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Imagine a star so big that if it replaced the Sun, it could engulf the Solar System as far out as the orbit of Saturn. Or one that produces as much energy in one second as our Sun does in a hundred days. These are hypergiant stars.

These might sound unreal, but both stars exist - they're two examples of hypergiant stars, the most massive stars in the universe. Hypergiants are stars that burn with the brilliance of millions of Suns. Though born from the same clouds of interstellar hydrogen gas as normal stars, 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 the nuclear fusion reactions in their core.

In 2005, an international team of astronomers discovered that dying red giant stars could act like a defibrillator and bring icy planets back from the dead.

In 2005, an international team of astronomers discovered that dying red giant stars could act like a defibrillator and bring icy planets back from the dead.


So while a star like the Sun can sustain itself on a relatively small amount of hydrogen fuel for a period of up to 10 billion years, a hypergiant star with perhaps a hundred times the available fuel will squander it in a million years or less, blazing away as a brilliant but comparatively short-lived cosmic beacon. 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 through their lifetimes. Astronomers on Earth detect these differences through 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 certainly don't change quickly enough for us to see them evolve significantly over the course of a human lifetime).

The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.

The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.

By plotting these properties for various stars on a Hertzsprung-Russell diagram, they can work out the relationships between them, and the likely paths by which one type of star changes into another.

Hypergiant stars, it's clear, 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; ie a cool, red star of a certain brightness must be significantly larger than a hot, blue star of the same brightness.

The term hypergiant stars describe a star's luminosity rather than its physical size, so blue hypergiants can actually 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. Perhaps the most famous is Mu Cephei in the northern constellation of Cepheus. Known as the Garnet Star on account of its deep red colour, it is large enough to engulf over a billion Suns within it.

The extremes which hypergiants display stem ultimately 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 stars 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 few tens of solar diameters enormous but still compact enough for its surface to remain searing hot and blue-white in colour. Once the core's supply of hydrogen is exhausted, it starts to burn fuel from surrounding shells in an attempt 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 own solar wind.

So-called Wolf-Rayet stars can shed perhaps 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 which is prone to sudden outbursts. LBVs are often surrounded by clouds of gas ejected from previous eruptions. Perhaps 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 major outburst saw Eta Carinae brighten from its usual position on the borders of naked-eye visibility, to become the second-brightest star in the sky. Today, the system is still surrounded by a cloud of gas and dust ejected from that explosion. There are many aspects of hypergiant evolution that astronomers still don't fully understand, but one thing that is certain is their fate. Lower-mass stars like our Sun have relatively sedate deaths, in which a short-lived second wind of helium fusion is followed by instability that hurls off the star's outer layers to form a planetary nebula with the burnt-out stellar core: a white dwarf at its centre.

Supergiant and hypergiant stars can keep burning elements to produce heavier ones right up 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.

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.

Star classes with the colours very close to those actually perceived by the human eye. The relative sizes are for main sequence or "dwarf" stars.

Star classes with the colours very close to those actually perceived by the human eye. The relative sizes are for main sequence or "dwarf" stars.

Catalogued as R136a1, this is a monster 8.7 million times more luminous than the Sun, and 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 a factor of 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 in the near future, but pair-instability supernovas do not give the same kind of advance warnings as their fainter cousins, so in 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 possibly erupt soon into a spectacular supernova.

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