If we were talking about people, I’d say there’s no such thing as a “normal” person. We’re all weird in our own way—that’s what makes us unique and ourselves.

However, there’s such a thing as a functional human—a human with a combination of functional organ systems and/or prosthetics that makes daily life navigable. And just as no star is exactly alike, there are functional stars.

Nature makes mistakes all the time. It is not intelligent—it doesn’t know the best way to do anything. It doesn’t know the path of least resistance or least effort. It just tries everything at random, and we get to observe what happens.

A “normal” star is what happens when nature stumbles upon the right conditions. But…what does that mean?

There are four basic laws of nature we can use to describe the functional structure of a star. That’s right, just four.

The law of hydrostatic equilibrium describes the balance between weight and pressure. In a star, it’s the balance between the energy produced within—which drives the star to expand—and the star’s own gravity, which drives it to contract.

The law of energy transport says that energy must flow from hot to cool regions. It’s an effect we see all the time. When you warm your hands over a fire, you expect the energy from the fire to flow to the cooler region of your hand.

The law of mass conservation means that the total mass of a star must equal the sum of the masses in all its layers.

Make sense? If you imagine a star as similar to an onion, each layer adds up to the star’s total mass. Or you could think of something you own. The mass of all your pencils, for instance, is equal to the mass of each one combined.

There’s just one added requirement to this law for stars: the mass has to be smoothly distributed throughout the star, no gaps allowed.

Last but not least, the law of energy conservation states that the amount of energy flowing out from the top of a star layer must be equal to the amount of energy coming in at the bottom, plus whatever energy is generated within the layer.

Makes sense, right? That’s simple math. It’s like cars in a factory. The total number of new cars produced must equal the sum of all cars manufactured inside. Cars can’t appear out of thin air…or disappear magically.

So, how do we use these laws to describe the inside of a star?

We can use these four laws to model the interior of any star. We simply divide a star’s interior into at least one hundred concentric “shells”—the “layers” I’ve been talking about—and we solve all four equations for each one.

Yeah. At least 400 equations for each star. It’s simple…with the help of computers.

The image above shows a much-simplified model of our own sun. There are only ten shells shown. But you can see how they’re defined by the layer’s radius—its distance from the center of the star.

The bottom layer has a radius of zero, since it’s literally the center. The top layer has a radius of 1, since it’s measured in units of solar radii. One solar radius, R_☉, is the radius of our sun. A star 2 R_☉ would have a radius twice the size of our sun.

The temperature of each layer is measured in 10⁶ Kelvins—that is, in Celsius degrees, plus 273 (to convert to Kelvin), times 10⁶.

The density of each layer is fairly self-explanatory—the units are grams per cubic centimeter, so it’s a measurement of how many grams are in each cubic centimeter of space.

The mass and luminosity of each layer are measured in solar units like the radii. One solar mass, M_☉, is the mass of our sun, and one solar luminosity, L_☉, is the luminosity of our sun. Luminosity, by the way, is the total energy a star produces—not just the visible light we can see.

Now, let’s get back to our original question. What is a “normal” star?

Like I said above, it’s a star that nature didn’t make a mistake with.

Stars form when a shock wave condenses the material in a giant molecular cloud—often the result of winds from much older stars. But what happens after that depends entirely on how much material was available and the strength of the shock.

Stars that form successfully take their place on what astronomers call the main sequence. Here it is on the H-R diagram, possibly the most fundamental diagram to stellar astronomy.

The H-R diagram organizes all stars according to temperature and luminosity. You can see these qualities on the horizontal and vertical axes.

You might notice that when stars are sorted in this way, they form distinct categories. Supergiants are large stars reaching the end of their lifespans, and giants are similar, smaller stars reaching the end of their lifespans.

White dwarfs are typically shown on the H-R diagram, but they are not really stars—not if we define a star as an object that generates energy by nuclear fusion. They are actually the dead husks of low-mass stars like our sun.

But…what’s up with that band of stars that neatly crosses the H-R diagram from corner to corner?

Those are “normal,” functional stars.

I’ve shown you what the interior of a star looks like. Most importantly, each star is a balance between internal pressure and its own gravity, both forces constantly fighting for dominance.

A star that is massive, but not very hot, cannot support its own weight. It doesn’t have enough internal pressure to balance its own gravity.

A star that is small, but extremely hot, will actually burst apart. Its internal pressure will be too strong for its gravity to counteract, and it won’t be able to hold itself together.

Main-sequence stars, though, have the perfect balance of mass and internal pressure.

They range from less than a tenth of a solar mass to seventy solar masses, but the more massive ones have the internal pressure to balance their own gravity—because they are the lucky few that resulted from nature’s success.

Main-sequence stars are also pretty good at maintaining that internal pressure. The pressure-temperature thermostat is a quirk of nature that allows that.

But, like living beings, or like any mechanical or electric device that needs power or fuel, no star lasts forever. Even main-sequence stars will eventually reach a point where their pressure-temperature thermostat will fail, and they will leave the main sequence to become giants or supergiants.

That evolution of stars is something we’ll explore in depth coming up.