Tuesday, March 21, 2023

3926 - RED DWARF STARS - live the longest? -

 

-   3926 - RED  DWARF  STARS   -  live the longest?    Red dwarf stars are more active and wild than the Sun.   Red dwarfs,  known as "M dwarfs", are the most common stars in the Milky Way and can remain placid for long periods of time before erupting with huge “superflares”.


------------  3926  -    RED  DWARF  STARS   -  live the longest?

-   These “supersolar flares” have previously been measured to be 100 to 1,000 times more powerful than similar flares from the sun, with young red dwarfs particularly tumultuous.

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                 -    These outbursts, as well as eruptions of scorching-hot plasma known as coronal mass ejections (CMES), can be incredibly destructive to planets orbiting red dwarfs, stripping their atmospheres and emitting enough radiation to boil away liquid water even in the so-called habitable zone around them. The “habitable zone” the region around a star in which liquid water can exist on a world's surface.

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-    Such high activity might therefore make it tough for life to take root around red dwarfs.    TRAPPIST-1, which contains seven roughly Earth-size planets, at least three of which are in the habitable zone.

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-   But TRAPPIST-1 is a red dwarf, and if it erupts with violent flares like fellow red dwarf Proxima Centauri, which in 2019 emitted a flare 14,000 times brighter than its pre-flare brightness, then TRAPPIST-1's life-hosting potential may be relatively low.

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-   Observations of 177 red dwarfs collected between 2003 and 2020 by the High Accuracy Radial velocity Planet Searcher (HARPS), an instrument on the 3.6-meter telescope at the European Southern Observatory's La Silla Observatory.

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-    All stars exhibit some degree of variability; for example, the sun has an activity cycle that lasts around 11 years. During this cycle, sunspots on our star's surface increase and decrease in frequency. An increased number of sunspots, driven by magnetic activity, brings an increase in solar flares and space weather intensity.

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-   Fortunately, life emerges and evolves on much longer timescales than solar cycles. Here on Earth life emerged about 4 billion years ago, roughly 500 million years after our planet's formation.

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-   If life elsewhere in the Milky Way takes a similar time frame to develop, then the long-term variability and ferocity of red dwarfs could make a significant difference to this process.

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-    “HARPS”  instrument is particularly useful in looking at how active stars are. This is because it looks at emissions from a star's chromosphere, the second layer of its atmosphere. Emissions from this layer are driven by magnetic activity just like flares, rather than the fusion that takes place in the stars' cores.

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-    The identification of a cycle requires measurements showing its repetition over several periods. This requires data taken over a long period of time.  Even if the time coverage is not sufficient for some stars, however, data can be used to estimate a minimum cycle period if present.

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-   To make up for the lack of long-term data, the team identified "seasons" for individual stars. The scientists defined these seasons as periods of 150 days during which at least five observations took place, with gaps between observations no longer than 40 days. The team chose 150 days as their season because that is the typical maximum limit for the rotation period of red dwarfs.

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-   This led the researchers to identify a subset of 57 stars from the total sample of 177 red dwarfs.  They found that most stars are significantly variable, even the quietest stars.    75% of the 57 stars in the sample displayed long-term variability. They found long-term timescales ranging from several years to more than 20 years.

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-    Red dwarf stars make up the largest population of stars in the galaxy, but they hide in the shadows, too dim to be seen with the naked eye from Earth. Their limited radiance helps to extend their lifetimes, which are far greater than that of the sun.

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-  Scientists think that 20 out of the 30 stars near Earth are red dwarfs. The closest star to the sun, Proxima Centauri, is a red dwarf.    The term "red dwarf" does not refer to a single kind of star. It is frequently applied to the coolest objects, including K and M dwarfs, which are true stars, and brown dwarfs, often referred to as "failed stars" because they do not sustain hydrogen fusion in their cores.

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-    “Red dwarf "generally refers to dwarf stars with a spectral type ranging from K5V to M5V.  Red dwarfs form like other main-sequence stars. First, a cloud of dust and gas is drawn together by gravity and begins rotating. The material then clumps at the center, and when it reaches the critical temperature, fusion begins.

