Supernovae are the brilliant, explosive deaths of stars. For a short time, these explosions can outshine an entire galaxy containing billions of stars.
A recently discovered rare class of supernovae, termed “super-luminous supernovae”, are the most luminous and most energetic of these explosions. They are 10 to 100 times brighter and more powerful than “normal” supernovae.
Such events actually warrant the over-use of superlatives – during their explosion, they emit more light than our sun will emit over its entire 10 billion year lifetime and even more energy.
And now, my colleagues and I have discovered a couple of these super-bright explosions in the early universe.
As part of our research program searching for supernovae in the early universe, we used our technique of combining hundreds of deep images to discover two super-luminous supernovae in the Canada-France-Hawaii Telescope Legacy Survey – an extremely deep image of a wide area of the night sky that took five years to create.
Using the giant 10-meter Keck telescopes near the summit of Mauna Kea in Hawaii, we confirmed that the supernovae occurred 10 and 12 billion years ago, way back when the universe was only 25% and 10% of its current age, respectively.
Keep in mind that the light from these supernovae travelled towards us that whole time. Light travels at roughly 300,000 kilometers (about 7.5 times around the Earth) every second. In a year, light covers almost 10 trillion kilometers (10,000,000,000,000 km).
Realising that the light from these supernovae had travelled for 10 to 12 billion years gives a real sense of the extreme distances involved.
Since the discovery of super-luminous supernovae about ten years ago, only a dozen or so super-luminous supernovae have been found, with nearly all of them relatively close (astronomically speaking) at 1 - 5 billion light years away.
This particular type of supernova results from the death of a very massive star (about 150 to 250 times the mass of our sun) and explodes in a completely different way compared to other supernovae.
During the star’s lifetime, gamma ray photons produced by the fusion process in their cores provide radiation pressure support to hold up the outer layers of the star from the crushing force of gravity.
The condition inside these stars evolves and becomes ideal for gamma ray photons to convert into electron-positron particle pairs. The conversion from outward radiation pressure into mass causes the star to collapse on itself from gravity.
This gravitational collapse then results in a runaway thermonuclear explosion that completely obliterates the star.
Pair-instability supernovae are thought to have occurred more often in the early universe and interestingly, one, if not both, of our super-luminous supernovae appear to be this type of event.
Life and death
In the early universe, many galaxies were physically quite small but vigorously forming stars. The enormous energy of a single super-luminous supernova could disrupt a significant fraction of such a galaxy and, in some cases, cause the star-formation process to come to a halt.
But, interestingly, the material that is blown off supernovae in larger galaxies (which survive such an event) provides the seeds for the next generation of stars. Furthermore, the shock wave from the explosions can help to compress gas in those galaxies to accelerate the star-formation process.
So super-luminous supernovae can be the bringers of death or the bringers of life to stars.
The first stars
Shortly after the Big Bang, only hydrogen, helium and trace amounts of lithium existed in the universe. All the other elements that we see around us today, such as carbon, oxygen, iron, and silicon, are manufactured in the cores of stars or during supernova explosions.
The first generation of stars born after the Big Bang was formed from this pristine gas which is believed to have generated a large fraction of very massive stars.
The supernova deaths of the first stars polluted the universe with heavier elements that subsequently cooled and condensed and formed the next generations of stars, including the sun’s generation.
Thus, the first generation was truly unique.
Those heavy elements that “polluted” the universe (carbon, oxygen, iron etc.) make up Earth, you, puppies, iPhones and the sandwich you’ve got for lunch today. All those atoms were once inside a star.
The first stars laid the framework for the long process of polluting the universe that eventually produced the diverse set of galaxies, stars, and planets we see around us today.
The theory goes that regions of the universe with clouds of pristine gas survived until as recently as 10 billion years ago.
In addition, two such gas clouds capable of forming first generation stars have been observed to exist.
So, for the first time, we are able to observe supernovae at times that overlap with the times we expect to find the first generation of stars.
Our discoveries mean that we now have the means to investigate the star-formation process from the beginning.
From analysing the spectroscopy (the intensity of light at different wavelengths from an object in which the unique signature of the elements can be seen), the two super-luminous supernovae we found are unlikely to have formed from pristine gas and hence were not the deaths of the first stars.
But, as more supernovae are detected in the early universe using our technique, discovering the deaths of the first stars will not only be possible, but entirely likely.