Two massive stars ‘touching’ each other will merge as black holes

Star being eaten away by its companion will first 'take revenge' on it after turning into black hole

Two massive stars “touching” each other in a neighbouring galaxy are on the course of becoming black holes that would merge to release huge amounts of energy.

Researchers, including those from the University College London in the UK, looked at a known binary star by analysing starlight from a range of ground- and space-based telescopes.

The most massive “touching stars” – or contact binaries – yet observed are located in a neighboring dwarf galaxy called the Small Magellanic Cloud.

The two stars in the system are in partial contact and are swapping material with each other, with one of them currently “feeding” off the other, said scientists.

“The smaller, brighter, hotter star, 32 times the mass of the Sun, is currently losing mass to its bigger companion, which has 55 times our Sun’s mass,” said Daniel Pauli, an author of the study from the University of Potsdam in Germany.

In the latest study, published in the journal Astronomy & Astrophysics, they found the star that is currently being fed on is on course to become a black hole and will feed on its companion star.

While the black holes are expected to form in only a couple of million years, the two stars are predicted to orbit each other for billions of years before colliding with a force so high it will generate ripples in the fabric of space-time that can be detected on Earth.

“Our best-fit model suggests these stars will merge as black holes in 18 billion years. Finding stars on this evolutionary pathway so close to our Milky Way galaxy presents us with an excellent opportunity learn even more about how these black hole binaries form,” study co-author Matthew Rickard said in a statement.

Scientists said such a cosmic event would get much rarer with time.

This is because the black hole mergers currently observed via telescopes by astronomers in faraway regions of space, formed billions of years ago at a time the universe had lower levels of iron and other heavier elements.

Since the proportion of these heavy elements has increased as the universe has aged, they said black hole mergers would become less likely.

Stars with a higher proportion of these heavy elements have stronger winds that blow themselves apart sooner, scientists explained.

In the new study, researchers analysed different bands of light coming from the binary star system obtained using instruments such as those on Nasa’s Hubble Space Telescope and the European Space Agency’s Very Large Telescope in Chile.

The analysis revealed much of the outer envelope of the smaller star had been stripped away by its larger companion.

It also suggested some of the smaller star’s material is overflowing and transferring to the companion star.

Using this data, obtained over multiple time periods, scientists could estimate the movement the stars made towards or away from Earth, as well as their masses, brightness, temperature and orbits.

They also matched these parameters with best prediction models of how the system could evolve.

Scientists predict the smaller star would become a black hole first in as little as 700,000 years, either via a “spectacular explosion” or by collapsing into a black hole with no outward explosion.

Then, about three million years later, the first black hole would start “taking revenge on its companion” by accreting mass from it.

After about 200,000 years – an instant in astronomical terms – the companion star will collapse into a black hole as well.

And finally, the two black holes are predicted to merge together in 18 billion years with a huge release of energy.