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Cloud Bound for Milky Way’s Black Hole Puzzles Astronomers

The views expressed are those of the author and are not necessarily those of Scientific American.


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A simulation of the G2 dust cloud approaching the black hole at the center of the Milky Way. Stellar orbits around the black hole are traced in blue. Credit: M. Schartmann and L. Calcada/ European Southern Observatory and Max-Planck-Institut fur Extraterrestrische Physik.

For the past year, astronomers around the world have been watching the center of the Milky Way in anticipation of a once-per-eon event. Right around now (or, technically, 24,000 years ago—that’s roughly how far away the galactic center is in light years), a cloud of gas and dust plummeting toward our galaxy’s supermassive black hole, Sagittarius A*, is expected to make its closest approach. The black hole’s tidal forces should then rip the cloud to shreds. Eventually, Sagittarius A* might even swallow some of those shreds. And if that happens? Fireworks. X-rays and other radiation should rain down on all those watchful telescopes back on Earth. For the first time, we will have witnessed a black hole eating in real time.

But we’re so far away from the galactic center that it’s difficult to know exactly what’s happening there, and the nature of this cloud, which astronomers call G2, has been the subject of debate for quite a while. When Stefan Gillessen, an astronomer with the Max Planck Institute for Extraterrestrial Physics, and his colleagues announced the discovery of G2 in Nature in 2011, they described it as a cold, dense cloud of gas and dust. Andrea Ghez at UCLA, meanwhile, has argued that G2 probably has some sort of star in its center. That sounds like a pretty academic distinction until you consider that if G2 is in fact a dusty, windy star, it will interact with the black hole in a much different way than a dust cloud. Namely, the fireworks could be cancelled. The star’s gravity will bind the cloud together tightly enough that the black hole can’t rip it to pieces. Instead, it will pull away the envelope of gas and dust around the central star and reveal what is inside.

On March 19th and 20th, Ghez and her colleagues watched the galactic center from the W. M. Keck Observatory on Mauna Kea, Hawaii. Before the observation, Ghez and her colleagues took bets on the outcome. “If it’s a dust cloud, it won’t be there,” Ghez said, because it will have been torn to pieces. “If it’s a star, it will.”

Both nights, there it was. “We saw G2, clear as day,” Ghez said.

Ghez and her colleagues describe the observation in a dispatch posted this afternoon to the online forum Astronomer’s Telegram. At the moment, all they can say is that G2 is still there, intact and approaching the black hole. “That doesn’t mean there’s no gas associated with it,” Ghez said. Astronomers know more in a month or so, after they’ve observed G2 in different wavelengths. “But the simple gas cloud hypothesis can be ruled out,” she said.

Ghez has been tracking stars in the galactic center for nearly two decades. In fact, her discovery that massive stars orbit a blank spot at the heart of the Milky Way much like planets orbit their sun is the strongest evidence to date that the galactic center contains a 4-million-solar mass black hole. She hypothesizes that G2 might be ultimately become another one of these stars.

The argument goes like this: The stars that Ghez has recorded orbiting around Sagittarius A* are so young that we don’t understand how they got to the galactic center; they should have had trouble forming there. One theory is that each of these stars (called S stars) was once a member of a binary system—a pair of stars orbiting around a single center of mass. When the pair fell into the galactic center, the black hole tore them asunder. “The black hole grabs one and flings the other out,” Ghez said. “You end up with one object that’s bound to the black hole in a very eccentric orbit. G2 has the absurdly high eccentricity that is predicted.” Perhaps, then, G2 is a binary on its way to becoming an S star.

Ghez’s argument continues like so: To explain what we’re seeing at the galactic center, you need an object that has gas at large distances from the central binding object—otherwise, the tidal forces from the black hole wouldn’t be able to shear away the gas. And G2 is definitely being tidally sheared, Ghez said. “Typical stars in that region are sufficiently compact that there should be no tidal interaction. It’s a competition between the black hole and the central object. If the gravity of the black hole wins, that material gets stripped off.” Scientists have proposed several theories that could explain this weird arrangement, Ghez said, but all of them predict that G2 would get brighter as it got closer to the black hole. “That’s not happening,” she said. “Not only do you see it and it’s compact, but it hasn’t changed brightness at all.”

