What’s Causing the Mysterious Radio Waves Coming From the Center of the Milky Way?

The center of the Milky Way is a mysterious place. Astronomers think there’s a supermassive black hole there, though it could be dark matter instead. The region is densely packed with stars, dominated by red giants. And because of all the dust between Earth and the galactic center, we can’t see anything with visible light, ultraviolet light, or low-energy x-rays.

But we can detect radio waves, and there are some unexplained ones coming from the center of the galaxy, and adding to the mystery.

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New Mosaic Shows the Galactic Core From Opposite Sides of the Electromagnetic Spectrum

The core of the Milky Way Galaxy (aka. Galactic Center), the region around which the rest of the galaxy revolves, is a strange and mysterious place. It is here that the Supermassive Black Hole (SMBH) that powers the compact radio source known as Sagittarius A* is located. It is also the most compact region in the galaxy, with an estimated 10 million stars within 3.26 light-years of the Galactic Center.

Using data from Chandra X-ray Observatory and the MeerKAT radio telescope, NASA and the National Research Foundation (NSF) of South Africa created a mosaic of the center of the Milky Way. Combining images taken in the x-ray and radio wavelengths, the resulting panoramic image manages to capture the filaments of super-heated gas and magnetic fields that (when visualized) shows the complex web of energy at the center of our galaxy.

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7% of the Stars in the Milky Way’s Center Came From a Single Globular Cluster That Got Too Close and Was Broken Up

The heart of the Milky Way can be a mysterious place. A gigantic black hole resides there, and it’s surrounded by a retinue of stars that astronomers call a Nuclear Star Cluster (NSC). The NSC is one of the densest populations of stars in the Universe. There are about 20 million stars in the innermost 26 light years of the galaxy.

New research shows that about 7% of the stars in the NSC came from a single source: a globular cluster of stars that fell into the Milky Way between 3 and 5 billion years ago.

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More Mysterious Space Blobs Have Been Found Near the Center of the Milky Way

At the center of our galaxy lies a region where roughly 10 million stars are packed into just 1 parsec (3.25 light-years) of space. At the center of this lies the supermassive black hole (SMBH) known as Sagittarius A*, which has a mass of over 4 million Suns. For decades, astronomers have been trying to get a better look at this region in the hopes of understanding the incredible forces at work and how they have affected the evolution of our galaxy.

What they’ve found includes a series of stars that orbit very closely to Sagittarius A* (like S1 and S2), which have been used to test Einstein’s Theory of General Relativity. And recently, a team from UCLA’s Galactic Center Orbits Initiative detected a series of compact objects that also orbit the SMBH. These objects look like clouds of gas but behave like stars, depending on how close they are in their orbits to Sagittarius A*.

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There are Strange Objects Near the Center of the Galaxy. They Look Like Gas, but Behave Like Stars

During the 1970s, astronomer became aware of a massive radio source at the center of our galaxy that they later realized was a Supermassive Black Hole (SMBH) – which has since been named Sagittarius A*. And in a recent survey conducted by NASA’s Chandra X-ray Observatory, astronomers discovered evidence for hundreds or even thousands of black holes located in the same vicinity of the Milky Way.

But, as it turns out, the center of our galaxy has more mysteries that are just waiting to be discovered. For instance, a team of astronomers recently detected a number of “mystery objects” that appeared to be moving around the SMBH at Galactic Center. Using 12 years of data taken from the W.M. Keck Observatory in Hawaii, the astronomers found objects that looked like dust clouds but behaved like stars.

The research was conducted through a collaboration between Randy Campbell at the W.M. Keck Observatory, members of the Galactic Center Group at UCLA (Anna Ciurlo, Mark Morris, and Andrea Ghez) and Rainer Schoedel of the Instituto de Astrofisica de Andalucia (CSIC) in Granada, Spain. The results of this study were presented at the 232nd American Astronomical Society Meeting during a press conference titled “The Milky Way & Active Galactic Nuclei”.

Pictured here are members of GCOI in front of Keck Observatory on Maunakea, Hawaii, during a visit last year. Credit: W.M. Keck Observatory

As Ciurlo explained in a recent W.M. Keck press release:

“These compact dusty stellar objects move extremely fast and close to our Galaxy’s supermassive black hole. It is fascinating to watch them move from year to year. How did they get there? And what will they become? They must have an interesting story to tell.”

The researchers made their discovery using 12 years of spectroscopic measurements obtained by the Keck Observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS). These objects – which were designed as G3, G4, and G5 – were found while examining the gas dynamics of the center of our galaxy, and were distinguished from background emissions because of their movements.

