A dying star situated 400 light years away from us exhibits an unusual and massive tail of heated gas that spreads for more than 13 light years.

The star, dubbed Mira A- mira meaning “wonderful” in Latin- is situated in the constellation Cetus (“the whale”) and is a part of a stellar system bearing the same name and which has been studied intensely for the last couple of years.

Astronomers from Caltech in Pasadena, US, using NASA's Galaxy Evolution Explorer (GALEX) satellite, have been puzzled in their observations by the fuzzy aspect of Mira A, so they’ve decided to take more detailed pictures of the dying red giant- which is slightly bigger than our Sun and far older- in the UV spectrum. Initially, they were only able to spot the “short” tail near Mira A, but successive shots taken afterwards showed the huge trail of heated gas that spreads for no less than 13 light years behind the star. To get a picture of how big the “comet tail” is, if we were able to see it in the night sky, it would span the width of four full Moons…

A short lesson of astronomy is needed before going further: red giants like Mira A usually end up becoming white dwarfs, because they lack the amount of stellar mass to become a supernova (our Sun will have the same fate).

What that means is that Mira is not large enough to ignite the mind-boggling supernova explosions which at their turn produce so much light that they outshine an entire galaxy. Instead, the maximum in terms dimensions Mira and our Sun can hope for is the so-called red giant phase (our Sun will achieve that in about 7 billion years), which is preceding the final, white-dwarf phase.

To put it simple: you have stars that produce energy by creating inside their core an almost endless chain reaction, in which lighter elements like hydrogen are “forced” to bond and form helium. This chain reaction can last for billions of years, but since the amount of hydrogen is limited, the core will start fusing helium into carbon and oxygen until the helium is depleted too.

The carbon and oxygen ash from the process will form a ball about the size of the Earth. It will contract and get incredibly hot. Helium outside the core, previously unavailable for fusion will start to fuse in a thin shell surrounding the core. This will dump vast amounts of heat into the outer part of the star, which will respond like any gas will when heated: it will expand and cool. This is how astronomers were able to spot the tail that Mira A left behind it: the heated gas expelled from the star meets the hydrogen molecules in surrounding gas- known as interstellar medium (ISM)- and the collision between the fast moving electrons and the ISM produces ultraviolet light.

"This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved," says team member Mark Seibert of the Carnegie Observatories in Pasadena, California, US. "We never would have predicted a turbulent wake behind a star that glows only with ultraviolet light."

It’s also the star’s speed that has helped scientists get an insight into its galactic trajectory and past: Mira A travels faster than other stars in our Milky Way, at about 130 kilometers per second (about 80 miles per second). The trail it created for 30,000 years (that is the amount of time it took for the star to travel 13 light years) has given scientists the occasion to study how the star behaved some 30 millennia ago.

Eventually (probably in the next hundreds of thousands of years, maybe even less) the thin layer that still covers Mira A’s hot core (which you can see in the right of the picture slaming into the ISM and creating a parabolic arc in front of the star, called bow shock) will remain behind the star, completely exposing the core to the freezing temperatures in the Universe. The star will then be officially classified as a white dwarf, beginning a process of slow cool-down that will go on for the next few hundred billion years.

Astronomers have also noted that Mira A’s little sister, dubbed Mira B, which is already a white dwarf, is grabbing some of the material expelled by Mira A and is forming an accretion disk (structure formed by diffuse material in orbital motion around a central body), which could yield some planets in the future.