Black holes have been the subject of intense interest ever since scientists began speculating about their existence. Originally proposed in the early 20th century as a consequence of Einstein’s Theory of General Relativity, black holes became a mainstream subject a few decades later. By 1971, the first physical evidence of black holes was found and by 2016, the existence of gravitational waves was confirmed for the first time.
This discovery touched off a new era in astrophysics, letting people know collision between massive objects (black holes and/or neutron stars) creates ripples in spacetime that can be detected light-years away. To give people a sense of how profound these events are, Álvaro Díez created the Black Hole Collision Calculator (BHCC) – a tool that lets you see what the outcome of a collision between a black hole and any astronomical object would be!
Stellar collisions are an amazingly rare thing. According to our best estimates, such events only occur in our galaxy (within globular clusters) once every 10,000 years. It’s only been recently, thanks to ongoing improvements in instrumentation and technology, that astronomers have been able to observe such mergers taking place. As of yet, no one has ever witnessed this phenomena in action – but that may be about to change!
According to study from a team of researchers from Calvin College in Grand Rapids, Michigan, a binary star system that will likely merge and explode in 2022. This is an historic find, since it will allow astronomers to witness a stellar merger and explosion for the first time in history. What’s more, they claim, this explosion will be visible with the naked-eye to observers here on Earth.
This binary star system, which is known as KIC 9832227, is one that Prof. Molnar and his colleagues – which includes students from the Apache Point Observatory and the University of Wyoming – have been monitoring since 2013. His interest in the star was piqued during a conference in 2013 when Karen Kinemuchi (an astronomers with the Apache Point Observatory) presented findings about brightness changes in the star.
This led to questions about the nature of this star system – specifically, whether it was a pulsar or a binary pair. After conducting their own observations using the Calvin observatory, Prof. Molnar and his colleagues concluded that the star was a contact binary – a class of binary star where the two stars are close enough to share an atmosphere. This brought to mind similar research in the past about another binary star system known as V1309 Scorpii.
This binary pair also had a shared atmosphere; and over time, their orbital period kept decreasing until (in 2008) they unexpectedly collided and exploded. Believing that KIC 9832227 would undergo a similar fate, they began conducting tests to see if the star system was exhibiting the same behavior. The first step was to make spectroscopic observations to see if their observations could be explained by the presence of a companion star.
As Cara Alexander, a Calvin College student and one of the co-authors on the team’s research paper, explained in a college press release:
“We had to rule out the possibility of a third star. That would have been a pedestrian, boring explanation. I was processing data from two telescopes and obtained images that showed a signature of our star and no sign of a third star. Then we knew we were looking at the right thing. It took most of the summer to analyze the data, but it was so exciting. To be a part of this research, I don’t know any other place where I would get an opportunity like that; Calvin is an amazing place.”
The next step was to measure the pair’s orbital period, to see it was in fact getting shorter over time – which would indicate that the stars were moving closer to each other. By 2015, Prof. Molnar and his team determined that the stars would eventually collide, resulting in a kind of stellar explosion known as a “Red Nova”. Initially, they estimated this would take place between 2018 and 2020, but have since placed the date at 2022.
In addition, they predict that the burst of light it will cause will be bright enough to be seen from Earth. The star will be visible as part of the constellation Cygnus, and it appear as an addition star in the familiar Northern Cross star pattern (see above). This is an historic case, since no astronomer has ever been able to accurately predict when and where a stellar collision would take place in the past.
What’s more, this discovery is immensely significant because it represents a break with the traditional discovery process. Not only have small research institutions and universities not been the ones to take the lead on these sorts of discoveries in the past, but student-and-teacher teams have also not been the ones who got to make them. As Molnar explained it:
“Most big scientific projects are done in enormous groups with thousands of people and billions of dollars. This project is just the opposite. It’s been done using a small telescope, with one professor and a few students looking for something that is not likely. Nobody has ever predicted a nova explosion before. Why pay someone to do something that almost certainly won’t succeed? It’s a high-risk proposal. But at Calvin it’s only my risk, and I can use my work on interesting, open-ended questions to bring extra excitement into my classroom. Some projects still have an advantage when you don’t have as much time or money.”