“We started this project thinking that if we looked carefully at the complicated structure of gas and dust near the supermassive black hole, we might detect some subtle changes to the shape and velocity,” explained Randy Campbell. “It was quite surprising to detect several objects that have very distinct movement and characteristics that place them in the G-object class, or dusty stellar objects.”

Astronomers first discovered G-objects in proximity to Sagittarius A* more than a decade ago – G1 was discovered in 2004 and G2 in 2012. Initially, both were thought to be gas clouds until they made their closest approach to the supermassive black hole and survived. Ordinarily, the SMBHs gravitational pull would shred gas clouds apart, but this did not happen with G1 and G2.

3-D spectro-imaging data cube produced using software called OSIRIS-Volume Display ( OsrsVol) to separate G3, G4, and G5 from the background emission. Credit: W.M. Keck Observatory

Because these newly discovered infrared sources (G3, G4, and G5) shared the physical characteristics of G1 and G2, the team concluded that they could potentially be G-objects. What makes G-objects unusual is their “puffiness”, where they appear to be cloaked in a layer of dust and gas that makes them difficult to detect. Unlike other stars, astronomers only see a glowing envelope of dust when looking at G-objects.

To see these objects clearly through their obscuring envelope of dust and gas, Campbell developed a tool called the OSIRIS-Volume Display (OsrsVol). As Campbell described it:

“OsrsVol allowed us to isolate these G-objects from the background emission and analyze the spectral data in three dimensions: two spatial dimensions, and the wavelength dimension that provides velocity information. Once we were able to distinguish the objects in a 3-D data cube, we could then track their motion over time relative to the black hole.”

UCLA Astronomy Professor Mark Morris, a co-principal investigator and fellow member of UCLA’s Galactic Center Orbits Initiative (GCOI), was also involved in the study. As he indicated:

“If they were gas clouds, G1 and G2 would not have been able to stay intact. Our view of the G-objects is that they are bloated stars – stars that have become so large that the tidal forces exerted by the central black hole can pull matter off of their stellar atmospheres when the stars get close enough, but have a stellar core with enough mass to remain intact. The question is then, why are they so large?

Chandra data (above, graph) on J0806 show that its X-rays vary with a period of 321.5 seconds, or slightly more than five minutes. This implies that the X-ray source is a binary star system where two white dwarf stars are orbiting each other (above, illustration) only 50,000 miles apart, making it one of the smallest known binary orbits in the Galaxy. According to Einstein's General Theory of Relativity, such a system should produce gravitational waves - ripples in space-time - that carry energy away from the system and cause the stars to move closer together. X-ray and optical observations indicate that the orbital period of this system is decreasing by 1.2 milliseconds every year, which means that the stars are moving closer at a rate of 2 feet per year.
A binary star system potentially on the verge of a stellar collision. Credit: Chandra

After examining the objects, the team noticed that there was a great deal of energy was emanating from them, more than what would be expected from typical stars. As a result, they theorized that these G-objects are the result of stellar mergers, which occur when two stars that orbit each other (aka. binaries) crash into each other. This would have been caused by the long-term gravitational influence of the SMBH.

The resulting single object would be distended (i.e. swell up) over the course of millions of years before it finally settled down and appeared like a normal-sized star. The combined objects that resulted from these violent mergers could explain where the excess energy came from and why they behave like stars do. As Andrea Ghez, the founder and director of GCOI, explained:

“This is what I find most exciting. If these objects are indeed binary star systems that have been driven to merge through their interaction with the central supermassive black hole, this may provide us with insight into a process which may be responsible for the recently discovered stellar mass black hole mergers that have been detected through gravitational waves.”

Looking ahead, the team plans to continue following the size and shape of the G-objects’ orbits in the hopes of determining how they formed. They will be paying especially close attention when these stellar objects make their closest approach to Sagittarius A*, since this will allow them to further observe their behavior and see if they remain intact (as G1 and G2 did).

This will take a few decades, with G3 making its closest pass in 20 years and G4 and G5 taking decades longer. In the meantime, the team hopes to learn more about these “puffy” star-like objects by following their dynamical evolution using Keck’s OSIRIS instrument. As Ciurlo stated:

“Understanding G-objects can teach us a lot about the Galactic Center’s fascinating and still mysterious environment. There are so many things going on that every localized process can help explain how this extreme, exotic environment works.”

And be sure to check out this video of the presentation, which takes place from 18:30 until 30:20:

Further Reading: Keck Observatory

Distance & Speed Of Sun’s Orbit Around Galactic Centre Measured

In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.

Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.

