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:

Astronomers have spotted orbiting around a young star a newborn planet that took only 3 million years to form – quite swift in cosmic terms – in a discovery that challenges the current understanding of the speed of planetary formation.

This infant world, estimated at around 10 to 20 times the mass of Earth, is one of the youngest planets beyond our solar system – called exoplanets – ever discovered. It resides alongside the remnants of the disk of dense gas and dust circling the host star – called a protoplanetary disk – that provided the ingredients for the planet to form.

The star it orbits is expected to become a stellar type called an orange dwarf, less hot and less massive than our sun. The star’s mass is about 70% that of the sun and it is about half as luminous. It is located in our Milky Way galaxy about 520 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

“This discovery confirms that planets can be in a cohesive form within 3 million years, which was previously unclear as Earth took 10 to 20 million years to form,” said Madyson Barber, a graduate student in the department of physics and astronomy at the University of North Carolina at Chapel Hill and lead author of the study published this week in the journal Nature.

“We don’t really know how long it takes for planets to form,” UNC astrophysicist and study co-author Andrew Mann added. “We know that giant planets must form faster than their disk dissipates because they need a lot of gas from the disk. But disks take 5 to 10 million years to dissipate. So do planets form in 1 million years? 5? 10?”

The planet, given the names IRAS 04125+2902 b and TIDYE-1b, orbits its star every 8.8 days at a distance about one-fifth that separating our solar system’s innermost planet Mercury from the sun. Its mass is in between that of Earth, the largest of our solar system’s rocky planets, and Neptune, the smallest of the gas planets. It is less dense than Earth and has a diameter about 11 times greater. Its chemical composition is not known.

The researchers suspect that the planet formed further away from its star and then migrated inward.

“Forming large planets close to the star is difficult because the protoplanetary disk dissipates away from closest to the star the fastest, meaning there’s not enough material to form a large planet that close that quickly,” Barber said.

The researchers detected it using what is called the “transit” method, observing a dip in the host star’s brightness when the planet passes in front of it, from the perspective of a viewer on Earth. It was found by NASA’s Transiting Exoplanet Survey Satellite, or TESS, space telescope.

“This is the youngest-known transiting planet. It is on par with the youngest planets known,” Barber said.

Exoplanets not detected using this method sometimes are directly imaged using telescopes. But these typically are massive ones, around 10 times greater than our solar system’s largest planet Jupiter.

Stars and planets form from clouds of interstellar gas and dust.

“To form a star-planet system, the cloud of gas and dust will collapse and spin into a flat environment, with the star at the center and the disk surrounding it. Planets will form in that disk. The disk will then dissipate starting from the inner region near the star,” Barber said.

“It was previously thought that we wouldn’t be able to find a transiting planet this young because the disk would be in the way. But for some reason that we aren’t sure of, the outer disk is warped, leaving a perfect window to the star and allowing us to detect the transit,” Barber added.
 

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