Low-mass stars like our own Sun start life as "first cores," which is the earliest point where gas and dust clump together to form a coherent structure. They're an essential part of stellar evolution, but we've never seen one...until now.
Stars like our Sun can last for many billions of years - the Sun is thought to be halfway through its ten billion year life - and fifty years of theoretical work and direct observation have given astronomers a good sense of how stars develop. But one of the very earliest stages has so far escaped detection: the first core, a short-lived but crucial stage that lasts only about ten thousand years and glows only very faintly in the infrared spectrum.
That understandably makes first cores hard to spot, but they're a crucial missing link in stellar evolution, the stage where nebular gas starts collapsing into itself to eventually form a hot, dense object: a protostar. Gas and dust particles are gravitationally attracted to each other to form dense clouds, and eventually enough materials come together to form a stable, dense structure at the cloud's center.
The entire first core is huge - if you replaced the Sun with a first core, it would stretch all the way out to the asteroid belt - but it's also very cold, only reaching temperatures of about -175 degrees Celsius. The formation of the first core slows down the further accumulation of material, but it does still continue, slowly ramping up the density and temperature of the core until it's ready to become a protostar, about when it reaches 1,700 degrees.
At that temperature, hydrogen atoms start separating out from their molecules, and the core's energy shifts from keeping the core hot to breaking molecules apart. This causes the core to collapse very quickly, forming a protostar, the immediate predecessor of a full-blown star.
Because they are so short-lived and they are so incredibly faint, no one has observed a first core. But now two different teams are reporting that they've spotted strong candidates for first cores. Both teams found their cores by searching through gaseous clouds that were thought to be starless, but still emitted apparently star-like radiation. They then discovered traces of infrared radiation in two separate locations, both of which fit well with possible first cores.
Their findings aren't yet confirmed, but if nothing else, they have almost certainly discovered protostars at the earliest point yet in their development, even if they haven't found actual first cores. For more on how they made their discovery, check out Scientific American.
Image of a protostar by Markus Seethaler