For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (~23,483 to 28,700 light years).

Astronomy Image Gallery
Infrared image from Spitzer Space Telescope, showing the stars at the center of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)

For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.

Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.

In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.

As Hunt explained in Dunlap Institute press release:

“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”

Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.

The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.

This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.

But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.

“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”

As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.

Further Reading: University of Toronto, The Astrophysical Journal Letters

One of the Milky Way’s Arms Might Encircle the Entire Galaxy

Given that our Solar System sits inside the Milky Way Galaxy, getting a clear picture of what it looks like as a whole can be quite tricky. In fact, it was not until 1852 that astronomer Stephen Alexander first postulated that the galaxy was spiral in shape. And since that time, numerous discoveries have come along that have altered how we picture it.

For decades astronomers have thought the Milky Way consists of four arms — made up of stars and clouds of star-forming gas — that extend outwards in a spiral fashion. Then in 2008, data from the Spitzer Space Telescope seemed to indicate that our Milky Way has just two arms, but a larger central bar. But now, according to a team of astronomers from China, one of our galaxy’s arms may stretch farther than previously thought, reaching all the way around the galaxy.

This arm is known as Scutum–Centaurus, which emanates from one end of the Milky Way bar, passes between us and Galactic Center, and extends to the other side of the galaxy. For many decades, it was believed that was where this arm terminated.

However, back in 2011, astronomers Thomas Dame and Patrick Thaddeus from the Harvard–Smithsonian Center for Astrophysics spotted what appeared to be an extension of this arm on the other side of the galaxy.

Star-forming region in interstellar space.  Image credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
Star-forming region in interstellar space. Image credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

But according to astronomer Yan Sun and colleagues from the Purple Mountain Observatory in Nanjing, China, the Scutum–Centaurus Arm may extend even farther than that. Using a novel approach to study gas clouds located between 46,000 to 67,000 light-years beyond the center of our galaxy, they detected 48 new clouds of interstellar gas, as well as 24 previously-observed ones.

For the sake of their study, Sun and his colleagues relied on radio telescope data provided by the Milky Way Imaging Scroll Painting project, which scans interstellar dust clouds for radio waves emitted by carbon monoxide gas. Next to hydrogen, this gas is the most abundant element to be found in interstellar space – but is easier for radio telescopes to detect.

Combining this information with data obtained by the Canadian Galactic Plane Survey (which looks for hydrogen gas), they concluded that these 72 clouds line up along a spiral-arm segment that is 30,000 light-years in length. What’s more, they claim in their report that: “The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum-Centaurus Arm into the outer second quadrant.”

Ilustration of our galaxy, showing our Sun (red dot) and the possible extension of the Scutum-Centaurus Arm. CREDIT: Modified from "A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant" by Yan Sun et al., in The Astrophysical Journal Letters, Vol. 798, January 2015; Robert Hurt. NASA/JPL-Caltech/SSC (background spiral).
Illustration of our galaxy showing the possible extension of the Scutum-Centaurus Arm. CREDIT: Yan Sun/The Astrophysical Journal Letters, Vol. 798/Robert Hurt. NASA/JPL-Caltech/SSC

This would mean the arm is not only the single largest in our galaxy, but is also the only one to effectively reach 360° around the Milky Way. Such a find would be unprecedented given the fact that nothing of the sort has been observed with other spiral galaxies in our local universe.

Thomas Dame, one of the astronomers who discovered the possible extension of the Scutum-Centaurus Arm in 2011, was quoted by Scientific American as saying: “It’s rare. I bet that you would have to look through dozens of face-on spiral galaxy images to find one where you could convince yourself you could track one arm 360 degrees around.”

Naturally, the prospect presents some problems. For one, there is an apparent gap between the segment that Dame and Thaddeus discovered in 2011 and the start of the one discovered by the Chinese team –  a 40,000 light-year gap to be exact. This could mean that the clouds that Sun and his colleagues discovered may not be part of the Scutum-Centaurus Arm after all, but an entirely new spiral-arm segment.

If this is true, than it would mean that our Galaxy has several “outer” arm segments. On the other hand, additional research may close that gap (so to speak) and prove that the Milky Way is as beautiful when seen afar as any of the spirals we often observe from the comfort of our own Solar System.

Further Reading: arXiv Astrophysics, The Astrophysical Letters

Where Have All the Pulsars Gone? The Mystery at the Center of Our Galaxy

The galactic center is a happening place, with lots of gas, dust, stars, and surprising binary stars orbiting a supermassive black hole about three million times the size of our sun. With so many stars, astronomers estimate that there should be hundreds of dead ones. But to date, scientists have found only a single young pulsar at the galactic center where there should be as many as 50.

