# At what wavelengths can black hole Sagittarius A* be observed from Earth?

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The supermassive black hole in the center of the Milky Way is blocked by dust and gas clouds as seen from where we are. Which wavelengths are blocked and which are most undisturbed for observation of Sagittarius A* and its immediate neighborhood? Does the matter between us change for example polarization at some wavelengths or cause disturbances other than blocking the light? Could galactic cosmic rays, heavy ions, from it be detected if they were emitted, or would they be deflected by magnetic fields?

By the way, at what distance from us is the most dust located? Is it concentrated in one region or are there several dust clouds in the way?

From Genzel et al. (2010), here's part of Fig. 7.7.1:

This is part of the spectral energy distribution of Sagittarius A*, a flot of $$u$$ (frequency) vs. $$u L_{ u}$$ (frequency times luminosity). For comparison, visible light is in wavelengths from $$sim4 imes10^{14} ext{ Hz}$$ to $$sim8 imes10^{14} ext{ Hz}$$, which happens to be around the bottom of the trough of nonthermal electron emission. Right away, this makes submillimeter and millimeter wavelengths good candidates, leading to studies using Very Long Baseline Interferometry. Likewise, infrared emissions are a good target, and so the Spitzer Space Telescope, for instance, has been used. X-ray flares of up to $$sim10^{36} ext{ erg/s}$$ also occur from time to time,1 so that part of the spectrum is sometimes used to observe this activity. Finally, of course, Sagittarius A* is a very strong radio source, and it was initially observed in radio wavelengths (and it still is!).

As Fish & Doeleman 2010 write,2

Interstellar scattering, which varies as $$lambda^2$$, dominates over intrinsic source structure at longer wavelengths, and the emission from Sgr A* transitions from optically thick to optically thin near $$lambda = 1 ext{ mm}$$ (Doeleman et al. 2001).

This means that the visible part of the spectrum is dimmed even more. Combine that with relatively low emission at those wavelengths, and you have a pretty poor target for optical telescopes, and a pretty good target (all other factors considering) at other wavelengths, especially near the emission peak.

1 Flares also are visible in other wavelengths, but I mention X-rays here because Sagittarius A* is typically much less luminous in X-rays.
2 I believe they mean $$lambda^{-2}$$. Power laws for scattering in the area vary, but they are generally given between as $$lambda^{-1.5}$$ and $$lambda^{-2}$$. At any rate, the index may differ over different regimes.

## “Next Up” –Event Horizon Telescope’s Picture of Milky Way’s Supermassive Black Hole

What will we actually see? When it’s completed, the picture of the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), is an image sure to equal the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968. The obvious target for the Event Horizon Telescope (EHT), the team hopes to get imagery of our supermassive black hole soon, said Shep Doeleman, Director, Event Horizon Telescope, following the first ever image of Galaxy M87’s gargantuan black hole (above).

The researchers looked at M87 below, first, because it’s an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. It’s a bit easier to resolve than Sagittarius A* because it’s less variable over short timescales, Doeleman explained. For comparison, Sgr A* is estimated to be about a thousand times less massive, with about 4 million times the mass of the sun.

M87 is an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. “At the small end of that range, M87 would be an impossible target for EHT, Doeleman observed. At the high end, it is possibly suitable. So M87 became a secondary target in the pursuit of Sagittarius A*.

Polarization of light — Reveals the Physics Behind the Image

This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy is able to launch energetic jets from its core”.

“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.

A Major Milestone

This work is a major milestone: the polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019

The EHT collaboration delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They discovered that a significant fraction of the light around the M87 black hole is polarized.

“The polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,” explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that “unveiling this new polarized-light image required years of work due to the complex techniques involved in obtaining and analyzing the data.”

Milky Way’s Sagittarius A* —The Last Photon Orbit

What we’ll see when the EHT actually sees Sagittarius A* is an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit.”

“Another cool aspect of the M87 EHT image was the degree of polarization in the accretion disk, revealing the structure of magnetic fields in the final orbits around the SMBH,” says Maxwell Moe, dailygalaxxy.com editor and NASA Einstein fellow at the U of Arizona. “We may also see polarized light in an EHT image of Sag A*.

Were you to float there, says astrophysicist Janna Levin , professor of physics and astronomy at Barnard College of Columbia University and author of Black Hole Blues , “you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face. Closer than that, all the light falls in.”

A Dark Shadow the Size of Our Solar System

The M87 EHT image is unmistakable — a dark shadow the size of our solar system, writes Levin, enveloped by a bright, beautiful blob.

