Stars
A star is a hot cloud of gas held together by its own gravitational pull. The Sun is the nearest example of a star to Earth, and like the Sun, most stars produce light because of the high temperature of the gases they contain. Like living things, stars are born, live, and die, though their lifespans are much longer than ours -- millions, billions, or even trillions of years.
We know these things because we can explain and predict the observed properties of stars using computer simulations based on known physics. Stars form through the collapse of cold, dense hydrogen gas clouds in the interstellar medium. When their centers become hot and dense enough, nuclear fusion reactionsA type of reaction in which atomic nuclei fuse together to make heavier nuclei. Through the equivalence of mass and energy predicted by Einstein's theory of special relativity, the combined nuclei lose some mass, which is released as energy. start to produce energy that keeps the gas from collapsing further.
A star spends most of its life in a state of balance between gravity and pressure (hydrostatic equilibriumA condition in which a fluid is kept at rest by a balance of forces, usually pressure and gravity.). During this main sequenceThe portion of a star's life during which it produces energy by fusing hydrogen into helium at its center. phase, nuclear reactions gradually convert hydrogen to helium in the star's core. Eventually enough hydrogen is used up that nuclear reactions stop at the very center of the star. However, they continue in a shell around the center, growing the helium-rich core and puffing the star up into an enormous red giantA late stage of stellar evolution in which a star fuses hydrogen into helium in a shell surrounding a core made of helium. A red giant is swollen well beyond the star's main-sequence size and has a reddish color because of its low surface temperature (around 3000 degrees)..
Eventually enough helium builds up in the core to allow it to begin fusing to make carbon and oxygen. For a "low-mass" star like our Sun, this is the last stage; its core will never become hot enough to fuse carbon and oxygen. Eventually the outer layers of the star are blown off into space as a planetary nebulaA brilliantly colored cloud of gas ejected from a low-mass star during its final years as a giant. The gas is lit up by ultraviolet radiation produced by the white dwarf that is left behind when the last of the gas outside the core is lost. Has nothing to do with planets, despite the name., leaving behind the carbon-oxygen core as a white dwarfA stellar corpse; the extremely dense core of a low-mass star, left behind when the star's outer layers are ejected as a planetary nebula. White dwarfs are typically about the size of the Earth but the mass of the Sun. They are kept from collapsing under their own weight by a quantum mechanical phenomenon called electron degeneracy..
Stars more massive than about eight times the Sun's mass have a very different fate. Their cores are able to get hot enough after repeated fusion cycles to fuse carbon and oxygen, neon, magnesium, silicon, all the way up to iron. Since iron is the most stable type of atomic nucleus, fusing it cannot release energy to keep the star hot, and the core collapses a final time, releasing a huge burst of energy that destroys the outer parts of the star in a supernova explosionThe explosive destruction of a star, either through release of gravitational energy (a "core collapse supernova") or unstable nuclear reactions (a "thermonuclear supernova").. What is left of the core becomes a neutron starA stellar corpse; the extremely dense core of a high-mass star, left behind after a core-collapse supernova. Neutron stars have roughly the mass of the Sun but are the size of a city. Because of their extremely high density, their constituent atomic nuclei have broken down to form a single giant "nucleus". or black holeA region of space in which matter has been compressed so much that light itself cannot escape the gravitational field. Stellar-mass black holes are left behind by supernova explosions in which enough material fell into the star's core that it could not remain stable as a neutron star..