The question thus arises: where are all those rapidly spinning, dense stellar corpses known as pulsars? Joseph Bramante of Notre Dame University and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters.

Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Basically, they disappeared into the fabric of space and time by becoming so massive that they punched a hole right through it.

Dark matter, as you may know, is the theoretical mass that astrophysicists believe fills roughly a quarter of our universe. Alas, it is invisible and undetectable by conventional means, making its presence known only in how its gravitational pull interacts with other stellar objects.

One of the more popular candidates for dark matter is Weakly Interacting Massive Particles, otherwise known as WIMPs. Underground detectors are currently hunting for WIMPs and debate has raged over whether gamma rays streaming from the galactic center come from WIMPs annihilating one another.

In general, any particle and its antimatter partner will annihilate each other in a flurry of energy. But WIMPs don’t have an antimatter counterpart. Instead, they’re thought to be their own antiparticles, meaning that one WIMP can annihilate another.

But over the last few years, physicists have considered another class of dark matter called asymmetric dark matter. Unlike WIMPs, this type of dark matter does have an antimatter counterpart.

Numerical simulation of the density of matter when the universe was one billion years old. Cosmic Infrared Background ExpeRIment (CIBER) Credit: Caltech/Jamie Bock
 Cosmic Infrared Background ExpeRIment (CIBER) simulation of the density of matter when the universe was one billion years old, as produced by large-scale structures from dark matter. Credit: Caltech/Jamie Bock

Asymmetric dark matter appeals to physicists because it’s intrinsically linked to the imbalance of matter and antimatter. Basically, there’s a lot more matter in the universe than antimatter – which is good considering anything less than an imbalance would lead to our annihilation. Likewise, according to the theory, there’s much more dark matter than anti-dark-matter.

Physicists think that in the beginning, the Big Bang should’ve created as much matter as antimatter, but something altered this balance. No one’s sure what this mechanism was, but it might have triggered an imbalance in dark matter as well – hence it is “asymmetric”.

Dark matter is concentrated at the galactic center, and if it’s asymmetric, then it could collect at the center of pulsars, pulled in by their extremely strong gravity. Eventually, the pulsar would accumulate so much mass from dark matter that it would collapse into a black hole.

The idea that dark matter can cause pulsars to implode isn’t new.  But the new research is the first to apply this possibility to the missing-pulsar problem.

If the hypothesis is correct, then pulsars around the galactic center could only get so old before grabbing so much dark matter that they turn into black holes. Because the density of dark matter drops the farther you go from the center, the researchers predict that the maximum age of pulsars will increase with distance from the center. Observing this distinct pattern would be strong evidence that dark matter is not only causing pulsars to implode, but also that it’s asymmetric.

“The most exciting part about this is just from looking at pulsars, you can perhaps say what dark matter is made of,” Bramante said. Measuring this pattern would also help physicists narrow down the mass of the dark matter particle.

    Artist's illustration of a pulsar that was found to be an ultraluminous X-ray source. Credit: NASA, Caltech-JPL
Artist’s illustration of a pulsar that was found to be an ultraluminous X-ray source.
Credit: NASA, Caltech-JPL

But as Bramante admits, it won’t be easy to detect this signature. Astronomers will need to collect much more data about the galactic center’s pulsars by searching for radio signals, he claims. The hope is that as astronomers explore the galactic center with a wider range of radio frequencies, they will uncover more pulsars.

But of course, the idea that dark matter is behind the missing pulsar problem is still highly speculative, and the likelihood of it is being called into question.

“I think it’s unlikely—or at least it is too early to say anything definitive,” said Zurek, who was one of the first to revive the notion of asymmetric dark matter in 2009. The tricky part is being able to know for sure that any measurable pattern in the pulsar population is due to dark-matter-induced collapse and not something else.

Even if astronomers find this pulsar signature, it’s still far from being definitive evidence for asymmetric dark matter. As Kathryn Zurek of the Lawrence Berkeley National Laboratory explained: “Realistically, when dark matter is detected, we are going to need multiple, complementary probes to begin to be convinced that we have a handle on the theory of dark matter.”

And asymmetric dark matter may not have anything to do with the missing pulsar problem at all. The problem is relatively new, so astronomers may find more plausible, conventional explanations.

“I’d say give them some time and maybe they come up with some competing explanation that’s more fleshed out,” Bramante said.