Over the course of the next year, Molnar and his colleagues will be monitoring KIC 9832227 carefully, and in multiple wavelengths. This will be done with the help of the NROA’s Very Large Array (VLA), NASA’s Infrared Telescope Facility at Mauna Kea, and the ESA’s XMM-Newton spacecraft. These observatories will study the star’s radio, infrared and X-ray emissions, respectively.
Molnar also expects that amateur astronomers will be able to monitor the pair’s orbital timing and variations in brightness. And if he and his team’s predictions are correct, every student and stargazer in the northern hemisphere – not to mention people who just happen to be out for a walk – will be privy to the amazing light show. This is sure to be a once-in-a-lifetime event, so stay tuned for more information!
Interestingly enough, this historic discovery is also the subject of a documentary film. Titled “Luminous“, the documentary – which is directed by Sam Smartt, a Calvin professor of communication arts and sciences – chronicles the process that led Prof. Molnar and his team to make this unprecedented discovery. The documentary will also include footage of the Red Nova as it happens in 2022, and is expected to be released sometime in 2023.
It’s known as HIP 85605, one of two stars that make up a binary in the Hercules constellation roughly 16 light years away. And if a recent research paper produced by Dr. Coryn Bailer-Jones of the Max Planck Institute for Astronomy in Heidelberg, Germany is correct, it is on a collision course with our Solar System.
Now for the good news: according to Bailer-Jones’ calculations, the star will pass by our Solar System at a distance of 0.04 parsecs, which is equivalent to 8,000 times the distance between the Earth and the Sun (8,000 AUs). In addition, this passage will not affect Earth or any other planet’s orbit around the Sun. And perhaps most importantly of all, none of it will be happening for another 240,000 to 470,000 years from now.
“Even though the galaxy contains very many stars,” Bailer-Jones told Universe Today via email, “the spaces between them are huge. So even over the (long) life of our galaxy so far, the probability of any two stars have actually collided — as opposed to just coming close — is extremely small.”
However, in astronomical terms, that still counts as a near-miss. In a universe that is 46 billion light years in any direction – and that’s just the observable part of it – an event that is expected to take place just 50 light days away is considered to be pretty close. And in the context of space and time, a quarter of a million to half a million years is the very near future.
The real concern is the effect that the passage of HIP 85605 could have on the Oort Cloud – the massive cloud of icy planetesimals that surrounds the Solar System. Given that it’s distance is between 20,000 and 50,000 AU from our Sun, HIP 85605 would actually move through the Oort cloud and cause serious disruption.
Many of these planetesimals could be blown off into space, but others could be sent hurtling towards Earth. Assuming humanity is still around at this point in time, this could present a bit of an inconvenience, even if it is spread over the course of a million years.
As it stands, such “close encounters” between stars are quite rare. Stellar collisions usually only occur within binaries, where white dwarfs or neutron stars are concerned. “The exception to this is physically bound binary stars in a tight orbit,” said Bailer-Jones. “It can and does happen that one star expands during its evolution and will then interfere with the evolution of the other star. Neutron-neutron star pairs can even merge.”
But of course, on an astronomical timescale, stars passing each other by as they perform their cosmic dance is actually a pretty common occurrence. As part of Bailer-Jones larger study of over 50,000 stars within our galaxy, this “close encounter” is one of several predicted to take place in the coming years.
Of all of them, only HIP 85605 is expected to come within a single parsec between 240 and 470 thousand years from now. He also indicates with (90% confidence) that the last time such an encounter took place was 3.8 million years ago when gamma Microscopii – a G7 giant which has two and a half times the mass of our Sun – came within 0.35-1.34 pc of our system, which may have caused a large perturbation in the Oort cloud.
On his MPIA webpage, in the study’s FAQ section, Bailer-Jones claims that his research into stellar close encounters was motivated by a desire to study the potential impacts of astronomical phenomena on Earth, and is part of a larger program named “astroimpacts”.
