Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.
It's easy enough to say what a star is. It's one of those bright pointy things that twinkle in the night sky. But beyond that, the actual definition of a star is as rich and colorful as, well, the stars themselves.
A star is born
First off, a decent enough astrophysical definition of a star is: any object that is sufficiently massive that it can ignite the fusion of elements in its core due to the gravitational pressures inside the object itself.
The smallest object that we know of today that is capable of such feats is around 10% the mass of our sun. In the far future, with more and more heavier elements adding to the mix and polluting the interstellar waterways, fusion will be possible in lower-mass objects, but that's not something we need to worry about today.
The smallest stars are called red dwarfs, because they are red and small. They only weakly and feebly burn hydrogen in their cores and emit radiation primarily in the infrared part of the electromagnetic spectrum, hence their dull red color. They are by far the most common star in the Milky Way galaxy, though they are so small and so dim that even our nearest neighbor star, Proxima Centauri, is completely invisible to the naked eye.
The next category up in stars are the ones like our sun. Medium mass, medium brightness, medium lives. They emit radiation throughout the visible spectrum, making them appear nice and white (yes, our sun is really white, but filtered through our blue atmosphere it appears slightly yellow).
Above that you have the giant stars, which are as big as they are rare. But because they are so intensely bright, they are easy to spot. For example, we see the spiral arms of galaxies not because they are that much more populated than the spaces in between, but because they are lit up like Christmas tree lights with bright stars.
Almost every star you see in the night sky is much larger than the sun. For the majority of their lives, the biggest stars are tinted blue. This is because they emit so much energy that the radiation that comes out is actually all the way over in the ultraviolet, with a little bit of the emission coming out in the blue end of our visible range.
The main sequence
Besides the small red stars, the medium white stars and the big blue stars, there are of course all the in-between stars, and some strange ones that are both large and red. A hundred years ago, when astronomers were first cataloging stars, this was absolutely a confusing mess, with apparently no rhyme or reason between a star's color and its brightness and size.
The solution came with what we now call the Hertzsprung-Russell diagram, which is the backbone of understanding how stars live even today. The Hertzsprung-Russell Russell diagram is a plot of the temperature of a star (which we can get from its color) and its brightness.
If you take a whole bunch of stars and plot their temperature and their brightness, with one point for each star on the diagram, you find something surprising. It turns out that stars don't have all sorts of color and brightness combinations. Instead there is a stripe running diagonally that the vast majority of stars live on. This stripe runs from the dim, red end to the bright, blue end.
This stripe is known as the main sequence, and stars that burn hydrogen in their cores (the primary fuel source for the vast majority of a star's life) will live somewhere on this stripe. As stars age, they slowly and gently move up the track along the main sequence, becoming steadily brighter and bluer as the eons go by.
How long they live on that track, burning hydrogen in their cores, depends on how massive they are. A low-mass red dwarf can spend trillions of years on the main sequence, while a giant star bigger than our sun may only last a few million years at best.
Once hydrogen fusion ends inside of the core of a star, it moves off the main sequence and evolves in different directions. Large stars become red giants, which occupy their own positions on the Hertzsprung-Russell diagram. Other stars might zigzag back and forth, alternating between blueness and redness as heavy elements attempt to fuse deep in their hearts.
Armed with the Hertzsprung-Russell diagram, we can see what truly defines a star: it's an object that lives on the main sequence of that diagram. It's an object that burns hydrogen and steadily evolves along that narrow strip connecting its brightness to its temperature. Things that exist outside that strip are either giants attempting to fuse heavier elements in a futile attempt to stay burning, or dead and decaying remnants like white dwarfs and neutron stars.
That Hertzsprung-Russell diagram is the unsung hero of observational astronomy. With it, astronomers can spot a star, measure its brightness and temperature, and know exactly where in its life cycle it is. This enables them to predict its future and its evolution. Nature rarely affords such straightforward insights, but stars are truly special cases in the universe.
The vast majority of matter in the universe is strung out in wispy nebulas. Stars are a special, unique breed — a temporary object powered by fusion. This fact makes them oddly easy to predict and understand.
Learn more by listening to the episode "What is a star?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Mitchell L. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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