Stellar Evolution Sancho Criado del Rey Stellar evolution is the study of how a star changes throughout its “lifetime”. The major changes which a star in evolution goes through are classified as: * Birth of the star, it’s the first phase of a star’s life. A “new-born” star can be a * Average star * Massive star * Maturity of the star, it’s the second phase of a star’s life. A “mature” star can be a * Red Giant * Red Supergiant * Stellar Remnants, it’s the third phase of a star’s life, just before its death. An “elder” star can be a * Planetary Nebula Supernova A star’s life begins with a Stellar Nebula and then it can either become an Average Star or a Massive Star. If it becomes an Average Star, the next phase will be a Red Giant, followed by a Planetary Nebula and ending on a White Dwarf. If it becomes a Massive Star, the next phase will be a Red Supergiant, followed by a Supernova that can develop in either a Neutron Star or a Black Hole. Now we will begin to go discuss more concretely every kind of star regarding its age. A Stellar Nebula is an interstellar cloud formed by gravitational force that pulled gases together.
Stars usually get formed in their centers. The ultraviolet radiation between the gases makes them visible and luminescent. Stellar nebulas are made of: * Hydrogen * Dust * Helium An Average Star is a luminous sphere of plasma that is held together by gravity. Average stars shine due to thermonuclear fusion between hydrogen and helium in their core. An average star’s characteristics are: * 1. 98 x 1030 Kg of mass aprox. (4. 37 x 1030 pounds) * 1. 412 x 1019 Km3 of volume aprox. (3. 11 x 1029 gallons) * 4-5 billion years of age. A Red Giant is a star on a late phase of stellar evolution.
When the hydrogen in the core of an average star is exhausted, the hydrogen in the outer shell begins to ignite due to thermonuclear fusion, causing the star to increase its size, temperature and luminosity. * They grow 0. 3 – 8 times bigger than an average star (Sometimes as big as a massive star). * They last at least a billion years. A Planetary Nebula is an interstellar cloud of ionized gases. When the hydrogen in the outside of a Red Giant is exhausted, the remaining layers are expelled by stellar winds and pulsations, then, the hot core emits ultraviolet radiation that ionizes the expelled layers, making the nebula luminescent.
Planetary Nebulas are essential on the creation of new heavy elements such as: * Carbon * Nitrogen * Oxygen A White Dwarf is the remaining core of a planetary nebula and the last phase on an average star’s life. It burns carbon and oxygen for a very short period of time. When the fuel is exhausted, it cools down during a very long period of time, until it becomes a black dwarf. For a stellar nebula to go through all the phases until it reaches the phase of “black dwarf” it takes at least 20 billion years; taking under consideration that our universe is only 17. billion years old, there is no existence of any black dwarfs in our universe. The characteristics of a White Dwarf are: * 1. 98 x 1030 Kg of mass aprox. (The mass of the sun) Really dense. * 1. 083 x 1012 Km3 of volume aprox. (The volume of the earth) A Massive Star is a larger version of an average star. Since massive starts are larger than average stars, they have more fuel and they stay burning longer. Their usual characteristics are: * 9. 99 x 1031 Kg of mass aprox. (2. 2 x 1031 pounds) * 1. 129 x 1020 Km3 of volume aprox. (2. 48 x 1031 gallons) * 10-20 billion years of age
A Red Supergiant is the biggest matter in the universe. When the hydrogen in the core of a massive star is exhausted, the hydrogen in the outer shell begins to ignite due to thermonuclear fusion, causing the star to increase its size, temperature and luminosity. Their usual characteristics are: * 3. 168 x 1031 Kg of mass aprox. (6. 98 x 1031 pounds) * 7. 06 x 1021 Km3 of volume aprox. (1. 55 x 1033 gallons) * 5-50 billion years of age A Supernova is an interstellar explosion. It occurs when the core of a Red Supergiant undergoes sudden gravitational collapse.
The usual characteristics of its extremely radioactive wave are: * Travels at a speed of 30,000 Km/s (6. 71 x 107 mph) * It lasts 1 week – 4 months A Neutron Star is the result of a supernova. It’s mainly composed by neutrons, so it’s neutrally charged. They are very hot and they can’t collapse, due to the Pauli exclusion, which states that no two protons can be in the same place with the same quantum state simultaneously. Neutron Stars are extremely radioactives and magnetic. The common characteristics of a neutron star are: * 2. 986 x 1030 Kg of mass aprox. (500,000 times the mass of the earth) * 7,238. 2 Km3 of volume aprox. (The size of Brooklin, NY) * The density of a neutron star can be compared to a sand grain with the mass of a Boeing-747. A Black Hole is a star with a very big gravitational force. Such a big gravitational force is due to its high density. Around a black hole there is a point of no return, which is also radioactive, that is determined by the following formula: Black holes get formed when the core of a supernova collapses into itself. The gravitational force of a black hole can absorb: * Light (Reason why it’s black) * Space * Time (According to Einstein’s Relativity Law) Time and space are directly related, as the formula of speed proofs. * When space is modified, or stretched, time is as well. * A black hole absorbs spaces (which is the distance between point A to point B), but point A in that space is closer to the black hole than point B, meaning that point A is going to get more and more separated from point B, at an increasing speed, that means that space, when it reaches the point of no return of a black hole, will be dilated; this would seem possible if the distance between point A and point B was covered by a rubber band, but it’s actually covered by nothing.
The tricky part comes when we add time. If the distance between point A and point B is 100 Km, when it gets in the point of no return it’s still 100 Km but further apart (it’s hard to visualize without a picture) so if before it took us 1 hour to cover 100 Km, now it’s going to take us 1. 5 hours to cover that same exact distance, so 1 hour it’s actually 1. 5 hours, making the time to go slower.
