This is the exact opposite of what has happened in each nuclear reaction so far: instead of providing energy to balance the inward pull of gravity, any nuclear reactions involving iron would remove some energy from the core of the star.
When nuclear reactions stop, the core of a massive star is supported by degenerate electrons, just as a white dwarf is. For stars that begin their evolution with masses of at least 10 M Sun , this core is likely made mainly of iron. For stars with initial masses in the range 8 to 10 M Sun , the core is likely made of oxygen, neon, and magnesium, because the star never gets hot enough to form elements as heavy as iron.
The exact composition of the cores of stars in this mass range is very difficult to determine because of the complex physical characteristics in the cores, particularly at the very high densities and temperatures involved. We will focus on the more massive iron cores in our discussion.
While no energy is being generated within the white dwarf core of the star, fusion still occurs in the shells that surround the core. As the shells finish their fusion reactions and stop producing energy, the ashes of the last reaction fall onto the white dwarf core, increasing its mass.
The core can contract because even a degenerate gas is still mostly empty space. Electrons and atomic nuclei are, after all, extremely small. The electrons and nuclei in a stellar core may be crowded compared to the air in your room, but there is still lots of space between them. The electrons at first resist being crowded closer together, and so the core shrinks only a small amount.
Ultimately, however, the iron core reaches a mass so large that even degenerate electrons can no longer support it. This transformation is not something that is familiar from everyday life, but becomes very important as such a massive star core collapses.
The core begins to shrink rapidly. More and more electrons are now pushed into the atomic nuclei, which ultimately become so saturated with neutrons that they cannot hold onto them. At this point, the neutrons are squeezed out of the nuclei and can exert a new force. As is true for electrons, it turns out that the neutrons strongly resist being in the same place and moving in the same way.
The force that can be exerted by such degenerate neutrons is much greater than that produced by degenerate electrons, so unless the core is too massive, they can ultimately stop the collapse. This means the collapsing core can reach a stable state as a crushed ball made mainly of neutrons, which astronomers call a neutron star. So if the mass of the core were greater than this, then even neutron degeneracy would not be able to stop the core from collapsing further.
The dying star must end up as something even more extremely compressed, which until recently was believed to be only one possible type of object—the state of ultimate compaction known as a black hole which is the subject of our next chapter.
This is because no force was believed to exist that could stop a collapse beyond the neutron star stage. The collapse that takes place when electrons are absorbed into the nuclei is very rapid. In less than a second, a core with a mass of about 1 M Sun , which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers.
The speed with which material falls inward reaches one-fourth the speed of light. The collapse halts only when the density of the core exceeds the density of an atomic nucleus which is the densest form of matter we know. A typical neutron star is so compressed that to duplicate its density, we would have to squeeze all the people in the world into a single sugar cube! The neutron degenerate core strongly resists further compression, abruptly halting the collapse.
The shock of the sudden jolt initiates a shock wave that starts to propagate outward. However, this shock alone is not enough to create a star explosion. The energy produced by the outflowing matter is quickly absorbed by atomic nuclei in the dense, overlying layers of gas, where it breaks up the nuclei into individual neutrons and protons.
These ghostly subatomic particles, introduced in The Sun: A Nuclear Powerhouse , carry away some of the nuclear energy. It is their presence that launches the final disastrous explosion of the star. Yet, this seemingly large sample of solar neighbors does not contain a single massive star. So rare are these heavyweights that the nearest one lies well over light-years away.
Massive stars easily rank as the most luminous in the cosmos — the brightest outshine the Sun by a million times. And these are the only stars bright enough for us to see in distant galaxies. So, understanding massive stars better will improve our knowledge of extragalactic astronomy.
How do massive stars form? The problem: Massive objects should start nuclear reactions long before they reach final form. The reactions emit intense radiation that should stop more matter from falling in. A slight alteration may save this idea. Scientists now suggest the largest fragments become midsize protostars, each surrounded by an accretion disk.
The growing stars gain weight by feeding off their disks. Observations have found at least one massive star, located in the Omega Nebula M17 , forming as this model predicts. Another group has performed simulations that show instabilities in the formation process create channels to funnel out the radiation while allowing the gas to accrete.
