Stars along the main sequence MS all have one thing in common: they are all burning hydrogen into helium in their cores. This process is called fusion. Fusion creates heavier elements from lighter elements for example, helium from hydrogen. Fusion also releases energy photons which travel outward from the core of the star where the fusion is taking place and eventually escape from the surface of the star this is the light we see from the star.
These photons exert a pressure on the stellar material as they travel outward from the core; this pressure is called Radiation Pressure. In the Main Sequence Phase of a star's evolution, radiation pressure pushing outward exactly balances the gravitational pressure pulling inward this balance is called Hydrostatic Equilibrium.
Because these two forces are exactly in balance, the star is stable this means it neither shrinks nor expands. The lifetime of a star on the MS is determined by its mass. 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. This is also the longest phase of a star's life. Our sun will spend about 10 billion years on the main sequence. However, a more massive star uses its fuel faster, and may only be on the main sequence for millions of years. Eventually the core of the star runs out of hydrogen.
When that happens, the star can no longer hold up against gravity. Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star. While the core collapses, the outer layers of material in the star to expand outward. At this point the star is called a red giant. When a medium-sized star up to about 7 times the mass of the Sun reaches the red giant phase of its life, the core will have enough heat and pressure to cause helium to fuse into carbon, giving the core a brief reprieve from its collapse.
Once the helium in the core is gone, the star will shed most of its mass, forming a cloud of material called a planetary nebula. The core of the star will cool and shrink, leaving behind a small, hot ball called a white dwarf. A white dwarf doesn't collapse against gravity because of the pressure of electrons repelling each other in its core. A red giant star with more than 7 times the mass of the Sun is fated for a more spectacular ending.
These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, but even bigger, forming a red supergiant. Next, the core starts to shrink, becoming very hot and dense. Then, fusion of helium into carbon begins in the core. The overall reaction can be summarised as:. The neutrinos are neutral and have extremely low rest masses. They essentially do not interact with normal matter and so travel straight out from the core and escape from the star at almost the speed of light.
Positrons are the antiparticle of electrons. Although the pp chain involves the fusion of hydrogen nuclei, the cores of stars still contain electrons that have been ionised or ripped off from their hydrogen or helium nuclei.
When a positron collides with an electron, an antimatter-matter event occurs in which each annihilates the other, releasing yet more high-energy gamma photons. In the ppII chain, a He-3 nucleus produced via the first stages of the ppI chain undergoes fusion with a He-4 nucleus, producing Be-7 and releasing a gamma photon. The Be-7 nucleus then collides with a positron, releasing a neutrino and forming Li This in turn fuses with a proton, splitting to release two He-4 nuclei.
A rarer event is the ppIII chain whereby a Be-7 nucleus produced as above fuses with a proton to form B-8 and release a gamma photon. B-8 is unstable, undergoing beta positive decay into Be-8, releasing a positron and a neutrino.
Be-8 is also unstable and splits into two He-4 nuclei. This process only contributes 0. These forms are summarised as:. Stars with a mass of about 1. 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.
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