The Brightest Thing in the Universe

December 25, 2014

Since ancient times people used to look up into the sky and wonder what that mysterious luminous disk was. A powerful god of light, bound in deadly eternal dance with his nemesis, god of darkness? A supernatural phenomenon whose presence gives warmth and allows the existence of life itself? Whatever its nature was, people felt it was important enough to have monuments erected to worship or to study it. The brightness of the Sun was so intolerable that we find references of blinding by prolonged looking at the Sun used as a penalty since old Persian and up to Byzantine times.

And still even the Sun was not that bright. A mere torch, or later a lightbulb, put in couple centimeters from your face would look overwhelmingly brighter than the Sun up in the sky.

If, for example, we observe sky relative to Earth, the apparent magnitude, and thus the brightest object would undoubtedly be the Sun with -26.74 (it’s a logarithmic scale where a smaller number means a greater brightness), followed by the Moon (−12.92), Venus (−4.89), Jupiter (−2.94), Mars (−2.91), Mercury (−2.45), Sirius (−1.46), Canopus (−0.72) and Saturn (−0.49).

Scale of brightness of stars and planets
A relative scale of apparent magnitude of the brightest celestial objects (with white gamma set as minimum and yellow as maximum)

But that’s not fair. The above-mentioned lightbulb, placed at the same distance as the Sun would look a septillion times dimmer. Due to the distance some of the objects in the night sky although being much bigger and brighter than our Sun are so far away that their light seems dimmer than it would be, given we observe it from the same distance.

The absolute visual magnitude scale was set to measure it, assuming that we look at every object from the distance of 32.5 lightyears away.

Now looking at the brightness of celestial objects, Sun suddenly looks like a looser with 4.83, all the Solar system planets don’t even make it to the list as objects not emitting their own light. The brightest star of the night sky, Sirius, shows 1.4. Alpha Centauri, the closest star system to ours and the third brightest star in the night sky is a little bit ahead of our Sun with just 4.38. The most massive star ever found, R136a1 in the Large Magellanic Cloud, is estimated to be 265 times more massive then our Sun and 8,700,000 times the luminosity of the Sun. Its absolute magnitude of −12.6 makes it the most luminous star known.

Comparison of size between a red dwarf, the Sun, a blue dwarf, and R136a1
Left to right: a red dwarf, the Sun, a blue dwarf, and R136a1. R136a1 is not the largest known star in terms of radius or volume, only in mass and luminosity.

We could probably stop here, but it won’t answer the question of the brightest thing in the Universe.

Even if we ignore supernovas that although can eject colossal flashes of radiation known as gamma ray bursts (which can emit more energy in the matter of minutes than the Sun in its whole 10 billion year lifetime), but still are brief events lasting minutes or sometimes even milliseconds, there are objects whose luminosity can sustainably dwarf that of any star.

Gamma ray burst
A gamma ray burst

And paradoxically to find it, you’ll have to look at the darkest places in the Universe — black holes.

Until the early 1980s, the nature of the brightest things in the Universe was controversial, but there is now a scientific consensus that supermassive black holes, billions of times the mass of our Sun, while eating things like stars create halos out of gas and debris, called the accretion disc (remember that bright disc in Interstellar?). All this matter is spinning around the black hole before crossing the event horizon at extreme speed and the friction in the accretion disc generates heat on the level hard to imagine. Quasars (and that’s how these objects are called) shine hundreds times brighter than galaxies, containing hundreds of billions of stars. Accretion disk

A supermassive black hole and its accretion disk

The quasar that appears brightest in the sky is 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (bright enough to be seen through a medium-size amateur telescope), but it has an absolute magnitude of −26.7. From a distance of 32.5 lightyears, this object would shine in the sky about as brightly as the Sun observed from Earth. This quasar’s luminosity is, therefore, about 4 trillion times that of our Sun.

Comparison of star and quasar
These two objects look quite similar. Only the one on the right is a star a hundred lightyears away, and the one on the left is a quasar 9 billion lightyears away.

Though the brightest object ever observed is a blazar (a type of quasar with a jet oriented toward Earth) 3C 454.3 with an absolute magnitude of 31.4.

Blazar 3C 454.3

Quasars are one of the most ancient things in our Universe. To keep itself a quasar a black hole has to consume around 10 stars a year, but with each star it consumes it becomes more massive and at some point it’s big enough to gobble the star without the creation of the accretion disc — just in one gulp. So quasars are not there for long and perhaps the majority of them don’t exist anymore, it’s just the imprints of their light that we still see.

What is really fascinating in all the search for the brightest thing in the Universe is its origin. Remember that ancient tale of a god of light, bound in deadly eternal fight with his nemesis, god of darkness? The brightest thing in the Universe exists due to the darkest thing. A beautiful dance of the Universe.

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