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.
The star turns on and becomes a main sequence star, powered by hydrogen fusion. Fusion produces an outward pressure that balances with the inward pressure caused by gravity, stabilizing the star.
How long a main sequence star lives depends on how massive it is. A higher-mass star may have more material, but it burns through it faster due to higher core temperatures caused by greater gravitational forces. While the sun will spend about 10 billion years on the main sequence, a star 10 times as massive will stick around for only 20 million years.
A red dwarf , which is half as massive as the sun, can last 80 to billion years, which is far longer than the universe's age of This long lifetime is one reason red dwarfs are considered to be good sources for planets hosting life , because they are stable for such a long time.
More than 2, years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness , according to Dave Rothstein, who participated in Cornell University's "Ask An Astronomer" website in In the early 20th century, astronomers realized that the mass of a star is related to its luminosity , or how much light it produces.
These are both related to the stellar temperature. Stars 10 times as massive as the sun shine more than a thousand times as much. The mass and luminosity of a star also relate to its color. As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of this material ends up as part of a star — the remaining dust can become planets, asteroids, or comets or may remain as dust. In some cases, the cloud may not collapse at a steady pace. In January , an amateur astronomer, James McNeil, discovered a small nebula that appeared unexpectedly near the nebula Messier 78, in the constellation of Orion.
When observers around the world pointed their instruments at McNeil's Nebula , they found something interesting — its brightness appears to vary. Observations with NASA's Chandra X-ray Observatory provided a likely explanation: the interaction between the young star's magnetic field and the surrounding gas causes episodic increases in brightness. A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood.
Our Sun will stay in this mature phase on the main sequence as shown in the Hertzsprung-Russell Diagram for approximately 10 billion years. Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors.
The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines. As shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of luminosities and colors, and can be classified according to those characteristics.
Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years. On the other hand, the most massive stars, known as hypergiants, may be or more times more massive than the Sun, and have surface temperatures of more than 30, K.
Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants. In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease.
Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core.
The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.
If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust.
What happens next depends on the size of the core. Stars Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. Star Formation Stars are born within the clouds of dust and scattered throughout most galaxies. Within the portal, all users can view and copy all storyboards. More options. How Do I Use This? Use these encyclopedias as a springboard for individual and class-wide projects!
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