A star is a celestial body that generates light and heat through nuclear fusion. When a star nears its end, it undergoes different transformations depending on its initial mass and current stage. Therefore, there isn’t one single name for a “dying star,” but rather specific names associated with its various phases. This article explores how stars evolve and the distinct stellar objects they become.
How a Star’s Mass Determines Its End
A star’s “death” does not happen suddenly but involves a series of transformations that begin when it exhausts its primary fuel source, hydrogen. The most important factor determining a star’s eventual fate and the names it acquires during its final stages is its initial mass. Stars are categorized into low-mass and high-mass types, each following a unique evolutionary path, leading to different “dying” phases and ultimate remnants.
The Final Stages of Low-Mass Stars
Stars with masses up to about eight times that of our Sun, including our own Sun, undergo a specific sequence of changes as they age. After billions of years of fusing hydrogen into helium, the star’s core begins to shrink and heat up. This causes its outer layers to expand dramatically and cool, transforming the star into a red giant. For instance, the Sun is expected to become a red giant in about 5 billion years, expanding enough to engulf Mercury and Venus.
As a red giant, the star continues to shed its outer layers into space. This expulsion of gas and dust forms a glowing shell around the exposed core, known as a planetary nebula. Despite their name, planetary nebulae have no direct connection to planets; the term originated because early astronomers observed their round, planet-like appearance through telescopes. The remaining core, now stripped of its outer layers, becomes a white dwarf, a dense, hot remnant roughly the size of Earth.
White dwarfs slowly cool and fade over billions of years. They no longer undergo nuclear fusion but radiate residual heat. This cooling process eventually leads them to become cold, dark bodies known as black dwarfs, though the universe is not old enough for any black dwarfs to have formed yet.
The Violent End of High-Mass Stars
Stars with masses greater than eight times that of our Sun experience a dramatic and rapid end. After exhausting their hydrogen fuel, these massive stars expand, becoming red supergiants that are larger and more luminous than red giants. They fuse heavier elements in their cores, eventually creating an iron core. Iron cannot be fused to produce energy, so the core collapses catastrophically under its own immense gravity.
This rapid core collapse triggers an explosion known as a supernova. Supernovae are highly energetic events, briefly outshining entire galaxies. The intense shockwave from a supernova expels the star’s outer layers into space at high speeds, enriching the cosmos with heavy elements essential for new stars and planets. What remains after this cataclysmic event depends on the original mass of the star’s core.
The Remnants of Stellar Death
After a star’s “dying” process, various stable objects are left behind as ultimate end states. These include white dwarfs, which are the remnants of low-mass stars like our Sun.
For more massive stars that undergo a supernova, the remnant can be a neutron star. These dense objects form when the core of a star, between 1.4 and 3 solar masses, collapses further than a white dwarf but is not massive enough to form a black hole. Neutron stars are only about 12 miles (20 kilometers) in diameter but contain more mass than the Sun, making them extremely compact.
If the original star’s core is extremely massive, over three solar masses, its gravitational collapse after a supernova is so intense it forms a black hole. A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape. These remnants represent the final, most extreme products of stellar evolution, with the star’s entire mass concentrated into an infinitely small point called a singularity.