The first ever photograph above of the black hole shadow in the very center of M87 galaxy was published in April 2019 by the Event Horizon Telescope. This figure shows the polarized image of M87 black hole, published on March 24, 2021.

The Anthropological Impact

“While the scientific implications will take time to unpack”, Levin says, “some of the anthropological impact feels immediate. The light EHT collected from M87 headed our way 55 million years ago. Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs. Looking at M87, I am reminded that scientific discoveries transcend those differences. We are all under the same sky, all of us bound to this pale blue dot, floating in the sparse local territory of our solar system’s celestial bodies, under the warmth of our yellow sun, in a sparse sea of stars, in orbit around a supermassive black hole at the center of our luminous galaxy.”

The Daily Galaxy, Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Quanta and New York Times . Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL).

Image Credit top of page: M87 Black Hole EHT, Victor Tangerman

Your free twice-weekly fix of stories of space and science –a random journey from Planet Earth through the Cosmos– that has the capacity to provide clues to our existence and add a much needed cosmic perspective in our Anthropocene epoch.

## At what wavelengths can black hole Sagittarius A* be observed from Earth? - Astronomy

We present the results of a 3.3 yr project to monitor the flux density of Sagittarius A* at 2.0, 1.3, and 0.7 cm with the Very Large Array. Between 2000.5 and 2003.0, 119 epochs of data were taken with a mean separation between epochs of 8 days. After 2003.0, observations were made roughly once per month for a total of nine additional epochs. Details of the data calibration process are discussed, including corrections for opacity and elevation effects, as well as changes in the flux density scales between epochs. The fully calibrated light curves for Sgr A* at all three wavelengths are presented. Typical errors in the flux density are 6.1%, 6.2%, and 9.2% at 2.0, 1.3, and 0.7 cm, respectively. There is preliminary evidence for a bimodal distribution of flux densities, which may indicate the existence of two distinct states of accretion onto the supermassive black hole. At 1.3 and 0.7 cm, there is a tail in the distribution toward high flux densities. Significant variability is detected at all three wavelengths, with the largest amplitude variations occurring at 0.7 cm. The rms deviation of the flux density of Sgr A* is 0.13, 0.16, and 0.21 Jy at 2.0, 1.3, and 0.7 cm, respectively. During much of this monitoring campaign, Sgr A* appeared to be relatively quiescent compared with results from previous campaigns. At no point during the monitoring campaign did the flux density of Sgr A* more than double its mean value. The mean spectral index of Sgr A* is α=0.20+/-0.01 (where S ν

ν α ), with a standard deviation of 0.14. The spectral index appears to depend linearly on the observed flux density at 0.7 cm with a steeper index observed during outbursts. This correlation is consistent with the expectation for outbursts that are self-absorbed at wavelengths of 0.7 cm or longer and inconsistent with the effects of simple models for interstellar scintillation. Much of the variability of Sgr A*, including possible time lags between flux density changes at the different wavelengths, appears to occur on timescales less than the time resolution of our observations (8 days). Future observations should focus on the evolution of the flux density on these time-scales.

## Contents

A study was done with the measured parallaxes and motions of 10 massive regions in the Sagittarius spiral arm of the Milky Way where stars are formed. Data was gathered using the BeSSeL Survey with the VLBA, and the results were synthesized to discover the physical properties of these sections (called the Galactocentric azimuth, around −2 and 65 degrees). The results were that the spiral pitch angle of the arms is 7.3 ± 1.5 degrees, and the half-width of the arms of the Milky Way were found to be 0.2 kpc. The nearest arm from the Sun is around 1.4 ± 0.2 kpc away. [1]

This feature is approximately 25 light-years in width and has the attributes of a supernova remnant from an explosive event that occurred between 35,000 and 100,000 BC. However, it would take 50 to 100 times more energy than a standard supernova explosion to create a structure of this size and energy. It is conjectured that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made a close approach to the central black hole. [2]

Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth. For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB stars have been identified so far) that also occupy the central parsec.

Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.

The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.

On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity, which is interpreted as a bubble blown inside the Northern Arm by the stellar wind of a massive star, which is not clearly identified.