All of this is speculation, Ghez admitted. “We’re trying to come up with a theory that passes Occam’s Razor.”

So far, evidence from other spring observations—at least what’s been reported so far—is ambiguous. Daryl Haggard at Northwestern University is leading this year’s observations of Sagittarius A* with the Chandra x-ray satellite telescope. “We are just not seeing G2 in the x-ray yet,” Haggard said in late March. “If we really don’t detect anything from G2 in the next couple months, that will probably support a stellar interpretation,” Haggard said.

A stellar interpretation—that is, G2 as a star surrounded by gas, rather than a pure dust cloud—still won’t fully solve the mystery of what the object is, and how it will interact with Sagittarius A* in the months and years ahead. Haggard pointed out that even if G2 is a star, it could still unleash x-rays as it plows through the got gases surrounding the black hole. G2 is “a cold little bullet slamming through hot diffuse medium” surrounding Sagittarius A*. That impact could create a shock wave that would accelerate particles until they emit x-rays. “It’s not entirely clear why a star wouldn’t drive one of those shocks,” she said.

Plenty of people will no doubt be disappointed if it turns out we don’t get to see Sagittarius A* shred dust. Ghez won’t be one of them.

“The reason I’m excited about this is that most stars in our galaxy are binaries,” she said. “So far we’ve completely ignored the impact of binaries.” That is a huge blind spot, she said. “It would be like modeling society as if every family unit were composed of one adult. You can’t understand the family unless you understand that there are two people involved.”

Ultimately it won’t matter much what G2 turns out to be—we still get to see something we’ve never seen before. “Our galaxy is the only place where we can understand how black holes interact with their environment,” Ghez said. “This is first time to track something like this going through closest approach.”

A side effect of the novelty is a little bit of mayhem in the astronomy community. “It’s because we’ve never been able to see this before,” she said, “that there’s confusion.”

About the Author: Seth is a senior editor. Follow on Twitter @seth_fletcher.

The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 12 Comments

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  1. 1. jtdwyer 2:15 pm 05/3/2014

    Regardless of what it is, if G2 (in large part) interacts with the SMBH event horizon we can hopefully collect a great deal of new and invaluable information about the nature of black holes – and the firewall conjecture!

    Link to this
  2. 2. SJCrum 7:31 pm 05/3/2014

    If it is interesting, black holes are not real at all, and they are just theory. And, the real attraction force that does exist in the core of our galaxy is truly the heat-attraction force of real gravity. This gravity science is like the enormous heat of the sun, and earth’s magma core heat, pulling on all colder objects.
    A proof of this gravity science is that earth’s continuous spin is caused entirely by this attraction force also. And, any mass-attracting-mass type of gravity theory is totally impossible to cause a spin at all. Instead, a magnet-like attraction would pull on both the left and right sides of earth the same, and this would totally prevent any spin.
    In real science, with heat-attraction, if you imagine a view of earth from the sun, the sun pulls on both sides of earth, but it pulls more on the left side. And that is the side that has just always come from the colder temperatures of night.
    There are other proofs of real gravity, but that is a truly good one.
    As for the viewed dust cloud in the galaxy core, it is an explosion cloud of gas that has occurred because a star from the constellation Andromeda streaked into the core and collided with a star there. And, this occurred because our galaxy is streaking through the universe at light speed, and Andromeda is stationary. So, the two stars collided.

    Link to this
  3. 3. lunatik96 5:23 pm 05/5/2014

    @SJ Scrum – not familiar with your terminology, however magnetism does explain spin. Otherwise how could an electric motor work? Your heat attraction theory explains wind, but not anything else as far as I can tell.

    Link to this
  4. 4. Michael Hanlon 6:54 pm 05/6/2014

    Something with enough of a gravity well to collect and hold together such a large dust/gas cloud together could be something a touch more massive than a star. It could be a travelling black hole! I hope that Andrea Ghez keeps that possibility on her option list of what is involved in the coming dynamic event. I look forward to hearing more, if it happens soon enough.

