Hypergiant Stars: The Most Massive Stars in the Universe

Hypergiant stars are the live-fast, die-young rock stars of the universe, burning millions times brighter than the Sun and burn out quickly.

Consider a star that emits as much energy in a single second as our Sun does in 100 days. These are not mere figments of imagination, but actual celestial entities. They are hypergiant stars, the most colossal stars in the cosmos, each with its own unique set of characteristics.

What are Hypergiant Stars?

Hypergiants are stars that burn with the brilliance of millions of Suns. They are born from the same clouds of interstellar hydrogen gas as normal stars. However, their enormous masses of tens or even hundreds of times that of the Sun create tremendous internal pressures that heat their interiors and accelerate the rate of nuclear fusion reactions in their core.

A star like our Sun can sustain itself on a relatively modest amount of hydrogen fuel for a staggering period of up to 10 billion years. In contrast, with its vast reserves of fuel, a hypergiant star will exhaust it in a million years or less, blazing forth as a brilliant but comparably short-lived cosmic beacon. The rapid and dramatic evolution of these stars is proof of their sheer magnitude and grandeur.

Like all stars, the physical characteristics of hypergiants depend on the delicate balance between the outward radiation pressure from energy escaping their cores and the inward pull of gravity from their enormous mass. As a result, hypergiants usually change their appearance throughout their lifetimes. Astronomers on Earth detect these differences by measuring the range of different luminosities and colours from star to star. Even though hypergiant stars live and die quickly on a cosmic timescale, they don't change quickly enough for us to see them evolve significantly throughout a human lifetime.

By plotting these properties for various stars on a Hertzsprung-Russell diagram, we can determine their relationships and the likely paths by which one type of star changes into another.

Evolution of Hypergiant Stars

Hypergiant stars spend most of their short lives as brilliant blue stars - with temperatures of perhaps 50,000 degrees Celsius (90,000 degrees Fahrenheit), compared to the Sun's 5,500 degrees Celsius (9,930 degrees Fahrenheit). But many later evolve towards the cooler red end of the colour range, with surface temperatures of perhaps just 3,000 degrees Celsius (5,430 degrees Fahrenheit). Because a star's surface temperature depends on the amount of energy escaping through each square metre of its surface, there's a direct link between a star's luminosity, colour and size; i.e. a cool, red star of a certain brightness must be significantly larger than a hot, blue star of the same brightness.

Hypergiant stars describe a star's luminosity rather than its physical size, so blue hypergiants can be smaller than the standard red giants formed by normal Sun-like stars towards the end of their lives despite being many times brighter. Rare red hypergiants, however, are the biggest stars in the universe. The most famous is Mu Cephei in the northern constellation of Cepheus. Known as the Garnet Star because of its deep red colour, it is large enough to engulf over a billion Suns.

The extremes that hypergiants display ultimately stem from their enormous mass. Like all stars, they spend their main sequence life shining through the fusion of hydrogen (the lightest element) into helium (the next lightest) in their cores. But while normal stars fuse hydrogen through relatively long-winded, inefficient chain reactions that rely on random collisions of atomic nuclei, the enormous pressures in a hypergiant's core allow it to use a much faster and more efficient set of reactions called the carbon-nitrogen-oxygen (CNO) cycle.

The rate of reactions in the hypergiant star's core generates an enormous outward radiation pressure that swells the star's outer layers. During the main-sequence phase, the inward pull of gravity stabilises the star at a few tens of solar diameters, enormous but still compact enough for its surface to remain searing hot and blue-white. Once the core's hydrogen supply is exhausted, it burns fuel from surrounding shells to keep shining. Perhaps surprisingly, this increases the hypergiant's luminosity still further, and the additional pressure of escaping radiation causes the star's outer surface to swell and cool, transforming it into a yellow, orange or red hypergiant depending on exactly where the balance is reached. However, many hypergiants never quite reach this stage, staying hot and relatively compact throughout their short lifetime. They do this by blowing away their outer layers on a stellar wind similar to, but much more powerful than, our Sun's solar wind.

