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---------------------------- 2173 - Supernovae - Stars that explode
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- Stars die and explode. Our Sun will die in another 5,000,000,000 years. It is just middle aged right now. I will not be around for this explosion. Our mother Earth will explode more gently as a planetary nebula. The hot envelope of the nebula will expand all the way out to cover the planet Earth. We’re toast.
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- If our star were 8 times more massive it would explode as a supernova then collapse into a neutron star instead of a planetary nebula. If it were 10 to 20 times more massive the end of the supernova explosion would collapse into a blackhole.
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- (October 13, 2018) Astronomers found a neutron star “2012au” in the galaxy 4790 that was 77 million lightyears from Earth. Astronomers measured the oxygen and sulfur atoms flying away from the star at 5,150,000 miles per hour. These heavier atoms would trail behind the lighter element hydrogen atoms that leave the supernova explosion first forming an inner shell of ejected gas.
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- (November 10, 2018) Supernova “iPTF 14gqr” occurred that was 930 million lightyears away. The star was 8 times the mass of the sun. It burned through its nuclear fuel that was keeping it from collapsing due to the compression of gravity. Its core collapsed into a neutron star. The explosion ejected an outer layer of gas as a bright flare lasting 20 days. Its visible light lasted for 7 days.
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- Supernova explosions are spotted as gamma ray bursts approximately once a day from all directions in the sky. These flashes of electromagnetic radiation are 100,000,000,000,000,000,000 times more energy than our Sun but lasting only few seconds.
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- What likely happens with a gamma ray burst that is not a supernova is that it starts out more massive. The stellar core collapses under its own gravity into a blackhole. Jets of material shootout from the magnetic poles that collide with the expanding layers of gas. The collisions of atoms cause the emission of high energy gamma rays.
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- A gamma ray is a high energy photon of light, higher energy than an X-ray. Gamma rays would penetrate 10 feet of lead without being absorbed.. The wavelength of a gamma ray is the smallest in the electromagnetic spectrum. Its wavelengths start at 0.01 * 10^-9 meters and is even smaller with higher energy. For comparison: The color violet in visible light is 380* 10-9 meters.
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- When the Sun dies it will leave behind a core of carbon and oxygen with a shell of helium. This phase is called a White Dwarf star. It has lost all of its hydrogen that has burned into helium. There is no more energy to sustain it and it will continue to cool and fade away.
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- The fate pf every star depends on its mass.
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- Larger stars balloon into Red Giants, after all the hydrogen is burned their gravity force begins forming atom nuclei into heavier elements The heavier elements become onion layers of carbon, oxygen, silicon, etc until the heaviest element iron is compressed into the core
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- Iron is too heavy to fuse any heavier elements, fusion stops. The star collapses. The electrons are driven into the nuclei of the iron atoms. The electrons combine with the protons to form neutrons and neutrinos.
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- The neutrinos have little mass and escape the core immediately with a tremendous amount of energy. The temperature increases to 100 billion degrees. The outward explosive pressure creates a shockwave traveling at 10% the speed of light. The supernova explodes. The core collapses into neutron star, or into a blackhole, depending upon the amount of mass left behind.
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- If the star starts out with a 25 solar mass, it can burn all of its hydrogen in a short time, about 7 million years. The temperature can reach 40 million degrees. The compression of gravity increases the temperature to 200 million degrees using helium fusion for another 500,000 years. Then, carbon fusion takes over and the temperature is 600 million degrees lasting for only 600 years.
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- Neon fusion at 1200 million degrees lasts for only one year. Oxygen fusion at 1500 million degrees lasts for only 6 months. Silicon fusion at 2700 million degrees lasts for single day. Then, in 2 seconds the core collapses when it reaches 5400 million degrees.
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- The core bounces and expands in 1 millisecond. The star explodes into a supernova reaching a temperature of 23,000 million degrees. The supernova explosion lasts for 10 seconds.
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- Over the supernova’s lifetime it expels 80% of its mass back into space. This shock wave is called a stellar wind that spreads the heavier elements out into the universe. Somehow these heavier elements condense back into planets and into people that are reading this.
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- An example of a supernova that completed this process is the Gum Nebula in the constellation Vela. It exploded as a supernova 11,000 years ago and is now has a diameter spreading out 2,300 light years. It spans 60 arc degrees across the sky. And , it is only 300 lightyears away from Earth.
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- The Crab nebula in the constellation Taurus the Bull, known as Messier, M1, is 6,000 lightyears from Earth. It was even visible from Earth with the naked eye in 1054 A.D. The event was recorded in American Indian and in Chinese history. These must have been our first astronomers.
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- The Crab Nebula is 230 arc seconds across and 5,872 lightyears away.
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--------------- Arc Seconds = 206,265 * diameter / distance
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------------- Diameter = 230 * 1800 / 206,265
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-------------- Diameter = parsecs * 3.262 lightyears / parsec = 6.55 lightyears.
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- The Crab Nebula is expanding at 1,400 kilometers / second. The radius is 3.2736 lightyears.
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------------------- Time = Distance / Velocity
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------------------- Time = 3.2736 LY / 1400 km / sec. * 9.461 * 10^12 km / LY = 2.2 * 10^10 sec / 3.16 sec / year = 700 years.
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-------------------- 700 years
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------------------- This calculation puts the event at 303 A.D. The actual date was 1054 A.D. My oversimplified calculation was only accurate to within 24%. Not bad for an amateur astronomer.
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- These supernova were classified as Type II supernova because hydrogen is present in their spectrum and the core collapse was the cause of their explosion.
