con't - Chemistry 101: Where did the elements come from?

Any medium-mass or larger star, from one about 40 percent the size of the Sun to one several times greater, relies on energy produced in its core to offset the enormous force of gravity pulling matter toward its center. Nuclear fusion produces this sustaining energy. When hydrogen nuclei -- a star's primary fuel supply -- fuse with one another, helium nuclei are created. Although helium is heavier than hydrogen, the mass of the single helium nucleus is less than the total mass of the two hydrogen nuclei from which it formed. The difference is released as energy. A star will blaze away as long as there is enough hydrogen to fuel the reactions. Depending on its mass, this can range from a few million years for the most massive stars to several billion years for less massive stars, including the Sun.

A star between 0.5 and 1.4 times the size of our Sun does not explode when it has used up its fuel supply. Instead, it evolves into a smaller, denser, and cooler body called a white dwarf. After one of the stars in a binary system has become a white dwarf and the second star exhausts its fuel, the white dwarf's gravity field draws in the gases escaping from its dying companion. The white dwarf increases in mass until its core can no longer support itself, and it collapses in a huge explosion.

Supernovae are rare -- so rare that only three or four can be expected to occur in our galaxy each century. One was last observed in the Milky Way in 1604. However, thanks to continual improvements in telescope and instrument technology, astronomers can more easily detect and monitor supernovae in other galaxies. And although they cannot predict when a star will explode, observing the event and its aftermath can provide important clues to the workings of the universe. All Type Ia supernovae fade in a characteristic manner: rapidly at first, then more slowly. By analyzing the light they emit, scientists can measure distances to faraway galaxies and the speed at which the universe is expanding. They can also identify the elements from which the stars were made. And as the explosion fades to obscurity in a year or two, a gas remnant continues to mark where it occurred. From recent Hubble research, we know that Type Ia supernovae are not all of the same brightness, as was previously postulated.[5]

Ingredients for Life - Carbon

The structure of the carbon atom allows for chemical bonding with up to four other atoms, which makes possible the vast array of chemical arrangements in organic molecules. All life on Earth depends on organic molecules, the primary components of which are also some of the most abundant elements in the universe: carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus.

Naturally occurring elements are produced in the cores of stars by a process known as nucleosynthesis. Just after the Big Bang, when the universe was very young, the only elements present were hydrogen, helium, and a trace amount of lithium. As stars formed and nuclear fusion ignited within their cores, other elements were created. These elements are all lighter than iron, and include carbon, oxygen, and nitrogen. As low-mass stars neared the ends of their lives, they lost their outer layers into space where the material became the interstellar medium -- the gas and dust between stars. Before the outer layers were expelled, convection enriched them by "dredging up" chemical elements from stellar interiors. It is thought that the majority of the carbon in the universe comes from this phase of stellar evolution. Elements heavier than iron were created in the much more dramatic endings of high-mass stars. The cataclysmic explosions of these supernovae created the intense conditions needed to form the heaviest elements, which were then also dispersed into the interstellar medium.

The interstellar medium is recycled to form new stars and planets. And because the relative abundances of the elements are the same throughout the universe, all planets, moons, asteroids, and comets should have the same basic ingredients available to them. In fact, observations of other stars and galaxies have shown similar chemical abundances: 98% of the mass is hydrogen and helium, and all other elements compose the remaining 2%. That 2% may not seem like much, but it is enough to create all living things on Earth. One of the most common of the remaining elements is carbon -- and organic molecules have even been observed in interstellar clouds and found in comets and meteorites.

While it is still not clear how life on Earth originated from basic organic molecules, the fact is that life exists. If basic organic molecules were able to create life on Earth, and they are available elsewhere in the universe, it is not unreasonable to wonder if life has also developed elsewhere.[7]

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