1000 Things You Didn’t Know About the Universe #4: Most Stars are Small and Red

Welcome to a new series of posts that will characterize 1000 amazing facts about the Universe.  There is so much out there that we have yet to learn, and every day, astronomers across the globe are using their research to reveal the deepest secrets of the cosmos.  This series will look at the strangest, coolest, most exciting facts that we have discovered in hundreds of years of modern science.

Fact #4: Most of the stars in the universe are red dwarfs smaller than our Sun.

There is a leap of understanding that happens when a child learns that our Sun is a star, just like all the others they can see in the night sky.  It means that every star is also a Sun, and could provide warmth for faraway planets.  Of course the major difference is that the Sun is much closer to us so it outshines all of the other stars in the sky combined.  The next question that is usually asked is whether all those other stars are the same as the Sun.  The answer is both yes and no.  Other stars are fundamentally the same as the Sun, but they do have things that make them different from each other, and these differences are all interrelated.

Bellatrix, a true blue star. Credit: Majik Phil

The three major variations that most stars have are size, colour, and temperature.  All of them depend on the mass of the star (how much stuff it’s made of) and how long it has been around (since stars change over time).  All stars undergo a process called fusion, where lighter elements are subjected to such incredible temperature and pressure that they combine to form heavier elements in the core.  This process releases a huge amount of energy, which is why stars give off such intense light.  The more massive a star is, the hotter it’s core temperature is, resulting in more rapid fusion of basic elements, or fusion of heavier elements later on.  More massive stars don’t live as long either, since they burn through their fuel much faster.  The most massive stars will live as little as 100 Million years, compared to small stars that live for many billions of years.

The size, colour, and surface temperature of a star are interrelated.  Called blackbody radiation, stars with a hotter surface temperature are more blue, and stars with a (relatively) cooler surface temperature are more red.  Let’s look at two stars with the same mass / energy output.  One is smaller and releasing it’s energy over that smaller surface area, and it’s blue.  The other star is much larger, and even though it radiates the same energy, it has a much larger surface area to spread out that energy, so it has a lower energy output per unit area, giving it a red colour.  Energy per unit area is the key to surface temperature and colour for stars.

So stars can vary in colour, size, and temperature all over the universe, though most fit in categories.

Life cycles of stars: Credit: Johnchristiangonzalez.com

Aside from the stars shown above, there are large stars that start out blue, yellow, or white and can evolve into blue giants and supergiants.  There is a lot of variation depending on how massive the star is when it forms.  In terms of star size, the Sun is actually pretty average. Some stars make the Sun look like a grain of sand next to a beach ball, but these stars are incredibly rare.

So it’s clear that most stars in the sky are not like our Sun, even though they all undergo fusion to produce energy.  But of all these different sizes, colours, masses, and temperatures, which stars are the most common?

The answer is Red Dwarf stars.

Red Dwarfs make up the vast majority of the stars in the universe.  They are generally much smaller than the Sun, but still large enough to fuse Hydrogen in their core, meaning they would still be at least 10 times as massive as Jupiter.  Their small size means that even though they are undergoing fusion, the process is so slow that it takes them many billions of years to burn through their fuel.  In fact, some of the smallest red dwarfs could live for up to 10 Trillion years! That’s 1000 times the current age of the universe.  They are hard to find due to their size, and are much dimmer than the Sun, making them very hard to see if you’re using human eyes. When red dwarfs finally do burn up, they will become white dwarfs, just like the Sun eventually will, but without a red giant phase.

This longevity for red dwarfs and short lifespan for massive stars means that as the universe continues to evolve, red dwarfs are surviving while massive stars are dying, and so red dwarfs are slowly making up a larger and larger fraction of stars in the universe.  Eventually, the universe will stop forming stars and all galaxies will become ancient collections of old, red stars with no gas, like some elliptical galaxies.

A red elliptical galaxy with little gas and no star formation.

And then, after trillions and trillions of years, even the red dwarfs will have finally burned out, and the white dwarfs will cool and become invisible, and the universe will become a cold, dark, empty place once more, with humanity existing in one tiny insignificant moment, in the bright period of its life.



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