“I am interested in the history of the Earth,” he says, “and astronomical phenomena have clearly played a role in this. But what role precisely, how significant, and what can we expect to happen in the future?” Whereas several studies have been conducted in the past, he feels that the methods – which include assuming a linear relative motion of stars – produces inaccurate results.”
In contrast, Bailer-Jones study relies on “more recent data or re-analyses of data to produce hopefully more accurate results, and then compensate more rigorously for the uncertainties in the data, so that I can attach probabilities to my statements.”
As a result of this, he predicts that HIP 85605 has a 90% chance of passing within a single parsec of our Sun in the next 240 to 470 thousands years. However, he also admits that if the astronomy is incorrect, the next closest encounter won’t be happening for another 1.3 million years, when a K7 dwarf known as GL 710 is predicted to pass within 0.10 – 0.44 parsecs.
Bailer-Jones also believes that the European Space Agency’s Gaia spacecraft will help make more accurate predictions in the future. By understanding and mapping the environment of the Milky Way Galaxy, measuring the gravitational potential and determining the velocity of stars, scientists will be able to see how their various orbits around the galaxy’s center could cause them to intersect.
But perhaps the most interesting question explored on his webpage is the possibility of using stellar close encounters as a shortcut for exploring exoplanets. According to current cosmological models, the majority of stars within our galaxy are believed to host exoplanets.
So if a star is passing us at just a few parsecs (or even with a single parsec) why not hop on over and investigate its planets? Well, as Bailer-Jones indicates, that’s not really a practical idea: “Traveling to a star passing our solar system at a distance of around 1 pc with a relative speed of 30 km/s is no easier than traveling the the nearby stars (the nearest of which is just over 1 pc away). And we would have to wait 10s of thousands of years for the next encounter. If we can ever achieve interstellar travel, I don’t suppose it would take that long to achieve, so why wait?”
Darn. Still, if there’s one thing this phenomena and Bailer-Jones study reminds us, it is that in the course of dancing around the center of the Milky Way, stars are not fixed in a single point in space. Not only do they periodically move within reach of each other, they can also have an affect on life within them.
Alas, the timescale on which such things happen, not to mention the consequences they entail, are so large that people here on Earth need not worry. By the time HIP 85605 or GL 710 come within a parsec or two of us, we’ll either be long-since dead or too highly evolved to care!
*Update: According to a new study posted by Erick E. Mamajek and associates on arXiv, the passage of the recently-discovered low mass star W0720 (aka. “Scholtz Star”) – roughly 70,000 years ago and at a distance of 0.25 Parsecs from our Sun – was the closest encounter our Solar System has had with another star. They calculate the possibility that it would have penetrated the System’s Outer Oort Cloud at 98%. However, they also estimate that the impact it would have had on the flux of long-period comets was negligible, but that the passage also highlights how “dynamically important Oort Cloud perturbers may be lurking among nearby stars”.
Having read the study, Bailer-Jones claims on the updated FAQ section of his MPIA webpage that their analysis appears to be correct. Based on the assumption that the star was moving on a constant velocity relative to the Sun prior to the encounter, he agrees that the calculations on the distances and timing of the passage are valid. While his own study identified a possible closer encounter (Hip 85605), he reiterates that the data on this star is of poor quality. Meanwhile, another close encounter took place involving Hip 89825; but here, the approach distance is estimated to have been 0.02 Parsecs larger. Hence, W0720 can be said to have been the closest encounter with some degree of certainty at this time.
Imagine a really bad day. Perhaps you’re imagining a day where the Sun crashes into another star, destroying most of the Solar System.
No? Well then, even in your imagination things aren’t so bad… It’s all just matter of perspective.
Fortunately for us, we live in out the boring suburbs of the Milky Way. Out here, distances between stars are so vast that collisions are incredibly rare. There are places in the Milky Way where stars are crowded more densely, like globular clusters, and we get to see the aftermath of these collisions. These clusters are ancient spherical structures that can contain hundreds of thousands of stars, all of which formed together, shortly after the Big Bang.
