At the centre of the Milky Way is a supermassive black hole called Sagittarius A*. It is roughly 27,000 light years from Earth and 23.5 million kilometres in diameter.

In a world first, a team of astronomers led by Florian Peißker from the University of Cologne, Germany, have discovered a binary star system orbiting this black hole.

The system is known as D9. Its discovery, announced in a new paper published today in Nature Communications, sheds light on the extreme environment at the centre of our Milky Way galaxy.

It also helps explain a long-running cosmic mystery about why some stars hurtle through space much faster than others.

What Is A Binary Star System?

A binary star system is simply two stars orbiting each other.

Our Sun is not part of a binary, which is a good thing: we wouldn’t want another star wandering through our Solar System. It would disrupt the orbit of the Earth; we’d fry or freeze.

Observations show about two thirds of the stars in the Milky Way are single stars, and the remainder are part of a binary or multiple star system. Larger stars are more likely to be paired.

Binary star systems are useful to astronomers because their motion contains a wealth of information. For example, the speed and distance of the orbits tell us about the masses of the stars.

For a single star, by contrast, we usually work out its mass from how bright it is.

A Technically Challenging Discovery

Although scientists have previously predicted that binary star systems exist near supermassive black holes, they have never actually detected one.

This recent discovery was technically quite challenging. We can’t simply look at the system and see two stars, because it’s too far away. Rather, the astronomers used the European Southern Observatory’s Very Large Telescope to measure the shifting of the starlight – known as the Doppler effect. This showed that the stellar system’s light had a characteristic wobble, indicating an orbit.

But the team did much more than that.

Because binary stars contain a wealth of information, the astronomers could calculate that this particular system is approximately 2.7 million years old. That is, 2.7 million years ago, these stars first ignited.

They probably weren’t born in the black hole’s extreme surroundings, so unless they only recently wandered into this neighbourhood, they have lasted about a million years in their current environment.

This, in turn, tells us about the black hole’s ability to disrupt stars in its orbit. Black holes are mysterious beasts, but clues such as this are helping us unravel their nature.

Circling A Black Hole

The situation the astronomers discovered is quite familiar.

Think of the Moon: it orbits the Earth, and the Earth and the Moon together orbit the Sun. Because gravity is an attractive force, it can pull multiple celestial objects into complicated orbits. The complexity of this scenario inspired the recent book and Netflix series, The Three Body Problem.

If they are complicated, could the whole thing drift apart? The Moon–Earth–Sun arrangement is stable because two of the three bodies – the Earth and Moon – are much closer together than the other body, the Sun. The Moon and Earth are close enough that, so far as the Sun is concerned, it’s effectively a two-body system, which is stable.

But if all three bodies interact, the system can come apart. It is even possible for two of the bodies to eject the third body entirely.

Stars Of Unusual Speed

This mechanism probably explains a cosmic mystery: hypervelocity stars.

Most stars in the night sky are in a typical, almost-circular orbit around the centre of our galaxy. Orbital speeds are about 200 kilometres per second: very fast on Earth, but nothing special in space.

However, since 2005 we have discovered about 20 hypervelocity stars, which are hurtling through our galaxy at more than 1,000 kilometres per second. How?

Our best current idea is that hypervelocity stars were once part of a binary system orbiting our supermassive black hole. In time, the stars got too close to the black hole, and a complicated orbit resulted. In the kerfuffle, with a black hole calling the shots, one of the stars got ejected. It escaped to the outer Milky Way, where we see it as a hypervelocity star.

Finding The Hypervelocity Factory

It’s an interesting theory.

Theoretical calculations show the mechanism works and the speeds are about right. Observations show many of the known hypervelocity stars appear to be shooting away from the galactic centre, which is another plus for the theory. But how else could we test this idea?

An obvious way is to look for binary stars around our supermassive black hole.

Astronomers have been keeping a close eye on our galactic centre for decades. It’s not too difficult to find in the night sky, as you can see from the image below.

Here are two reliable methods to find Sagittarius A*. First, find Antares (bright and red), which is the centre of the back of Scorpio, and then follow the scorpion’s body to the tip of the tail, and that’s close-ish to the black hole. Alternatively, get a good night sky app on your phone; they’re amazing.

In the context of these theories, this recent discovery is very important. Astronomers found a binary star system around our supermassive black hole. An important piece of the hypervelocity puzzle falls into place.

(Author: Luke Barnes, Lecturer in Physics, Western Sydney University)

(Disclosure Statement: Luke Barnes does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment)

This article is republished from The Conversation under a Creative Commons license. Read the original article.
 

Washington:

The conventional wisdom among astronomers is that black holes – those exceptionally dense objects with gravity so powerful that not even light can escape – form in the violent explosion, called a supernova, of a massive dying star. But some, it turns out, may be born in a gentler fashion.

Researchers have identified a black hole that appears to have come into being through the collapse of the core of a large star in its death throes, but without the usual blast. It was observed gravitationally bound to two ordinary stars.

Black holes have previously been spotted orbiting with one other star or one other black hole in what are called binary systems. But this is the first known instance of a triple system with a black hole and two stars.

This system is located about 7,800 light-years from Earth in the constellation Cygnus. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

This black hole, called V404 Cygni, has been extensively studied since being confirmed in 1992. It previously was believed to be orbiting with only one other star, but data from the European Space Agency’s Gaia space observatory showed it instead has two companions.

