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.”

Black Hole Secrets: Revealing The S-Star

Deep in the heart of the Milky Way resides a black hole. However, that is not the mysterious object which scientists Fabio Antonini, of the Canadian Institute for Theoretical Astrophysics, and David Merritt, of the Rochester Institute of Technology, have been endeavoring to explain. The objects of their attention are the orbits of massive young stars which attend it. They are called “S-stars”.

No. That’s not a stutter. S-Stars are a legitimate phenomenon which enable researchers to even more closely examine black hole activity. Their very presence causes astronomers to question what they know. For example, how is it possible for these massive young stars to orbit so close to a region where it would be highly unlikely for them to form there? The sheer force of the strong gravity near a black hole means these stars had to have once been further away from their observed position. However, when theoreticians created models to depict how S-stars might have traveled to their current orbital positions, the numbers simply didn’t match up. How could their orbits be so radically removed from predictions?

Today, Dr. Antonini offered his best explanation of this enigma at the annual meeting of the Canadian Astronomical Society (CASCA). In “The Origin of the S-star Cluster at the Galactic Center,” he gave a unified theory for the origin and dynamics of the S-stars. It hasn’t been an easy task, but Antonini has been able to produce a very viable theory of how these stars were able to get in close proximity to a supermassive black hole in only tens of millions of years since their formation.

“Theories exist for how migration from larger distances has occurred, but have up until now been unable to convincingly explain why the S-stars orbit the galactic center the way they do,” Antonini said. “As main-sequence stars, the S-stars cannot be older than about 100 million years, yet their orbital distribution appears to be ‘relaxed’, contrary to the predictions of models for their origin.”

3-dimensional visualization of the stellar orbits in the Galactic center based on data obtained by the W. M. Keck Telescopes between 1995 and 2012. Stars with the best determined orbits are shown with full ellipses and trails behind each star span ~15-20 years. These stars are color-coded to represent their spectral type: Early-type (young) stars are shown in teal green, late-type (old) stars are shown in orange, and those with unknown spectral type are shown in magenta. Stars without ellipses are from a statistical sample and follow the observed radial distributions for the early (white) or late (yellow/orange) type stars. These stars are embedded in a model representation of the inner Milky Way provided by NCSA/AVL to provide context for the visualization.
3-dimensional visualization of the stellar orbits in the Galactic center based on data obtained by the W. M. Keck Telescopes between 1995 and 2012. Stars with the best determined orbits are shown with full ellipses and trails behind each star span ~15-20 years. These stars are color-coded to represent their spectral type: Early-type (young) stars are shown in teal green, late-type (old) stars are shown in orange, and those with unknown spectral type are shown in magenta. Stars without ellipses are from a statistical sample and follow the observed radial distributions for the early (white) or late (yellow/orange) type stars. These stars are embedded in a model representation of the inner Milky Way provided by NCSA/AVL to provide context for the visualization.

According to Antonini and Merritt’s model, S-stars began much further away from the galactic center. Normal? Yep. Normal mode. Then these seemingly normal orbiting stars encountered the black hole’s gravity and began their spiral inward. As they made the inexorable trek, they then encountered the gravity of other stars in the vicinity which then changed the S-stars orbital pattern. It’s a simple insight, and one that verifies how the galactic center evolves from the conjoined influence of a supermassive black holes relativistic effects and the handiwork of gravitational interactions.

“Theoretical modeling of S-star orbits is a means to constrain their origin, to probe the dynamical mechanisms of the region near the galactic center and,” says Merritt, “indirectly to learn about the density and number of unseen objects in this region.”

Although the presence of supermassive black holes at the center of nearly all massive galaxies isn’t a new concept, further research into how they take shape and evolve leads to a better understanding of what we see around them. These regions are deeply connected to the very formation of the galaxy where they exist. With the center of our own galaxy – Sagittarius A – so near to home, it has become the perfect laboratory to observe manifestations such as S-stars. Tracking their orbits over an extended period of time has validated the presence of a supermassive black hole and enlightened our thinking of our own galaxy’s many peculiarities.

Original Story Source: Canadian Astronomical Society Press Release

Dust Complicates Determinations of the Distance to Galactic Center

Obtaining an accurate distance between the Sun and the center of our Galaxy remains one of the principal challenges facing astronomers. The ongoing lively debate concerning this distance hinges partly on the nature of dust found along that sight-line. Specifically, are dust particles lying toward the Galactic center different from their counterparts near the Sun? A new study led by David Nataf asserts that, yes, dust located towards the Galactic center is anomalous. They also look at accurately defining both the distance to the Galactic center and the reputed bar structure that encompasses it.