What are Wolf-Rayet Stars?

Wolf-Rayet stars, often abbreviated as WR stars, are a select group of supermassive stars. They are distinguished by their robust, broad emission lines of helium, nitrogen, carbon, and oxygen. Their distinctive emission lines make them easily identifiable even in nearby galaxies, adding to their allure and rarity.

Wolf-Rayet stars can shed a solar mass of material every 100,000 years, exposing their even hotter interior layers. Towards the end of its life, such a star may become unstable, evolving into a Luminous Blue Variable or LBV star, prone to sudden outbursts. LBVs are often surrounded by clouds of gas ejected from previous eruptions. The most famous example is Eta Carinae, a double-star system containing a blue LBV of around 100 solar masses, orbited by a blue supergiant of about 30 solar masses.

In the early 1840s, a significant outburst transformed Eta Carinae from a star on the fringes of naked-eye visibility to the second-brightest star in the sky. Today, the system is still enveloped in a cloud of gas and dust expelled from that cataclysmic event. The evolution of hypergiant stars, while still not fully comprehended by astronomers, is undeniably a spectacle. Unlike the relatively calm demise of lower-mass stars like our Sun, hypergiants experience a turbulent end, marked by a brief resurgence of helium fusion followed by instability that ejects the star's outer layers, leaving behind a planetary nebula with a white dwarf at its core.

Supergiant and hypergiant stars can keep burning elements to produce heavier ones until they reach iron, the first element whose fusion absorbs more energy than it releases. At this point, the star's central power supply is abruptly cut off, and its outer layers collapse inwards before rebounding off the core.

The resulting shockwave ignites a tremendous burst of nuclear fusion in the star's upper layers, producing a supernova explosion that dwarfs even the brightest hypergiant and may even briefly outshine an entire galaxy. In some cases, the shockwave from the explosion can ignite clouds of material ejected from the star thousands of years before, creating an exceptionally bright supernova explosion known as a hypernova.

What is the Largest Hypergiant Star?

Hypergiant stars are the live-fast, die-young rock stars of the cosmos, but recently, astronomers have discovered what may be the biggest, baddest star of them all.

Catalogued as R136a1, this monster is 8.7 million times more luminous than the Sun, with roughly 256 times its mass. R136a1 lies at the heart of the Tarantula Nebula, an enormous star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. Discovered in 2010, this distant star tests the limits of how big a star can get without blowing itself apart. It is also undergoing mass loss at a tremendous rate and is thought to have shed more than 50 solar masses of material during its million-year lifespan. When this cosmic giant ends its life, it could detonate in a rare pair-instability supernova, outshining normal supernovas by 50 and becoming the brightest star in Earth's skies for several months. When this will happen is anybody's guess. Eta Carinae is often suggested as the bright star that is most likely to go supernova shortly, but pair-instability supernovas do not give the same kind of warnings as their fainter cousins, so theory, such an outburst might well happen at any time,

5 Famous Hypergiant stars

• Garnet Star This naked-eye star in Cepheus is one of the reddest stars in the sky and one of the largest stars in our galaxy.
• VY Canis Majoris This red hypergiant in the constellation of Canis Major may be the largest known star, with some estimates putting its diameter larger than Saturn's orbit around the Sun.
• S Doradus One of the most luminous stars, this blue variable in the Large Magellanic Cloud (a satellite galaxy of the Milky Way) is stable for long periods between sporadic outbursts.
• Pistol Star Hidden behind dense star clouds near the centre of the Milky Way, this blue hypergiant is surrounded by a nebula of material thrown off during outbursts a few thousand years previously.
• Eta Carinae This binary star system, embedded in the twin-lobed Homunculus Nebula, may erupt soon into a spectacular supernova.