Tag: Sagittarius A*

The spiraling magnetic field surrounding the Milky Way’s central supermassive black hole

9:19 am

The magnetic field lines surrounding Sagittarius A*
Click for original image.

Astronomers have now produced the first detailed image of polarized light surrounding the Milky Way’s central supermassive black hole, dubbed Sagittarius A* (pronounced “Sagittarius A-star”) which in turn maps out the spiraling field lines of that black hole’s magnetic field.

The image to the right, reduced to post here, shows that image. From the press release:

“What we’re seeing now is that there are strong, twisted, and organized magnetic fields near the black hole at the center of the Milky Way galaxy,” said Sara Issaoun, CfA NASA Hubble Fellowship Program Einstein Fellow, Smithsonian Astrophysical Observatory (SAO) astrophysicist, and co-lead of the project. “Along with Sgr A* having a strikingly similar polarization structure to that seen in the much larger and more powerful M87* black hole, we’ve learned that strong and ordered magnetic fields are critical to how black holes interact with the gas and matter around them.”

Light is an oscillating, or moving, electromagnetic wave that allows us to see objects. Sometimes, light oscillates in a preferred orientation, and we call it “polarized.” Although polarized light surrounds us, to human eyes it is indistinguishable from “normal” light. In the plasma around these black holes, particles whirling around magnetic field lines impart a polarization pattern perpendicular to the field. This allows astronomers to see in increasingly vivid detail what’s happening in black hole regions and map their magnetic field lines.

Despite this similarlity, it still remains a mystery why the much larger M87 black hole is very active while Sagittarius A’ remains generally quiet.

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The orbits of the nearest stars orbiting the Milky Way’s central black hole are impossible to predict

8:13 am

The uncertainty of science: Using a computer program developed in 2018 that can predict with accuracy the orbits of more than three interacting objects, scientists have found that the orbits of the 27 nearest stars orbiting the Milky Way’s central black hole, Sagittarius A* (pronounced A-star) are impossible to predict after only a very short time.

“Already after 462 years, we cannot predict the orbits with confidence. That is astonishingly short,” says astronomer Simon Portegies Zwart (Leiden University, the Netherlands). He compares it to our solar system, which is no longer predictable with confidence after 12 million years. “So, the vicinity of the black hole is 30,000 times more chaotic than ours, and we didn’t expect that at all. Of course, the solar system is about 20,000 times smaller, contains millions of times less mass, and has only eight relatively light objects instead of 27 massive ones, but, if you had asked me beforehand, that shouldn’t have mattered so much.”

According to the researchers, the chaos emerges each time in roughly the same way. There are always two or three stars that approach each other closely. This causes a mutual pushing and pulling among the stars. This in turn leads to slightly different stellar orbits. The black hole around which those stars orbit is then slightly pushed away, which in turn is felt by all the stars. In this way, a small interaction between two stars affects all 27 stars in the central cluster. [emphasis mine]

To my mind, the quote by the scientist above should be considered the most absurd statement by a scientist ever spoken, except that nowadays scientists make such idiotic statements all the time. To think that such different conditions wouldn’t produce different results suggests a hubris that is astonishing for a person supposedly trained in the scientific method.

Regardless, these results suggest that acquiring an understanding of the dynamics that created these stars is going to be very difficult, if not impossible. The conditions change so rapidly, and in an unpredictable manner, that any theory proposed will be simply guessing.

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Scientists: Stars orbiting close to Milky Way’s supermassive black hole do it alone

10:07 am

The stars orbiting Sag A*
The stars orbiting Sag A*. Click for original image.

Based on a ten year study of the motions of nine stars orbiting close to Sagittarius A* (pronounced A-star), the supermassive black hole at the center of the Milky Way, scientists have concluded that they are single stars, not binaries as would be expected.

