In 2002, astronomers detected a new ‘star’ in the Monoceros constellation, some 3,300 light-years away from Earth. The star is called V838 Monocerotis and was initially classified as a variable star — a star with varying brightness. However, it became apparent that the star was rather unusual.

Astronomers observed that the light intensity of this star resembled a nova — an explosive star that’s not quite as cataclysmic as a supernova. However, three months later, the star started emitting massive amounts of infrared light, so it didn’t really seem to be a nova after all. Ultimately, V838 Monocerotis was finally classified as a luminous red nova — a stellar explosion that occurs when two stars merge.

Now, researchers have captured new details about this mysterious star.

A cascading stellar event

When the merging happened, it produced one of the most spectacular images you can imagine. As the gases and dust traveled outward from the epicenter of the event, they scattered light from the explosion itself. The scattered light was then deflected by the molecular cloud, taking a little longer to reach us compared to the light coming directly to Earth — a phenomenon called a ‘light echo’.

After the stars merged, the remnant left behind is likely a red supergiant that’s dozens or even hundreds of times the size of the sun — big enough to fill Mars’s entire orbit. However, because the event took place very far away, it took years for us to observe the formation of ions from the dust ejected by the merger. The ejected material expelled during the collision traveled through space and encountered another star in the system, a third companion B-type star – this one, in particular, is a BV3 star which is nearly 8 times more massive than the Sun.  

In a recent study, astronomers found direct evidence of this third star for the first time, 17 years after they observed the red nova going boom. They used observations from the Atacama Large Millimeter/submillimeter Array (ALMA) interferometer from 2019. ALMA’s data helps scientists ‘see’ what is happening in the system in terms of dust, gases, and gathers information about the stars themselves. When the material was close enough to the giant’s companion, it became ionized by the photons emitted from this star, and that helped the researchers to learn details about the B star.

Their results show that the B-star companion’ gravity pulls some of the gas away from us, making them appear redshifted. They also learned that this companion is embedded in the ejected cloud. It orbits its giant sibling over a 1000 year period from a distance greater than 230 times our distance from the Sun so that the gas only reached it 3 years after the nova event. 

Researchers have also learned that the molecular cloud is traveling at 200 km per second (approximately 124.3 miles per second). With the help of spectroscopy, scientists can determine the chemical composition of the cloud because it is the preferred absorption of radiation observed by ALMA’s instruments. It is made of carbon monoxide, silicon monoxide, sulfur monoxide, sulfur dioxide, and aluminum monohydroxide.

Future observations will provide more evidence of novas ejected material and their formation through mergers thanks to millimeter/submillimeter observations, something scientists didn’t have access to 20 years ago. 

The study was published at Astronomy & Astrophysics.