the nuclei can undergo radioactive decay. This is the process by which heavier elements are constructed in the star from already present elements like iron.

An Asymptotic giant branch (AGB) star is the last major phase of life for stars with a mass nine times smaller than that of the sun, occurring after the Red Giant phase and past the Horizontal Branch. As a star passes through this phase, it gets larger, cooler and brighter and it continues to burn hydrogen.


Planetry Nebula (artist representation)

When a star with a lower mass gets to the end of its life, it becomes a white dwarf after the red giant phase. These stars are small - about the size of a planet - and very dense. They are created when a small star loses its outer layers as a planetary nebula and hydrogen fusion stops entirely, allowing gravity to take over. When it does so, it compresses the star, which accounts for its small, dense state. The white dwarf star is so dense that its electrons become degenerate, which is what supports it after gravity takes over.

Electron degeneracy is a stellar application of the Pauli exclusion principle, so no two electrons can occupy the same energy state. In white dwarf stars, electrons are packed together, making it so some must occupy higher energy levels, which creates pressure in the star. Because the pressure is created this way it is insensitive to temperature changes in the star. The pressure created through electron degeneracy can hold the white dwarf stable against its own gravity.

After either the red giant or the white dwarf phase, a star can become a supernova. This is the explosion of a star at the end of its life, which causes its brightness to increase as much as one billion times, as a huge amount of energy is released. There are two types of supernovae, depending on what the original size of the star was. Type I occurs in white dwarf stars, which is caused by the rapid fusion of carbon and oxygen within the star. Type II occur when the core of a massive star that has turned into a red supergiant collapses.


Supernova (infared image)

End of life

During a supernova the star begins to use the r-process to build up massive nuclei, which is similar to the s-process just much faster. This process captures neutrons at a rate faster than they can undergo radioactive decay. It can only occur during the collapse of a star during a supernova because that’s the only time there’s a density of neutrons large enough for it to take place. As this process occurs, some of the elements created are projected into space where they can help form other things. This is important because it’s the process by which heavier elements are created; we need this process to make these elements so they’re present in the solar system.

When red supergiant collapses it can turn into a neutron star. In these stars, protons and electrons combine to form neutrons, which make up the majority of the star. In these stars, the neutrons are degenerate. Similar to electron degeneracy, each neutron must occupy it’s own energy level in the star. In a neutron star, the neutrons are packed together so tightly that some must occupy higher energy states. The pressure this creates supports the neutron star, and because the pressure is created in this way it doesn’t fluctuate when the star’s temperature changes (otherwise it would decrease with a drop in temperature). The pressure holds the size of the star stable, and because the neutron is more massive than the electron, neutron degeneracy pressure can hold much larger, more massive, stars stable than electron degeneracy pressure can.

There are several types of neutron stars, one of those is the magnetar, a highly magnetized neutron star that emits bursts of gamma rays. A pulsar is another type of neutron star, which is a rapidly rotating star that gives off regular pulses of radio waves and other electromagnetic radiation at 1,000 pulses per second.   

RR Lyrae stars are stars that change brightness on a regular basis every few days. It is a member of the class of giant pulsating stars, all of which have pulsation periods of about one day. Classical cepheid variable stars are part of a class of stars whose pulsation periods of variation tend to be proportional to their luminosity and thus are useful when measuring interstellar and intergalactic distances.

A black hole contains large amounts of matter packed into a very small space. This results in a gravitational pull so strong that even light cannot escape from it. Black holes are what remains of  massive stars that went supernova and were taken over by the force of gravity instead of turning into a neutron star, because the gravitational force was too great (because they are very massive) for the degenerate particles to hold off as is the case in a neutron star.

Neutron star (artist rendering)
Black hole

HR-diagram:

  • evolutionary track: The path in an HR-diagram followed by the point representing the changing luminosity and temperature of a star as it evolves.
  • main sequence: The region of the HR-diagram that contains the most stars, which runs diagonally from hot, luminous stars to cool, dim stars.
  • red giants: The top of the HR-diagram, where cool, luminous giants live. All stars with masses greater than the Sun's (or equal to) expand and become a red giant.
  • white dwarfs: The hot but dim stars that lie in the lower left of the diagram.
  • birth line: The diagonal line where stars first appear on the diagram, before they reach the main sequence. It runs diagonally above the main sequence stars.
  • instability strip: The region of the HR-diagram occupied by pulsating stars, including Cepheid variables and RR Lyrae stars. This runs diagonally along where the red giants are.
  • asymptotic giant branch: The portion of the HR-diagram occupied by enormous, cool stars with helium-burning shells. View image for location.

Sources

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