The researchers said the black hole, with an estimated mass nine times greater than our sun, is in the process of eating one of its companions, a star about seven-tenths as massive as the sun. That star orbits the black hole every 6-1/2 days at a distance only about one-seventh of that separating Earth and the sun.

The black hole appears to be siphoning material off this star, which had puffed up in what is called a red giant phase as part of its natural aging process.

The researchers detected another star about 1.2 times as massive as the sun gravitationally bound to these two but rather far away, orbiting them every 70,000 years at a distance 3,500 times greater than that separating Earth and the sun.

The reason the researchers suspect a gentle birthing process for the black hole is simple. The triple system would have broken apart, they said, if the star that became a black hole had exploded.

A black hole is thought to form when a large star exhausts the nuclear fuel at its core and collapses inward due to its own gravitational pull, triggering an immense explosion that blows off its outer layers into space. The resultant crushed core forms the black hole.

But some astronomers have proposed another path to black hole formation called “direct collapse” in which the star caves in after expending all its fuel but does not explode.

“We call these events a ‘failed supernova.’ Basically, the gravitational collapse just acts too quickly for the supernova to be able to trigger and you get an implosion instead – which sounds super dramatic and awesome but it’s ‘gentle’ in the sense that you don’t expel any matter,” said Massachusetts Institute of Technology astronomer Kevin Burdge, lead author of the study published in the journal Nature.

The researchers estimated that the members of this triple system first formed about 4 billion years ago as ordinary stars.

“The triple system could not have survived if the black hole was born with a natal kick, so this discovery tells us that at least some black holes form without a kick – implying a quiet implosion rather than an explosive supernova,” added Caltech astronomer and study co-author Kareem El-Badry.

This system will not have three members forever, considering that the black hole is consuming its closer neighbor. That suggests that some known binary systems with a black hole and an ordinary star originally may have formed as a triple system, only to have the black hole gobble up one of its partners.

“People have actually predicted that black hole binaries might form mostly through triple evolution, but there was never any direct evidence until now,” El-Badry said.
 

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

Washington:

In 1995, astronomers confirmed the discovery for the first time of a brown dwarf, a body too small to be a star and too big to be a planet – sort of a celestial tweener. But it turns out that was not the full story.

Researchers now have taken a fresh look at that brown dwarf and learned that it actually is not a single brown dwarf but rather two of them orbiting astonishingly close to each other while circling a small star. This was documented in two new studies using telescopes in Chile and Hawaii.

These two brown dwarfs are gravitationally locked to each other in what is called a binary system, an arrangement commonly observed among stars. So the brown dwarf that three decades ago was named Gliese 229B is now recognized as Gliese 229Ba, with a mass 38 times greater than our solar system’s largest planet Jupiter, and Gliese 229Bb, with a mass 34 greater than Jupiter.

They are located 19 light-years from our solar system – rather close in cosmic terms – in the constellation Lepus. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

An artist’s illustration shows the nearest brown dwarf to Earth. ESO-I. Crossfield-N. Risinger/Handout via REUTERS

Binary brown dwarfs are a rarity. These two orbit each other every 12 days at a distance of only 16 times the separation between Earth and the moon. Only one other pair of brown dwarfs are known to orbit as close to each other as this twosome.

Brown dwarfs are neither a star nor a planet, but something in between. They could be considered wannabe stars that during their formative stages did not reach the mass necessary to ignite nuclear fusion at their core like a star. But they are more massive than the biggest planets.

“A brown dwarf is an object that fills the gap between a planet and a star. They are formally defined as objects that can burn a heavy form of hydrogen, called deuterium, but not the most common basic form of hydrogen,” said Sam Whitebook, a graduate student in Caltech’s division of physics, mathematics and astronomy and lead author of one of the studies, published in the Astrophysical Journal Letters.

“In practice, this means they range in mass from approximately 13 to 81 times the mass of Jupiter. Because they can’t fuse hydrogen, they can’t ignite the fusion channels that power most stars. This causes them to just glow dimly as they cool down,” Whitebook said.

The year 1995 was big for astronomers, with the discovery of the first planet beyond our solar system – an exoplanet – also being announced. Until Gliese 229B’s discovery, the existence of brown dwarfs had only been hypothesized. But there were anomalies about Gliese 229B, particularly after its mass was measured at about 71 times that of Jupiter.

“This didn’t make any sense since an object of that mass would be much brighter than Gliese 229B,” said Caltech astronomer Jerry Xuan, lead author of one of the studies, published in the journal Nature. “In fact, some models predict that objects with masses above 70 Jupiter masses fuse hydrogen and become stars, which was clearly not happening here.”

The new observations were able to discern two separate brown dwarfs. They orbit a common type of star called a red dwarf with a mass about six-tenths that of our sun. While both brown dwarfs are more massive than Jupiter, their diameter is actually smaller than the gas giant planet because they are more dense.

“We still don’t really know how different brown dwarfs form, and what the transition between a giant planet and a brown dwarf is. The boundary is fuzzy,” Xuan said. “This finding also shows us that brown dwarfs can come in weird configurations that we were not expecting. This goes to show how complex and messy the star formation process is. We should always be open to surprises.”
 

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)