Astronomers now have evidence that there is a supermassive black hole at the center of the galaxy. [4] Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible candidate for the location of this supermassive black hole. The Very Large Telescope and Keck Telescope detected stars orbiting Sgr A* at speeds greater than that of any other stars in the galaxy. One star, designated S2, was calculated to orbit Sgr A* at speeds of over 5,000 kilometers per second at its closest approach. [5]

A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the event horizon, as theorists predicted would happen. Rather, it disintegrated, suggesting that G2 and a previous gas cloud, G1, were star remnants with larger gravitational fields than gas clouds. [6] [7]

In September 2019, scientists found that Sagittarius A* had been consuming nearby matter at a much faster rate than usual over the previous year. Researchers speculated that this could mean that the black hole is entering a new phase, or that Sagittarius A* had stripped the outer layer of G2 when it passed through. [8]

## Black Holes: Sagittarius A*

Identifying our galaxy’s supermassive black hole by tracking stars’ orbits.

A black hole is a region of space packed with so much mass that its own gravity prevents anything from escaping—even a ray of light. Although we can’t see a black hole, telescopes can observe the material around it. Matter swirling around a black hole, which can be made up of gas and dust, heats up and emits radiation that can be detected. In some cases, telescopes can observe the gravitational influence of a black hole on the motions of nearby individual stars.

Although a point-like radio source, known as Sagittarius A* (pronounced Sagittarius A-star), was detected at the center of our Milky Way galaxy in the 1970s, researchers could not accurately determine enough of the characteristics of this compact object to precisely describe it. Beginning in the 1990s, astronomers began using new techniques to measure the orbits of the fast-moving stars orbiting this area. By 2008, they not only tracked complete orbits, but also gained enough information to determine the mass of and distances to the stars. At this point, they could confirm that the mass of the object at the center of those orbits is 4.6 million times the mass of our sun and its size can be no bigger than the orbit of Pluto. That meant it could be nothing other than a supermassive black hole. Researchers still track these stars, but for new reasons: to test fundamental physics, including Einstein’s general theory of relativity, which continues to hold up.

## Sagittarius A*

Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near to Sagittarius A* for a period of ten years, and obtained evidence that Sagittarius A* is a highly massive compact object Ώ] . From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ± 0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU). Later observations ΐ] determined the mass of the object to be about 3.7 million solar masses (our sun's mass is approximately 2吆 30 kg).

This is compatible with, and strong evidence in support of, the hypothesis that Sagittarius A* is a supermassive black hole.

Several teams of researchers, including groups at the National Radio Astronomy Observatory and at the Shanghai Astronomical Observatory, have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry. The images obtained have been consistent with the Sagittarius A* radio emissions being associated with the accretion disc and polar jets of a supermassive black hole.

Sagittarius A* is "associated" with the supermassive black hole what is seen is not strictly the black hole itself. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself emits only Hawking radiation. In the near future, astronomical interferometers may allow the direct imaging of the event horizon.

Coordinates (J2000): RA 17h 45m 40.045s Dec. 󔼥.00775 degrees (About 10 degrees to the west of the center of the constellation Sagittarius, towards Scorpius)

## Telescopes unite in unprecedented observations of famous black hole

To better understand the black hole at the core of galaxy M87, the EHT Collaboration mounted a multi-wavelength observing campaign. Observations across the electromagnetic spectrum in radio, visible-light, ultraviolet, X-ray, and gamma-ray revealed the far-reaching impact of the supermassive black hole on its surroundings. Credit: EHT Collaboration NASA/Swift NASA/Fermi Caltech-NuSTAR CXC CfA-VERITAS MAGIC HESS

In April 2019, scientists released the first image of a black hole in galaxy M87 using the Event Horizon Telescope (EHT). However, that remarkable achievement was just the beginning of the science story to be told.

Data from 19 observatories released today promise to give unparalleled insight into this black hole and the system it powers, and to improve tests of Einstein's General Theory of Relativity.

"We knew that the first direct image of a black hole would be groundbreaking," says Kazuhiro Hada of the National Astronomical Observatory of Japan, a co-author of a new study published in The Astrophysical Journal Letters that describes the large set of data. "But to get the most out of this remarkable image, we need to know everything we can about the black hole's behavior at that time by observing over the entire electromagnetic spectrum."

The immense gravitational pull of a supermassive black hole can power jets of particles that travel at almost the speed of light across vast distances. M87's jets produce light spanning the entire electromagnetic spectrum, from radio waves to visible light to gamma rays. This pattern is different for each black hole. Identifying this pattern gives crucial insight into a black hole's properties—for example, its spin and energy output—but is a challenge because the pattern changes with time.

Scientists compensated for this variability by coordinating observations with many of the world's most powerful telescopes on the ground and in space, collecting light from across the spectrum. These 2017 observations were the largest simultaneous observing campaign ever undertaken on a supermassive black hole with jets.

