Supermassive black hole ejects hyper-velocity star

It’s hard for most of us to comprehend some of the enormous speeds that are related to space exploration, travel or discovery.

The average flight speed of a 747 passenger aircraft, one that many people have experienced, is about 900 km/h. The fastest any human has ever travelled was by the Apollo 10 astronauts, as they soared around the back of the Moon at 39,897 km/h heading back to Earth. Whilst, in November 2018, NASA’s Parker Solar Probe, was clocked in at 343,180 km/h – setting the Guinness World Record.

We picture ourselves trying to imagine how fast this would feel or even look like from a drivers point of view, by mentally comparing our experiences of riding in cars and aeroplanes.

But our brains can’t compute the velocity observed of a new massive object, a star bigger than our Sun, moving across our Milky Way galaxy at over 6,000,000 km/h. At the speed that this newly discovered star is travelling - you could circle Earth’s equator almost 150 times in the same hour or fly from Sydney to London in under 10 seconds.

How could any such object, receive enough energy to blast it across space at extreme speeds? And what could be bigger to give it such a kick?

Australian astronomers, working as part of an international collaboration, have discovered the fastest moving main-sequence star on its journey to depart the Milky Way, after encountering the enormous forces of the Galaxy’s supermassive black hole 4.8 million years ago.

“This discovery is very exciting!” said Geraint Lewis, Professor of Astrophysics at the University of Sydney in addition to being one of the S⁵ project leaders.

“A star that spent its life at the centre of the Milky Way has been spat out in a violent interaction with our supermassive black hole and is now speeding out of the Galaxy. What secrets of the inner parts of the Galaxy will it reveal?” he said.

The star, called S5-HVS1, is a hot (~9,600K) A-Class main-sequence star (meaning hydrogen is still fusing in its core), making it is a little larger than the Sun with 2.35 times its mass. It’s traversing across the southern sky, currently in the constellation of Grus (the Crane) at a mind-boggling 6 million km/h.

Tracing its trajectory and velocity backwards, the researchers have indicated that S5-HVS1 is likely to have been part of a binary system which wondered close to Sgr A* (pronounced “Sagittarius A-Star”) the supermassive black hole at the centre of our galaxy. Unfortunately for S5-HVS1’s binary partner, the close proximity to Sgr A* means it was captured whilst the surviving star received an enormous kick of energy – propelling it away from the Galactic Centre.

“I was really surprised when we found a star going so fast. It only sank in when we got follow-up observations that confirmed the speed,” said Dr. Jeffrey Simpson, a post-doctoral research fellow at the UNSW science school of physics, a co-author on the paper.

Researchers from a number of Australian universities, organisations and divisions were involved in the international collaboration, including Australian National University, University of Sydney, Macquarie University, University of NSW, Australian Astronomical Optics (Macquarie University), Sydney Institute of Astronomy (University of Sydney), ASTRO3D.

“When we started this project I never thought we would accidentally find a star escaping the Milky Way,” said co-author Associate Professor Sarah Martell, Scientia Fellow at UNSW Science’s School of Physics.

“It highlights the importance of the investment by UNSW and other Australian universities in the continuing operation of the Anglo-Australian Telescope and Siding Spring Observatory.”

This discovery was made by the Southern Stellar Stream Spectroscopic Survey collaboration, (also known as the S⁵ project).

“I am so excited this fast-moving star was discovered by S⁵,” says S⁵ Executive Committee member Kyler Kuehn, at Lowell Observatory and also affiliated with Australian Astronomical Optics. “While the main science goal of S⁵ is to probe the stellar streams — disrupting dwarf galaxies and globular clusters — we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for ‘free.’ With our future observations, hopefully, we will find even more!”

The S⁵ project began in 2018 and is a spectroscopic survey that uses the 3.9m Anglo-Australian Telescope located in Central North-West NSW, at the Siding Springs Observatory.

The telescope is mounted with the 2dF (2-Degree Field) positioning system – a robot which places 392 optical fibres on a metal plate to simultaneously collect their light to an accuracy of 0.3 arcseconds - before sending the data 50m below the telescope to the AAOmega spectrograph, the instrument which then splits the light into different channels for analysis.

“S⁵ shows us how modern science works. An international team of excellent scientists with the goal of understanding how our Galaxy has grown over cosmic time, whilst revealing the amount of dark matter is holding the Milky Way together.” Said Professor Lewis.

