Saturday, June 18, 2022

3604 - BROWN DWARF PLANETS - before becoming a star?

  -  3604 -   BROWN  DWARF  PLANETS  - before becoming a star?  Brown Dwarfs were only discovered a few years ago.  Much more needs to be learned to understand their origins.  Learning more will help us understand the origins of stars as well as planets. 


--------  3604  -  BROWN  DWARF  PLANETS  - before becoming a star?   

-  November 3, 2018, playing the video game Counter-Strike, when he made astronomy history. The astronomer was working with as part of the “Backyard Worlds: Planet 9 project“. 

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-   An exceedingly faint object 50 light-years away blazing through the galaxy at 200 kilometers per second. It was given the name “WISE 1534-1043“.

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-  Astronomers think they found a “brown dwarf“.   A brown dwarf is failed star that lacks the necessary bulk to begin nuclear fusion in its core.  It forms like a star,  however, it never gains enough mass to fuse hydrogen into helium and start burning anything.

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-  The discovery highlighted how we still have much to learn about brown dwarfs. These objects range in mass from an estimated 13 times the mass of Jupiter to 75 times or more, but exactly where those two boundaries lie is an ongoing dilemma. 

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-   While 13 Jupiter masses is roughly the mass at which deuterium fusion can take place, which is the characteristic that differentiates brown dwarfs from gas giant planets, the boundary can vary. 

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-  Brown dwarfs also vary greatly in temperature. The hottest ones have surface temperatures of around 2,000 degrees Celsius which is about that of a candle flame. 

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-  The coldest are below 200 degrees. As they do not have their own source of heat, brown dwarfs will gradually cool over billions of years to these lower temperatures. 

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-    “Subdwarfs“, which blur the boundary further between planets and brown dwarfs, can be cooler still. An object called WISE 0855-0714 is below freezing. It’s the coldest object we know of outside of our solar system.

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-     What a brown dwarf might look like up close is also unclear. Despite their name which was proposed by astronomer Jill Tarter in 1975,  they are likely not brown. They’re more orange or red. 

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- They also have atmospheres, and those atmospheres may show some kind of banding and spotlike storms, like on Jupiter. In 2021 astronomers used these storms to measure the wind speed on a brown dwarf about 34 light-years away. 

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-  They first watched features in its atmosphere come into and out of view as they rotated, and then compared this speed to a measurement of the object’s interior rotation speed gleaned from its magnetic field. Comparing the two values, the researchers calculated a wind speed of over 2,300 kilometers per hour.  This is more than five times that of Jupiter’s winds.

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-  Because brown dwarfs bridge the gap between stars and planets, they can help us understand both. At the upper end of the mass scale, the boundary between the largest brown dwarfs and the smallest stars can give us insights into how nuclear fusion begins.

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-   An object needs to reach temperatures of around 3 million degrees Celsius in its core to kick-start nuclear fusion.  This ignites a chain reaction that turns hydrogen into helium. But no one is exactly sure how much mass is needed for that to happen, and at what point a brown dwarf becomes a star. 

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-  Astronomers have identified five high-mass brown dwarfs with masses between 77 and 98 times that of Jupiter.    Some brown dwarfs may even be so starlike that they could host their own planets. 

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-  There’s every indication that there are probably brown dwarfs that have their own exoplanets in orbit around them as well. A holy grail is to find a brown dwarf with a transiting exoplanet.

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-  On the opposite end of the brown dwarf mass scale lies a new discovery. It’s an extremely small, cold and faint object, just barely at the level where we could detect it. Astronomers are eager to work out what the difference is between a low-mass brown dwarf and a high-mass gas giant planet. 

-  This brown dwarf appears to be made of some strange stuff. As the universe ages, supernovas spit out lots of heavier elements such as carbon and oxygen (what astronomers call “metals).   Because of this, old objects that formed early in the universe’s history tend to have few metals, while new objects have more. 

