What’s a Star and What Are They Made Of?

What are stars made of? It’s a question almost everyone has asked at some time or another. The answer is very hot gas; specifically, hydrogen and helium, but that barely begins to tell the full story.

So let’s take a step back for a minute and consider how stars are formed in the first place. After we answer that question, we’ll delve a little deeper and resolve some other stellar mysteries too.

How Are Stars Formed?

Stars are formed out of vast clouds of gas (hydrogen and helium) and dust, called nebulae. These clouds are almost incomprehensibly large and can be hundreds of light years in diameter. (The Tarantula Nebula, embedded within the Large Magellanic Cloud, is thought to be about a thousand light-years across.)

This close-up view of the Tarantula Nebula from Hubble shows a large star-forming region within the cloud. Credit: NASA/ESA
This close-up view of the Tarantula Nebula from Hubble shows a large star-forming region within the cloud

To put things into perspective, in normal, “empty” space, there’s roughly one atom per cubic centimeter, but in a nebula, there could be a million atoms per cubic centimeter. In photographs, from our Earthly vantage point, they can appear as beautiful, colored clouds.

You might think it would look pretty amazing to live on a planet within a nebula, but, unlike sci-fi movies and TV shows, if you could move through one you’d probably hardly see anything at all.

That’s because the particles of gas and dust are so small and so far apart that you wouldn’t even notice them (although any planet within a nebula would almost certainly have some fine meteor showers!)

So how do these tiny particles come together to form a star? That literally requires a force of nature. In this case, something needs to stir up the cloud so that the particles collide with one another. It could be the shockwave from a nearby supernova, a passing star or even a collision between two galaxies.

As the particles collide, they stick together and grow larger, gaining mass as they increase in size. After about ten million years or so, the protostar has gained enough mass that its gravitational pull begins to drag particles down toward its center.

This artist's impression shows a protostar pulling material in from the surrounding nebula. Credit: NASA/JPL-Caltach/R. Hurt (SSC)
This artist’s impression shows a protostar pulling material in from the surrounding nebula.

This causes energy to be released and for the pressure at the core to increase. The hydrogen atoms then fuse together, forming helium, and more energy is released as a by-product of the process. This energy is what causes a star to shine and, as a result, you could say the star is born.

The whole process can take anywhere between hundreds of thousands to tens of millions of years, depending upon the mass of the star. The higher the mass, the less time it takes – but also the shorter the lifespan of the star. A typical sun-like star would take about ten million years to reach this stage.

It’s also at this point that the star blows out a stellar wind, preventing more gas and dust from falling in toward it. The remaining gas and dust continues to collide and can form planets, moons, asteroids, and comets.

What’s Inside a Star?

Once it’s shining, the nuclear reactions continue at the star’s core. Hydrogen atoms continue to fuse together, forming helium and generating vast amounts of thermal radiation, in the form of heat and light. In our own solar system, the core makes up about 20%-25% of the Sun’s radius.

Surrounding the core is the radiative zone, through which the thermal radiation passes on its way to the surface. The radiative zone extends to about 70% of the Sun’s radius and carries the energy in the form of particles known as photons.

Next is a relatively narrow transition layer known as the tachocline. This acts as a buffer between the radiative zone – where energy travels in waves – and the convective zone, where that energy is mixed and churned together.

The energy cools as it travels toward the surface, causing its density to increase and for it to fall back down toward the tachocline. As it sinks, it heats up and expands, causing its density to decrease and for the energy to then rise again. In this way, the convection cycle begins again.

The visible surface of the Sun is known as the photosphere. This is where sunspots form and appear as black marks against the Sun’s blinding surface. These spots appear dark because they’re cooler than the surrounding solar surface and are formed by the Sun’s magnetic field inhibiting convection.

The photosphere is also the lowest layer of the Sun’s atmosphere and extends for about 300 miles or roughly 500 kilometers. The atmosphere is made up the chromosphere, the transition region, the corona, and the heliosphere and extends for roughly nine million miles (over fourteen million kilometers) in total – that’s nearly a tenth of the distance from the Earth to the Sun.

layers and features of the Sun.
layers and features of the Sun. Credit: Kelvinsong

You can actually see the corona during a total solar eclipse. That’s the white halo that appears to surround the darkened silhouette of the Moon as it passes in front of the Sun.

Curiously, the temperature in the atmosphere is hotter than the surface of the Sun, but no one quite knows why.

Why Are Some Stars Larger Than Others?

Another previously unsolved mystery is why some stars are larger than others. For example, the star Rigel, in the winter constellation of Orion the Hunter, is thought to be about 21 times more massive than the Sun and has a radius of about 79 times that of the Sun. If it were placed at the center of the solar system, it would nearly reach Mercury.

How did it get to be so large? Up until early 2009, no one quite knew. In theory, the pressure of the energy radiating out from a massive star should blow away the gas and dust surrounding it, preventing any further material from falling inward. As a result, the star should stop growing.

But according to computer simulations reported in early 2009, that’s not what happens. As the dust falls in toward the star, instabilities form channels that allow the energy to radiate out. Meanwhile, other channels form that allows the surrounding dust to fall in. Consequently, the star continues to grow.

This image of Jupiter is overlayed by the orbits of the planets within the solar system.
This image of Jupiter is overlayed by the orbits of the planets within the solar system. Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Other stars are larger simply because they’re reaching the end of their lives. For example, the star Betelgeuse, neighbor to Rigel in Orion, is probably the largest star easily seen with the naked eye. A red supergiant, if you were to place it at the center of our solar system it would probably extend past the orbit of Jupiter and could possibly reach halfway to Saturn. It’s so large it could contain a billion Suns within it.

