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---------------------------- - 2328 - Brown Dwarfs - too small to be a star.
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- All stars are destined to evolve into one of these three:
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------------------------------- (1) White Dwarfs
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------------------------------ (2) Neutron Stars
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------------------------------ (3) Black Holes
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- Review 625 explained how “ Neutron Stars” fit into this evolution of stars depending on their size. Where do “Brown Dwarfs” fit into this star evolution? Technically, Brown Dwarfs are not stars. They are not quite big enough to start hydrogen burning, the thermonuclear reaction, that makes a star shine.
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- Brown Dwarfs range in size from 12 times to 75 times the mass of Jupiter. Jupiter would have to be 80 times larger to cause the nuclear fusion of hydrogen. That is the tipping point for Brown Dwarfs as well as planets. Our Sun is 1,000 times the mass of Jupiter.
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-------------------------- Mass of Sun = 1,989 * 10^27 kilograms
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------------------------- Mass of Jupiter = 1.899 * 10^27 kilograms
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-------------------------- Sun is 1,047 times more massive than Jupiter.
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- Brown Dwarfs were only discovered 2 dozen years ago. Some 24 years ago astronomers from Vanderbilt University at Kitt Observatory discovered a binary pair of Brown Dwarfs in mutual orbit. This orbit is edge-on and allows the dwarfs to eclipse each other. This in turn causes dimming and brightening when one dwarf eclipses the other. Astronomers have made over 1,600 measurements of these occultation’s and their mutual orbit over 300 nights.
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- The existence of Brown Dwarfs was first proposed in 1980, but, astronomers did not concede that Brown Dwarfs actually existed until 2000. The reason is that these Brown Dwarfs are about the same size as big planets and it is difficult to tell the difference. The difference is how they are formed.
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- Planets are formed in the accretion disk of stars causing them to have a higher density than stars of the same size. On the other hand, Brown Dwarfs are formed just like stars, through the concentration of interstellar gas through the constant compression of gravity. In the star evolution process the Brown Dwarf would be mostly hydrogen gas throughout and would be far less dense than a planet the same size.
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- Jupiter is a gaseous planet but it was formed in the accretion disk of our Sun, like the Earth, and Jupiter has an iron core. We believe this because Jupiter has a magnetic field 19,000 greater than the magnetic field surrounding Earth.
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- The Sun is a gaseous star, mostly hydrogen gas, but its mass is large enough, to have enough gravity, to create enough pressure at its center, to have a high enough temperature, to start a thermonuclear reaction fusing hydrogen into helium.
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- Our Sun fuses 600,000,000 tons of hydrogen gas into helium gas every second. When this fusion occurs a small amount of the mass of hydrogen is converted directly into energy, E=mc^2, radiating gamma rays out from the center. It is these gamma rays that power the Sun.
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- When gamma rays make their way to the surface they loose energy and escape from the Sun’s surface at the frequency of visible light. We see the Sunlight. Brown Dwarfs have the compressed hydrogen gas but never gather enough mass to have enough gravitational pressure to start thermonuclear fusion.
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- From the outside a failed star and a giant planet would look the same. We really need to know the density of the object before we can decide which it is. To know the density we need to know the mass and the volume so we can calculate density.
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- This is where our breakthrough comes in. Measurements of these binary Brown Dwarfs in the Orion Nebula some 1,500 lightyears away have yielded this data. By timing the orbit and timing the eclipse as one dwarf moves in front of the other we have measured the diameter of one dwarf to be 70% the diameter of the Sun and the diameter of the other to be 30% the diameter of the Sun.
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- The mass is calculated from the measurements of the mutual orbit. Using Kepler’s formula that the square of the period is proportional to the cube of the radius.
Period^2 = 4*pi^2*(radius)^3 / G * (m+M)
This formula allows the calculation of the sum of the two masses. But, using their relative sizes you can deduce the mass of each dwarf.
Their masses were found to be 55 times the mass of Jupiter and 35 times the mass of Jupiter, but, only 5.5% and 3.5% the mass of the Sun. The density of the larger Brown Dwarf is only 16% the density of Jupiter so it definitely qualifies.
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- But, my calculations show the density of the smaller object to be the same as Jupiter, so that qualifies as a planet. There must be a mistake somewhere?