Three observatories managed by the Center for Astrophysics | Harvard & Smithsonian participated in the landmark campaign: the Submillimeter Array (SMA) in Hilo, Hawaii the space-based Chandra X-ray Observatory and the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in southern Arizona.

Beginning with the EHT's now iconic image of M87, a new video takes viewers on a journey through the data from each telescope. Each consecutive frame shows data across many factors of ten in scale, both of wavelengths of light and physical size.

The sequence begins with the April 2019 image of the black hole. It then moves through images from other radio telescope arrays from around the globe (SMA), moving outward in the field of view during each step. Next, the view changes to telescopes that detect visible light, ultraviolet light, and X-rays (Chandra). The screen splits to show how these images, which cover the same amount of the sky at the same time, compare to one another. The sequence finishes by showing what gamma-ray telescopes on the ground (VERITAS), and Fermi in space, detect from this black hole and its jet.

Each telescope delivers different information about the behavior and impact of the 6.5-billion-solar-mass black hole at the center of M87, which is located about 55 million light-years from Earth.

"There are multiple groups eager to see if their models are a match for these rich observations, and we're excited to see the whole community use this public data set to help us better understand the deep links between black holes and their jets," says co-author Daryl Haggard of McGill University in Montreal, Canada.

The data were collected by a team of 760 scientists and engineers from nearly 200 institutions, spanning 32 countries or regions, and using observatories funded by agencies and institutions around the globe. The observations were concentrated from the end of March to the middle of April 2017.

"This incredible set of observations includes many of the world's best telescopes," says co-author Juan Carlos Algaba of the University of Malaya in Kuala Lumpur, Malaysia. "This is a wonderful example of astronomers around the world working together in the pursuit of science."

The first results show that the intensity of the light produced by material around M87's supermassive black hole was the lowest that had ever been observed. This produced ideal conditions for viewing the 'shadow' of the black hole, as well as being able to isolate the light from regions close to the event horizon from those tens of thousands of light-years away from the black hole.

The combination of data from these telescopes, and current (and future) EHT observations, will allow scientists to conduct important lines of investigation into some of astrophysics' most significant and challenging fields of study. For example, scientists plan to use these data to improve tests of Einstein's Theory of General Relativity. Currently, uncertainties about the material rotating around the black hole and being blasted away in jets, in particular the properties that determine the emitted light, represent a major hurdle for these General Relativity tests.

A related question that is addressed by today's study concerns the origin of energetic particles called "cosmic rays," which continually bombard the Earth from outer space. Their energies can be a million times higher than what can be produced in the most powerful accelerator on Earth, the Large Hadron Collider. The huge jets launched from black holes, like the ones shown in today's images, are thought to be the most likely source of the highest energy cosmic rays, but there are many questions about the details, including the precise locations where the particles get accelerated. Because cosmic rays produce light via their collisions, the highest-energy gamma rays can pinpoint this location, and the new study indicates that these gamma-rays are likely not produced near the event horizon—at least not in 2017. A key to settling this debate will be comparison to the observations from 2018, and the new data being collected this week.

"Understanding the particle acceleration is really central to our understanding of both the EHT image as well as the jets, in all their 'colors'," says co-author Sera Markoff from the University of Amsterdam. "These jets manage to transport energy released by the black hole out to scales larger than the host galaxy, like a huge power cord. Our results will help us calculate the amount of power carried, and the effect the black hole's jets have on its environment."

The release of this new treasure trove of data coincides with the EHT's 2021 observing run, which leverages a worldwide array of radio dishes, the first since 2018. Last year's campaign was canceled because of the COVID-19 pandemic, and the previous year was suspended because of unforeseen technical problems. This very week, for six nights, EHT astronomers are targeting several supermassive black holes: the one in M87 again, the one in our Galaxy called Sagittarius A*, and several more distant black holes. Compared to 2017, the array has been improved by adding three more radio telescopes: the Greenland Telescope, the Kitt Peak 12-meter Telescope in Arizona, and the NOrthern Extended Millimeter Array (NOEMA) in France.

"With the release of these data, combined with the resumption of observing and an improved EHT, we know many exciting new results are on the horizon," says co-author Mislav Balokovic of Yale University.