There are four science surveys being conducted under the S⁵ project:

• S⁵ Streams: The main survey of the collaboration with goals to measure the radial velocity and metallicities of stream members
• S⁵ Halo: Surveying the Milky Way halo for interesting objects like Hyper-Velocity Stars, extremely metal-poor stars, RR Lyraes and White Dwarfs
• S⁵ Lowz: Observing and documenting low-redshift galaxies to increase the database of these nearby faint galaxies, so that researchers can better train algorithms to build up a larger sample of these objects
• S⁵ Hires: Using the MIKE instrument located at the Las Campanas Observatory in Chile, this survey aims to capture high-resolution spectra of bright stream members to derive stellar parameters and precise elemental abundances, helping determine the chemical evolution of stars

In Astronomy, a metal is anything that is not Hydrogen or Helium.

An overview of the S⁵ project can be found in this paper published in MNRAS.

Astronomers had long suspected a gargantuan black hole resided in the centre of our Galaxy, and in 2002 astronomers traced the highly eccentric orbit of a star called “S2” around a central massive, unseen object – confirming the enormous mass of 4 million Suns jammed into a small region of space – a supermassive black hole. In fact, a cluster of stars has been observed to orbit around Sgr A* in a range of different orbits – some with moderate and some with higher eccentricities.

In this latest paper, astronomers have described how S5-HVS1’s original binary pair could have the same origin as the young stellar disc surrounding Sgr A*, given the ejected star flies within the planes of the young stars orbiting the galactic centre. 

The following animation video, created by James Josephides (Swinburne Astronomy Productions) demonstrates this process.

This in itself provides an interesting opportunity: the ability to study a star who originated within the central region of our Galaxy and not have to deal with the complexities of extinction (where light is blocked between our view and the Galactic Centre by interstellar dust and clouds). Astronomers could learn more about the origins of the stars surrounding the supermassive black hole through this discovery.

Are they all captured stars? Are they old binary pairs that have been ripped apart, much like S5-HVS1? Or did they form from a large cloud of gas that was surrounding Sgr A*?

“It’s a weird, bizarre place, and very hard for us to probe because there is a lot of dust between us and it; we can see things with infrared and radio waves but not necessarily with optical light,” says Daniel Zucker, who is one of the leaders of the international project and Associate Professor of the Department of Physics and Astronomy at Macquarie University.

“Now we’ve got a star that seems to have formed within the region and has escaped from it and, at 29,000 light-years from Earth, is now close enough for us to study in relative detail.

“And it seems to be perfectly normal, so that should tell us something about how stars are being formed near the Galactic centre and about the conditions there.”

Supermassive black holes have a ‘regulatory’ role to play in star formation in the central regions of galaxies – from limiting the star formation rate due to the disruptive tidal pull on the tenuous molecular clouds that give birth to stars, to blasting the same clouds with powerful winds when the black hole is feeding and active, once again prohibiting the opportunity for stars to form.

However, the presence of young massive stars orbiting in a range of eccentric rings and orbits around Sgr A* has presented a number of different theories to how they came to be. One states that the stars formed elsewhere and migrated inwards, whilst another theory implies that multiple discs of molecular clouds (with different planes to each other) fragmented into stars during the early days and formation of the supermassive black hole.

The central cubic parsec surrounding the galactic centre contains approximately 10 million stars – most of which are older, red giant stars – in addition to young, massive OB and Wolf-Rayet stars.

“The centre of the Galaxy is a maelstrom of objects circling and falling into a massive black hole, Sagittarius A*, and yet there seems to be stars forming there,” says Associate Professor Daniel Zucker.

New star formation can occur near the centre of the galaxy, and in 2002 scientists from the Harvard Smithsonian Centre of Astrophysics (using the south pole telescope) mapped the gas density within a 400 light-year region and found that an accumulating ring was developing, with a mass many million times that of the Sun. It was noted that this ring would near the critical density for star formation to occur in about 200 million years from now – triggering an epoch of starburst for the Milky Way.

In 2009 – two protostars were discovered within 10 light-years of this region – suggesting molecular clouds in the region might be denser than previously expected (to allow self-gravitational triggers to overcome the tidal and external forces that limit the rate of formation). Then in 2017, scientist using the Atacama Large Millimetre/Submillimetre Array (ALMA) found 11 low-mass stars forming within three light-years in the turbulent region.

The small cluster of stars surrounding Sgr A* is known as ‘S-Stars’ – a pack of high-velocity stars that have been measured to be young, massive and with highly eccentric orbits that have close proximity to the supermassive black hole. It is thought that these stars were once binary pairs as well much like S5-HVS1.

