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STARS

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Formality and Evolution …

Formality and Evolution
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  • 1. • a massive, luminous ball of plasma. • The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. •Other stars are visible in the night sky, when they are not outshone by the Sun. • For most of its life, a star shines because thermonuclear fusion in its core releases energy that traverses the star's interior and then radiates into outer space. • Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.
  • 2. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
  • 3. An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun
  • 4. For the Sun, this is estimated to be about 1010 years. Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer. However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no such stars are expected to exist yet.
  • 5. Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a starforming region is the Orion Nebula.[As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region. When stars form they are composed of about 70% hydrogen and 28% helium, as measured by mass, with a small fraction of heavier elements
  • 6. Age Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula. As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region.   The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years
  • 7. Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds
  • 8. Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun —about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun
  • 9. In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. The most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of coorbiting binary stars. Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe. While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered. Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe.
  • 10. The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (1012) kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, including in the vicinity of the solar system. Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
  • 11. GIANTS AND DWARFS Medium-sized stars like our Sun become white dwarfs. White dwarfs can explode. The outside gas layers blow off and make clouds called nebulas. The core keeps shrinking. A spoonful of white dwarf core could weigh more than a dump truck. After several billion years, the star loses all its energy and becomes a cold black dwarf. Really big stars become supergiants. Supergiants become supernovas, which are big exploding stars. The explosion sends gas and dust into space to make new stars. The core gets packed tighter and tighter. Some cores then turn to iron and become neutron stars. Some supernova cores turn into black holes, which swallow everything around them in space. Not even light can escape from a black hole.
  • 12. COULD ASTRONAUTS VISIT A STAR? Stars beyond our solar system are too far away for a spacecraft to reach. The closest star is Proxima Centauri. It is more than 4 light-years from Earth. A light-year is the distance light travels in a year, about 6 trillion miles (about 9 trillion kilometers) Most stars are much farther away than Proxima Centauri. No spaceship can travel fast enough to reach even the nearest star during an astronaut’s lifetime. It takes billions of years for even light to reach the most distant stars.