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-   Red dwarfs include the smallest of the stars, weighing between 7.5% and 50% the mass of the sun. Their reduced size means that they burn at a lower temperature, reaching only 6,380 degrees Fahrenheit.

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-   The Sun, by comparison, has a temperature of 9,900 F. The low temperatures of red dwarfs mean they are far, far dimmer than stars like the sun.  Their low temperature also means that they burn through their supply of hydrogen less rapidly.

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-    More massive stars burn through only the hydrogen at their core before coming to the end of their lifetimes, red dwarfs consume all of their hydrogen, inside and outside their core. This stretches out the lifetime of red dwarfs to trillions of years; far beyond the 10-billion-year lifetime of sun-like stars.

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-    Scientists occasionally have difficulty distinguishing a red dwarf star from a brown dwarf. Brown dwarfs are cool and dim, and likely form the same way red dwarfs do, but brown dwarfs never reach the point of fusion because they're too small, and therefore, they're not considered stars.

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-    When we observe a red dwarf and measure its atmosphere, we don't necessarily know whether it's a brown dwarf or a star.  Young brown dwarfs look almost exactly like ultracool stars. 

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-    To figure out whether a celestial object is a brown or red dwarf, scientists measure the temperature of the object's atmosphere. Fusion-free brown dwarfs are cooler than 2,000 Kelvin while hydrogen-fusing stars are warmer than. In between, a star could be classified as a red dwarf or brown dwarf.

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-   Sometimes, chemicals in the object's atmosphere can reveal clues about what's happening at its heart.  The presence of molecules like methane or ammonia, which can only survive at cold temperatures, suggests that an object is a brown dwarf. Lithium in the atmosphere also suggests that a red dwarf is a brown dwarf rather than a true star.

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-    Planets form from the material left over in a disk after their star has been created. Many red dwarfs have been found with planets surrounding them, though enormous gas giants are rare.

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-     Because red dwarfs are dimmer than stars like the sun, it is easier to find small planets that may surround these dimmer objects, making red dwarfs a popular target for planet hunting. NASA's Kepler space telescope which operated between 2009 and 2018 and Transiting Exoplanet Survey Satellite, or TESS which started operations in 2018, have surveyed many red dwarf stars for possible Earth-like planets.

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-   Since the planets examined by TESS are near bright stars that tend to be close to Earth, it's easier for ground telescopes to follow up on the observations. In April 2019, TESS investigators announced they had found their mission's first Earth-size planet, although its conditions are not ideal for life as we know it.

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-    Scientists thought red dwarfs were uninhabitable. Their limited light and heat meant that the habitable zone, or the region where liquid water could form on planets around a red dwarf, would be very close to the star, putting the planets in range of harmful radiation from the star. Other planets may be tidally locked to the star, with one side constantly facing the sun, causing one side to be too warm, and the other to be too cold.

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-   In 2016, a potentially habitable planet was found orbiting Proxima Centauri (Earth's closest star). And in 2019, astronomers announced the possibility of a second planet orbiting far outside the star's habitable zone. At least seven Earth-size planets orbit the red dwarf TRAPPIST-1, and many studies suggest at least some of those planets could host life.

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-    Tiny red dwarfs may have an extended lifetime, but like all other stars, they'll eventually burn through their supply of fuel. When they do, the red dwarfs become white dwarfs, dead stars that no longer undergo fusion at their core. Eventually, the white dwarfs will radiate away all of their heat and become black dwarfs.

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-    But unlike the sun, which will become a white dwarf in a few billion years, red dwarfs will take trillions of years to burn through their fuel. This is significantly longer than the age of the universe, which is less than 14 billion years old.

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-    Red dwarfs may be a bit dim, but like the tortoise, they slowly but surely win the survival race.

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                   March 21, 2023           RED  DWARF  STARS    live the longest?              3926                                                                                                                          

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--------------------- ---  Tuesday, March 21, 2023  ---------------------------

 

 

 

 

         

 

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