"I'm really excited to see these results come out, along with my fellow colleagues working on the SMA, some of whom were directly involved in collecting some of the data for this spectacular view into M87," says co-author Garrett Keating, a Submillimeter Array project scientist. "And with the results of Sagittarius A*—the massive black hole at the center of the Milky Way—coming out soon, and the resumption of observing this year, we are looking forward to even more amazing results with the EHT for years to come."

## At what wavelengths can black hole Sagittarius A* be observed from Earth? - Astronomy

Astronomers have found strong evidence that Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is producing a jet of high-energy particles.

Jets of high-energy particles are found throughout the universe, on large and small scales. They are produced by young stars and by black holes a thousand times larger than the Milky Way's black hole. They play important roles in transporting energy away from the central object and, on a galactic scale, in regulating the rate of formation of new stars.

Previous studies, using a variety of telescopes, suggested there was a jet, but these reports, including the orientation of the suspected jets, often contradicted each other and were not considered definitive. The latest study used results from NASA's Chandra X-ray Observatory and the National Science Foundation's Very Large Array (VLA) radio telescope.

"For decades astronomers have looked for a jet associated with the Milky Way's black hole. Our new observations make the strongest case yet for such a jet," said lead author Zhiyuan Li of Nanjing University in China.

"We were very eager to find a jet from Sgr A* because it tells us the direction of the black hole's spin axis. This gives us important clues about the growth history of the black hole," said Mark Morris of the University of California at Los Angeles, a co-author of the study.

The study shows the spin axis of Sgr A* is pointing in one direction, parallel to the rotation axis of the Milky Way, which indicates to astronomers that gas and dust have migrated steadily into Sgr A* over the past 10 billion years. If the Milky Way had collided with large galaxies in the recent past and their central black holes had merged with Sgr A*, the jet could point in any direction.

New evidence has been uncovered for the presence of a jet of high-energy particles blasting out of the Milky Way's supermassive black hole known as Sagittarius A* (Sgr A*). This image of Sgr A* and the region around it contains some of the data used in the study, with X-rays from Chandra (purple) and radio emission from the Very Large Array (blue). Jets of high-energy particles are found throughout the Universe on large and small scales. The likely discovery of a jet from Sgr A* helps astronomers learn more about the giant black hole, including how it is spinning.

(Photo Credit: X-ray: NASA/CXC/UCLA/Z. Li et al Radio: NRAO/VLA)

The jet appears to be running into gas near Sgr A*, producing X-rays detected by Chandra and radio emission observed by the VLA. The two key pieces of evidence for the jet are a straight line of X-ray emitting gas that points toward Sgr A* and a shock front -- similar to a sonic boom -- seen in radio data, where the jet appears to be striking the gas. Additionally, the energy signature, or spectrum, in X-rays of Sgr A* resembles that of jets coming from supermassive black holes in other galaxies.

Scientists think jets are produced when some material falling toward the black hole is redirected outward. Since Sgr A* is presently known to be consuming very little material, it is not surprising that the jet appears weak. A jet in the opposite direction is not seen, possibly because of gas or dust blocking the line of sight from Earth or a lack of material to fuel the jet.

The region around Sgr A* is faint, which means the black hole has been quiet in the past few hundred years. However, a separate Chandra study announced last month shows that it was at least a million times brighter before then.

"We know this giant black hole has been much more active at consuming material in the past. When it stirs again, the jet may brighten dramatically," said co-author Frederick K. Baganoff of the Massachusetts Institute of Technology in Cambridge, Mass.

Astronomers have suggested the giant bubbles of high-energy particles extending out from the Milky Way and detected by NASA's Fermi Gamma Ray Telescope in 2008 are caused by jets from Sgr A* that are aligned with the rotation axis of the galaxy. The latest results from Chandra support this explanation.

The supermassive black hole at the center of the Milky Way is about four million times more massive than our Sun and lies about 26,000 light-years from Earth. The Chandra observations in this study were taken between September 1999 and March 2011, with a total exposure of about 17 days.

## Earth is 2,000 light-years closer to supermassive black hole Sagittarius A*

Earth faster, closer to black hole in new map of galaxy.

VERA (VLBI Exploration of Radio Astrometry, by the way, “VLBI” represents Very Long Baseline Interferometry) started in 2000 to map three-dimensional velocity and spatial structures in the Milky Way. Using a methodology called interferometry, VERS combines data from radio telescopes scattered across the Japanese archipelago to achieve the same resolution as a 2300 km diameter telescope would have.

This year, the First VERA Astrometry Catalog was published containing data for 99 objects.

Based on the VERA Astrometry Catalog and recent observations by other groups, astronomers constructed a position and velocity map. From this map, they calculated the center of the Galaxy, the point that everything revolves around.