Using W. M. Keck Observatory on Maunakea, Hawaiʻi Island, Devin Chu of Hilo, an astronomer with the UCLA Galactic Center Orbits Initiative, led a 10-year survey that found these ‘S-stars,’ where ‘S’ stands for Sagittarius A*, the name of the monster black hole at our galaxy’s core, are all single.

The result is surprising given the S-stars Chu’s team observed included young, massive main-sequence stars that are only about six million years old. Normally, stars at this age that are 10 times more massive than our Sun spend their childhood years paired with a twin in a binary system, or sometimes even as triplets.

This finding suggests that the black hole’s massive gravitational field causes the binaries to be pulled apart, or somehow to merge during their formation.

This data point and the questions it raises pales before the more fundamental question that astronomers have been asking since these stars were first discovered in the 1990s: How is it possible for any stars to form so close to such a disruptive gravitational field?

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First radio image of event horizon of Milky Way’s central black hole

9:31 am

Sagittarius A*
Click for full image.

Using an array of eight radio telescopes worldwide, dubbed the Event Horizon Telescope because its purpose is to study black holes, scientists have obtained the first radio image of the event horizon of Sagittarius A* (pronounced “A-star”), the supermassive black hole at the center of the Milky Way.

The image to the right, reduced to post here, is that photo.

The image is a long-anticipated look at the massive object that sits at the very centre of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact, and very massive at the centre of the Milky Way. This strongly suggested that this object — known as Sagittarius A* (Sgr A*, pronounced “sadge-ay-star”) — is a black hole, and today’s image provides the first direct visual evidence of it.

Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun.

This is the second supermassive black hole that the Event Horizon array has imaged. In 2019 it captured the central black hole of the galaxy M87, 55 million light years away. Like that first image, much of what we see here is created by computer, since the data from the eight radio telescopes needs to be massaged to create something as smooth and as complete as this.

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99.9% of all mass at center of Milky Way is found in central black hole

10:46 am

New measurements of the orbits of several stars circling the Milky Way’s central supermassive black hole, Sagittarius A* (pronounced A-star), have confirmed that 99.9% of all mass at the galaxy’s center is concentrated in that black hole.

Astronomers have measured more precisely than ever before the position and velocity of four stars in the immediate vicinity of the supermassive black hole that lurks at the center of the Milky Way, known as Sagittarius A* (Sgr A*) [1]. These stars — called S2, S29, S38, and S55 — were found to be moving in a way that shows that the mass in the center of the Milky Way is almost entirely due to the Sgr A* black hole, leaving very little room for anything else.

The measurements, which further refine the mass of Sagittarius A* as 4.3 million times the mass of the Sun, show that very little of this mass is found in the surrounding space as gas or dark matter. It is all in the black hole, which might also help explain why the Milky Way’s central black hole is so quiescent. It has very little gas or other stars to feed it and thus produce emissions.

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New Chandra mosaic of galactic center reveals spider-web of magnetism

10:19 am

Magnetic field line at the galactic center
Click for full image.

Scientists today released a spectacular panorama of the center of the Milky Way using X-ray data from the Chandra X-ray Observatory and radio data from the MeerKAT radio telescope in South Africa. The panorama reveals a complex web of magnetic field lines emanating out from the supermassive black hole at the center, Sagittarius A* (pronounced A-star).

Below the fold are reduced versions of the full panorama, unlabeled on the left and labeled on the right. The image to the right, reduced to post here, shows just one single example of those magnetic field lines, dubbed G0.17-0.41 and about 20 light years long. This particular filament is the subject of a paper just published in connection with the release of this panorama. From the press release.

A new study of the X-ray and radio properties of this thread by Q. Daniel Wang of the University of Massachusetts at Amherst suggests these features are bound together by thin strips of magnetic fields. This is similar to what was observed in a previously studied thread. (Both threads are labeled with red rectangles in the [full labeled panorama]. The newly studied one in the lower left, G0.17-0.41, is much farther away from the plane of the Galaxy.) Such strips may have formed when magnetic fields aligned in different directions, collided, and became twisted around each other in a process called magnetic reconnection. This is similar to the phenomenon that drives energetic particles away from the Sun and is responsible for the space weather that sometimes affects Earth.

