There are many kinds of stars out there in the big, wide Universe. We have a whole system for categorizing them according to temperature, size, and brightness.
Even so, a recently discovered object is suggesting that we're far from knowing everything.
It's been nicknamed 'The Accident', and it's a type of object called a brown dwarf, also known as failed stars. But it's unlike any brown dwarf we've ever seen before, with a confusing spectrum – suggesting that it may be nearly as old as the Universe.
Since all of the other brown dwarfs discovered to date are considerably younger, this means there could be an entire population of very old ones out there that we've simply not spotted because they don't look like we expect them to.
"This object defied all our expectations," said astrophysicist Davy Kirkpatrick of Caltech.
Brown dwarfs occupy the space between the most massive planets and the smallest stars, forming from the same top-down cloud-collapse model as stars, rather than the bottom-up accretion process planets undergo.
They're what happens when the star formation process ends before the object obtains enough mass to ignite the fusion of hydrogen in the core.
However, unlike planets, they are massive enough to fuse something, and that something is deuterium, aka 'heavy' hydrogen.
Deuterium is an isotope of hydrogen with a proton and a neutron in the nucleus instead of just a single proton. Its fusion temperature and pressure are lower than the fusion temperature and pressure of hydrogen.
As a result, brown dwarfs tend to be smaller, cooler, and dimmer than most stars. Their mass range is between about 13 and 80 times the mass of Jupiter, and they cool as they age.
We have, therefore, a pretty good grasp of what a brown dwarf ought to look like, and look for them based on this set of characteristics.
That faint, moving object in the bottom left is The Accident. (NASA/JPL-Caltech/Dan Caselden)
So far, around 2,000 of these objects have been found in the Milky Way. The Accident – whose real name is WISEA J153429.75-104303.3 – wasn't picked up in brown dwarf surveys because it doesn't match those characteristics.
Its detection by NASA's Near-Earth Object Wide-Field Infrared Survey Explorer, and subsequent discovery, were, as the name suggests, an accident.
It really is a most peculiar object. In some wavelengths, it is very faint, suggesting that it is also very cool – below the boiling point of water, in fact – and therefore quite old. In other wavelengths, it glows more brightly, which in turn suggests a higher temperature.
To unravel the mystery, the team turned to a different infrared wavelength range, since infrared wavelengths reveal thermal radiation. But observations taken using the terrestrial W. M. Keck Observatory didn't reveal The Accident at all, once more suggesting cooler temperatures.
The Accident's distance from the Solar System could have been a clue – if it were far away, that could explain its faintness. But it turned out to be not very far at all, relatively for space distances, at around 53 light-years away.
Oddly, it's zooming around the galaxy very fast, at speeds of around 207.4 kilometers (128.9 miles) per second. That's over 25 percent faster than any other star of its kind.
Like The Accident's temperature, this speed suggests that the star has been around for a very long time, picking up and accumulating speed boosts from gravitational interactions with other objects in the galaxy.
The Universe is around 13.8 billion years old. Kirkpatrick's team calculated that The Accident could be between 10 and 13 billion years old – double the median age of the known population of brown dwarfs.
"It's not a surprise to find a brown dwarf this old, but it is a surprise to find one in our backyard," said astrophysicist Federico Marocco of Caltech.
"We expected that brown dwarfs this old exist, but we also expected them to be incredibly rare. The chance of finding one so close to the Solar System could be a lucky coincidence, or it tells us that they're more common than we thought."
This venerable age suggests that The Accident's composition might be quite different from other brown dwarfs, too – which is supported by the spectrum of light it emits.
That's because, in the very early Universe, the range of elements was much lower. Just after the Big Bang, most of the matter was hydrogen and helium, with very little else.
It took a few generations of stars for more elements to proliferate. They fused atomic nuclei in their cores, producing heavier elements, then died, spreading those elements throughout space. Supernova explosions produced even heavier elements through processes that can only be found in such energetic events.
If The Accident was around before these elements (including carbon) were more common through the Universe, then its light would be stronger in specific wavelengths that would normally be absorbed by methane (made of carbon and hydrogen) in a brown dwarf's atmosphere. This is exactly what the researchers observed.
"This discovery is telling us that there's more variety in brown dwarf compositions than we've seen so far," Kirkpatrick said.
"There are likely more weird ones out there, and we need to think about how to look for them."
The research has been published in The Astrophysical Journal Letters.