Science 7

Star

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky. The closest star to Earth is a triple-star system called alpha Centauri. The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are about 4.35 light-years from Earth, Where one light year equals to 9.461 trillion kilometres. Stellar evolution is the process by which a star undergoes a sequence of radical changes during a star's lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. Scientists classify stars by temperature and the elements they absorb, which are called their spectra. They have divided stars into seven main types. There are seven main types of stars: O, B, A, F, G, K and M. The O stars are the bright, hot, blue stars and the M stars are the dimmer, cooler. Supergiant stars are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K .The hottest known star, WR 102, is a star which has a surface temperature more than 35 times hotter than the Sun. The star Betelgeuse in the constellation of Orion is a red supergiant. Red supergiant stars evolve from large main sequence stars. These are stars which contain more than 8 times the mass of our Sun. Some stars are born with more than 200 times the mass of the Sun. Supergiants develop when massive main-sequence stars run out of hydrogen in their cores, at which point they start to expand, just like lower-mass stars. Unlike lower-mass stars, however, they begin to fuse helium in the core smoothly and not long after exhausting their hydrogen. Stars roughly eight to 40 times more massive than the sun, for example, go through a "red supergiant" phase. Their cores get hot enough to burn carbon, which our sun never will, and they eventually die in powerful supernova explosions. Some of the brightest and best-known stars in the night sky are red supergiant stars. A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Neutron stars have a radius on the order of 10 kilometres (6 mi) and a mass of about 1.4 solar masses. Neutron stars have intense gravitational and magnetic fields. The gravity of a neutron star is about a thousand billion times stronger than that of the Earth. Thus the surface of a neutron star is exceedingly smooth; gravity does not permit anything tall to exist. The temperature inside a newly formed neutron star is from around 10¹¹ to 10¹² kelvins. However, the huge number of neutrinos it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around 10⁶ kelvins. A neutron star is the compressed core of a massive star the super dense cinders left over after a supernova. It has the mass of the sun, but squeezed into a space the width of a city. In a newly born neutron star, neutrinos are temporarily trapped in the opaque stellar core, but they diffuse out in a matter of seconds, leaving most of their energy to heat the matter in the core to more than 500 billion kelvin. Deeper, the protons inside nuclei start turning into neutrons, which cluster so close together that they start to overlap. A star like our sun lives for about 10 billion years, while a star which weighs 20 times as much lives only 10 million years, about a thousandth as long. Stars begin their lives as dense clouds of gas and dust. A teaspoon of neutron star material would weigh 4 billion tons. In about 5 billion years, the sun will run out of hydrogen. Our star is currently in the most stable phase of its life cycle and has been since the formation of our solar system, about 4.5 billion years ago. Once all the hydrogen gets used up, the sun will grow out of this stable phase. Our star will grow to be larger than we can imagine so large that it will envelope the inner planets, including Earth. That's when the sun will become a red giant, which it will remain for about a billion years. Then, the hydrogen in that outer core will deplete, leaving an abundance of helium. Once all the helium disappears, the forces of gravity will take over, and the sun will shrink into a white dwarf. While the full death of the Sun is still trillions of years away, some scientists believe the current phase of the Sun's life cycle will end as soon as 5 billion years from now. At that point, the massive star at the center of our Solar System will have eaten through most of its hydrogen core. The hottest stars have temperatures of over 40,000 K, and the coolest stars have temperatures of about 2000 K. Our Sun's surface temperature is about 6000 K; its peak wavelength color is a slightly greenish-yellow. But in some cases the star burns through the last of its fuel, the object may collapse, or fall into itself, But when a larger star collapses, it continues to compress and creates a stellar black hole. A black hole is a region of spacetime where gravity is so strong that nothing no particles or even electromagnetic radiation such as light can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. At the center of a black hole, it is often postulated there is something called a gravitational singularity, or singularity. This is where gravity and density are infinite and space-time extends into infinity. Near a black hole, the slowing of time is extreme. From the viewpoint of an observer outside the black hole, time stops. Inside the black hole, the flow of time itself draws falling objects into the center of the black hole. No force in the universe can stop this fall. The life span of a black hole is shockingly a long time for a black hole to shed all of its mass as energy via Hawking radiation. Stephen Hawking an astrophysicist eventually squared two ideas in 1974, showing that black holes could have entropy and emit radiation over very long timescales if their quantum effects were taken into account. This phenomenon was dubbed “Hawking radiation” and remains one of the most fundamental revelations about black holes. Black holes are freezing cold on the inside, but incredibly hot just outside. The internal temperature of a black hole with the mass of our Sun is around one-millionth of a degree above absolute zero. The black hole evaporates. The most massive black holes in the Universe, the supermassive black holes with millions of times the mass of the Sun will have a temperature of 1.4 x 10^-14 Kelvin. That's low. Almost absolute zero. It would take 10 to the 100th power of years which is 1 followed by 100 zeros, or a googol years, for a supermassive black hole to fully disappear. The biggest black hole in the universe is the Ton 618, which is 66 billion times bigger than the sun, Ton 618 has a Schwarzschild radius (the radius of a black hole based on its mass) of 1,949 × 10¹⁴ meters. That's a black hole with a diameter of 389.8 billion kilometers, or 242.2 billion miles, which is 77 times the distance from the sun to pluto. It is believed to be the active galactic nucleus at the center of a galaxy, the engine of which is a supermassive black hole feeding on intensely hot gas and matter in an accretion disc. The light originating from the quasar is estimated to be 10.8 billion years old. Most stars take millions of years to die. After puffing off its outer layers, the star collapses to form a very dense white dwarf. One teaspoon of material from a white dwarf would weigh up to 100 tonnes. Over billions of years, the white dwarf cools and becomes invisible. In some case stars can turn into a planet, but this transformation only happens for a very particular type of star known as a brown dwarf. Some scientists do not consider brown dwarfs to be true stars because they do not have enough mass to ignite the nuclear fusion of ordinary hydrogen. Multiple-star systems are called triple, ternary, or trinary if they contain 3 stars; quadruple or quaternary if they contain 4 stars; quintuple or quintenary with 5 stars; sextuple or sextenary with 6 stars; septuple or septenary with 7 stars. It's estimated up to 85 percent of all stars could be in binary pairs, or even triple or quadruple systems; and over 50 percent of all Sun-like stars are in binary pairs. Our Sun is a solitary star, all on its ownsome, which makes it something of an oddball. There are approximately 200 billion trillion stars in the universe. Or, to put it another way, 200 sextillion. That's 200,000,000,000,000,000,000,000. However, if we once again assume that our own Milky Way Galaxy represents an average type of galaxy, we can calculate that there are roughly 150 billion stars born per year in the entire Universe. This corresponds to about 400 million stars born per day or 4800 stars per second. We estimate at about 100 billion the number of galaxies in the observable Universe, therefore there are about 100 billion stars being born and dying each year, which corresponds to about 275 million per day, in the whole observable Universe. A star twinkle as light from a star races through our atmosphere, it bounces and bumps through the different layers, bending the light before you see it. Since the hot and cold layers of air keep moving, the bending of the light changes too, which causes the star's appearance to wobble or twinkle. Jupiter is called a failed star because it is made of the same elements like hydrogen and helium like the Sun, but it is not massive enough to have the internal pressure and temperature necessary to cause hydrogen to fuse to helium, the energy source that powers the sun and most other stars.

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Data source - Google and Nasa