Nevertheless, the idea is worth pursuing, says Haibo Yu of the University of California, Riverside. If anything, this analysis is a good example of how scientists can understand dark matter by exploring how it may influence astrophysical objects. “This tells us there are ways to explore dark matter that we’ve never thought of before,” he said. “We should have an open mind to see all possible effects that dark matter can have.”

There’s one other way to determine if dark matter can cause pulsars to implode: To catch them in the act. No one knows what a collapsing pulsar might look like. It might even blow up.

“While the idea of an explosion is really fun to think about, what would be even cooler is if it didn’t explode when it collapsed,” Bramante said. A pulsar emits a powerful beam of radiation, and as it spins, it appears to blink like a lighthouse with a frequency as high as several hundred times per second. As it implodes into a black hole, its gravity gets stronger, increasingly warping the surrounding space and time.

Studying this scenario would be a great way to test Einstein’s theory of general relativity, Bramante says. According to theory, the pulse rate would get slower and slower until the time between pulses becomes infinitely long. At that point, the pulses would stop entirely and the pulsar would be no more.

Further Reading: APS Physics, WIRED

What’s At The Center Of Our Galaxy?

Dr. Andrea Ghez has spent much of her career studying the region right around the center of the Milky Way, including its supermassive black hole. In fact, she helped discover it in the first place. Dr. Ghez speaks about this amazing and dynamic region.

“Hi, I’m Dr. Andrea Ghez, and I’m a professor of physics and astronomy at UCLA. I study the center of our galaxy. The original objective was to figure out if there’s a supermassive black hole there, and in doing this, we’ve actually uncovered more questions than answers.”

What are you looking for at the center of the galaxy?

“We are tremendously privileged to be able to study the center of the galaxy, and have this exquisite laboratory to play with, to get insight into the fundamental physics of black holes, and also their astrophysical role in the formation and evolution of galaxies. You can also ask what kinds of phenomena do you expect to see around a black hole, and we have a lot of predictions about our thoughts about how galaxies form and evolve, and our ideas suggest that there’s a feedback between the galaxy and the black hole. But many of these models predict things that we simply don’t see, which again provides yet another playground.”

What’s it like around the supermassive black hole at the center of the galaxy?

“If you could get into a spaceship and get right down to the black hole, it would be a very busy place. Stars would be zooming around, like the sun, but you’d have a very busy day. You wouldn’t survive – I guess that would be another problem! You’d get torn apart. It’s just a very extreme place. The analogy that often gets made with the center of the galaxy is that it’s like the urban downtown, and we live out in the suburbs, so we live in a very calm place whereas the center of the galaxy is a a very extreme place, in almost every way you can describe an environment.”

What are some of the discoveries?

Astronomy Image Gallery
Stars at the Galactic Center. Credit: Astronomy Image Gallery

“The observations at the center of the Milky Way have taught us that one, it’s really normal to have a black hole at the center of the galaxy. I mean, our galaxy is completely ordinary, garden-variety, nothing-special-about-us, so if we have one, presumably every galaxy harbors a supermassive black hole at it’s center. We’ve also learned that the idea that a supermassive black hole should be surrounded by a very dense concentration of very old stars is not true. And that prediction is often used in other galaxies to find their black holes, because we can’t do the kinds of experiments we’ve done at the center of our own – that you look for this concentration of light, but in our galaxy we’re not seeing that, so you have a case where’s there’s absolutely clearly a supermassive black hole, yet you don’t see this collection of old stars. That’s a puzzle.

“Another puzzle that we’ve found that’s illuminating our ideas about other galaxies is that people predicted that you shouldn’t see young stars being formed near a black hole. In fact, in the early 1980’s, when people recognized that there were young stars found in the vicinity of a black hole, that was used to argue that perhaps you couldn’t possibly have a black hole because of these young stars. And yet again, we have a supermassive black hole – we know it, and those young stars are still exist, and we’ve even found stars even closer. And it’s the tidal forces that make it even more difficult to understand why the young stars should be there. The tidal forces pull the gases apart, and for star formation, you need a very fragile balls of gas and dust to collapse, so something’s amiss.”

How might those young stars get formed?

“There are so many ideas about how young stars could form at the center of the galaxy, but the one that has the most support is the idea that, at the time that these stars were being formed, that there was a much denser concentration of gas than there is today, and in that denser concentration you can get the collapse of those little clouds. We think that because as we continue to study the orbits of those stars, and what we’ve seen is that those orbits outside a certain distance start to fall into an ordered plane, like the planets orbiting the sun. We see a substantial fraction of them having a common orbital plane, and that looks very reminiscent to the solar system. The same way the planets formed out of a gas disc in the early days, that’s the same idea that is being invoked for these young stars, on a very different scale.”