The team argues that characterizing the nature of small dust particles is key to establishing the correct distance to the Galactic center, and such an analysis may mitigate the scatter among published estimates for that distance (shown in the figure below).  Nataf et al. 2013 conclude that dust along the sight-line to the Galactic center is anomalous, thus causing a non-standard ‘extinction law‘.  

The extinction law describes how dust causes objects to appear fainter as a function of the emitted wavelength of light, and hence relays important information pertaining to the dust properties.

The team notes that, “We estimate a distance to the Galactic center of [26745 light-years] … [adopting a] non-standard [extinction law] thus relieves a major bottleneck in Galactic bulge studies.”

Various estimates for the distance to the Galactic center tabulated by Malkin 2013. The x-axis describes the year, while the y-axis features the distance to the Galactic center in kiloparsecs (image credit: Fig 1 from Malkin 2013/arXiv/ARep).

Nataf et al. 2013 likewise notes that, “The variations in both the extinction and the extinction law made it difficult to reliably trace the spatial structure of the [Galactic] bulge.”  Thus variations in the extinction law (tied directly to the dust properties) also affect efforts to delineate the Galactic bar, in addition to certain determinations of the distance to the Galactic center.  Variations in the extinction law imply inhomogeneities among the dust particles.

“The viewing angle between the bulge’s major axis and the Sun-Galactic centerline of sight remains undetermined, with best values ranging from from  13  to …  44 [degrees],” said Nataf et al. 2013 (see also Table 1 in Vanhollebekke et al. 2009).  The team added that, “We measure an upper bound on the tilt of 40 [degrees] between the bulge’s major axis and the Sun-Galactic center line of sight.”

However, the properties of dust found towards the Galactic center are debated, and a spectrum of opinions exist.  While Nataf et al. 2013 find that the extinction law is anomalously low, there are studies arguing for a standard extinction law.  Incidentally, Nataf et al. 2013 highlight that the extinction law characterizing dust near the Galactic center is similar to that tied to extragalactic supernovae (SNe), “The … [extinction] law toward the inner Galaxy [is] approximately consistent with extra-galactic investigations of the hosts of type Ia SNe.”

The delineation of the bar at the center of our Milky Way galaxy by Nataf et al. 2013. The bar is closer toward the Sun in the 1st Galactic quadrant. The center line represents the direction toward the constellation of Sagitarrius (image credit: Fig 17 from Nataf et al. 2013/arXiv/ApJ).
Left, the delineation of the bar at the center of the Milky Way by Nataf et al. 2013. The centerline represents the direction towards Sagittarius (image credit: Fig 17 from Nataf et al. 2013/arXiv/ApJ).  Right, a macro view of the Galaxy highlighting the general orientation and location of the Galactic bar (image credit: NASA/Wikipedia).  The Galactic bar is not readily discernible in the distribution of RR Lyrae variables.

Deviations from the standard extinction law, and the importance of characterizing that offset, is also exemplified by studies of the Carina spiral arm.  Optical surveys reveal that a prominent spiral arm runs through Carina (although that topic is likewise debated), and recent studies argue that the extinction law for Carina is higher than the standard value (Carraro et al. 2013Vargas Alvarez et al. 2013).  Conversely, Nataf et al. 2013 advocate that dust towards the Galactic center is lower by comparison to the standard (average) extinction law value.

The impact of adopting an anomalously high extinction law for objects located in Carina is conveyed by the case of the famed star cluster Westerlund 2, which is reputed to host some of the Galaxy’s most massive stars.  Adopting an anomalous extinction law for Westerlund 2 (Carraro et al. 2013Vargas Alvarez et al. 2013) forces certain prior distance estimates to decrease by some 50% (however see Dame 2007).  That merely emphasizes the sheer importance of characterizing local dust properties when establishing the cosmic distance scale.

In sum, characterizing the properties of small dust particles is important when ascertaining such fundamental quantities like the distance to the Galactic center, delineating the Galactic bar, and employing distance indicators like Type Ia SNe.

