Let them fight! Beautiful gas cloud represents the aftermath of a massive cosmic battle

Astronomers have determined that a stunning gas cloud represents the aftermath of a ‘stellar fight’ between two stars.

This new ALMA image shows the outcome of a stellar fight: a complex and stunning gas environment surrounding the binary HD101584. The colours represent speed, going from blue — gas moving the fastest towards us — to red — gas moving the fastest away from us. Jets, almost along the line of sight, propel the material in blue and red. The stars in the binary are located at the single bright dot at the centre of the ring-like structure shown in green, which is moving with the same velocity as the system as a whole along the line of sight. Astronomers believe this ring has its origin in the material ejected as the lower mass star in the binary spiralled towards its red-giant partner.

Using the Atacama Large Millimeter/submillimetre Array astronomers have observed a gas cloud surrounding the binary star system HD101584, located within a complex nebula in the constellation of Centaurus. The cloud is the result of an interaction between two stars which resulted in one dying star becoming a massive red giant and engulfing the other. In retaliation, the smaller star spiralled towards its companion — violently ripping away its outer layers leaving its core exposed in what researchers have described as a ‘stellar fight.’

Their research is published in the journal Astronomy & Astrophysics.

This wide-field view shows the region of the sky, in the constellation of Centaurus, where HD101584, a gas cloud surrounding a binary star recently studied with ALMA and APEX, is located. This view was created from images forming part of the Digitized Sky Survey 2. (ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)

Stars ‘die’ when they leave what astronomers call the main sequence, something that occurs when hydrogen burning ceases in the core. This triggers the transformation into a red giant with the star swelling and its outer layers ‘puffing out.’ Stars of a similar mass to our Sun that undergo this process will eventually lose their outer-layers leaving behind a white dwarf — a small dense stellar remnant.

However, this red giant evolution is often complicated by the presence of a companion star within a binary system. This is of particular interest to astronomers when the two stars — the primary swelling to a red giant — and the secondary, smaller companion star, end up sharing a common envelope of gas. In order to study this common envelope evolution, a team of astronomers turned their attention to the circumstellar environment of the binary system HD101584.

“The star system HD101584 is special in the sense that this ‘death process’ was terminated prematurely and dramatically as a nearby low-mass companion star was engulfed by the giant,” says Hans Olofsson of the Chalmers University of Technology, Sweden, who led the study.

Tale of the tape: meet the fighters

This video starts by showing a wide-field view of a region of the sky in the constellation of Centaurus. It then zooms in to show HD 101584, a gas cloud surrounding a binary star recently studied with ALMA and APEX. (ALMA (ESO/NAOJ/NRAO), Olofsson et al. Acknowledgement: Robert Cumming; N. Risinger (skysurvey.org), Digitized Sky Survey 2. Music: Astral Electronic)

Olofsson and his team were able to assess what happened to the binary system in question by collecting observations made by ALMA combining it with data harvested from the Atacama Pathfinder EXperiment (APEX). The team compare what happened in HD101584 to a ‘stellar fight’ — with the primary star swelling as a red giant swallowing its lower mass companion. As this occurred the smaller star spiralled towards the core of the red giant but avoided colliding with it. Instead, this spiralling-in process triggered an outburst from the red giant with scattered its outer layers, thus leaving its core exposed.

The complex structure of the gas cloud around the HD101584 is a result of the secondary star’s spiral towards its high-mass companion and jets of gas that it created. These gas jets shot through ejected material forming rings of gas and the red and blue ‘blobs’ of material seen through the nebula.

The team believe that their findings should help astrophysicists to better understand the formation and evolution of stars — including that of the Sun. “Currently, we can describe the death processes common to many Sun-like stars, but we cannot explain why or exactly how they happen,” says Sofia Ramstedt from Uppsala University, Sweden. “HD101584 gives us important clues to solve this puzzle since it is currently in a short transitional phase between better studied evolutionary stages.

“With detailed images of the environment of HD101584 we can make the connection between the giant star it was before, and the stellar remnant it will soon become.”

A remarkable image of the antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), set against the splendour of the Milky Way. (ESO/ Babak Tafreshi)

Elizabeth Humphreys from the European Southern Observatory (ESO) describes the importance of the organisation’s projects ALMA and APEX located in the Atacama desert, Chile, in alowing the team to investigate the chemistry and physics acting in unison in the gas cloud: “This stunning image of the circumstellar environment of HD101584 would not have been possible without the exquisite sensitivity and angular resolution provided by ALMA.”

Unfortunately, as the stars in the binary system are so close together the current generation of telescopes are neither powerful or sensitive enough to be individually resolved. This situation will likely change upon the completion of the ESO’s Extremely Large Telescope (ELT) currently being constructed in the Atacama Desert.

“[The ELT] will provide information on the ‘heart’ of the object,” concludes Olofsson, thus allowing astronomers to take a closer look at these battle-scarred stellar pugilists.