Stars come in a wide range of masses and
sizes. Some are stellar behemoths up to 300 times the mass of our Sun. Others are
similar in mass to our Sun and represent our stellar cousins. But the lowest mass
stars are the red dwarfs. They're small, dim, and slow-burning. Their lifespans are so
long that they will be the last stars ever to shine in our universe, and will
be the place where life will make its final stand. Welcome back to Launch Pad,
I'm Christian Ready, your friendly neighborhood astronomer, and in this
video we're going to learn about the smallest, dimmest stars in the universe,
the red dwarfs. As their name implies, red dwarfs are smaller and cooler than
our Sun, shining brightest at infrared wavelengths. They're also much less
massive, about 40% the mass of our Sun all the way down to 8%. In fact, the
smallest and least massive red dwarfs are barely stars at all; they're just massive
enough to sustain hydrogen fusion in their cores. When compared against other
stars' luminosities and temperatures, the red dwarfs occupy the low, dim end of the
main sequence of hydrogen burning stars. But red dwarfs are the most common type
of star in our galaxy, making up to 85% of the hundred billion stars in our
galaxy alone. In fact, the nearest star to our Sun Proxima Centauri is a red dwarf
just 4.2 light years away, but barely noticeable in a time exposure image. Not
only that, but most of the nearest stars to our Sun are red dwarfs as well. Like
any main sequence star, red dwarfs shine by fusing hydrogen into helium in their
cores and releasing energy in the process. But the rate at which stars burn
their hydrogen fuel depends on their mass and how much pressure they can
exert on their cores. High mass stars squeeze their cores harder so they fuse
their hydrogen fuel much more quickly. But low mass stars are lighter so their
cores are under less pressure and therefore burn their hydrogen fuel much
longer. But red dwarfs have the smallest cores, so it might seem that they should
run out of their hydrogen fuel after a relatively short amount of time. But red
dwarfs- particularly those that are under 35% the Sun's mass - have
another tremendous advantage. The core of our Sun is surrounded by a radiative
zone which effectively isolates it from the rest of the star. As a result, our Sun
only has about 10% of its hydrogen supply to fuse over its lifetime. But a
red dwarf's interior is fully convective so it carries away the helium that's
produced in the core and mixes it with the hydrogen in the rest of the star. As
it cools, it sinks back down to the core, bringing fresh hydrogen down with it.
That means red dwarfs can use 100% of their hydrogen fuel. So a red
dwarf with 10% the Sun's mass has the same amount of fuel to work with that
the Sun does, only it burns it a thousand times slower. The result is a star that
can last anywhere from 1 to 12 trillion years. But don't let this picture of an
efficient, slow burning star fool you into thinking that red dwarfs are
somehow mild-mannered stellar citizens. On the contrary, these little stars can
pack an enormous punch. Their convective interiors create enormous star spots
that can cover as much as 40 to 50 percent of the entire star's surface,
giving the star a blotchy, uneven appearance. And the younger a star is, the
faster rotates. This further intensifies the magnetic activity on the star,
resulting in enormous stellar flares that are more than 10,000 times
those found on the Sun. Exactly how long red dwarfs remain violent like this is
not well understood, but suffice to say these are not the sort of stars we'd
expect to find life around - at least not while they're young. But maybe life has a
better chance later on as the red dwarf evolves. We're going to talk about how
red dwarfs evolve in a moment ,but first I want to thank Patti Broussard, Neil from
crypto cruising, Eric from modern day tech, John Cannonm Erynn Wilson, Leszek Skrzypczynski,
Jo A Walters, Jennifer Brozek, Neils Christensen, and I want
to welcome my two newest patrons, Chris Konkle Vlogs and Rasmus Borup. Thank you
so much for your support, and if you want to help support Launch Pad, please head
over to my Patreon page. Red Dwarfs spend several trillion years on the main
sequence fusing hydrogen to helium. Gradually that helium starts
to build up in the star, increasing its density. The core shrinks and heats up,
speeding up the fusion rate by a factor of 10. This is why red dwarfs with 10%
the Sun's mass can "only" last for seven trillion years rather than ten trillion
years...hey, what are you gonna do? As the red dwarf ages, its color changes from
red to white, becoming as hot as the present-day Sun. If the red dwarf is a
little bit more massive, its temperature will continue to climb to become hotter
than the Sun, becoming a blue dwarf. In fact, blue dwarfs with about 16% the
sun's mass will maintain a steady brightness for about five billion years.
By that point the planets orbiting farther away from the star will thaw out,
allowing perhaps for atmospheres and even running water on their surfaces. Five
billion years is enough time for at least one intelligent civilization to
evolve in the universe, so maybe around this time, trillions of years from now
there will be a renaissance of all kinds of intelligent civilizations teeming
throughout the universe. If we do not destroy ourselves,
maybe our descendants will witness this new era. But first they'll have to
survive the demise of our Sun, seven billion years from now. Maybe they'll
migrate to another Sun-like star, perhaps one that will form 5 billion years from
now when the Andromeda and Milky Way galaxies collide. For the next several
billion years, they'll watch as new stars are formed and massive stars explode as
supernovae. But this new era of star formation will only last for so long. As
our merged galaxy takes on a new elliptical shape, it will literally run
out of gas, and star formation will effectively cease. By the time the first
red dwarf begins to evolve, our Sun and those that will come after it will have
long been dead; their white dwarf cores will have long faded away as black
dwarfs. For hundreds of billions of years, the only stars shining in the universe
will be the red dwarfs...albeit shining very dimly. That is, until the first new
pinpoints of light appear in the sky once again. This will be the emergence of
blue dwarfs along, perhaps, with their attendant civilizations. Maybe
our descendants will be able to welcome these newcomers to the universe and
possibly tell them tales of their ancient ancestors who, trillions of years
ago, lived under a sky full of bright, dazzling stars. But eventually the last
of the hydrogen is burned and the star becomes a solid ball of helium. Without
any radiation pressure to hold itself up, the star contracts under its own weight,
squeezing down to the size of Earth in a ball of degenerate helium. It will spend
the next few billion years radiating away its heat, first as a white dwarf,
then eventually, slowly fading away as a black dwarf. Perhaps our descendants will
witness this final act of the universe, as the last star fades to black. [silence] There's some interesting footnotes to all this. First, we deferred our
discussion about life around a red dwarf until far in their future when they
evolved. However, red dwarfs are home to Earth-
sized planets, many of which already live in their stars'
"Goldilocks" or "habitable" zones where water can exist on their surfaces. Maybe
life can and already does exist around such stars? We're gonna find out more
about the challenges that life faces in a future video. Footnote number 2: all red
dwarfs are rich in metals. But those metals had to be forged in the previous
generation of stars because the universe was pure hydrogen and helium at the time.
Without metals to act as a kind of braking mechanism against runaway cloud
collapse, the first generation of stars were very massive. Now obviously, we can't
go back in time to see those first massive stars forming, but we can use one
of our satellite galaxies, the Large Magellanic Cloud, as a kind of a natural
laboratory to understand how massive stars form in low metal environments. In
fact, I just did a video about this very recently and I'd like to invite you to
go check that out. And our final footnote: our understanding
of red dwarf evolution is purely theoretical. The universe simply isn't
old enough to have ever seen such a star evolve. However, it's plenty old enough for stars like our Sun to evolve, and we're going
to take what we've learned about Sun-like star evolution and understand
how our own Sun will evolve and eventually die .So I invite you to
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new videos. Until next time, stay curious, my friends.
