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Besides being a point of light, a star is a luminous, spherical mass of plasma, enough to hold itself together under its own gravity. On its own, though, gravitational rounding isn’t enough. What differentiates a star from even the largest, hottest gas giant is fusion. How does that work, and how do stars form in the first place? Let’s take a tour of one of the universe’s most defining, beautiful, and enduring objects—including our own Sun.
Fusion: How Stars Hold Together
Most of an atom’s volume is empty space. What holds atom away from atom is the mutual repulsion between electrons, a force sometimes called the Coulomb barrier, so strong that it can only be broken at incredible temperatures and pressures. That’s why fusion is so hard to do on Earth’s surface—but stars don’t care about that.
Inside the core of a star, the crushing weight of its own mass is enough to overcome the fundamental forces holding individual atomic nuclei apart. Electrostatic repulsion gives way to the pressure, breaking the Coulomb barrier and fusing two nuclei into one in a process called stellar nucleosynthesis. The energy that had been holding the atoms apart is released in the form of light.
The smallest amount of matter that will reliably collapse into a star is about 13 times the mass of Jupiter. These cool, dim stars are known as brown dwarfs or ‘failed stars.’
Credit: NASA/JPL-Caltech
But even the mass of a brown dwarf isn’t enough to overcome the Coulomb barrier and start up stellar fusion. Fusion takes so much energy that some elements, up to the atomic mass of iron, are only made by fusion inside the core of a star, and the absence of those elements is one way astronomers conclude whether fusion is happening or not. A star needs to be about 80 times the mass of Jupiter to spark up a fusion engine in its core.
How Do Stars Form?
Stars are born in stellar nurseries, molecular clouds of cold gas and dust. These clouds contain many times the mass of a star, but they’re so huge that their average density is just a few dozen molecules per cubic centimeter. Compare that with the best Earthly vacuum chambers, which can get down to perhaps a million molecules per cubic centimeter.
The cold molecular cloud our Solar System formed out of is believed to have contained about triple the mass of the sun. Gravitational attraction between atoms in the cloud is enough to pull it together into an eddy with an axis of rotation. As matter concentrates, it heats up due to compression. The resulting spheroid of hot, dense matter is called a protostar. More matter in the same volume makes a deeper gravity well, and infalling grains of dust and ice that don’t fall into the protostar can clump together, forming an accretion disk from which planets can form. Material that doesn’t become a planet may be flung outward, beyond a growing star’s gravitational influence. In our own Solar System, the outer boundary of the Oort Cloud marks the place where the Sun’s gravitational influence can no longer overcome the galactic tide.
The theoretical upper limit for a star is about 300 solar masses (abbreviated M☉), but in reality, there are precious few stars above 120 M☉. The most massive stars, suspected or confirmed, are Wolf-Rayet stars, of which only a handful have been catalogued. (More on these ultra-high-energy stars below.) Above 200 M☉, a molecular cloud may collapse into a binary star system or even a black hole, instead of a single star.
Younger, metal-rich stars like our Sun are called population I stars, because they formed out of the ashes of their predecessors, a generation of stars (population II) that formed so early there hadn’t been supernovas to make heavy elements yet. There’s a hypothesized group of still older stars, which would have collapsed directly from primordial hydrogen in the early universe. However, none have yet been discovered. One reason: population III stars would have been absolutely huge, and their great mass would make them short-lived: just a few million years from cradle to grave.
One of the James Webb Space Telescope’s major goals is to investigate stellar nurseries like this one in Rho Ophiuchus.
Credit: NASA/ESA/James Webb Space Telescope
It’s not clear whether gravity alone is enough to make stars coalesce from a molecular cloud, or whether more of an impetus is required, such as being shocked by a nearby supernova. However, there’s supporting evidence for that theory: granules of an uncommon isotope of iron, found in Antarctic ice. Iron-60 or ⁶⁰Fe is formed late in a star’s life, too deep for convection to bring it to the surface, so it’s only released in a supernova. There’s too much ⁶⁰Fe for it to have just drifted in from the interstellar medium, suggesting that it was present in the molecular cloud from which the Solar System evolved.
Types of Stars
Stars are classified according to many factors, including their color, temperature, brightness, chemistry, and mass. Astronomers relate these properties to one another in graphs known as Hertzsprung-Russell diagrams, which show that most observed stars conform to certain combinations of vital stats. The most prominent feature of a Hertzsprung-Russell diagram is the main sequence: a single bold streak that reaches almost from corner to corner, which encodes the life story of most known stars.
Credit: ESO
There’s a lot going on in these diagrams, so if you aren’t already familiar, here’s a less-than-two-minute video that explains it, visually.
All stars spend the majority of their lives as so-called main-sequence stars. Consequently the vast majority of stars—90% or so—are undergoing hydrogen-helium fusion at any given time. This is because stars spend most of their lifespan simply burning through their fuel supply.
As stars get bigger, broadly speaking, they get hotter and brighter. Low-mass stars turn into giant stars at the end of their lives, and high-mass stars become supergiants or even hypergiants. At the same time, the brightest stars have the shortest lifespans. It may be counterintuitive because embers are hot and icebergs are blue, but cooler stars are redder in color, while the hottest stars burn a clear bluish-white.
To understand how it all pieces together, check out this outstanding “cheat sheet” from Atlas of the Universe:
Sol, our sun, is a smallish yellow G-type main-sequence star, about halfway through its lifespan.
Credit: Atlas of the Universe
Stellar Evolution: Going Out With A Bang
Since all stars spend time on the main sequence, how stars live is almost less interesting than how they die. So, here’s how it ends, from the smallest failed stars to the biggest, brightest, hottest, and widest hypergiants:
A star below about 8 M☉ will never restart fusion after it has fused its carbon into oxygen. Instead, the star will eject its outer layers, forming a planetary nebula with the core of the star exposed. This exposed core is called a white dwarf.
On the other end of the continuum, Wolf-Rayet stars are an exotic blue-white inferno, formed as a matter of course, a brief phase late in the lives of the most massive stars. Against the backdrop of a star’s lifetime, the Wolf-Rayet stage looks like the flutter of a mayfly. Fewer than 1,500 Wolf-Rayet stars are thought to exist in the whole of the Local Group, with fully a third of them contained within the Milky Way.
When a star with more than eight times the Sun’s mass runs out of fuel, its core collapses, rebounds, and explodes as a supernova. What’s left behind depends on the star’s mass before the explosion. If what remains has a mass greater than about 0.7 M☉ it forms a neutron star. Above two or three solar masses, the exposed core collapses into a stellar-mass black hole.
Red dwarf stars are tiny, as stars go, but they’re the most common type of star in the universe. The coolest, dimmest red dwarf stars are barely twenty percent the mass of the Sun, and less than one percent as bright. It’s thought that at the end of their lives, red dwarfs cast off their mantles, leaving only the naked remnant of the stellar core slowly cooling into the void. However, their lifespan would be on the order of hundreds of billions or even trillions of years, much longer than the current age of the universe, and no one has found a red dwarf old enough to check.
Even odds that by the time your average solo DPS queue pops, though, we’ll have figured it out.