The image below is fascinating to study because of the wealth of detail it includes, not only of magnetic filaments but of other nearby gas clouds and Sagittarius A* itself.
» Read more

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“Spots” orbiting Milky Ways central black hole

9:19 am

Using the ALMA ground-based telescope array in Chile, astronomers have detected two energetic “spots” that appear to be orbiting Sagittarius A* (pronounced A-star), the super-massive black hole at the center of the Milky Way.

The spots appear to be regions in the accretion disk surrounding the black hole that are emitting energy.

Their scenario is as follows. Hot spots are sporadically formed in the disk and circle around the black hole, emitting strong millimeter waves. According to Einstein’s special relativity theory, the emission is largely amplified when the source is moving toward the observer with a speed comparable to that of light. The rotation speed of the inner edge of the accretion disk is quite large, so this extraordinary effect arises. The astronomers believe that this is the origin of the short-term variation of the millimeter emission from Sgr A*.

The team supposes that the variation might affect the effort to make an image of the supermassive black hole with the Event Horizon Telescope. “In general, the faster the movement is, the more difficult it is to take a photo of the object,” says Oka. “Instead, the variation of the emission itself provides compelling insight for the gas motion. We may witness the very moment of gas absorption by the black hole with a long-term monitoring campaign with ALMA.” The researchers aim to draw out independent information to understand the mystifying environment around the supermassive black hole.

Everyone please repeat after me: Though this scenario makes sense, based on the facts and our knowledge, there is a lot of uncertainty about these conclusions.

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Strange things at center of Milky Way

9:31 am

Astronomers have discovered an additional four more weird objects orbiting the supermassive black hole, dubbed Sagittarius A* (pronounced A-star) for a total of six, all of which display behavior that is inexplicable.

Part of a new class called G objects, they look compact most of the time and stretch out when their orbits bring them closest to the black hole. Those orbits range from about 100 to 1000 years. “These objects look like gas and behave like stars,” says Andrea Ghez, director of the Galactic Centre Group at the University of California, Los Angeles (UCLA) and co-author of a paper in the journal Nature.

The new discoveries are known simply as G3 to G6. G1 was discovered by Ghez’s research group back in 2005, and G2 by astronomers in Germany in 2012. “The fact that there are now several of these objects observed near the black hole means that they are, most likely, part of a common population,” says co-author Randy Campbell, from the Keck Observatory in Hawaii.

It is not surprising that the intense gravitational field of Sagittarius A* rips these objects into elongated stretched objects as their orbits bring them close to the black hole. What is very very puzzling is their apparent ability to spring back to compact form as their orbits take them away from the black hole.

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Is the pole of the Milky Way’s central black hole pointing directly at us?

9:42 am

The uncertainty of science: New data obtained using a constellation of Earth-based telescopes, working as a unit, strongly suggests that the pole of the Milky Way7s supermassive central black hole, dubbed Sagittarius A* (pronounced A-star), is pointing directly at us.

The high quality of the unscattered image has allowed the team to constrain theoretical models for the gas around Sgr A*. The bulk of the radio emission is coming from a mere 300 milllionth of a degree, and the source has a symmetrical morphology. “This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” explains Sara Issaoun, graduate student at the Radboud University Nijmegen in the Netherlands, who leads the work and has tested several computer models against the data. “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us”.

The German astronomer Heino Falcke, Professor of Radio Astronomy at Radboud University and PhD supervisor of Issaoun, calls this statement very unusual, but he also no longer rules it out. Last year, Falcke would have considered this a contrived model, but recently the GRAVITY team came to a similar conclusion using ESO’s Very Large Telescope Interferometer of optical telescopes and an independent technique. “Maybe this is true after all”, concludes Falcke, “and we are looking at this beast from a very special vantage point.”

