One
day, the universe has to come to an end, and the last star will stop shining
and die. It will probably be a red dwarf. Unlike the other celestial bodies
that have clear classification standards, red dwarfs fall into a variety of
luminosities, masses and fusion activities, making them really interesting to
study. They are also considered our best bets to find life forms outside of our
solar system. The term “red dwarf” was first coined in 1915, but doesn’t
actually hold a clear meaning, astronomically speaking. The appellation was set only to distinguish
them from their cousins, the blue dwarfs. So what do we know about red dwarfs,
and what makes them our last hope for survival?
1. Red dwarfs are little stars
To fully understand why red dwarfs are
classified as stars, we first need to comprehend what makes any other star a
star. With so many bodies out there in the universe, astronomers struggled to
trace down a separating line between what could possibly be a really enormous
planet, or an excessively small star. Dupuy finally solved the puzzle and broke
it down to a single property: the mass. Dupuy stated the following: “Mass is the
single most important property of stars because it dictates how their lives
will proceed”. Obviously, there is a mass limit that separates stars from planets. This
mass limit permit nuclear reactions that continuously converts hydrogen atoms
into helium in the core of the star, thus resulting in its brightness. “Below this limit there's not to
replenish the energy that's constantly being radiated into space," as
Dupuy explained. "Objects with a given mass below this limit would simply
cool forever."
Let’s now return to our red dwarfs. They are
from 7 to 50% of the mass of our sun. The fact that the sun is considered by
astronomers as an average star, in the matters of mass, makes red dwarfs look
pretty small, not that much bigger than the planet Jupiter, barely crossing the
mass limit defined by Dupuy. In fact, red dwarfs are right in the verge of
being stars at all, and that’s exactly why they are called dwarfs. Any less
than their actual mass and they would be brown dwarfs, or in other friendly words,
failed stars, not able to perform nuclear fusion. Brown dwarfs lie right
between stars and planets, in astronomical classification.
2. Red dwarfs are dim
Do not be fooled by the appellation “red
dwarfs”. While red usually refers to fire, light, brightness, red dwarfs are
actually dim and not so luminous. So why on Earth do we call them red? Have
astronomers gone mad? In fact, there is quite a strong reason why we do so, and
that’s because they are actually red. Remember, red dwarfs are not very hot
compared to other stars like our beloved sun, and yet they are red. To solve
this riddle, think of fire. As known, the base of the fire is the hottest part,
and it cools down all the way up to the top of the flame. Coincidentally, the
top happens to be red whereas the bottom blue. Finally, red is not that hot and
astronomers not that stupid, after all.
Being red and not very hot, red dwarfs can’t be
seen by the naked eye. Ultra-modern observatories and sophisticated equipment
turn out to be rather mandatory to observe one of them.
3. Red dwarfs are the longest-living
bodies in the universe
Once again, do not be fooled by appearances.
Unlike humans, where healthy and stronger people are more likely to live longer
than the fragile and the dim, there is a completely different rule that
dictates who should live longer up in space. In fact, it is all the other way
around; the largest and most massive stars die quicker and burst into gigantic
supernovas and enigmatic black holes, where red dwarfs, the smallest stars,
live to what we can safely call forever, given our modest awareness of time.
As any other star, red dwarfs fuse hydrogen
into helium, but instead of keeping the fused helium in the core of the star,
red dwarfs are highly convective, meaning that hydrogen and helium remain mixed
together, slowing the consumption of fuel -hydrogen- by a great deal, making it
both dim, not very hot, not very bright, and live longer at the same time. Red
dwarfs burn so slowly that the average timespan is estimated between one to ten
trillion years. In comparison, the entire universe has lived only for 13.75
billion years, the thing that make all the red dwarfs still babies and in the
early state of their development. The sun is estimated to die in one billion
years, which makes red dwarfs more likely to outlive the universe.
4. Red dwarfs outnumber all the other
stars
Although it is impossible to observe a red
dwarf with naked eyes, this does not imply any kind of scarcity whatsoever. All
in all, astronomical observations showed that, at least 70% of the total number
of stars in the universe are actually red dwarfs. Further, it is believed that
20 out of the 30 closest stars to Earth are no other than, guess what, red dwarfs.
The great importance of these little stars has
risen a huge interest in studying them much more deeply, to the point of giving
them names. Proxima Centauri is the designation of the closest one to the solar
system, with 4.25 light-years far, followed by Bernard’s Star, a 5.978
light-years distant red dwarf, and Wolf359 with 7.86 year-light lying between
it and the sun.
