Red Dwarfs
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.
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| Red Dwarfs Source: ESA/Hubble / CC |
Welcome back to my blog "Mystery of Galaxy" and in this article 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.
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| Proxima Centauri Source: ESA/Hubble / CC BY |
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 , 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. 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.
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| Source: ESO/M. Kornmesser / CC BY |
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.
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| Andromeda |
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| Milky Way Galaxy with starry sky |
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| White dwarf |
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| black dwarf |
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.
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 next article. 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. 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 .That's all. Thank you
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