The Nataf et al. 2013 findings have been accepted for publication in the Astrophysical Journal (ApJ), and a preprint is available on arXiv.  The coauthors on the study are Andrew Gould, Pascal Fouque, Oscar A. Gonzalez, Jennifer A. Johnson, Jan Skowron, Andrzej Udalski, Michal K. Szymanski, Marcin Kubiak, Grzegorz Pietrzynski, Igor Soszynski, Krzysztof Ulaczyk, Lukasz Wyrzykowski, Radoslaw Poleski.  The Nataf et al. 2013 results are based partly on data acquired via the Optical Graviational Lensing Experiment (OGLE).  The interested reader desiring additional information will find the following pertinent: Udalski 2003Pottasch and Bernard-Salas 2013Kunder et al. 2008Vargas Alvarez et al. 2013Carraro et al. 2013Malkin 2013Churchwell et al. 2009, Dame 2007Ghez et al. 2008Vanhollebekke et al. 2009.

The Nataf et al. 2013 results are based partly on observations acquired by the OGLE survey (image credit: OGLE team).

Galactic Gas Cloud Could Help Spot Hidden Black Holes

The heart of our Milky Way galaxy is an exotic place. It’s swarming with gigantic stars, showered by lethal blasts of high-energy radiation and a veritable cul-de-sac for the most enigmatic stellar corpses known to science: black holes. And at the center of the whole mélange is the granddaddy of all the black holes in the galaxy — Sagittarius A*,  a supermassive monster with 4 million times more mass than the Sun packed into an area smaller than the orbit of Mercury.

Sgr A* dominates the core of the Milky Way with its powerful gravity, trapping giant stars into breakneck orbits and actively feeding on anything that comes close enough. Recently astronomers have been watching the movement of a large cloud of gas that’s caught in the pull of Sgr A* — they’re eager to see what exactly will happen once the cloud (designated G2) enters the black hole’s dining room… it will, in essence, be the first time anyone watches a black hole eat.

But before the dinner bell rings — estimated to be sometime this September — the cloud still has to cover a lot of space. Some scientists are now suggesting that G2’s trip through the crowded galactic nucleus could highlight the locations of other smaller black holes in the area, revealing their hiding places as it passes.

In a new paper titled “G2 can Illuminate the Black Hole Population near the Galactic Center” researchers from Columbia University in New York City and the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts propose that G2, a cloud of cool ionized gas over three times more massive than Earth, will likely encounter both neutron stars and other black holes on its way around (and/or into) SMBH Sgr A*.

Estimated number of stellar-mass black holes to be encountered by G2 along its trajectory (Bartos et al.)
Estimated number of stellar-mass black holes to be encountered by G2 along its trajectory (Bartos et al.)

The team notes that there are estimated to be around 20,000 stellar-mass black holes and about as many neutron stars in the central parsec of the galaxy. (A parsec is equal to 3.26 light-years, or 30.9 trillion km. In astronomical scale it’s just over 3/4 the way to the nearest star from the Sun.) In addition there may also be an unknown number of intermediate-mass black holes lurking within the same area.

These ultra-dense stellar remains are drawn to the center region of the galaxy due to the effects of dynamical friction — drag, if you will — as they move through the interstellar material.

Of course, unless black holes are feeding and actively throwing out excess gobs of hot energy and matter due to their sloppy eating habits, they are very nearly impossible to find. But as G2 is observed moving along its elliptical path toward Sgr A*, it could very well encounter a small number of stellar- and intermediate-mass black holes and neutron stars. According to the research team, such interactions may be visible with X-ray spotting spacecraft like NASA’s Chandra and NuSTAR.

Read more: Chandra Stares Deep Into the Heart of Sagittarius A*

NuSTAR X-ray image of a flare emitted by Sgr A* in July 2012 (NASA/JPL-Caltech)
NuSTAR X-ray image of a flare emitted by Sgr A* in July 2012 (NASA/JPL-Caltech)

The chances of G2 encountering black holes and interacting with them in such a way as to produce bright enough x-ray flares that can be detected depends upon a lot of variables, like the angles of interaction, the relative velocities of the gas cloud and black holes, the resulting accretion rates of in-falling cloud matter, and the temperature of the accretion material. In addition, any observations must be made at the right time and for long enough a duration to capture an interaction (or possibly multiple interactions simultaneously) yet also be able to discern them from any background X-ray sources.

Still, according to the researchers such observations would be important as they could provide valuable information on galactic evolution, and shed further insight into the behavior of black holes.

