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THE MOST LUMINOUS OBJECTS IN THE KNOWN UNIVERSE
Your luminosity is intrinsic, but your brightness will depend on who is looking at you. In astronomy, the luminosity of an object is the total amount of energy that it emits across all wavelengths, measured over time. It is often used when referring to stars, whose luminosity will depend on their size, and mass, and temperature. Brightness (formally known as apparent brightness), although related to luminosity, varies wildly depending on the location, positioning, or proximity of the observer. Something with great luminosity might, to us, seem nothing more than a fleck of dust, only because it’s sitting and burning and minding its own business unthinkably far away. For a human standing on the surface of the planet, the brightest object is the nighttime star Sirius, primarily because it is a mere 8.6 light-years away. It is by no means the most luminous star, though, and even within the constellation containing Sirius, Canis Major, at least three other stars are thousands of times more luminous; they only appear fainter as they are so much farther away. Even the most ordinary of stars seem noteworthy from where we are, and so we point at those pins of ancient light, nod at the brightness, assign them names and neighbors. In February 1963, a Dutch astronomer named Maarten Schmidt was analyzing an unusually bright speck in the sky, slowly realizing that while he had thought it might be a nearby star, it actually was something entirely different: not close at all, but rather 2 billion light-years away, and in order for it to still be so bright at that distance it would have to be more than anything known at the time. Schmidt named this object a “quasar,” which is short for “quasi-stellar object,” or QSO. Named 3C 273, it is located in the constellation Virgo, and optically speaking is the brightest of the bunch. In the fifty or so years since this discovery, hundreds of thousands of quasars have been observed. They remain some of the most astonishing things in the universe, and are perhaps the most luminous of all. Lying in the middle of galaxies, galaxies with vast black holes that can be billions of times larger than the sun, the temperature of a quasar can reach tens of millions of degrees, and their immense radiation means they outshine everything around them, drowning out all nearby stars. But they are not unchanging, and while one minute a quasar might be blinding, ten years later it can have become just another average galaxy. In astronomical terms, ten years is the briefest of moments, but it is events and observations such as this that lead to a better understanding of a black hole’s appetite: how they can be ravenously hungry one moment and completely disinterested the next.
