When Chandrasekhar took these relativistic effects into account, something spectacular happened. He found a firm upper limit for the mass of any body which could be supported by electron degeneracy pressure. Once this limit—the Chandraskehar limit—was exceeded, the object could no longer resist the force of gravity, and it would begin to collapse. When Chandrasekhar published these results in , he set off a battle with one of the greatest astrophysicists of the era, Sir Arthur Eddington, who believed that the white dwarf state was the eventual fate of every star.
Various accidents may intervene to save a star, but I want more protection than that. I think there should be a law of Nature to prevent a star from behaving in this absurd way! Chandrasekhar and Eddington remained friends, went to the Wimbledon tennis tournament together and went for bicycle rides in the English countryside. I do not believe, for example, that he ever thought harshly of anyone. That was why it was so easy to disagree with him on scientific matters.
You can always be certain he would never misjudge you or think ill of you on that account. Vindication would eventually come to Chandrasekhar when he was awarded the Nobel Prize in for his work. The Chandrasekhar Limit is now accepted to be approximately 1. In so doing, the star itself dies but furthers the growth process of the universe—it both generates and distributes the elements on which life depends.
The life of a star is characterized by thermonuclear fusion; hydrogen fuses to helium, helium to carbon, and so on, creating heavier and heavier elements. The supermassive black hole at the center of the Milky Way galaxy , for example, is about 7. Such a huge number is a bit harder to imagine than if you were to say the black hole is as massive as 4 million suns. Thanks to Sir Isaac Newton , calculating the sun's mass isn't too hard, either. The sun's mass determines how strong its gravity is.
And its gravity determines the orbital distance and speed of a planet like Earth. For example, if the sun were more massive with a stronger gravitational pull, and if Earth were at the same distance from the sun, our planet would have to orbit faster or it would fall into the sun. If the sun were less massive with a weaker gravitational pull, Earth would have to orbit slower or it would be flung out of the solar system.
Newton's equations will calculate the sun's mass as long as we know the speed of Earth's orbit and the distance to the sun. Astronomers use basic geometry to calculate those two constants. In the late s, Newton computed the relative masses of the sun and other planets.
The bipolar nature of many planetary nebulae may be due to the parent star being in a binary system. Strong magnetic fields of remnant cores may also influence the shape of the nebulae. Colour of the nebulae reveals information about their composition. The characteristic blue-green colour is from the doubly-ionised oxygen emissions, OIII. Oxygen, carbon and some s -process elements ejected by AGBs and found in planetary nebulae may eventually seed the ISM for the next generation of star formation.
Some of the carbon and oxygen in our bodies may have come from such nebulae, the rest probably came from supernovae explosions. Planetary nebulae do not exist for long. An expanding shell of dust and gas may only be visible for a few 20, years or so before dispersing into the ISM. There are, however, over 1, known in our galaxy and others are visible in nearby galaxies. They are useful as one method for determining distances to these galaxies. Although a planetary nebula is only short-lived, the exposed core remains.
We shall now see what happens to it. The exposed, remnant core that ionised the planetary nebula material is basically an extremely hot, dense sphere of carbon and oxygen. Any hydrogen not ejected quickly fuses via shell-burning.
The stellar remnant becomes a white dwarf or wd with a surface temperature of about 10 4 K. White dwarfs have unusual properties. Firstly, they are very small but the more massive white dwarfs are actually smaller than less massive ones. With their fuel used up no fusion takes place so there is no outward radiation pressure to withstand gravitational collapse.
More massive stellar cores experience stronger gravitational force so actually compress more. A white dwarf is composed of carbon and oxygen ions mixed in with a sea of degenerate electrons. It is the degeneracy pressure provided by the electrons that prevents further collapse. A white dwarf, with a mass roughly that of the Sun packed into a volume not much greater than the Earth must have an extremely high density. At 10 9 kg m -3 its density is one million times greater than that of water.
Although its surface temperature is about 10, K, the core temperature may be as high as 10 7 K. The heat trapped within a white dwarf will gradually be radiated away by it but with its small radius, a white dwarf has only a small surface area.
Heat therefore cannot escape quickly. In fact it will take tens to hundreds of billions of years for a white dwarf to radiate away its heat and cool down to a black, inert clump of carbon and degenerate electrons. As the Universe is not yet old enough for this to have happened, all the white dwarfs that have ever formed in single-star systems are still white dwarfs. Interestingly, not only are the more massive dwarfs smaller than less massive ones, they are also less luminous for the reason explained above.
Typical luminosities are less than 10 -3 that of our Sun.
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