A dramatic collision between two stars scatters their remains in outer space in the most impressive way – and we can really see it with our own eyes.
Here's how it happened: When the dying star turned into a red giant, it swallowed up another star of lesser mass. She, in turn, began to move in a spiral towards the red giant, forcing him to prematurely shed the outer layers and suspend its evolution.
Result? A complex gas cloud that has puzzled astronomers for decades. Now, using the powerful capabilities of the Atacama Large Millimeter / Submillimeter Array (ALMA), the secrets of the HD 101584 star system are being revealed.
Stars with masses roughly similar to our Sun, have a known evolutionary path. When they burn all the hydrogen in their cores with helium, nuclear fusion inside stops and the core begins to shrink. This brings more hydrogen into the area immediately around the core, forming a hydrogen envelope; then fusion begins again, dumping helium into the core. This is called burning the hydrogen envelope.
During this time, the outer layers of the star expand significantly. For example, when this eventually happens to the Sun, it will expand beyond the orbit of Mars. It is a red giant branch of stellar evolution.
When all of the hydrogen is burned, the star will start to melt the helium. And when all the helium in the core has merged with oxygen and carbon, the core contracts again, and the star begins to expand again. This is the asymptotic branch of the giant.
A 1995 paper describing HD 101584 as a 'kind of supergiant' concludes that the star is on the giant's post-asymptotic arm. But ALMA observations suggest that the star's evolution was halted on the branch of the red giant before it moved on to a new round of development.
“The star system HD 101584 is special in the sense that this 'evolutionary process' was prematurely and abruptly terminated as a nearby companion star was captured by a giant,” said astronomer Hans Olofsson of Chalmers University of Technology in Sweden.
Based on the team's reconstruction of what happened, the event was very tough.
The dying star has evolved, expanding into the surrounding space. However, its smaller companion star also fell into this space. When the red giant's shell swallowed the smaller star, the smaller star spun towards the red giant's core – but did not collide with it.
This made the red giant go crazy. It flared up prematurely, blasting out its layers of gas, and is currently in a bare core state. It was an amazing cosmic dance, leaving behind a magnificent nebula, carved out by the death spiral of a smaller star.
Astronomers also managed to identify two jets emanating from the poles of the red giant. They blew into the already ejected gas and dust, creating the gas rings you see in the above image.
“We can currently describe evolutionary processes that are common to many solar-like stars, but we cannot explain why or how they occur,” said astronomer Sophia Ramstedt of Uppsala University in Sweden.
'HD101584 provides us with important clues for solving this puzzle, as it is currently in a short transitional phase between the evolutionary stages studied.
'With HD101584's detailed images, we can establish a link between the giant star it once was and the stellar remnant it will soon become.'
The study was published in Astronomy and Astrophysics.
Sources: Photo: ALMA (ESO / NAOJ / NRAO), Olofsson et al. Acknowledgment: Robert Cumming