However, S5-HVS1 is a middle-aged A-class star (white) that is not massive like the OB-class stars nor an older red giant stars (M-class range). It’s somewhere down the middle of the range of ordinary stars, if anything, slightly on the larger side.  

In 1988, J.G. Hills proposed a mechanism in which hyper-velocity stars (+1,000km/s) could obtain their massive speeds by being part of a binary system that interacts with a supermassive black hole. In this scenario, the energy of the binary pair is unleashed when one of the stars becomes bound to the supermassive black hole, thereby ditching its original partner.

 “It has to do with the gravitational binding energy,” Zucker says of Hills’ theory. “In a binary system, a very massive object takes the place of the other star in the binary, then the remaining star is flung out. It’s like switching dance partners, but where the initial partner is thrown out at high speed. If the object replacing one of the stars is way, way, way bigger, then it will give an enormous kick to the one that’s escaping, and that is what’s happened in this case.”

The discovery of S5-HSV1 confirms that the Hills Mechanism is a method of hyper-velocity star ejection from the Galactic Centre, in addition to adding to the supporting evidence that a supermassive black hole does indeed reside in the heart of the Milky Way.

Unfortunately for S5-HSV1, this means that the star is now on a trajectory that will exit the galaxy in about 100 million years from now entering into intergalactic space, isolated and alienated from its former island universe of stars – never returning.

By studying S5-HVS1’s motion and modelling its history, the research team were also able to place constraints over the velocity of our own Sun as it orbits the galaxy, and its radial distance from the Galactic Centre.

It turns out that the Sun’s velocity as it whips around the galaxy is about 246 km/s, a result which is similar to that found by the Gravity Collaboration et al. team in 2019, which measured it at approximately 247 km/s. In addition to this, it was found that the distance between the Sun and the centre of the milky way is about 8.12 kiloparsecs (which translates to about 26,484 light-years).

In terms of where we are geographically, the Sun (and the solar system) reside in the Orion arm (sometimes referred to as the local arm) of the Milky Way galaxy. The Orion arm is about 3,500 light-years across and about 10,000 light-years in length. It’s named after the prominent constellation Orion which has many bright stars in Earth’s sky from about November onwards. In fact – when we look at the constellation Orion, we are looking into the very spiral arm of our galaxy that we live in.

The search and study of high-velocity stars is not a new practice – in 1926 Oort wrote his Ph.D. thesis on the topic, placing a boundary limit of what is a high-velocity star and what is not (63 km/s).

In the 1950s and 60s, other studies started appearing that classified any star as a high-velocity star where its radial velocity could be measured to be equal or greater than 100km/s, which at the time were found to be mostly stars that resided in the Milky Way halo – a spherical population and region surrounding our Galaxy which contains globular clusters.

Further research in 1974 announced that even faster stars were found in the Milky Way halo, this time traveling up to 200 km/s, and at these velocities, the progenitor event proposed to give such powerful kicks was the expulsion of a binary through a supernova event or ‘sling-shot’ encounters experienced within star clusters.

It wasn’t until Hills published his work in 1988 in which hyper velocities (> 1,000km/s) could be attributed to stellar dealings with supermassive black holes, as the mechanism to provide such massive energy boosts to stars.

S5-HSV1 was ripped away from its partner about 5 million years ago when it strayed too close to Sgr A* and is now on a one-way departure ticket out of the Galaxy, exiting about 100 million years from now. It holds the record for the third fastest hyper-velocity star ever observed.

The discovery is one that has shed new light on our understanding of the mechanisms that create runaway stars and given us that little bit more of knowledge about what lies at the centre of our Galaxy.

It has also opened up several questions that we are bound to explore more about, such as resolving any connection between S5-HSV1 and the disc of young stars around Sgr A* (in particular, did the star get ejected around the same time the disc formed)?

Or what the star can tell us about the spectral and elementary properties of the young disc, if it was indeed formed as part of the same cluster.            

More mysteriously, has S5-HVS1 been travelling at a constant speed over time? And where are all the other hyper-velocity stars that would have also experienced the same expulsion, gaining such high velocities?

As astronomers and researchers pour over these questions through further observations, one thing remains constant and clear: never get close to the dark heart of our Galaxy.

Sagittarius A* is a powerful, merciless and unforgiving entity. S5-HSV1 was the lucky one that got away. It’s partner, forever imprisoned by the monster.