our simulation begins with stars plotted ~ above the main sequence. ~ above theHR diagram, the main sequence is an nearly straight line from the top leftto the reduced right region of the diagram. We carry out not worry ourselves with showingany of the very short-lived proto-stellar evolution; stars begin on the mainsequence whereby they spend most of their lives. It is right now in a star"slife that it is in hydrostatic equilibrium and also is turning hydrogen into heliumand energy in its main point in a process known together thermonuclear fusion. The staris really stable once it will this point and will certainly exist top top the main sequenceuntil the core hydrogen combination ceases. During this secure hydrogen fusion process, a star"sproperties will evolve an extremely little. A low-mass star will increasein luminosity and temperature together it fuses that is hydrogen, causing it come moveslightly up along the key sequence (to the upper left). However, a high-mass star willincrease in luminosity but decrease in temperature, bring about it to slowlymove off the main sequence come the right on the HR chart (Hurley et al. 2000).There is also a correlation between astar"s mass and its initial location on the main sequence. The greater a star is onthe main sequence (i.e. The higher its luminosity and temperature), thehigher its mass will certainly be.Hertzsprung space or Subgiant Branch
After most of the hydrogen in a star"s core has been fused to helium,fusion stops, and also the new helium core begins to contractdue come its self-gravity. Hydrogen fusion still continues ina slim layer over the core. The star starts to evolve off of the main sequence atthis point. As the helium core contracts, the layers abovethe main point fuse hydrogen at a more rapid rate, due to the fact that of raised temperature.This rise in hydrogen-shell burning causes the outerlayers that the star to expand and cool and also thus redden. Atthis suggest the star is coming to be a red gigantic star (Chaisson and McMillan). As thisoccurs, the star move to the best of the key sequence on the HR diagram. Together the star leaves the main sequence, and justbefore it becomes a red giant, the will get in the Hertzsprung gap orsubgiant branch. This occurs really rapidly, commonly in much less than a million years, and at an almost consistent luminosity.Thus, that is an overwhelming to actually observe stars in this step of evolution(Hurley et al. 2000).
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~ the star leaves the short-lived subgiant branch, the enters the huge branchas the core proceeds to collapse and the external layers continueto expand. Eventually, the helium core of the star will reach a criticaltemperature and also pressure at which the helium will begin to fuse into carbonand oxygen. Because that a star the mass much less than about 2 solar masses, the star will havea degenerate helium main point at the moment of helium combination and will start this newfusion procedure rather violent in what is well-known as the helium flash. Thehelium flash deserve to be watched on the HR diagram together a spike in the motion of thelower mass stars" evolutionary tracks. Instantly after the helium flash, astar promptly moves down the HR diagram (towards much less luminosity) and also thenback towards the key sequence in a horizontal fashion (towards highertemperatures) right into a duration of steady helium combination known together the horizontalbranch. This change onto the horizontal branchis an ext gradual because that stars the mass higher than around 2 solar masses. For stars of mass greater than about 12 solar masses, it ispossible because that the main point temperatures of this stars to increase quicklyenough that helium fusion begins with nearly no large branch step ofevolution. This can be viewed on the HR diagram as stars that this massive movehorizontally far from the main sequence without significantlyincreasing luminosity (Chaisson and also McMillan, Hurley et al. 2000).Asymptotic huge Branch and Red Supergiants
The new helium fusion process does not last really long compared to the initial hydrogen fusion process that emerged on the main sequence. The greater temperature in a star"s core present at the time of helium fusion causes therate the helium burning to be greater than the of the previoushydrogen core burning (Chaisson and also McMillan). The luminosity is comparable to what the is ~ above the main sequence, even thoughhelium combination only release 1/10 of the power per unit mass that hydrogen combination does. After every one of the helium has been fused in the core of a star, the star undergoes changes similar to the alters it went through after the key sequence phase. The now carbon/oxygen core begins to contract under that owngravity as the outer layers the the star again expand and also cool. This phase of stellar evolution is known as the asymptotic-giant branch and also can be seen on the HR diagram together the course of the star together it moves far from the horizontal branch towards a an ar of lower temperature and also in the situation of reduced mass stars, greater luminosity. The star is now becoming a red supergiant (Chaisson and McMillan, Hurley et al. 2000).Carbon/Oxygen White Dwarfs - the Fate that MediumMass Stars
