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13.22 Stellar Mass Loss

While all stars are undergoing regular mass loss, the amount of mass loss is related to the size of star and its stage of evolution. Our Sun, for instance, is losing roughly 7 billion tons per hour. While this may seem like a lot, the Sun has only lost about 0.01% of its total mass since its formation. More massive stars, however, experience a more rapid and violent evolution. Stars of more than a few solar masses are very hot and evolve rapidly. When their atmospheres expand, these stars leave the main sequence with more luminosity than that of most giants. They are called super giants because at a given temperature they must have a larger surface area to emit a larger amount of energy. Super giants are the largest and most luminous stars on the H-R diagram. The collapsing core generates energy that drives the outer atmosphere of the star into space, resulting in mass loss. Because massive stars evolve quickly, mass is lost only a few million or tens of millions of years after the star’s formation.


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Hertzsprung-Russell diagram showing color and size of stars. Click here for original source URL

The most massive stars return a lot of material to the space between stars. A hot supergiant emits many ultraviolet photons that can drive gas away from the star. For example, a blue supergiant might lose one solar mass of gas about every 100,000 years. The most massive stars can therefore lose a third to a half of their total material into interstellar space. Red super giants have cooler atmospheres in which the gas is not ionized and molecules can form. Tiny solid particles called dust grains can also form, taking the visible light from the star and re-radiating it as infrared emission. Massive stars can seed space with dust as well as gas.

More modest stars can also lose mass late in their lives. The gas being blown out of mass-shedding stars expands into space — it is often called a stellar wind. It may cool enough for grains of dust, such as carbon grains, to condense in it. It may collide with other nearby gas clouds at high speed, creating glowing shock waves. Often, however, it is shed in nearly spherical bubbles of gas that surround the central star. Ultraviolet light from the star excites and ionizes the gas atoms, causing them to glow. Clouds of gas in space are called nebulae, and decades ago these particular nebulae came to be called planetary nebulae because the pale glowing bubbles looked like disks of planets in small telescopes. This term is a misnomer, however, because they have nothing to do with planets. They mark the end of a low mass star’s life and are among the most beautiful celestial features, with wispy symmetry and delicate colors.

The glowing gas that surrounds evolved stars is not just a pretty light show. When stars eject gas into space they are participating in an ongoing cycle of life and death. Massive stars create elements up to and heavier than iron when they explode at the end of their lives. The gas ejected at high speeds by the explosion is returned to interstellar space to be incorporated into new stars. Lower mass stars return material to space much more quietly and slowly but, because they are much more numerous than massive stars, they contribute more total material than the massive stars. The glowing gas of a planetary nebula shows emission lines due to nitrogen, oxygen, carbon, neon, sulfur, chlorine, and iron. Chemically enriched interstellar material then becomes the raw material for a new generation of stars. This is what Geoffrey and Margaret Burbidge, Fred Hoyle, and William Fowler meant when they wrote: "The history of matter is hidden in the abundance distribution of the elements." Notice that this includes the carbon, nitrogen, and oxygen atoms that are essential for life. Each of the heavy atoms in your body was once a part of another star. Without the creation and cosmic recycling of elements in stars, we would not exist!

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Eta Carinae is about 100 times more massive than the Sun. Click here for original source URL.

", nitrogen, and oxygen atoms that are essential for life. Each of the heavy atoms in your body was once a part of another star. Without the creation and cosmic recycling of elements in stars," we would not exist!


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NGC 7293, The Helix Nebula. Click here for original source URL.