See gallery: “Space superlatives: The universe’s extreme performers“
A journey towards the hottest climes of the cosmos must start by passing the sun, the fiery centre of our solar system. With a surface temperature of 5800 kelvin, our star is far from chilly, but it is no cosmic record breaker either. Blue supergiants, whose greater mass compresses their cores and stokes the nuclear fires within, run at more than 50,000 K.
Even that is surpassed by some white dwarfs, compact spheres of heat left behind when a smallish star burns out. One such stellar cinder, called HD62166, measures a scorching 200,000 K and lights up a vast nebula with its painfully bright atmosphere.
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Plunging deep inside a star will take you to even more hellish realms. The largest supergiant stars may have core temperatures of more than a billion kelvin. For a stable star, the theoretical upper limit is about 6 billion kelvin. At this temperature, matter within the star starts to emit photons that are so dangerously energetic they can create pairs of electrons and positrons when they collide. The result is a runaway reaction that obliterates the star in a colossal explosion.
The first suspected sighting of such a “pair-instability supernova” came in 2007, when a brilliant and exceptionally long-lasting stellar explosion was observed, suggesting the existence of a star far bigger than had previously been thought possible ().
During a supernova, stellar temperatures can briefly rise far above 6 billion kelvin. In 1987, a star was seen exploding in the Large Magellanic Cloud, a satellite galaxy of our own Milky Way some 160,000 light years away from us. Neutrinos from its heart detected on Earth revealed an internal temperature of about 200 billion kelvin.
That’s nothing, though, compared with whatever produces a gamma-ray burst. These brief flashes of ultra-high-energy light are spotted once or twice a day by specially tuned telescopes. CRBs are thought to mark the birth of black holes, either when a giant star’s core collapses or when two ultra-dense neutron stars collide. Somehow the gravitational energy is turned into a tight beam of gamma rays and other radiation. While the details of this process are currently unknown, it must involve a fireball of relativistic particles heated to something in the region of a trillion kelvin (1012 K).
Closer to home is a place that is even hotter: not a natural inferno, but a detector cavity 100 metres or so beneath the generally temperate outskirts of Geneva in Switzerland. There, between 8 November and 6 December 2010, for the first time at CERN’s Large Hadron Collider in an attempt to mimic some of the universe’s opening moments. The result was the highest temperatures ever recorded on Earth, a subatomic fireball registering several trillion kelvin.
That experiment gives us a clue to where the universe’s extreme of heat lies. Not in the here and now, but the way back when. Looking into the heart of the big bang, the singularity of temperature and density in which our universe began, the maximum temperature is just a matter of how many zeros you can write before our understanding of physics breaks down. That’s probably somewhere in the region of 32.
Read more: “Extreme universe: Eight cosmic record-breakers“