Stellar Evolution In 5 Minutes

Depending on a star’s mass, it follows one of several pathways during its life. Brown dwarfs barely get going and then cool. Red dwarfs keep burning until they’ve transformed all their hydrogen into helium, turning into a white dwarf. Sun-like stars swell into red giants before puffing away their outer shells into colorful nebula while their cores collapse into a white dwarf. The most massive stars collapse abruptly once they have burned through their fuel, triggering a supernova explosion or gamma-ray burst, and leaving behind a neutron star or black hole. ESO

Stars are born from cold clouds of dust and gas that collapse under the force of gravity. How a star goes through life depends on its initial mass. Mass is how much matter a star has which is often related to its size. Hydrogen, the simplest of the elements, is the fuel stars “burn” to radiate the heat and light that make them stars. In a star’s core, where the pressure and heat is enormous, the core of a hydrogen atom, called a proton, fuses with another proton to create helium. Energy is released in the process, and this is what powers the star. Small stars burn hydrogen slowly; large stars, where the heat and pressure are turned w-a-y up, burn it rapidly.

A star with too little mass never gets hot enough to ignite hydrogen in its core and simply cools off and fades from view. These are called brown dwarfs with masses between 15 and 75 times that of Jupiter. Brown dwarfs are weird hybrids, somewhere between real stars and giant planets.

Most brown dwarfs are only slightly larger than Jupiter (10–15%) but up to 80 times heavier due to greater density. The low-mass star illustrated is a typical red dwarf. NASA/JPL-Caltech/UCB

Red dwarfs, the most common type of true star, have masses between .075 and half that of the sun. Their low core temperature means they burn through their supply of hydrogen slowly, making the stars extremely long-lived. A red dwarf keeps its candle burning for 10 trillion years or more, slowly transforming hydrogen into helium before gravity compresses the now helium-rich core into an Earth-sized white dwarf star. White dwarfs, unless paired closely with another stellar companion, quietly cool to become black dwarfs, a process that takes trillions more years.

Sun-like stars burn hydrogen for most of their lives, but the helium “ash” created in the process ignites under the added pressure of gravitational contraction late in the star’s life, causing it to balloon into a red giant. Red giants can measure from 62 million to 620 million miles across or from 70 to 700 times the size of the current sun. Because the star’s energy is spread across a huge area, the surface temperature drops and the star glows orange-red, hence the name.

A selection of planetary nebulae photographed by the Hubble Space Telescope. All were formed from the former outer envelopes of their stars and set to glow by ultraviolet light radiated by the hot white dwarfs in their centers. NASA / ESA

Eventually, helium burning stops and the star collapses. But the mounting pressure and heat from the collapse fire up a new round of helium burning in a shell around the now-inert core of carbon and oxygen. Once again, heat causes the star to expand outward, but this time the entire envelope is blown off. A white dwarf, born as the core of the star contracts under the force of gravity, energizes the gases in the puffed-away atmosphere to glow red, blue and green. Called planetary nebulae, they look like expanding bubbles and some are strikingly colorful through amateur telescopes.

Stars 8 to 15 times more massive than the sun burn hydrogen like the champ scarfing down hot dogs in a hot dog eating contest. They live short lives by cosmic standards — only about 20 million years — and then explode as supernovas. Even though sun-mass stars create carbon and oxygen by burning helium, they’re not hot enough to use those elements as fuel to make more energy. Massive stars have the wherewithal to heat and compress those and additional elements like neon and silicon and create energy from their fusion. But once silicon burns to iron, time’s up!

Model of the inside of a supergiant star just before it explodes as a supernova. The tremendous heat and pressure in its core not only fuses hydrogen to make helium but helium to make carbon and so on in a series of nested shells all the way up to iron … the end! R.J. Hall

Iron can’t be fused to into heavier elements. Now out of energy (and the push-back pressure from burning), gravity takes over and the star collapses rapidly. The infall spawns a shock wave that rips the star apart in an explosion 10 billion times more luminous than the sun called a supernova. The magnitude of the blast makes supernovae visible in galaxies billions of light years distant from Earth. Once the “smoke” clears, astronomers often find the star’s super-compressed core still intact, either in the form of a city-sized neutron star or black hole. But sometimes the detonation leaves nary a nugget, only an expanding cloud of debris.

Now you see that the fate of a single star has almost all to do with a simple fact: how much stuff it started out with.

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