    Link to this
  5. 5. jtdwyer 7:31 am 05/7/2014

    SJCrum,
    Wow – are you related to the famous/infamous cartoonist?
    <%)

    Link to this
  6. 6. xmiinc 1:15 am 05/9/2014

    @SJCrum
    You appear to have a Newtonian grasp of gravity, where things ‘pull’ on each other, ie ‘mass attracting’. But if you think in terms of a gravity ‘well’, where things with velocity and mass follow a circumscribed path around any object with greater mass, you’d realize that, indeed, any object following such a path has a side that is always closer to that path than the opposing side. All else being equal, this helps maintain existing spin. How do bodies obtain spin in the first place? Everything, from the angular momentum of smaller objects accreting into larger ones, to the affect of impacts from other bodies, will induce spin in a vacuum over the lifetime of that object.

    Link to this
  7. 7. FleetAngel 4:14 pm 05/9/2014

    This all has the making for an opportunity for some good science. It also sounds like we are in good hands to take advantage of the opportunity. Exciting Times!!!!

    Link to this
  8. 8. kebil 9:32 pm 05/9/2014

    JT – I don’t think that we will be able to answer the firewall question with these observations – as object get closer and closer to the event horizon, the appear to approach it slower and slower. This is part of the Bob and Alice scenario. As Bob (or Alice) falls into the black hole, Bob notices nothing and moves at an ever increasing speed towards the horizon. He will either pass the horizon without incident or burn up, so he will have the answer to the firewall question. However, Alice, watching him, will never get her answer because the closer Bob gets, the slower he appears to be moving through time (relative to Alice, that is). Besides, even if that was not the case, we would never see what happens to something if it actually passes the horizon (it just is gone), or if it is burnt up, it is smeared infinitely thinly over the horizon, approaching it ever more slowly.
    This is how I have read it explained, and had it explained to me. If you have some other insight please fill me in.

    Link to this
  9. 9. Lucyhaye 10:04 am 05/10/2014

    To SJCRUM
    Too complicated and purely descriptive. Gravitation is a *Pushing* by Gravitons, which developed a new Celestial Mechanics that explain what Newton-Einstein cannot,
    See please:
    Quantum Universal Gravitation
    http://autodynamicslborg.blogspot.com/2011/05/quantun-universal-gravitation_06.html

    Link to this
  10. 10. edprochak 3:45 pm 05/14/2014

    Hi SJ,

    A simple test of your theory will show that it is wrong.

    Perform the standard (Cavendish) gravity experiment. Then cool the small weights, heat the large weights, and perform it again. You will find NO difference in the deflection. IOW, gravitational attraction is not dependent on temperature.

    You also misunderstand the source and nature of the earth’s spin. It is due to the conservation of angular momentum. It needs nothing to keep it spinning.

    Link to this
  11. 11. hkraznodar 10:48 am 05/22/2014

    @SJCrum: You make a lot of statements as if they were facts and yet provide no actual peer reviewed proof. I don’t know enough to agree or disagree with most of your comment. I do however, challenge your statement that Andromeda is stationary and that the Milkyway travels at light speed. First; Andromeda would only appear stationary if you were in the Andromeda galaxy and using that as your central point of reference.

    As far as I’m aware no one has ever identified the exact center of the universe. Even if they did they would be subject to so much challenge as to never have their assessment accepted by the majority of astronomers.

    The speed of light is 299 792 458 m / s or 299,792.458 kilometers per second. The speed at which the Milky Way and Andromeda are approaching each other is 300 kilometers per second. If you are bad at math let me clarify, 299,792 is significantly greater than 300.

    Link to this
  12. 12. kebil 12:54 pm 06/10/2014

    Crum, how do you explain the gravitational attraction of the distant dwarf planets and asteroids, many of which have moons or satellites? These are very cold objects. In addition, as others have said, angular momentum is what keeps something spinning, not gravity. The fact that both sides experience the same force does nothing to stop spin.

    Link to this

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