- 3787 - PLANETARY NEBULAE - what our Sun will become. If all the stars were about the same size of our Sun I could not be telling you this. Life would not be possible. Our Sun will live 10 billion years and become a Planetary Nebula. (That would be the year 5,000,002,008 A.D.)
-------- 3787 - PLANETARY NEBULAE - what our Sun will become?
- Planetary Nebulae are the most beautiful expanding stars in the heavens, but, it takes a larger star and an exploding supernova to create the elements carbon, iron, and other heavy elements. Our Sun is considered a low mass star and it will not become a supernova. To see what our Sun will look like in 5 billion years see the Hubble Space Telescope images:
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----------- Ring Nebula
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----------- Eskimo Nebula
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----------- Spirograph Nebula
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----------- Hourglass Nebula
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- Our Sun is about half way in its lifetime to becoming one of these Planetary Nebulae. This event happens when stars burn all their hydrogen and they start to expand and end their lives as White Dwarfs, Neutron Stars, or Black Holes. Which of these depends on what is left of the core star after it has expanded or exploded. The bigger the star the shorter its lifetime.
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-------------- ½ Solar Mass -------------- 50 billion years
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-------------- 1 Solar Mass --------------- 10 billion years
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-------------- 10 Solar Mass -------------- 20 million years
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-------------- 30 ½ Solar Mass ------------ 3 million years
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- At the end of its lifetime, the nuclear fuel is all burned up, and, how the star ends up depends upon how much mass explodes or expands away and how much is left in the core.
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------------------------------------- Solar Mass = Ms
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------------- Beginning star ------------ Core -------------------- Remaining
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------------- ½ to 6 Ms ------------- 0.2 to 1.4 Ms --------------- White Dwarf
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------------- 6 to 25 Ms ------------ 1.4 to 3 Ms ---------------- Neutron Star
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------------- 26 to 150 Ms -------- 3 to 10^10 Ms -------------- Black Holes
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- Once a Solar Mass star has burned all of its hydrogen, converting it into helium, the core gets smaller because the helium is heavier and burns at a higher temperature. With rising core temperatures the remaining hydrogen in the shells surrounding the helium core begin to burn.
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- We’re talking nuclear burning, hydrogen fusing into helium, which is now in the outer shells of the star causing the outer shells to go through a huge expansion. The star increases in brightness up to 5,000 times, at the same time it increases its radius, up to 50 times, lowering the stars effective temperature.
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- The result , the star evolves into a Red Giant. Red is a lower temperature than white or yellow. This Red Giant phase only lasts 1 billion years for Solar Mass stars. It lasts only 1 million years for 10 Solar Mass stars.
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- As the Red Giant phase progresses the helium core contracts and gets even hotter. When the outer envelop of the star expands to the point that it is completely blown away from the surface of the dying star it enters the Planetary Nebula stage. This stage lasts only 10,000 years for a Solar Mass star.
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- The ultraviolet light from the core lights up the expanding shell. That is the beautiful images you see in the Hobble telescope pictures. The core that is left is called a White Dwarf. Nuclear fusion has ended in the White Dwarf and the star slowly cools as it radiates away its heat.
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- The density in a White Dwarf is 1,000 kilograms/ centimeter^3 at a temperature of 10,000,000 Kelvin. The greater the mass of the White Dwarf the smaller its radius and the greater its density.
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- However, there is an upper limit to how massive a White Dwarf can be. If it exceeds 1.4 Solar Mass it will collapse into a Neutron Star. It is the degenerative pressure of the electrons that are overcome by gravity that collapses the star into neutrons.
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- If the Neutron Star then exceeds 3.0 Solar Mass it will collapse into a Black Hole. So, 3.0 Solar Mass is the upper limit for a star core that no longer has nuclear thermo pressure to resist gravity. Astronomers have not yet found a White Dwarf less than 0.25 Solar Mass because the star to produce it would have a lifetime older than the Universe, older than 13.7 billion years.
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- It takes a core temperature of 10,000,000 Kelvin to have hydrogen nuclear fusion. For the pressure of gravity to create that temperature you need a mass of at least 70% Solar Mass, or about 70 times the mass of Jupiter. When a mass gets down to 50% Solar Mass and 50 times the mass of Jupiter the interior pressure due to temperature and the pressure due to gravity are in balance. In this case the star is a Brown Dwarf.
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- Brown Dwarf are the only size star that wins over gravity. They are called Brown but they are actually deep red, or magenta, in color. The first Brown Dwarf was not discovered until 1995. Today, with infrared telescopes 1,000’s of Brown Dwarfs have been discovered in our galaxy. Their very low surface temperatures range from 2,200 to 1,400 Kelvin. If you get below 1,400 Kelvin surface temperature it should be called a planet, not a Brown Dwarf. Planet Earth is about 300 Kelvin.
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- Our galaxy is littered with the corpses of dead stars. The balance that stars must maintain to stay alive is thermal nuclear pressure versus gravity pressure. Pressure is force per unit area. Therefore Force = pressure * area. The calculations in the example below are for our star, the Sun.
----------------- The force of gravity = G*m*M /r^2.
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- The gravity at the center of the star is the force due to each half of the mass separated by the distance “r”. The force at the center = G*M^2/4*r^2
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----------------- where: M = 2*10^30 kg.
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----------------- where : r = 7*10^8 m
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----------------- where: G = 6.67*10^-11 m^3/(kg*sec^2)
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----------------- The force of gravity = 1.36 *10^32 kg*m/sec^2
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- The force of gravity between masses is directly proportional to the masses (M) and indirectly proportional to the square of the distance between them (r^2). The “r^2” makes sense because the force of gravity radiates out as an expanding sphere. The surface area of a sphere is 4*pi*r^2. A better formula for the force of gravity would be:
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----------------------- Force = G’m*M / 4*pi^r^2.
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----------------------- Where G’ now becomes 5.3*10^-1 m^3/ (Kg * sec^2).
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---------------- The force due to thermal nuclear pressure = n*k*T * the surface area.
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---------------- where: n = the particle density = M/m / (4/3*pi*r^3) = 10^29 / m^3
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----------------- where m = mass of a proton = 1.67*10^-27
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--------------- where: k = Boltzman’s Constant = 1.38 * 10^-23 kg*m^2/sec^2*K
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--------------- where T = 15*10^6 Kelvin.
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---------------- The force due to thermal nuclear pressure = .3185* 10^32 kg*m/sec^2
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- Force = mass * acceleration, so the units are correct, k*m/sec^2. The two forces, thermal nuclear and gravity are very close to being equal, and balanced, with the core temperature of the Sun at 15,000,000 degrees Kelvin.
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- For every star that is not expanding or exploding, that is stable, these two forces are equal opposite and in balance. If we wanted to learn the temperature of the core of the Sun we could have set these two equations equal to each other and solved for “T”.
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December 15, 2022 PLANETARY NEBULAE - our Sun.? 878 3787
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--------------------- --- Thursday, December 15, 2022 ---------------------------
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