Within one of these clusters, stars average about a light year apart, and at their core, they can get as close to one another as the radius of our Solar System. With all these stars buzzing around for billions of years, you can imagine they’ve gotten up to some serious mischief.
Within globular clusters there are these mysterious blue straggler stars. They’re large hot stars, and if they had formed with the rest of the cluster, they would have detonated as supernovae billions of years ago. So scientists figure that they must have formed recently.
How? Astronomers think they’re the result of a stellar collision. Perhaps a binary pair of stars merged, or maybe two stars smashed into one another.
Professor Mark Morris of the University of California at Los Angeles in the Department of Physics and Astronomy helps to explain this idea.
“When you see two stars colliding with each other, it depends on how fast they’re moving. If they’re moving at speeds like we see at the center of our galaxy, then the collision is extremely violent. If it’s a head-on collision, the stars get completely splashed to the far corners of the galaxy. If they’re merging at slower velocities than we see at our neck of the woods in our galaxy, then stars are more happy to merge with us and coalesce into one single, more massive object.”
There’s another place in the Milky Way where you’ve got a dense collection of stars, racing around at breakneck speeds… near the supermassive black hole at the center of the galaxy.
This monster black hole contains the mass of 4 million times the Sun, and dominates the region around the center of the Milky Way.
“The core of the Milky Way is one of those places where you find the extremes of nature. The density of stars there is higher than anywhere else in the galaxy,”Professor Morris continues. “Overall, in the center of our galaxy on scales of hundreds of light years, there is much more gas present than anywhere else in the galaxy. The magnetic field is stronger there than anywhere else in the galaxy, and it has it’s own geometry there. So it’s an unusual place, an energetic place, a violent place, because everything else is moving so much faster there than you see elsewhere.”
“We study the stars in the immediate vicinity of the black hole, and we find that there’s not as many stars as one might have expected, and one of the explanations for that is that stars collide with each other and either eliminate one another or merge, and two stars become one, and both of those processes are probably occurring.”
Stars whip around it, like comets dart around our Sun, and interactions are commonplace.
There’s another scenario that can crash stars together.
The Milky Way mostly has multiple star systems. Several stars can be orbiting a common center of gravity. Many are great distances, but some can have orbits tighter than the planets around our Sun.
When one star reaches the end of its life, expanding into a red giant, It can consume its binary partner. The consumed star then strips away 90% of the mass of the red giant, leaving behind a rapidly pulsating remnant.
What about when galaxies collide? That sounds like a recipe for mayhem.
Surprisingly, not so much.
“That’s actually a very interesting question, because if you imagine two galaxies colliding, you’d imagine that to be an exceptionally violent event,’ Professor Morris explains. “But in fact, the stars in those two galaxies are relatively unaffected. The number of stars that will collide when two galaxies collide is possibly counted on the fingers of one or two hands. Stars are so few and far between that they just aren’t going to meet each other with any significance in a field like that.”
“What you see when you see two galaxies collide, however, on the large scale, is that the tidal forces of the two galaxies will rip each of the galaxies apart in terms of what it will look like. Streams of stars will be strewn out in various directions depending on the precise history of the interaction between the two galaxies. And so, eventually over time, the galaxies will merge, the whole configuration of stars will settle down into something that looks unlike either of the two initially colliding galaxies. Perhaps something more spheroidal or spherical, and it might look more like an elliptical galaxy than the spiral galaxy that these two galaxies now are.”
Currently, we’re on a collision course with the Andromeda Galaxy, and it’s expected we’ll smash into it in about 4 billion years. The gas and dust will collide and pile up, igniting an era of furious star formation. But the stars themselves? They’ll barely notice. The stars in the two galaxies will just streak past each other, like a swarm of angry bees.
So, good news! When you’re imagining a worse day, you won’t have to worry about our Sun colliding with another star. We’re going to be safe and sound for billions of years. But if you live in a globular cluster or near the center of the galaxy, you might want to check out some property here in the burbs.