If this is true, it might explain why Sgr A* is generally observed to be one of the quietest central supermassive black holes known. Compared to many others, its flux of emissions is far less.

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Astronomers find evidence for thousands of black holes near galaxy center

8:46 am

The uncertainty of science: Using data from the Chandra X-Ray Observatory, astronomers have found evidence suggesting that thousands of stellar-mass black holes might exist circling Sagittarius A* (pronounced A-star), the super-massive black hole at the center of the Milky Way.

Essentially, they found a dozen likely black hole candidates in what they think are X-ray binaries system. From this they extrapolate the number of potential stellar-massed black holes at the center of the galaxy. However,

While the authors strongly favor the black hole explanation, they cannot rule out the possibility that up to about half of the observed dozen sources are from a population of millisecond pulsars, i.e., very rapidly rotating neutron stars with strong magnetic fields.

In other words, this conclusion is very uncertain. Nonetheless, even if half of their candidates are not stellar-mass black holes, the results do suggest that there are a very large number of black holes circling Sagittarius A*. Using this information astronomers will be able to better refine their theories on the formation process for such super-massive black holes.

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Baby stars at center of galaxy

2:40 pm

New observations of the region surrounding Sagittarius A* (Sgr A*), the super-massive black hole at the center of the Milky Way, has confirmed earlier research by finding what appears to be eleven newly formed baby stars.

Prior observations of the region surrounding Sgr A* by Zadeh and his team had revealed multiple massive infant stars but the finding was not conclusive. These objects, known as proplyds, are common features in more placid star-forming regions, like the Orion Nebula. The new measurements provide more conclusive evidence for young star formation activity. Though the galactic center is a challenging environment for star formation, it is possible for particularly dense cores of hydrogen gas to cross the necessary threshold and forge new stars.

The new ALMA observations, however, revealed something even more remarkable, signs that 11 low-mass protostars are forming within one parsec – a scant three light-years – of the galaxy’s central black hole. Zadeh and his team used ALMA to confirm that the masses and momentum transfer rates – the ability of the protostar jets to plow through surrounding interstellar material – are consistent with young protostars found throughout the disk of our galaxy. “This discovery provides evidence that star formation is taking place within clouds surprisingly close to Sagittarius A*,” said Al Wootten with the National Radio Astronomy Observatory in Charlottesville, Virginia, and co-author on the paper.

They have several theories on how new stars could coalesce in such a violent and turbulent region, but none appears that convincing. Essentially, this is a mystery that does not yet have an answer. It does tell us however that star formation can occur almost anywhere.

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Milky Way’s central black hole is getting active

1:03 pm

The uncertainty of science: Sagittarius A* (pronounced A-Star), the Milky Way’s supermassive central black hole, has shown signs of increased activity in recent months.

The new study reveals that Sagittarius A* (Sgr A* for short) has been producing one bright X-ray flare about every ten days. However, within the past year, there has been a ten-fold increase in the rate of bright flares from Sgr A*, at about one every day. This increase happened soon after the close approach to Sgr A* by a mysterious object called G2.

“For several years, we’ve been tracking the X-ray emission from Sgr A*. This includes also the close passage of this dusty object” said Gabriele Ponti of the Max Planck Institute for Extraterrestrial Physics in Germany. “A year or so ago, we thought it had absolutely no effect on Sgr A*, but our new data raise the possibility that that might not be the case.”

G2 was first thought to be a cloud that would be ripped apart as it passed close to Sgr A*, causing an outburst of activity. When it wasn’t ripped apart and there was no immediate increase in activity astronomers concluded that G2 was a star surrounded by dust which was generally unaffected by its close fly-by of the black hole.

The timing of this new activity now is puzzling. It comes much later than it should have if it was caused by G2, but astronomers don’t have any other explanation for it. It might be because of G2’s fly-by, or maybe the activity is just the natural variability of this poorly understand object. Either way it illustrates how little we really know about the behavior of giant black holes.

