Astronomers have seen gas and dust from a dead star create an enormous shock wave – something that they thought shouldn't be possible.
The star in question is RXJ0528+2838, a lifeless celestial body that shouldn't have the necessary structures surrounding it for the shock wave to occur.
Scientists say the shock wave, beautiful as it is, it something of a mystery, and is challenging current theories as to how dead stars interact with their surroundings.
Capturing the dead star
Shock waves around stars are nothing new. Astronomers already know that gas and dust flowing from stars can smash into the star's surroundings and create a shock wave.
The problem is that it shouldn't be possible for the phenomena to occur around a dead star.
This discovery was made by a team using using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile.
And they've even been able to capture an image of the shock wave around the dead star.
More stellar science
"We found something never seen before and, more importantly, entirely unexpected," says Simone Scaringi, associate professor at Durham University, UK, and co-lead author of the study.
"Our observations reveal a powerful outflow that, according to our current understanding, shouldn’t be there," says Krystian Iłkiewicz, postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland and study co-lead.
Here, ‘outflow’ refers to the material ejected from objects in space.
Why RXJ0528+2838 is so strange
RXJ0528+2838 is 730 lightyears away, making it very close to Earth in cosmic terms.
And just like our own Sun, RXJ0528+2838 rotates around the centre of our Galaxy.
As it does so, it interacts with the interstellar gas that's present between stars, creating a bow shock, a type of shock wave. This is similar to the wave that forms as a ship plows through water.
In space, bow shocks are generated as a star moves through space, its stellar material flowing outwards into the cosmos.
RXJ0528+2838 is a dead star. Specifically, it's a white dwarf, which is the dense remnant of a low-mass star that has used up all its fuel.
What's more, it's a binary star, meaning it has a Sun-like companion orbiting it.
In binary systems like this, material from the second star is sucked up by the white dwarf, forming a disk of material around it.
Some of that material is ejected into space, but RXJ0528+2838 doesn't seem to have a disk, which makes its outflow a mystery.
"The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare ‘wow’ moments," says Scaringi.
Investigating the mystery
The team behind this study discovered the strange nebula – the cloud of material surrounding it – in images captured by the Isaac Newton Telescope in Spain.
They then turned to the Very Large Telescope and its MUSE instrument for a closer look.
"Observations with the ESO MUSE instrument allowed us to map the bow shock in detail and analyse its composition," says Iłkiewicz.
"This was crucial to confirm that the structure really originates from the binary system and not from an unrelated nebula or interstellar cloud."
The team calculate the white dwarf has been expelling material for at least 1,000 years.
So what could be causing this outflow? Astronomers say the white dwarf has a magnetic field that could be channelling material from the companion star onto the white dwarf, but without forming a disc.
"Our finding shows that even without a disk, these systems can drive powerful outflows, revealing a mechanism we do not yet understand. This discovery challenges the standard picture of how matter moves and interacts in these extreme binary systems," says Iłkiewicz.
However, the team say their observations show the magnetic field should only be able to power a bow shock lasting a few hundred years.
They say more binary systems need to be studied, if they're to get to the bottom of the mystery of this mysterious, disk-less outflow.
Read the full paper (PDF)
