--------- #1357 - Betelgeuse, Algol, and Epselon Aurigae.
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- Betelgeuse, Algol, and Epselon Aurigae are Variable Stars. Each presented a mystery as to why its brightness should vary. This review presents a story for each one.
- Betelgeuse is a super-giant star that is orange or reddish in color located at the right shoulder of Orion the Hunter, everyone’s favorite Constellation in the southern night sky.
Rigel is a blue-white star that is the left foot of Orion the Hunter. Rigel’s surface temperature is 10,300 K which makes it blue. Betelgeuse is 3,400 C which makes it red. Our Sun is 5,800 K and is yellow.
- Betelgeuse is a variable star whose brightness changes by 50% over a period of years. At its brightest it has the luminosity of 38,000 Suns. Its brightness is changing because it is huffing and puffing as it reaches its final stages of nuclear burning. It has been burning all the elements from hydrogen up to iron. We have know way of knowing at what stage of burning resides in its core. If iron is piling up, and iron will not fuse releasing energy, than it could go supernova any minute. Or, it may burn remaining fuel for another thousand years. We could be the witness of one of the most dramatic events that occurs in the Universe. Maybe.
- A stars brightness is a function of both temperature and surface area. Betelgeuse’s surface area is immense. Luminosity = Constant * (4*pi*r^2) * ( T^4). Luminosity is proportional to the 4th power of the temperature. The Constant is 5.7*10^28 watts / (m^2 * Kelvin^4)
---------------------- L = Constant * (4*pi*r^2) * ( T^4).
----------------------- The Luminosity of Betelgeuse is 38,000 Suns.
---------------------- The Luminosity of the Sun is 3.8*10^26 watts.
--------------------- Therefore, the Luminosity of Betelgeuse is 1.4*10^31 watts
---------------------- The temperature of Betelgeuse is 3,400 Kelvin
- We can use this equation to calculate the size of Betelgeuse. Solving for the radius:
-------------------- r^2 = L // 4*pi * Constant * T^4
--------------------- r = 1.4*10^31 // 4 * 3.14* (5.7*10^-8) * (3.4*10^3)^4
---------------------- r = 3.8 *10^11 meters
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- The radius of Betelgeuse is 380 billion meters. If Betelgeuse replaced our Sun its surface would engulf the Earth and reach beyond the orbit of Mars.
- Algol is a variable star, but it has a different cause. Algol is an eclipsing binary system. Algol , the “ Demon Star”, marks the head of Medusa the gorgon carried by Persus who had decapitated her in one of the most famous Greek Myths.
- Algol’s brightness regularly recurs every 2 days, 21 hours. In just 5 hours it can dim brightness by 33%. 5 hours later it will be full brightness again. One star is blue-white and 25 times brighter than the other. It is 3.7 Solar Mass and a Main-Sequence Star. The companion star is orange and slightly larger in diameter. It is 0.8 Solar Mass and a Sub-giant Star At minimum brightness 80% of the brighter star is eclipsed from our point of view.
- Binary stars are born at the same time. More massive stars have shorter lives. Therefore, more massive stars exhaust their core hydrogen and become sub-giants sooner than the less massive stars. In Algol’s system of binaries the opposite is occurring. Why is this?
- When the massive star exhausts its hydrogen first it swells in size and becomes a Red Giant Star. Its massive size allows the companion star to pull gas away from its huge tidally distorted outer layers. This results in a mass exchange. As the gas spills over from the Red Giant to the companion, the companion gains mass at the giant’s expense.
- The 0.8 Solar Mass star used to be much more massive. As the more massive star it was the first to begin expanding into a Red Giant Star. So much of its matter spilled over onto its companion that it is now the less massive star.
- The 3.7 Solar Mass star is still gaining mass from Sub-giant companion. Its lifecycle is actually accelerating as its increasing gravity raises its core hydrogen fusion rate. It will eventually exhaust its hydrogen and begin expanding into a Red Giant itself. At that point the cycle could start all over again.
- Epsilon Aurigae is a variable star with still another story. It is in the Constellation Capella the Charioteer. The constellation is directly overhead at this time. Capella is the brightest star in the 5 star pentagon shape. The “ Three Kids” is the triangle of stars next to Capella. Epsilon is the next orange star as you go around the Pentagon vertices.
Epsilon is a Super Giant Star 20 Solar Mass, 300 times the diameter, and 60,000 times the luminosity of our Sun. It is 3,000 lightyears away. Epsilon is a variable star caused by an eclipsing binary. However, the eclipse occurs once every 27 years and lasts for 2 years. The last eclipse started in 2009 and ends this year. If this is a star passing in front of another star it must be enormous size. In fact, impossibly big to block out the sun for 2 years.
- Infrared Space Telescopes have given us the answer. It is not a star it is a cloud of dust. Only the dust is not what we expect. This is a pancake of countless solid particles the size of gravel. The diameter of this gravel patch is 430 times the diameter of our Sun. (4 AU).
- When you look at Epsilon Aurigae tonight it is getting brighter and you can visualize this giant patch of gravel floating by in front of it. Awesome!
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707-536-3272, Monday, December 26, 2011
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