7 Types of Stars That Exist in the Universe: From White Dwarfs to Neutron Stars and Hypergiants

About 90 percent of all stars in the observable universe are doing the same thing right now: fusing hydrogen into helium in their cores. These are main sequence stars, and our Sun is one of them. The Sun sits in the middle of the main sequence, not especially large, not especially small. Main sequence stars are classified by colour and temperature, running from hot blue O-type stars down to cool red M-type stars. A red M-dwarf can burn steadily for trillions of years. A massive blue O-type star burns through its fuel in a few million. The Sun has about 5 billion years left. Stellar mass is the single variable that determines almost everything else about a star's life and death.

When a Sun-like star exhausts the hydrogen in its core, the core contracts and heats up while the outer layers expand enormously. The star becomes a red giant, cooler at the surface, but vastly larger. Betelgeuse, the red star at Orion's left shoulder visible from Indian skies every winter, is a red supergiant with a radius roughly 700 times that of the Sun. If placed at the centre of our solar system, its surface would extend past the orbit of Mars. The Sun will become a red giant in about 5 billion years, expanding to engulf Mercury and Venus, and possibly Earth.

After a mid-sized star sheds its outer layers as a planetary nebula, what remains is the core: a white dwarf. No fusion happens inside a white dwarf. It is simply the hot, dense remnant of a stellar core cooling over billions of years. A white dwarf packs roughly the mass of the Sun into a sphere about the size of Earth. One teaspoon of white dwarf material would weigh several tonnes on Earth. The nearest white dwarf to our solar system is Sirius B, the faint companion to Sirius, the brightest star in the night sky, prominent in Indian winter skies from November through February. White dwarfs are among the most common stellar objects in the Milky Way.

When a star between roughly 8 and 20 solar masses exhausts its fuel, the core collapses so violently that protons and electrons are forced together into neutrons. The result is a neutron star: an object about 20 kilometres across that contains more mass than the Sun. A neutron star's density is comparable to packing all of humanity into a sugar cube. Some neutron stars rotate hundreds of times per second while emitting beams of radio waves, these are pulsars. The first pulsar was detected in 1967 by Jocelyn Bell Burnell at Cambridge. The fastest known pulsars spin at over 700 rotations per second, faster than a kitchen blender on its highest setting. Neutron stars are also the objects that, when two merge, produce gravitational waves, the ripples in spacetime first directly detected by LIGO in 2015.

Above roughly 20 solar masses, the core collapse after a supernova does not stop at a neutron star. Gravity wins completely. The core crushes to a singularity, a point of infinite density, and a black hole forms. The boundary around it, the event horizon, is the point of no return: nothing, including light, escapes from inside it. Black holes are not vacuum cleaners that suck in surrounding matter randomly; they only capture what crosses the event horizon. Stellar-mass black holes range from about 3 to 100 solar masses. Supermassive black holes at galactic centres are a different category entirely, the one at the centre of the Milky Way, Sagittarius A*, has a mass of about 4 million suns. The Event Horizon Telescope captured the first image of a black hole shadow in 2019, targeting M87*, a supermassive black hole 6.5 billion solar masses in size.

Supergiants are the most luminous stars in the universe that are still fusing elements in their cores. They sit at the top of the main sequence and beyond, massive stars that have evolved off it while remaining extraordinarily bright. Rigel, the blue-white star at Orion's right foot, is a blue supergiant about 120,000 times more luminous than the Sun and roughly 860 light-years away. Supergiants burn fast and die young on cosmic timescales, typically living only a few million years before a supernova. They are rare precisely because of this: the Milky Way contains perhaps a few hundred true supergiants against hundreds of billions of main sequence stars.

Hypergiants are the largest and most luminous stars known. They are so massive, some exceeding 100 solar masses, that they lose material at extraordinary rates through stellar winds, shedding mass continuously just to remain stable. R136a1, located in the Large Magellanic Cloud, has a mass estimated at around 170 to 230 solar masses and is among the most luminous objects in the Milky Way's neighbourhood. UY Scuti, a red hypergiant in the constellation Scutum, was once listed as the largest known star by volume, with a radius about 1,700 times that of the Sun. Hypergiants are transient objects on stellar timescales. They live millions of years at most, making them among the rarest stellar objects catalogued by astronomy.

The seven types are not a tidy ladder from small to large, they are different endings to the same beginning. Every star starts by fusing hydrogen, and what it becomes depends almost entirely on how much mass it started with. A star one-tenth the Sun's mass will outlive the current age of the universe as a dim red dwarf. A star 100 times the Sun's mass will tear itself apart in a supernova before the first complex life could evolve on any planet it warmed. Mass is the variable. Everything else, the colour, the density, the eventual neutron star or black hole, is just that variable playing out.