Tuesday, February 8, 2022

3452 - SIRIUS - Our Sun Becomes a White Dwarf

  -  3452  - SIRIUS  - Our Sun Becomes a White Dwarf.  Our Sun will become a White Dwarf some day.  It would be nice to learn more about White Dwarfs.  Our Sun is half way there.  Is there another sun that is already a white dwarf the we could study.  


-------------  3452  -  SIRIUS  - Our Sun Becomes a White Dwarf

-  Yes, you can Sirius most winter nights in the southern sky.  It is the brightest star in the night sky, Sirius,ly  you can’t miss it.  The brilliant blue-white “Dog Star“.  Find the constellation Orion.  The three stars in Orion’s belt point south and east to Sirius in the constellation Canis Major.  Procyon, the 8th brightest star, is nearby in the constellation Canis Minor.

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-  The problem is that Sirius is a binary star.  Sirius A is 2.1 times larger than our Sun and 26.1 times brighter.  Orbiting Sirius A is the white dwarf Sirius B.  The two together appear as one bright star although they are relatively nearby.  They are only 8.61 lightyears away which ranks number nine as the nearest star. 

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-   The white dwarf is in a 50 year elliptical orbit just 20 times the Earth-Sun distance away Sirius A.    It is hard for astronomers to measure light from the white dwarf when it is swamped out by the glare of Sirius A, which is 10,000 times brighter.

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-  Sirius B is 98% the mass of our Sun but only 7,500 miles in diameter.  So the density of a white dwarf is extreme.  A sugar cube size of Sirius B would weigh 4.2 tons here on Earth.  I would weigh 50,000,000 pounds if I were standing on Sirius B.  (See Review 387 on Sirius).

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-  According to Johan Kepler the period of the orbit squared times the mass is equal to radius of orbit cubed.  

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-   So the mass is equal to (20 AU)^3 / ( 50 years)^2.  8000/2500 = 3.2 solar masses. 

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-    Sirius A and Sirius B have a combined mass of 3.2 times the mass of the Sun. ( Later calculations get 2.98 times).

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-  Kepler’s laws of physics do not tell us enough.  But just recently astronomers using the Hubble Telescope have been able to use Einstein’s general relativity theory on the calculations.  Einstein predicted that gravity having the effect of warping space-time would cause light to lose a fraction of its energy leaving the intense gravitational field.  

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-  Sirius has a gravitational field 350,000 times greater than Earth’s.  Astronomers would see light shifted to longer, redder wavelengths.  This is called a gravitational redshift effect.

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-  This is how astronomers calculated that the white dwarf is 98% the mass of the Sun with a surface temperature of 44,000 Fahrenheit and Sirius A is 200% the mass of the Sun, with a diameter of 1,500,000 miles.  The surface temperature is 18,000 Fahrenheit.

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-  The White Dwarf has no nuclear reactor anymore so its 44,000 F temperature will continue to cool down. The star cools down but does not shrink over time due to degenerative pressure of the electrons separated from their nuclei under such intense gravity.  

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-  This cooling process slows over time, so the dimmer a White Dwarf gets the slower it gets dimmer.  It will take billions of years for it to radiate away the remaining heat and become a Black Dwarf.  The core of the remaining Black Dwarf is crystallized carbon, pure diamond.  Diamonds in the sky, great title for a song.

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-  Studying White Dwarfs are not just helping us understand our Sun.  White Dwarfs have a defined size no greater than 1.44 solar mass ( called the Chandrasekhar limit).  So, when a white dwarf gets swallowed up by the other binary star and a supernova explosion results astronomers know approximately how big the explosion is going to be.  It is called a Type 1a Supernova.

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-   If we know the absolute brightness of the Supernova than we can measure its distance.  By measuring many Type 1a Supernovae we can calculate the rate of expansion of the Universe.  This in turn we help us understand the Dark Energy that is causing the expansion to occur.

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February 8, 2022         SIRIUS  - Our Sun Becomes a White Dwarf          604       3453                                                                                                                                               

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