5. Red dwarfs host planetary systems
If we search for planets upon which we want to
put our feet, we’d better head toward red dwarfs. Yes of course, red dwarfs can
be the center of complete systems. Further, they are more likely to have planets
orbiting around them than standard stars. For this matter, the astronomer West
declares: “We now know for the first time in human history that almost every star
has at least one planet, and it turns out that the red dwarfs appear to host
more planets than other stars. Small
planets are more abundant around small stars. The most common stars in the
galaxy appear to be the most common planet hosts, including planets we could
envision walking on. If you were to put money around where we would find the
most habitable planet, it's likely to be around a red dwarf.”
6. Red dwarfs are most likely not habitable
Only 5% of the red dwarfs in the milky way
could be considered habitable and that for the following reasons: First, for a
planet to be life-friendly, it must remain in the habitable zone. Astronomers
call habitable zone the area around a star where water could hold its liquid
form. However, even if a planet happens to be in this zone, it needs to be much
closer to the red dwarf than Earth is to the sun, since the red dwarf is
smaller, less hot and less bright.
And then, two other problems rise. First, if a
planet was this close to a star, it would be tidally locked, meaning that while
the planet orbits the red dwarf, it will keep the same side facing the star,
resulting in a continuous hot day on one face, and a perpetual icy night on the
other. Second, this small distant separating the planet from the star will make
water evaporate completely, and the heat should alter the planet, sealing it to
the same fate as Venus, a hot burning hell.
7. Red dwarfs vary in their energy
output
Red dwarfs are far more variable, violent and
unstable than normal stars. They can output their emitted energy in many
different ways. Red dwarfs can be covered by star spots that can dim the
emitted light by up to 40%, for months in a row. At other times, the energy
could take the form of gigantic burning flames, known as solar flares, that can
make the star double its brightness in a matter of minutes. This dangerous
phenomenon can be very damaging for life and planets’ atmosphere.
Nevertheless, astronomers estimate that a red
dwarf flaring period is only the first 1.2 billion years of its existence. So,
if a planet happens to be in the habitable zone, though far enough to avoid
tidal locking of a red dwarf after its flaring phase, life can be possible up
there. Even writers and movie producers are optimistic about the perspective of
life forms in red dwarf systems, as the iconic superman is thought to come from
Krypton, orbiting a tar called Rao, a red dwarf!
8. Red dwarfs can help understand
cosmology
Given their very long lifespan, and the astonishing
snail speed with which they burn their fuel, the mass of red dwarfs hardly
decreases. In comparison with standard stars, the mass of the dwarfs feels
pretty much constant and not evolving through time at all. This interesting
property allows humans to learn, or at least estimate the mass limit at which
more massive stars derail off the main sequence, and thus calculate the age of
star clusters.
They might also help us understand how stars
form, how the universe will be like after the death of our sun, and more
importantly answer the big question: Are we alone in the universe?
9. All red dwarfs contain metals
All known red dwarfs contain metals.
Astronomically speaking, metals are elements heavier than Hydrogen and Helium.
They form due to the fusion of lighter elements giving Carbon, Oxygen up to
Iron. This is rather strange and defy some physics rules and properties. Given
the slow motion of red dwarfs and the relatively little time of the universe
existence, our beloved dwarfs should not contain any element heavier than
Helium since it requires a certain amount of speed far greater than what red
dwarf elements react with. To this day, no metal-proof red dwarfs have been
found, and the mystery always continue to elude humanity’s brightest minds.
10.
Everything
has to come to an end, even red dwarfs
Everything mentioned about red dwarfs and them
being humanity last hope after the sun burst is quite beautiful in its own
terms, but everything has to die at some point, even red dwarfs.
After what seems like trillions of years, the
life of the last star in the universe will come to an end as it blasts in a
final show, closing off the curtains. When a red dwarf consumes up all its
hydrogen, it will become what we call blue dwarfs, burning up completely. Then
it transforms into white dwarfs, very dense bodies of the size of Earth, that
mainly consist of Helium atoms. With the absence of any source of energy, white
dwarfs will cool down extremely slowly to finally enter its final phase: black
dwarfs.
By Yacine GACI
Resources:
- · https://phys.org/news/2014-06-red-dwarf-stars-planets.html
- · https://en.wikipedia.org/wiki/Red_dwarf
- http://www.dailygalaxy.com/my_weblog/2012/09/there-may-be-trillions-of-earths-orbiting-red-dwarfs-todays-most-popular.html
- https://en.wikipedia.org/wiki/Habitability_of_red_dwarf_systems
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