A while ago my sister asked me about stars, because my niece asked my sister. I’m a huge astronomy buff (my dog, Arcturus Vega, is named for two stars and my puppy is also destined for a star name, already selected) and I promised I’d write about stars. So, Camille and Zoe, this is for you.
What is a star?
A star, loosely defined, is a sphere of plasma which is held together by its own gravity. It used to be that we believed the plasma had to be hot and luminous, but recent discoveries of a star as cold as ice put a bit of a wrench in that assembly.
Plasma is not a separate type of matter, like wood or water or lead, but a state of matter. We recognize four states of matter: solid, liquid, gas, and plasma. Plasma is weird stuff. It has no real shape of its own, but takes the shape of whatever container it’s in like a liquid does. It’s intensely reactive to electromagnetism, like metal. It’s sort of fluid, sort of gaseous, and sort of solid. It flows like a liquid but flows so slowly that it behaves in the short term more like a solid. Stellar matter is so dense and thick that it takes light generated in the core of a star millions, even billions, of years to escape depending on the star’s size.
On top of this, stars are what we consider to be the basic fundamental components of galaxies. So they’re kind of a big deal.
Our star, the sun, is called Sol.
Where do stars come from?
Stars form in nebulae, (singular nebula, large interstellar clouds of gas and dust) from gas that essentially collapsed under the force of its own gravity, triggering nuclear fusion and thereby creating a star.
It’s much simpler than it sounds.
Gravity is basically matter attracting matter the way fabric stores draw quilters. It’s a fairly weak force, but when you’re talking about clouds of dust and gas that stretch for hundreds to millions of light years (which is what a nebula is)… that’s a lot of matter and a lot of attractive force, though it’s diffuse at first. It would only take a little bit of that gas and dust clumping together to start attracting more and more dust and gas into a relatively small area at an increased rate. At some point there would be enough force and pressure in the center of the clump that hydrogen would fuse at the atomic level into helium, which would release a massive amount of energy and would start a chain fusion reaction.
And thus you have a screaming, chaotic newborn star. From there it will grow up to become a great number of different types of stars.
What kinds of stars are there?
Most stars are dwarf stars. Some scientists estimate that 75% of stars in our galaxy are red or brown dwarves. These stars are small, 50% the size of Sol at the largest (dwarf stars that large are very rare), cool and dim, emitting as little as 1/10,000th the light of Sol. Brown dwarves aren’t even large enough to have gravity sufficient for nuclear fusion, so they’re a little more like Jupiter than a star the way we usually think about them.
There are stars like our sun. These stars make up (if I remember correctly) 10-12% of stars in the Milky Way and are technically classified as yellow dwarves. Yellow dwarves are called main sequence stars, meaning they are on the smaller side and are powered by nuclear fusion in their core. They’re mature, luminous, and will stay in their mature state for about 5 billion years and live for 10-15 billion years.
There are blue giants and blue supergiants. These are massive stars, usually the size of our entire solar system, that burn hot and fast. They’re bright, anywhere from 40 to 1.4 million times as bright as the sun. Where Sol will live for about 10 billion years, the average blue giant or supergiant will live anywhere from 100 to 500 million years. These are the stars responsible for most of the heavy metals that exist because they’re the ones large enough to go out in the biggest blaze of glory in the universe, a supernova.
Last but not least are red giants and red super-giants. These are old, old stars who are dim and getting ready to expel their outer layers like a snake shedding its skin, or else they already have. They’re dying, gasping for life, bloated and burning the last vestiges of their nuclear fuel before their cores collapse into supernovae or white or black dwarves.
What do stars do?
Stars do all kinds of cool stuff.
Our sun is a star, as you know, and without it life on Earth as we know it wouldn’t be possible. Plants rely on solar radiation, otherwise known as sunlight, to make their food through a process known as photosynthesis. Herbivores eat the plants, carnivores eat the herbivores, and that way the sun’s energy is cycled and recycled through the air and the land on Earth.
Solar radiation is responsible for all the weather on Earth. Uneven heating between water and land causes air currents to swirl around the planet in an attempt to even out the temperature over the entire globe. Too little sunlight and all the water on Earth would freeze solid and we’d all die. Too much sunlight and all the water on Earth would boil away and we’d all die.
Probably the coolest thing stars do is manufacture heavy metals in their core. Every metal in the universe heavier than hydrogen was made in the core of a star, and expelled into interstellar space when that star died. Without stars there would be no gold for jewelry, no iron or aluminum to make our cars, and no calcium to make our bones.
How does that work? Well, consider that hydrogen is the simplest, lightest thing in the universe and its atomic number is one. So when you start learning basic math, you learn that one plus one equals… two. When you combine one hydrogen with another hydrogen, both with a number of one, you get two. And two is helium. Mix hydrogen and helium, two plus one, and you get three, which is lithium. Combine two helium, two plus two, gets you four and four is beryllium. You can keep mixing and matching larger numbers all the way up to the top of the ladder. Mix to make 26 and you’ll make iron. 47 is silver. 79 makes gold.
The higher the number, the heavier the stuff and the older the star. Stars burn through their hydrogen first, because it’s light and plentiful and fuses easily, and then move on to making heavier things.
So how do stars die?
There are a few ways a star can die, but star death mostly falls into two categories so I’ll just touch on those. Some stars go out with a whimper and some go out with a bang.
Most stars start dying the same way. As they start fusing heavier and heavier metals, they start to bloat like a balloon. After a while they blow their outer layers off into space while the remaining core continues to swell. Heavier elements take more energy to fuse and give off less plentiful energy, in large part because they’re simply less plentiful, so eventually fusion grinds to a halt.
Once that stage is reached, the star does one of two things.
Stars around the size of Sol and smaller go out with a whimper. As the energy from fusion in the center of the star runs out, the balance between it and gravity is upset. Gravity yanks the stellar matter back in on itself, like an over-stretched rubber band. What you’re left with is a white dwarf or a black dwarf. White and black dwarves are essentially the corpses of larger stars. A black dwarf isn’t luminous at all, and a white dwarf is dead but some remaining traces of light created by its predecessor’s nuclear fusion is still escaping the body of the star.
Some stars go out with a bang, and what a bang it is. In these cases the sheer size of the star means it has enough gravity to create a supernova when the core of the star collapses. A supernova is the most energetic thing in the entire universe, and often lets off what’s called a gamma ray burst. A single gamma ray burst creates more energy in its very short life (a few minutes to a few hours) than the rest of the galaxy creates in its entire lifespan of billions and trillions of years. If a gamma ray burst were ever to hit Earth we’d all die before we knew what happened and life would probably never return to our planet. After a supernova the remnant usually turns into either a neutron star or a black hole.
A neutron star is a creepy, undead thing. It’s a zombie star. Neutron stars are dense. Crazy dense. The amount of matter from a neutron star that would fill a teaspoon would weigh as much as the entire island of Manhattan. They spin rapidly, sometimes millions of rotations a second, and lash their surroundings with radio waves, deadly radiation, and other unpleasant business.
The only thing worse to be standing by than a neutron star is a black hole. A black hole is a single point so dense that its gravitation is so strong not even light can escape, and this is what becomes of the cores of some supermassive stars. They’re terrifying and I won’t go into a great amount of detail here about what they are and what they do but suffice it to say there’s no escape from spaghettification if you’re caught too close to a black hole.
So! That’s a brief look into how stars work.