For sixteen long years, the “Blue Ring Nebula” puzzled astronomers trying to piece together its original story. The mystery is finally solved. The structure is believed to be the result of a fusion of two stars produced just thousands of years ago.
The mystery of the “blue ring”
The Blue Ring Nebula was first spotted in 2004 by the Galaxy Evolution Explorer (GALEX), a telescope designed to probe the sky in the ultraviolet. At the time, the object was unlike anything scientists had seen before. It indeed appeared like a gigantic ball of incandescent gas with a star in its center. Note that while the structure appears blue to us, in reality it does not emit any color visible to the human eye. Its invisible ultraviolet light has simply been “coded” in blue by the telescope images.
Initially, astronomers hypothesized that this “blue ring”, found 6,300 light years away in the constellation of Hercules, could be the afterglow of a supernova explosion. However, this nebula offers a very living star in its center. Additionally, supernova remnants radiate in several light wavelengths outside the UV range. Yet that was not the case here. The structure only radiates in UV. So we had to look elsewhere.
To try to find out more, in 2006, researchers from the Carnegie Institution for Science and member of the GALEX team at Caltech observed the nebula using two powerful telescopes: the Hale telescope of the Palomar Observatory. (California) and the WM Keck Observatory in Hawaii. This work revealed that the gas making up the nebula had been expelled following a shock wave. From then on, we knew that a violent event had occurred there, but what event? The mystery persisted.
The afterglow of a recent collision
Most recently, the team relied on four space telescopes and four ground-based telescopes to collect new data. They also looked at historical observations of the star dating back to 1895, to isolate any changes in its luminosity. Armed with this enormous wealth of information, they were finally able to solve this mystery.
The researchers found evidence to suggest that the Blue Ring Nebula was in fact the result of the merger between two stars. One was about the same size as our sun, and the other was about a tenth the size.
How did it happen ? A few thousand years ago, towards the end of its life cycle, the star similar in size to the sun began to swell. In doing so, it came closer to a smaller and closer star. Due to the gravitational pull of the big star, the smaller one eventually “spiraled” towards the larger companion, before being “swallowed” entirely. In the meantime, a disc of debris had formed around the large star.
Their fusion then set off a cloud of hot debris, emitted into space and cut in half by the gas disc in the middle. As a result, two cone-shaped clouds formed, moving in opposite directions, one of which was heading towards Earth as seen below:
A missing link
Although merged star systems are fairly common, they are almost impossible to study immediately after formation, as they are obscured by the debris that the collision triggers. Once the debris is cleared (at least hundreds of thousands of years later), it is also difficult to identify as it ultimately looks like single stars.
The Blue Ring Nebula thus presents itself as the “missing link”. Astronomers only noticed the presence of this star system a few thousand years after the merger. The debris clouds were therefore scattered enough so that we could isolate the presence of the star resulting from this union and at the same time still close enough that we could notice them.