Other observations imply massive stars are born when smaller objects collide — the second theory of high-mass star formation. Most big stars live in clusters, and the more stars a cluster contains, the more massive the largest stars are. X-ray observations reveal lots of low-mass stars near the massive objects. Could they be a food reservoir? Computer simulations show collisions can be an effective method of creating massive stars.
Nevertheless, not all massive stars belong to clusters. Were these solitary gems born in a different way, or were they violently ejected from their birthplaces?
Still, determining how much massive stars weigh proves difficult. Statistical surveys of big clusters have shown that no star with a mass larger than to solar masses exist. The only reliable method involves studying binary-star orbits. To date, two binary pairs appear most promising. However, more massive stars may be out there. For example, the star HD may contain solar masses — plus or minus 46 solar masses; the star WR 25 may contain 75 solar masses.
This computer simulation shows a central density concentration that will grow into one of these heavyweights. What role did massive stars play after the Big Bang? These missing links between the Big Bang and today will help scientists understand the current face of the universe. How do massive binary stars form? Astronomers have developed several scenarios. In a cluster, stars continually move around and occasionally graze each other.
When stars meet, a couple can form, although astronomers debate how frequently it occurs. Crank the rotational speed high enough, and it can break in two. Unfortunately, only close binaries can form by fission, and some massive binaries have wide separations. Can planets form around massive stars? What happens between the red giant phase and the supernova explosion is described below. From Red Giant to Supernova: The Evolutionary Path of High Mass Stars Once stars that are 5 times or more massive than our Sun reach the red giant phase, their core temperature increases as carbon atoms are formed from the fusion of helium atoms.
Gravity continues to pull carbon atoms together as the temperature increases and additional fusion processes proceed, forming oxygen, nitrogen, and eventually iron. The two supernovae, one reddish yellow and one blue, form a close pair just below the image center to the right of the galaxy nucleus Image Credit: C. Hergenrother, Whipple Observatory, P. Garnavich, P. Berlind, R. Kirshner CFA. When the core contains essentially just iron, fusion in the core ceases.
This is because iron is the most compact and stable of all the elements. It takes more energy to break up the iron nucleus than that of any other element. Creating heavier elements through fusing of iron thus requires an input of energy rather than the release of energy. Since energy is no longer being radiated from the core, in less than a second, the star begins the final phase of gravitational collapse.
The core temperature rises to over billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in a shock wave , which we see as a supernova explosion.
Compared with the main-sequence lifetimes of stars, the events that characterize the last stages of stellar evolution pass very quickly especially for massive stars. But the energy yield of these reactions is much less than that of the fusion of hydrogen to helium. And to trigger these reactions, the central temperature must be higher than that required for the fusion of hydrogen to helium, leading to even more rapid consumption of fuel.
Clearly this is a losing game, and very quickly the star reaches its end. As it does so, however, some remarkable things can happen, as we will see in The Death of Stars. In stars with masses higher than about 8 solar masses, nuclear reactions involving carbon, oxygen, and still heavier elements can build up nuclei as heavy as iron. The creation of new chemical elements is called nucleosynthesis. The late stages of evolution occur very quickly.
Ultimately, all stars must use up all of their available energy supplies. In the process of dying, most stars eject some matter, enriched in heavy elements, into interstellar space where it can be used to form new stars. Each succeeding generation of stars therefore contains a larger proportion of elements heavier than hydrogen and helium.
This progressive enrichment explains why the stars in open clusters which formed more recently contain more heavy elements than do those in ancient globular clusters, and it tells us where most of the atoms on Earth and in our bodies come from. Skip to main content. Stars from Adolescence to Old Age. Search for:. The Evolution of More Massive Stars Learning Objectives By the end of this section, you will be able to: Explain how and why massive stars evolve much more rapidly than lower-mass stars like our Sun Discuss the origin of the elements heavier than carbon within stars.
Key Concepts and Summary In stars with masses higher than about 8 solar masses, nuclear reactions involving carbon, oxygen, and still heavier elements can build up nuclei as heavy as iron. Licenses and Attributions. CC licensed content, Shared previously.
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