Astrophysics Fleet Mission Chart. Spacecraft Paper Models. Related Content Mysteries of the Sun. Death of Stars video. Life Cycles of Stars. More About Stars.
Stellar Evolution. Recommended Articles. August 19, July 29, July 06, Nancy Grace Roman's Legacy. May 20, Ask a Question. Average Stars Become White Dwarfs For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf.
White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers - why didn't they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation.
Pressure from fast moving electrons keeps these stars from collapsing. The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! These paradoxical stars are very common - our own Sun will be a white dwarf billions of years from now. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down.
This fate awaits only those stars with a mass up to about 1. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate as described below.
White Dwarfs May Become Novae If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. Nova is Latin for "new" - novae were once thought to be new stars. Today, we understand that they are in fact, very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter - mostly hydrogen - from the outer layers of that star onto itself, building up its surface layer.
When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs those near the 1. Supernovae Leave Behind Neutron Stars or Black Holes Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova.
A supernova is not merely a bigger nova. White dwarfs no longer burn fusion at their center, but they still radiate heat. Eventually, white dwarfs should cool into black dwarfs , but black dwarfs are only theoretical; the universe is not old enough for the first white dwarfs to sufficiently cool and make the transition. Larger stars find their outer layers collapsing inward until temperatures are hot enough to fuse helium into carbon.
Then the pressure of fusion provides an outward thrust that expands the star several times larger than its original size, forming a red giant. The new star is far dimmer than it was as a main sequence star.
Eventually, the sun will form a red giant, but don't worry — it won't happen for a while yet. If the original star had up to 10 times the mass of the sun, it burns through its material within million years and collapses into a super-dense white dwarf.
More massive stars explode in a violent supernova death , spewing the heavier elements formed in their core across the galaxy. The remaining core can form a neutron star , a compact object that can come in a variety of forms. The long lifetime of red dwarfs means that even those formed shortly after the Big Bang still exist today.
Eventually, however, these low-mass bodies will burn through their hydrogen. They will grow dimmer and cooler, and eventually the lights will go out. Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community space. Nola Taylor Tillman is a contributing writer for Space. She loves all things space and astronomy-related, and enjoys the opportunity to learn more.
CNO stands for carbon, nitrogen and oxygen as nuclei of these elements are involved in the process. As its name implies, this process is cyclical. It requires a proton to fuse with a C nuclei to start the cycle. The resultant N nucleus is unstable and undergoes beta positive decay to C This then fuses with another proton to from N which in turn fuses with a proton to give O Being unstable this undergoes beta positive decay to form N When this fuses with a proton, the resultant nucleus immediately splits to form a He-4 nucleus and a C nucleus.
This carbon nucleus is then able to initiate another cycle. Carbon thus acts like a nuclear catalyst, it is essential for the process to proceed but ultimately is not used up by it.
As with the various forms of the pp chain, gamma photons and positrons are released in the cycle along with the final helium and carbon nuclei. All these possess energy. Why does the CNO cycle dominate in higher-mass stars? The answer has to do with temperature. The first stage of the pp chain involves two protons fusing together whereas in the CNO cycle, a proton has to fuse with a carbon nucleus.
As carbon has six protons the coulombic repulsion is greater for the first step of the CNO cycle than in the pp chain. The nuclei thus require greater kinetic energy to overcome the stronger repulsion. This means they have to have a higher temperature to initiate a CNO fusion. Higher-mass stars have a stronger gravitational pull in their cores which leads to higher core temperatures. The CNO cycle becomes the chief source of energy in stars of 1.
Core temperatures in these stars are 18 million K or greater. As the Sun's core temperature is about 16 million K, the CNO cycle accounts for only a minute fraction of the total energy released. The relative energy produced by each process is shown on the plot below. How do astronomers calculate such a value? A first order approximation for this value is surprisingly easy to derive. You will recall that the mass of a helium-4 nucleus is slightly less than the sum of the four separate protons needed to form it.
A proton has a mass of 1. A helium-4 nucleus has a mass of 4. From equation 6. The production of each helium nucleus releases only a small amount of energy, 10 J which does not seem a lot. We know though measurement that the Sun's luminosity is 3. To produce this amount of energy, vast numbers of helium, 3.
Each second, million tons of hydrogen fuse to form million tons of helium. This means 4 million tons of matter is destroyed and converted into energy each second. The high temperature needed for hydrogen fusion is only found in the core region of the Sun. The energy potentially available from this mass of hydrogen is roughly:. Given that the Sun's energy output is currently 3. As it is currently about about 5 billion years old this means it is half way through its main sequence life.
We have now seen how energy is produced in a star such as the Sun. How, though, does this energy escape from the star? Two processes, radiation and convection, play a vital role. The Sun's interior comprises three main regions. High-energy gamma photons produced in the core do not escape easily from it.
The high temperature plasma in the core is about ten times denser than a dense metal on Earth. A photon can only travel a centimeter or so on average in the core before interacting with and scattering from an electron or positive ion. Each of these interactions changes both the energy and travel direction of the photon. The direction a photon travels after an interaction is random so sometimes it is reflected back into the core. Nonetheless over many successive interactions the net effect is that the photon gradually makes its way out from the core.
The path it takes is called a random walk. Photons lose energy to the electrons and ions with each interaction creating a range of photon energies. This process is known as thermalisation and results in the characteristic blackbody spectrum that forms the continuum background spectrum of stars. Interactions between ions and electrons also produce many additional photons of various energies. These also contribute to the blackbody spectrum. The electrons and nuclei formed in fusion reactions also carry kinetic energy that they can impart to other particles through interactions, raising the thermal energy of the plasma.
Neutrinos produced by the various fusion and decay reactions travel out from the core at almost the speed of light. They are effectively unimpeded by the dense matter in the core of main sequence stars. Here, convection currents are responsible for transporting energy to the surface. Deep cells, 30, km across are responsible for supergranulation. The cells just below the photosphere are only 1, km across and are responsible for the granulation seen on the surface of the Sun as in the image below.
What happens when a main sequence star runs out of hydrogen, the fuel in its core? This leads us to evolution off the main sequence which is discussed on the next page. Skip to main content. Australia Telescope National Facility.
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