Despite only being about 8 million years old, it’s already used up all the hydrogen at its core, which has caused it to swell and expand. At some point in the future – it could be tonight, tomorrow or a thousand years from now – it will explode as a supernova.

Why Are Some Stars Brighter Than Others?

This, again, relates to a star’s mass. If you could place all the stars at an equal distance from the Earth, the largest, most massive stars would shine the brightest. It’s not because they’re bigger and – theoretically – easier to see.

Massive stars radiate more light because there’s a lot more pressure bearing down upon the core. In order for the star to remain stable, it needs to push more thermal energy out to counteract the gravity pushing in.

That energy (a result of the fusion of hydrogen into helium) radiates out as heat and light. So the more massive the star, the more heat and light it produces. Hence, the star is brighter.

Why Are Stars Different Colors?

A star’s energy output also explains why the stars are of different colors. The hottest, brightest stars appear blue. This is because the thermal energy emits most of its light at the blue end of the spectrum. The more energy is produced, the more blue light is emitted.

As red light appears at the other end of the spectrum, it stands to reason that red stars are typically cooler and dimmer. Since many of these red stars are smaller and less massive, less energy is needed to counteract the gravity pushing inward. With less thermal energy being produced, the star’s surface temperature is lower and there’s little light being emitted at the blue end of the spectrum.

So why are there no green or purple stars? They exist, but our eyes can’t really see them. To help explain, let’s consider our own Sun.

In reality, our Sun is not yellow, but white. As a result, our eyes have evolved to be sensitive to white light. A green star primarily emits light right in the middle of the spectrum but it’s actually emitting light in all colors, across the entire spectrum. So we don’t see a green star, we see a white star.

It’s a little bit different with purple stars. Their light is emitted close to the blue end of the spectrum and is emitting blue light as well as purple. Since our eyes are more naturally adapted to see blue, rather than purple, we see that color.

What happens When Stars Die?

A star will continue to shine for as long as it can fuse hydrogen into helium. Since the hydrogen isn’t being replenished, once it’s burned through its hydrogen supply there’s no more fuel and the star reaches the end of its life.

What happens next really depends upon its mass. A single star about ten times more massive than the Sun will typically explode as a supernova. This happens because it’s no longer able to radiate enough energy to counteract the force of gravity pushing inward. Gravity pulls matter down toward the core, which then implodes under pressure, causing a supernova.

(In binary systems, one star may be cannibalizing matter from its partner. In this situation, the cannibal star gains so much mass that it can no longer produce enough energy to counteract the matter pushing down on it. Hence, the star explodes.)

The first ever image of a black hole. This one lies at the heart of the galaxy M87 and is thought to be about 6.5 billion times the mass of the Sun.
The first-ever image of a black hole. This one lies at the heart of the galaxy M87 and is thought to be about 6.5 billion times the mass of the Sun.

If the star is large enough, the core might continue to collapse and form a black hole. These stellar black holes initially have a mass of about ten Suns but are only about ten miles in diameter. The “hole” then continues to grow more massive as its immense gravity pulls other material in toward it.

Other stars with less mass may form neutron stars. In this situation, the core continues to collapse, with the resulting pressure causing electrons and protons to combine into neutrons. These stars are typically only about 10 miles (16 kilometers) in diameter but have a mass of several Suns.

They also spin incredibly fast – thousands of times a minute – a byproduct of the supernova that created them.

What about stars like our Sun? How will the Sun die? Once the Sun has burned through the hydrogen at its core, it’ll continue to burn the hydrogen found in its outer layers. As a result, it will expand as its density decreases and its light will turn red because less energy is being emitted.

As this happens, it will move into the red giant phase and become many times larger than it is now. At some point it will swallow the planets Mercury, Venus and, more than likely, the Earth too. (But don’t worry. We have about five billion years left to go.)

The Ring Nebula is formed from the discarded shell of a dying star. The star itself can be seen at the center of the image.
The Ring Nebula is formed from the discarded shell of a dying star. The star itself can be seen at the center of the image. Credit: NASA/ESA/C. Robert O’Dell

In each of these scenarios, shells of gas and dust are expelled and expand outward into space as the star dies. Many of these shells can actually be seen with a telescope from Earth. The Ring Nebula in the summer constellation of Lyra, is a fine example. A large telescope will reveal the tiny remains of the star at the center of the ring.

For the Sun, the outer shell of gas and dust will eventually dissipate, leaving behind the remains of the core – a white dwarf. In this state, the Sun could exist for billions, maybe even trillions of years before it finally cools and ceases to emit either light or heat. At that point, it becomes a black dwarf.

(Black dwarf stars are theoretical as the universe has not existed for long enough for a white dwarf to actually evolve this far!)

In many ways, it’s wrong to think of stars as dying – or at least, not in the way that we think of death. If anything, you could think of stars like the Sun as being almost embryonic, with the vast majority of their lives spent in another form throughout the billions or trillions of years after the red giant stage.

What about the gas and dust that was ejected during that red giant stage (or the occasional supernova)? In a perfect example of cosmic recycling, those particles will eventually go on to form new nebulae, new stars, and even new solar systems.

To quote the late Carl Sagan, “we are made of star stuff.” Everything you see around you, whether it be living or inanimate, is made of atoms that once existed within a star. When we are gone, a tiny part of us will live on in new Suns and maybe even new forms of life.

What are stars made of? Yes, they’re made from hydrogen and helium, but they’re also made from the ashes of the past. And someday they’ll be made from the ashes of our future.

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