By pinpointing the location and velocity of around 99 specific points in our Galaxy, VERA has concluded that the supermassive black hole Sagittarius A, at the center of our Galaxy, is 25,800 light-years from Earth — almost 2,000 light-years closer than what we previously believed.

This doesn’t mean that Earth is plunging towards the black hole. Instead, it’s merely the result of a more accurate model of the Milky Way based on new data.

The model also calculated that Earth is moving faster than we believed. The older model indicated that Earth is traveling at 220 km/s as it orbits around the Galactic Center. But the new model suggests that Earth just got seven km/s faster and traveling at 227 km/s as it orbits around the Galactic Center.

Now VERA hopes to observe more objects, particularly ones close to the central supermassive black hole, to characterize the Galaxy’s structure and motion better.

The center of our Galaxy, as seen in the radio.

As we zoom into the very core of the Galactic Center, our field of view shrinks to a mere 5 arcseconds (one thousandth of a degree). At radio wavelengths, the brightest feature of this region is the point-like radio source Sagittarius A* (pronounced "Sag A star"). This source is a compact object, and approximately one Astronomical Unit (1 AU is about 93 million miles) in size, which is much smaller than our solar system (Neptune is 2.8 billion miles from the Sun). At near-infrared wavelengths, this point source in the radio is not clearly seen. Astronomers have seen pulsation near the radio position of Sgr A* in the near-infrared, which they attribute to this radio source flaring.

In 1974, Sir Martin Rees proposed the idea that supermassive black holes could exist within the centers of active galactic nuclei or quasars. In that same year, Balick and Brown made the conenction between their radio detection of Sgr A* and other known active galactic nuclei

In the past 20 years, astronomers have collected enough evidence through the observed motions of gas and stars to convince ourselves that something very massive lurks at the center of our galaxy. The first dynamical evidence came from the motions of the ionized gas streamers of the mini-spiral orbiting Sgr A*. Using the velocities of the gas estimated from the Doppler shift of spectral lines, astronomers estimated that a mass of six million solar masses must lie within 10 arcseconds of Sgr A*. This did not explicitly prove the existence of a black hole since that amount of matter could be accounted for by a high density of stars within such a large volume.

Since 1995, high-resolution near-infrared studies have observed a compact cluster of early-type stars surrounding the radio position of Sgr A*. These stars have very large proper motions (they are moving across the sky very quickly) considering their 24 million light year distance from the Earth. The two main groups devoted to tracking these stars include Andrea Ghez and others at UCLA, who use the 10-m Keck telescope on Mauna Kea, Hawaii, and Reinhard Genzel and Andreas Eckart who use the 8-m VLT telescopes in Chile. Both groups take advantage of the high spatial resolution and sensitivity of these large telescopes to track the positions of the stars within the cluster using near-infrared images collected once or twice a year.

Despite the large diameters of the Keck and VLT telescopes, air turbulence in the Earth's atmosphere blurs the images taken at the telescopes. The atmosphere has a lot of molecules that are colliding into each other and getting heated up. On really hot days, we can see the heat waves coming up off the ground, or if you look at a flame, you can see the heat influencing the air around it. This is what happens in our atmosphere. In order to correct for it, astronmers are now using Adaptive Optics (AO) systems, which increases the sensitivity of observations. AO systems use a deformable mirror that mimics the shape of the incoming lightwave and corrects for the atmospheric turbulence before the data is recorded.

Very accurate stellar positions can be estimated in order to keep track of the motions of the stars in the compact central cluster, which are zipping around Sgr A* at speeds up to 3 million miles per hour! Using Kepler's laws of motion, the orbital velocities and the positions of the bright stars an be used to estimate the mass that must be contained within their orbits. The resulting enclosed mass is 4.6 ± 0.7 X 10^6 solar masses--4.6 million times the mass of our Sun! This large mass combined with the minute size of Sgr A* in radio emission suggests taht the stars must be swiftly circling around a supermassive black hole.

An active galactic nucleus (Cen A). There is a lot of activity at long wavelengths and short,
energetic wavelengths. This is completely different than our galactic nucleus, Sgr A*.
Sgr A* may have been more energetic in the past. Credit: NASA

Recent observations of nearby galaxies reveal that such supermassive black holes are not unique to the MIlky Way. The formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood. In the case of Sgr A*, there is a mysterious absence of the high energy emission (X-rays and UV radiation) often observed from active galactic nuclei.