Physical Science 8th grade (1).pdf

The old age of sunlike stars the formation of a red

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The old age of sunlike stars The formation of a red giant Eventually, the core of a star runs out of hydrogen. Gravity then causes the core to contract, raising the temperature. At higher temperatures, other nuclear fusion reactions occur that combine helium to make carbon and oxygen. The hotter core radiates more energy, pushing the outer layers of the star away. The star expands into a red giant as the outer layers cool. In its red giant phase, our sun will expand to beyond the orbit of Mars, and the inner planets, including Earth, will be incinerated. Fortunately, this event is still 4 billion years in the far future. White dwarf stars Sunlike stars don’t have enough mass (gravity) to squeeze their cores any hotter than what is needed to fuse helium into carbon and oxygen. Once the helium is used up, the nuclear reactions essentially stop. With no more energy flowing outward, nothing prevents gravity from crushing the matter in the core together as close as possible. At this stage, the core glows brightly and is called a white dwarf . It is about the size of Earth, yet has the same mass as the sun. Because of its high density, a spoonful of matter from a white dwarf would weigh about the same as an elephant on Earth. Planetary nebulae During the white-dwarf stage, the outer layers of the star expand and drift away from the core. In the most extravagant stars this creates a planetary nebula (Figure 16.15) . The planetary nebula contains mostly hydrogen and helium, but also some heavier elements that were formed in the core. Over time, the matter in a planetary nebula expands out into the rest of the universe and becomes available for forming new stars. Planetary nebulae are one of nature’s ways of recycling the matter in old stars and distributing new elements.
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347 16.2 T HE L IFE C YCLES OF S TARS C HAPTER 16: T HE S UN AND S TARS Supernovae and synthesis of the elements The origin of the elements Scientists believe the early universe was mostly hydrogen, with a small amount of helium and a trace of lithium. Heavier elements such as carbon and oxygen did not exist. So, where did they come from? All the heavier elements are created by nuclear fusion inside the cores of stars — including the elements in your body. Every carbon atom in your body, which is 53 percent of the solid matter of your body, was once inside a star. The creation of elements Stars of more than 12 times the mass of the sun have a violent end. As the core runs out of helium, gravity compresses and heats the core hot enough for other types of nuclear fusion to start. The new fusion reactions combine carbon and oxygen into neon, sodium, magnesium, sulfur, silicon, and even heavier elements up to iron. Nuclear fusion reactions are exothermic , releasing energy only up to iron (Fe, atomic number 26). After that, the reactions become endothermic , using energy rather than releasing it. When the core of the star contains mostly iron, nuclear fusion stops.
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