- 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.
-
-
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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
-
“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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- “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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- Red dwarfs
may be a bit dim, but like the tortoise, they slowly but surely win the
survival race.
-
March 21, 2023 RED
DWARF STARS live the
longest? 3926
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever
is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
---------------------
--- Tuesday, March 21, 2023 ---------------------------
-
No comments:
Post a Comment