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The race to map the space around the supermassive black hole at the center of our galaxy

9:30 am

The uncertainty of science: Using a new generation of telescopes, in space and on the ground, astronomers hope to better confine Einstein’s General Theory of Relativity by studying the distortion in light and energy produced by the powerful gravitational field surrounding Sagittarius A* (pronounced A-star), the 4 million solar mass black hole at the center of the Milky Way.

This is an excellent article explaining both the limits of our ability to study black holes as well as what we do know about Sagittarius A*.

Correction: Thanks to commenter Mike Nelson for noticing that I had mistakenly written “billion” instead of “million” for the mass of Sagittarius A* above.

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G2 survives fly-by of Milky Way’s supermassive black hole

9:02 am

The uncertainty of science: The mysterious object G2, thought by astronomers to be either a cloud or a star, has survived its close fly-by of Sagittarius A* (pronounced A-star), the supermassive black hole at the center of the Milky Way, without telling scientists whether it is a cloud or a star.

Not only do astronomers still not know clearly what G2 is, the Milky Way’s supermassive black hole continues to behave in ways that baffle them.

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It turns out that the object dubbed G2 that is zipping past the Milky Way’s central black hole is behaving not like a cloud but more like a star.

10:24 pm

The uncertainty of science: It turns out that the object dubbed G2 that is zipping past the Milky Way’s central black hole is behaving not like a cloud but more like a star.

The latest observations by the Keck Observatory in Hawaii show that the gas cloud called “G2” was surprisingly still intact, even during its closest approach to the supermassive black hole at the center of our Milky Way galaxy. Astronomers from the UCLA Galactic Center Group reported today that observations obtained on March 19 and 20, 2014 show the object’s density was still “robust” enough to be detected. This means G2 is not just a gas cloud, but likely has a star inside.

When I wrote a piece about this object for Sky & Telescope I found that among astronomers there was great skepticism about it being just a gas cloud that would be ripped apart when it flew past the black hole. The early data was not conclusive, but enough of it suggested G2 was a star, not a cloud. It turns out here that the skeptics were right.

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Part of the gas cloud being ripped apart by the supermassive black hole at the center of the Milky Way have already swung past the black hole.

3:54 pm

Part of a gas cloud, being ripped apart by the super massive black hole at the center of the Milky Way, has already swung past the black hole.

“The ionised gas at the head of the cloud is now stretched over more than 150 light-hours (about 160 billion kilometres) at the pericentre of the orbit around the black hole, with the closest approach being about 25 light-hours (or a bit more than 25 billion kilometres)”, explains Stefan Gillessen from MPE, who led the observing team. “The pericentre approach however is not a singular event but rather a process that will be stretching over a period of at least one year.”

The black hole, dubbed Sagittarius A* (pronounced A-star), is more than 4 billion times the mass of our Sun, but emits very little energy for its size. (Most super massive black holes emit energy as they swallow the mass around them.) Astronomers are hoping that they will see some action when it eats this cloud sometime next year.

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The supermassive black hole in the center of the Milky Way is about to get a snack.

6:16 pm

The supermassive black hole in the center of the Milky Way is about to get a snack.

Update: The recently launched NuStar telescope in July detected its first flare from the central black hole (which by the way is called Sagittarius A* and is pronounced Sagittarius A-star). If the gas cloud produces any fireworks as it whips past the black hole in the coming year then NuStar should see it.

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Astronomers have discovered a star that circles the supermassive black hole at the center of the Milky Way in only 11.5 years.

5:57 pm

Dizzy: Astronomers have discovered a star that circles the supermassive black hole at the center of the Milky Way in only 11.5 years.

This newly discovered star joins another that is 15 times brighter and has a 16 year orbit. The combined orbital data from both will allow astronomers to measure precisely the size of the black hole while also measuring the distortion of space caused by its intense gravitational field.

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