Read the full report here, and watch an ESO news video about the anticipated behavior of the G2 gas cloud around the SMBH Sgr A* below:

This research was conducted by Imre Bartos, Zoltán Haiman, and Bence Kocsis of Columbia University and Szabolcs Márka of the Harvard-Smithsonian Center for Astrophysics. 

Herschel Telescope Sees a Twisted Ring at Our Galaxy’s Center

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From a Herschel Observatory press release:

Observations with Herschel have revealed unprecedented views of a ring in the centre of our Milky Way galaxy. The ribbon of gas and dust is more than 600 light years across and appears to be twisted, for reasons which have yet to be explained. The origin of the ring could provide insight into the history of the Milky Way.

Professor Bruce Swinyard of the Rutherford Appleton Laboratory said “Herschel’s detectors are ideally suited to see through the dust lying between us and the center of our galaxy, and to find the relatively cold material, at only 15 degrees above absolute zero, which we have learned makes up the ring.” The new results are published in a recent issue of the Astrophysical Journal Letters.

Warmer gas and dust from the center of our galaxy is shown in blue in the above image, while the colder material appears red. The ring, in yellow, is made of gas and dust at a temperature of just 15 degrees above absolute zero. The bright regions are denser, and include some of the most massive and active sites of star formation in our galaxy.

An annotated view of the 'twist' in the galactic center as seen by the Herschel telescope. Credit: ESA/NASA/JPL-Caltech

“Hints of this feature were seen in previous images of the Galactic Centre made from the ground, but no-one realised what it was,” explained Dr. Mark Thompson of the University of Hertfordshire. “It was not until the launch of Herschel, with its unparalleled wavelength coverage, that we could measure the temperature of the dust clouds and determine its true nature.”

The central region of our galaxy is dominated by a bar-like structure, which stirs up the material in the outer galaxy as it rotates over millions of years and is thought to be responsible for its spiral structure. The ring seen by Herschel lies right in the middle of this bar, encircling the region which harbors a super-massive black hole at the center of our galaxy. Professor Glenn White of The Open University and The Rutherford Appleton Laboratory said that “although bars have been seen in other galaxies, this ring of cold material revealed by Herschel, and the way it twists around the Galactic Centre, were completely unexpected, revealing several surprises.”

Firstly, the ring of gas is twisted, so from our vantage point we see two loops which appear to meet in the middle. These are seen in yellow in the image above, tilted slightly such that they run from top-left to bottom-right. Secondly, it seems to be slightly offset from the very center of our Galaxy, where a super-massive black hole lurks. “This is what is so exciting about launching a new space telescope like Herschel,” said Sergio Molinari of the Institute of Space Physics in Rome, Italy, lead author of the new paper. “We have a new and exciting mystery on our hands, right at the center of our own galaxy.”

The reason for the ring’s twist and offset are unknown, but understanding their origin may help explain the origin of the ring itself. Computer simulations indicate that bars and rings such as those we see in the center of our galaxy can be formed by gravitational interactions. It is possible that the structures in the heart of the Milky Way were caused by interactions with our largest neighbor, the Andromeda Galaxy.

“Like all good science experiments, Herschel is creating as many questions as it answers”, said Professor Matt Griffin, of the University of Cardiff, and Principle Investigator on one of Herschel’s detectors used in this study. “Unravelling the mystery of this ring could help us to explore the processes which have taken place deep in the heart of our Galaxy over billions of years.”

See the “twist” in Chromoscope or Google Sky.

How Many Stars are There in the Milky Way?

When you look up into the night sky, it seems like you can see a lot of stars. There are about 2,500 stars visible to the naked eye at any one point in time on the Earth, and 5,800-8,000 total visible stars (i.e. that can be spotted with the aid of binoculars or a telescope). But this is a very tiny fraction of the stars the Milky Way is thought to have!

So the question is, then, exactly how many stars are in the Milky Way Galaxy? Astronomers estimate that there are 100 billion to 400 billion stars contained within our galaxy, though some estimate claim there may be as many as a trillion. The reason for the disparity is because we have a hard time viewing the galaxy, and there’s only so many stars we can be sure are there.

Structure of the Milky Way:

Why can we only see so few of these stars? Well, for starters, our Solar System is located within the disk of the Milky Way, which is a barred spiral galaxy approximately 100,000 light years across. In addition, we are about 30,000 light years from the galactic center, which means there is a lot of distance – and a LOT of stars – between us and the other side of the galaxy.