Stellar evolution simulator
Our stellar evolution simulation solves the equations that govern the internal structure of stars, allowing us to predict the behavior of different kinds of stars over the course of their lives. The results are plotted on the Hertzsprung-Russell diagramA diagram showing stars as individual points on a plot with temperature or color on the horizontal axis and luminosity or brightness on the vertical axis. Usually the axes are logarithmic, so that a given distance along an axis corresponds to multiplication rather than addition, and the horizontal axis is backwards, so hotter stars appear to the left., which plots the luminosity (brightness) against the surface temperature. Individual stars trace out curves on this diagram as they evolve. Check out some of our precomputed stellar evolution simulations below.
| Name of Video | Description |
|---|---|
| The Sun | This simulation shows where the Sun lies and moves on the HR diagram as it evolves over time. |
| Intermediate Mass Stars | This simulation shows the evolution of 12 intermediate mass stars, ranging in mass from 0.9 to 2 solar masses. |
| One to Eight Solar Masses | This simulation shows the evolution of 8 stars, ranging in mass from 1 to 8 solar masses. |
| Stars Become White Dwarfs | This simulation shows the evolution of 7 stars that will eventually become white dwarf stars. Their masses range from 5 to 8 solar masses. |
| Stars Become Neutron Stars | This simulation shows the evolution of stars that will eventually become neutron stars. |
| High Mass Stars | This simulation shows the evolution of 7 high mass stars ranging from 20 to 80 solar masses. |
| Intermediate Mass vs. High Mass | This simulation shows the difference in the evolution stages and evolution times of an intermediate mass star of 3 solar masses versus a high mass star of 15 solar masses. Note that the evolution tracks are on in this simulation. |
| Intermediate Mass Star | This simulation shows the evolution of one 2 solar mass star, with attention focused on the star's main sequence, subgiant branch, giant branch, horizontal branch, and asymptotic giant branch. The star's death is also shown in this simulation, as it dies in a planetary nebula, leaving a dense white dwarf behind. Note that the evolution tracks are on in this simulation. |
| High Mass Star Life | This simulation shows the evolution of 3 stars, of masses 10, 15, and 20 solar masses. The simulation focuses attention on the main sequence, subgiant, and the various red giant/supergiant stages of high mass star evolution. Note that the evolution tracks are on in this simulation. |
| High Mass Star Death - Supernova | This simulation shows the evolution of one 15 solar mass star. The main focus of this particular simulation is that high mass stars such as this one die in a violent explosion known as a type II supernova. Note that the evolution tracks are on in this simulation. |
| 999 Stars | This simulation shows the evolution of 999 stars, with their masses determined by an initial mass function. Note that the evolution tracks are not on in this simulation. |
Binary star evolution simulator
More than half of all stars are found in binary star systemsA system in which two stars orbit each other because of their mutual gravitational pull. . If the stars in a binary are close enough and/or are producing strong stellar windsA flow of gas that escapes from the surface of a star due to radiation pressure. Very massive, hot stars and very cool stars like red supergiants have the strongest winds., their interaction may significantly alter their evolution, particularly if mass transferA situation in which one star, the donor, loses mass and another star, the accretor, gains some or all of that mass. is involved.
In the type of mass transfer known as Roche lobeThe region of space around a star within which its gravitational pull exceeds the pull of any companion stars or planets and the centrifugal effect due to its orbit around those objects. overflow, the donorThe mass-losing star in a binary system undergoing mass transfer. star loses mass to its companion through the system's inner Lagrange pointThe point in space between two stars at which their gravitational pulls exactly balance. . In Case A mass transfer, the donor fills its Roche lobe while on the main sequence. Typically, these systems evolve through a very slow mass transfer phase. In Case B, the donor fills its Roche lobe just after leaving the main sequence, but before it undergoes core helium ignition. These systems evolve on a much faster timescale dependent on the expansion of the donor star. In Case C, the donor fills its Roche lobe after core helium ignition. Often these systems undergo unstable mass transfer, and the transfer occurs shortly before the end of the donor’s life.
Image credits: "Afternoon sky from Rockgrove Road" - Jim Simonson (CC 2.0); "The Sun as a red giant" - Tablizer at English Language Wikipedia (CC 3.0); "M57: The Ring Nebula" - NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
A typical star.
A red giant (artist's conception).
M57, a planetary nebula.