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-  Yet despite being found in our local solar neighborhood, which is home mostly to young, metal-rich stars, this brown dwarf appears to be metal-poor.   It is probably an older brown dwarf, probably one that was created before the Milky Way had all the metal enrichment it does now.  It was likely “one of the first brown dwarfs formed” in our galaxy, originating in the outer galactic halo surrounding the Milky Way and then migrating inward.

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-    Planets congeal out of debris floating around newborn stars.  They are the leftover scraps of the star formation process.  These recent discoveries of planetary bodies are 12 to 75 times the size of Jupiter, our biggest planet.

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-    They are not quite big enough to have enough gravity to ignite hydrogen fusion at their cores.  Brown Dwarfs are not big enough to fuse hydrogen into helium they are big enough to fuse deuterium into helium.  They briefly start to shine as stars with this process before they quickly exhaust their supply of deuterium and start to cool down like a planet.

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-  Astronomers have found hundreds of these Brown Dwarfs floating throughout our galaxy and some even orbiting other stars like planets.  Brown Dwarfs are very faint and hard to find but estimates from astronomers are that they may be as numerous as the stars we see in our galaxy.

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-  Planets are born in the disks of gas and dust orbiting a newborn star.  It is believed that Brown Dwarfs can not form this same way that planets do. This process is limited to about 10 to 15 times Jupiter mass.   Also, most are found as independent stars not orbiting other stars. 

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-   Astronomers think they are likely failed stars, no different than full blown stars at birth but for some reason get separated from the molecular cloud that feeds them.  They end up stuck in the embryo stage and never mature.

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-   Stars evolve depending on whatever mass they accumulate.  We know stars for two distinct properties, their brightness and their color.  Brightness is luminosity, or the intensity of their radiation.  If we us the luminosity of our Sun as the reference equal to 1, stars have luminosities form 1/10,000 that of our Sun, called ‘Red Dwarfs“; to 1,000,000 that of our Sun, called “Supergiants“. 

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-  Their brightness depends on their size.  The bigger stars are the brighter they are and the shorter their lives.  We are fortunate that our Sun is in the middle of the pack and should live for 10,000,000,000 years.

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-  Also, the bigger the star, the more gravity that compresses it, the hotter it gets.  The color of the star tells us its surface temperature.

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-  “Color” is another word for frequency, or wavelength of electromagnetic radiation in the visible range. Our eyes detect color because of the different wavelengths that reach the cones in our retina.  The temperature of a radiating body is equal to .0029/ wavelength, T = .0029 / w 

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-   Our Sun is yellow light.  Its maximum intensity of radiation occurs at a wavelength of about 500 nanometers.  From the equation, known as Wien’s law, the temperature is .0029 / 5*10^-7 meters = 5,800 degrees Kelvin.  

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-  Kelvin is the same as Centigrade but add 273 degrees because the Kelvin scale starts zero at absolute zero, -273C.

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-  Using this measure of temperature stars range from 2500K to 50,000K:


---------  Color         Maximum Temperature, K Example star

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---------  Blue-violet 50,000 Mintaka

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---------  Blue-white 30,000 Spica, Rigel

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---------  White 11,000 Sirius, Vega

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---------  Yellow-white          7,500            Canopus, Procyon

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---------  Yellow           5,900 Sun, Capella

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---------  Orange           5,200 Arcturus, Aldebaran

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---------  Red-orange 3,900 Antares, Beltegeuse

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---------  Brown less than 2,500 Brown Dwarfs

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-  Brown Dwarfs were only discovered a few years ago.  Much more needs to be learned to understand their origins.  Learning more will help us understand the origins of stars as well as planets. 

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-   These times are great for astronomy.  So much is being learned every day.

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June 18, 2022     BROWN  DWARF  PLANETS  - becoming a star?      3604                                                                                                                                           

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--------------------- ---  Saturday, June 18, 2022  ---------------------------






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