The Milky Way Galaxy. Astronomer Michael Hart, and cosmologist Frank Tipler propose that extraterrestrials would colonize every available planet. Since they aren't here, they have proposed that extraterrestrials don't exist. Sagan was able to imagine a broader range of possibilities. Credit: NASA
Artist’s impression of the Milky Way Galaxy. Credit: NASA

To complicate matter further, when astronomers look out at all of these stars, even closer ones that are relatively bright can be washed out by the light of brighter stars behind them. And then there are the faint stars that are at a significant distance from us, but which elude conventional detection because their light source is drowned out by brighter stars or star clusters in their vicinity.

The furthest stars that you can see with your naked eye (with a couple of exceptions) are about 1000 light years away. There are quite a few bright stars in the Milky Way, but clouds of dust and gas – especially those that lie at the galactic center – block visible light. This cloud, which appears as a dim glowing band arching across the night sky – is where our galaxy gets the “milky” in its name from.

It is also the reason why we can only really see the stars in our vicinity, and why those on the other side of the galaxy are hidden from us. To put it all in perspective, imagine you are standing in a very large, very crowded room, and are stuck in the far corner. If someone were to ask you, “how many people are there in here?”, you would have a hard time giving them an accurate figure.

Now imagine that someone brings in a smoke machine and begins filling the center of the room with a thick haze. Not only does it become difficult to see clearly more than a few meters in front of you, but objects on the other side of the room are entirely obscured. Basically, your inability to rise above the crowd and count heads means that you are stuck either making guesses, or estimating based on those that you can see.

a mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release.
A mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release. Credit: NASA/JPL

Imaging Methods:

Infrared (heat-sensitive) cameras like the Cosmic Background Explorer (aka. COBE) can see through the gas and dust because infrared light travels through it. And there’s also the Spitzer Space Telescope, an infrared space observatory launched by NASA in 2003; the Wide-field Infrared Survey Explorer (WISE), deployed in 2009; and the Herschel Space Observatory, a European Space Agency mission with important NASA participation.

All of these telescopes have been deployed over the past few years for the purpose of examining the universe in the infrared wavelength, so that astronomers will be able to detect stars that might have otherwise gone unnoticed. To give you a sense of what this might look like, check out the infrared image below, which was taken by COBE on Jan. 30th, 2000.

However, given that we still can’t seem them all, astronomers are forced to calculate the likely number of stars in the Milky Way based on a number of observable phenomena. They begin by observing the orbit of stars in the Milky Way’s disk to obtain the orbital velocity and rotational period of the Milky Way itself.

Estimates:

From what they have observed, astronomers have estimated that the galaxy’s rotational period (i.e. how long it takes to complete a single rotation) is apparently 225-250 million years at the position of the Sun. This means that the Milky Way as a whole is moving at a velocity of approximately 600 km per second, with respect to extragalactic frames of reference.

"This dazzling infrared image from NASA's Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view. Credit: NASA/JPL-Caltech
Infrared image of the Milky Way taken by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech

Then, after determining the mass (and subtracting out the halo of dark matter that makes up over 90% of the mass of the Milky Way), astronomers use surveys of the masses and types of stars in the galaxy to come up with an average mass. From all of this, they have obtained the estimate of 200-400 billion stars, though (as stated already) some believe there’s more.

Someday, our imaging techniques may become sophisticated enough that are able to spot every single star through the dust and particles that permeate our galaxy. Or perhaps will be able to send out space probes that will be able to take pictures of the Milky Way from Galactic north – i.e. the spot directly above the center of the Milky Way.

Until that time, estimates and a great deal of math are our only recourse for knowing exactly how crowded our local neighborhood is!

We have written many great articles on the Milky Way here at Universe Today. For example, here are 10 Facts About the Milky Way, as well as articles that answer other important questions.

These include How Big Is The Milky Way?, What is the Milky Way?, and Why Is Our Galaxy Called the Milky Way?

Astronomy Cast did a podcast all about the Milky Way, and the Students for the Exploration and Development of Space (SEDS) have plenty of information about the Milky Way here.

And if you’re up for counting a few of the stars, check out this mosaic from NASA’s Astronomy Picture of the Day. For a more in-depth explanation on the subject, go to How the Milky Way Galaxy Works.