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- There is another type of supernova called a Type I where there is no absorption lines or emission lines present in the spectrum for hydrogen. These special supernova are created in binary stars. The heavier of the two orbiting stars will convert hydrogen to helium more rapidly. It runs out of fuel more rapidly and becomes a Red Giant star dumping its outer layers onto its companion star.
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- The Red Giant eventual ejects its outer layers and becomes a White Dwarf star consisting mostly of carbon and oxygen. The second star eventually catches up to the first star and evolves into a Red Giant too. It begins dumping its outer layers into the White Dwarf star.
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- When the White Dwarf star reaches the critical mass of 1.4 Solar Mass its temperature climbs to 4 billion degrees igniting the carbon and detonating the star becoming a Type I supernova. The second Red Giant star gets flung out into space as action must equals reaction as Kepler taught us.
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- The 1.4 Solar Mass limit is called the Chandrasekhar limit. Sugrhmanyan Chandrasekhar was an India- American astronomer born 1910 in India. He graduated Cambridge University in 1933. He received the Noble Prize for the theoretical calculation that determined if a star was more massive than 1.4 Solar Mass it would explode as a supernova.
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- Subatomic particles have a finite volume of their own and can only be compressed a certain amount. Chandrasekhar calculated that this limit would be 1.4 Solar Mass.
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- Because Type I supernovae at 1.4 Solar Mass have the same tremendous runaway configuration each time, their brightness should be the same each time. By comparing this absolute brightness to the apparent brightness from Earth astronomers can calculate how far the supernova is from us. Astronomers call this their “standard candle” when the absolute magnitude of brightness is known.
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- When astronomers measured hundreds of these Type 1 supernova they calculated that their luminosity was10% to 20% fainter than they should be at their “known” distances. If they were further away that meant that the universe is expanding faster than expected. Even today astronomers can not explain what forces are accelerating the expansion of the Universe. So, we simply call the unknown force Dark Energy.
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- If you know the brightness of a star you can calculate the distance the star is away from us. A potential error in measuring this brightness is the effect of interstellar dust dimming the light before it reaches us. Interstellar dust is very fine submicroscopic haze made up mostly of carbon and silicon. The size of the dust is such is that it absorbs or scatters blue light more than it does red light.
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- This is the same reason why the sky is blue and the sunset is red. If we take a spectrum of the light from the supernova , and , the blue light and red light have the same intensity and the same decaying light spectrum, then, the dust absorption can be ruled out as a possible factor for dimness.
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- If we start with a Type 1 supernova in a nearby galaxy were we already know the distance using other means of calculation , then, the Type 1 supernova that is more distant will appear dimmer by the inverse square law of the distance.
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- To understand this think of a light source emitting from a point source into a sphere. One meter from the light source shines on a rectangular area one meter on a side, i.e. 1 square meter. Two meters from the light source expands to 2 meters on a side , 4 square meters, 3 meters to 9 square meters, etc. As the distance increases the same light is spread over a greater area which is equal to the square of the distance and the light is dimmer by that amount. A supernova 3 times the distance away will be 9 times as dim.
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- The Universe today is expanding at 70 kilometers per second per mega parsec distance. A mega parsec is 3,262,000 lightyears distance. If we measure the redshift of the light coming from the supernova we can calculate the distance the light has traveled to reach us.
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- As the galaxy is moving away from us its wavelength of light becomes longer, stretched toward the red end of the light spectrum
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------------------- Redshift = change in wavelength / wavelength emitted
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------------------ Redshift = z
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--------------- Velocity of the galaxy / Velocity of light = (z^2 + 2z) / ( z^2 + 2z +2)
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- A redshift of 1 means the receding velocity of the galaxy is 60% that of the speed of light. This is not your common sense velocity. It rather is a measurement of the expansion of space that has taken place while the light from the galaxy is on its way to us.
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- However, if we measure the same distance to the galaxy using its luminosity we find that the supernova is actually 5% dimmer than expected. Because the light is dimmer the galaxy must be further away. Therefore, the Universe must not be expanding at a “constant” 70 km/sec/mps. It must be accelerating even faster than that. This measured acceleration occurs up to 5 billion lightyears away.
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- When you see through a telescope the further distant you look the further back in time you see. Back 5 billion years we see us accelerating faster and faster. When we observe supernova that is more than 7 billion lightyears distant it appears 25% brighter than expected. It appears brighter because the Universe is decelerating its expansion at that point in our cosmic history. The supernova is closer than expected.
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- The coasting point that marks the transition from decelerating due to gravity and accelerating due to Dark Energy occurs at the redshift of z = 0.7. Using the formula above this corresponds to a receding velocity of 49% the speed of light. 49% of 299,800 km / sec / mps is 195,600 km / sec / mps. If the velocity were a constant the distance would be 200 km per mega parsec.
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- Each mega parsec is 3.262million lightyear. The distance is 7 billion lightyears back to half the age of the Universe.
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- Over 200 supernova explosions have been measured in this way. The best measurements to date put the age of the Universe to be 13.862 billion years. These measurements allow the calculation for the composition of the Universe to be 30% matter and 70% Dark energy.
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- Today’s expansion rate is 74.2 km/sec/mps = 49,306 miles per hour per million lightyears. The coasting point from deceleration to acceleration occurred 7 billion years go , half way back to the Big bang.
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- Supernova erupt every 100 years in a galaxy the size of our Milky Way Galaxy. The last supernova seen with the naked eye occurred in 1987. It was in our southern neighbor galaxy, the Large Magellanic Cloud.
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- If galaxies were the size of a dinner plate the Observable Universe would be 20 miles in every direction. If we knew which way to look we would see a supernova explosion every day.
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- November 19, 2018 5 , 54, 504 2171 2173
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