What happens next to the star depends critically on its mass. If the star"s massive is much less than around 8 and greater than about0.3 solar masses, the star end its life together a carbon/oxygen white dwarf (Hurley et al. 2000). Together the carbon/oxygen core contracts, the external layers become much more and more unstable, eventually widening off the the star fully and producing what is well-known as a planetary nebula. The continuing to be carbon/oxygen core contracts to the point at which electron degeneracy push counteracts the inward traction of gravity. This degeneracy pressure is not any kind of kind that gas pressure and is live independence of temperature, and only occurs at an extremely highdensity (>106g/cm3). That is in ~ this point that the core of the currently dead star has come to be a white dwarf, very dense star that has a high temperature however a low luminosity. The white dwarf an ar is at the reduced left that the HR diagram,below the key sequence (Chaisson and also McMillan, Hurley et al. 2000).Helium White Dwarfs - the Fate of short Mass Stars
A star the mass much less than around 0.3 solar masses never reaches high enough central temperatures to fuse helium into carbon and oxygen. However, after ~ a longtime, the star"s external layers will certainly dissipate into a planetary nebula due to the fact that of the short gravitational pull, revealing a greatly helium core that is recognized as a helium white dwarf (Chaisson and McMillan).Supernovae - the Fate the High mass Stars
at the various other extreme, a star that mass better than around 8 solar masses will have a main point too substantial to be supported by electron degeneracy push after the carbon/oxygen core contracts. The star is enormous enough that it go through number of more blend processes (carbon and also oxygen can fuse to neon, etc.). Each new blend process givesless energy and lasts for much less time, till the main point has come to be primarilyiron. As the star goes v these different combination processes, it moves on the HR diagram somewhat horizontally far from and also to the right of the key sequence. As soon as the main point becomes iron, no fusion process can counteract the gravitational pull of the main point on itself. Combination of iron is an endothermic process, soaking up energy instead of release it, do it difficult for iron to it is in fused together an energy source for the star. Thus, the iron core collapses because of its very own gravity, until the central densities with 1014 g/cm3, close to the densities of atomic nuclei. At this densities, a neutron degeneracy press (stronger than the electron degeneracy pressure) avoids further collapse. The inner main point is now a proto spirit star, and the neutron degeneracy push rebounds the external material. This reboundsends a shockwave up v the remainder of the star, exploding the outer layers of the core as well as the outer layers the the star chin in a violent occasion known together a type II supernova (Hurley et al. 2000).
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What happens to the continuing to be core after this explosion relies on the continuing to be core"s mass. If the core"s fixed is much less than about 3 solar masses, the core"s spirit degeneracy pressure continues to against the inward pull of gravity, stabilizing itinto a neutron star. Ghost degeneracy press occurs in ~ much greater densities 보다 electron degeneracy pressure, making neutron stars also denser than white dwarfs. Spirit stars are likewise dimmer than white dwarfs, placing them also lower on the HR diagram, yet still in the same general region below the key sequence. Numerous neutron stars have strong remnant magnetic fields. Several of these neutron stars collection infalling issue at and also radiate the end of their magnetic poles, which we observe together pulsars. If the core"s continuing to be mass is better than around 3 solar masses, not even neutron degeneracy push will be able to withstand the inward pull of gravity, and the core will collapse into a black color hole indigenous which no electromagnetic radiation have the right to escape (Chaisson andMcMillan).naked Helium Stars
Stars that mass greater than around 15 solar masses go through one additional phase the evolution. This stars shed a far-reaching amount of their initial massive to solid stellar winds. These stars may even lose their whole outer envelope during helium combination orsometimes throughout the rapid subgiant phase. If this occurs, the stars" helium burning cores will certainly be revealed before they have end up being white dwarfs or neutron stars, making stars recognized as naked helium stars. They show up to the left the the main sequence atthe really highest temperature (>50,000 K).