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------------------------ The diameter of Jupiter = 1.42984 * 10^5 kilometers
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------------------------ The diameter of the Sun = 13.92 * 10^5 kilometers
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------------------------ 70% the diameter of the Sun = 9.74 * 10^5 kilometers
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------------------------ 30% the diameter of the Sun = 4.14 * 10^5 kilometers
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------------------------------------------- The volume of a sphere = 4/3*pi* (radius)^3
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------------------------ Volume of 70% Dwarf = 484.3 * 10^24 m^3
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------------------------ Volume of 30% Dwarf = 38.13 * 10^24 m^3
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--------------------------------------- The mass of the Sun = 1,989 * 10^27 kilograms
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------------------------ Larger Dwarf 5.5% the mass of the Sun = 109 * 10^27 kilograms
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------------------------ Smaller Dwarf 3.5% the mass of the Sun = 70 * 10^27 kilograms
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-------------------------------------- The mass of Jupiter = 1.899 * 10^27 kilograms
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---------------------- 55 times the mass of Jupiter = 104.4 * 10^27 kilograms
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---------------------- 35 times the mass of Jupiter = 66.5 * 10^27 kilograms
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------------------------------------------- Density = mass / volume
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--------------------- The density of the larger Brown Dwarf = 216 kilograms / meter^3
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--------------------- The density of the smaller Brown Dwarf = 1,744 kilograms / meter^3
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--------------------- The density of Jupiter = 1,244 kilograms / meter^3
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- Astronomers put the density of Jupiter at 1,330 kilograms / meter^3. Our calculation is too low because we assume Jupiter to be a perfect sphere and actually it bulges at its equatorial diameter.
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---------------------- The density of water is 1,000 kilograms / meter^3
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---------------------- The density of the Sun is 1,410 kilograms / meter^3
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- Brown Dwarfs are dim red in color. They do have a temperature and do radiate infrared energy. It was surprising to learn that in this case the smaller Brown Dwarf had a higher temperature than the larger Brown Dwarf, 2,790 Kelvin versus 2,650 Kelvin. We would expect the more massive the star the hotter it gets ( See Review 167 ).
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- This discovery has lead astronomers to believe that the two Brown Dwarfs were not formed together out of the same gas and dust. They are likely not the same age but formed at different times in different places and somehow later were captured and locked into their mutual orbits.
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- Many astronomers believe the brown dwarf is the most common product of the star formation process. In other words, there many more failed stars in the Universe than there are stars. Today, astronomers have cataloged hundreds of small, faint objects that they think are Brown Dwarfs.
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- It is only in recent years that our telescopes have gotten powerful enough to find these small dim objects. They are all about the same size as planets have a reddish cast and probably look like a big Jupiter with weather clouds, bands and spots.
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- At the high end of their mass range, 60 to 80 Jupiter mass, the volume of Brown Dwarfs is governed primarily by electron degeneracy pressure. (See Review 625 ).
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- At the low end of their mass range, 1 to 10 Jupiter mass, the volume is governed primarily by Coulomb pressure, the same as the volume of planets. Throughout this wide range of masses the size, or radius, of Brown Dwarfs varies only by 10 to 15%. They are about the same size as planets which makes them so hard to differentiate.
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- Density is the clear giveaway. At the low end of the mass range, under 13 Jupiter mass, they are never hot enough to undergo even deuterium fusion. At the high end of the mass range deuterium fusion can occur but it can only last on the order of 10,000,000 years.
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- Some Brown Dwarfs emit X-rays and all Brown Dwarfs continue to glow in the red and infrared spectra until they cool to planet like temperatures under 1,000 Kelvin. Brown Dwarfs range in temperature from 300 to 3,000 Kelvin. The minimum temperature for stars is 4,000 Kelvin. Stars warm continuously with steady internal fusion. Brown Dwarfs cool quickly.
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- One of the more recent tests is called the Lithium Test. If a star starts to fuse hydrogen it rapidly depletes the Lithium that is present. A collision of Lithium-7 and a hydrogen nucleus ( a proton ) fuse to produce Helium-4 nuclei. This occurs just below the temperature at which Hydrogen will fuse into Helium. Measurements of the presence of Lithium become a good indicator if the object is a Brown Dwarf or a light star.
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- Brown Dwarfs are providing much more information about how stars were formed. And, we have just begun to study them. Each new instrument, new technology, new capability opens up a new window of knowledge and new discoveries. We do live in interesting times.
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- April 23, 2019. 626
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--------------------- Tuesday, April 23, 2019 -------------------------
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