# Sagittarius A*

Observation data Epoch J2000      Equinox J2000 Constellation Sgr A* (center) and two light echoes from a recent explosion (circled) Sagittarius 17h 45m 40.0409s −29° 0′ 28.118″[1] Mass (4.154±0.014)×106[2] M☉ Distance 26673±42[2] ly (8178±13[2] pc) SIMBAD data

Sagittarius A* (pronounced "Sagittarius A-Star", abbreviated Sgr A*) is a bright and very compact astronomical radio source at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic,[3] visually close to the Butterfly Cluster (M6) and Shaula. Sagittarius A* is the location of a supermassive black hole,[4][5][6] similar to massive objects at the centers of most, if not all, spiral and elliptical galaxies.

Observations of several stars orbiting Sagittarius A*, particularly star S2, have been used to determine the mass and upper limits on the radius of the object. Based on mass and increasingly precise radius limits, astronomers have concluded that Sagittarius A* is the Milky Way's central supermassive black hole.[7] The current value of its mass is slightly in excess of 4 million solar masses.

Reinhard Genzel and Andrea Ghez were awarded the 2020 Nobel Prize in Physics for their discovery that Sgr A* is a supermassive compact object, for which a black hole is the only currently known explanation.[8]

## Observation and description

ALMA observations of molecular-hydrogen-rich gas clouds[9]

Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth.[10] Several teams of researchers have attempted to image Sgr A* in the radio spectrum using very-long-baseline interferometry (VLBI).[11] The current highest-resolution (approximately 30 μas) measurement, made at a wavelength of 1.3 mm, indicated an overall angular size for the source of 50 μas.[12] At a distance of 26,000 light-years, this yields a diameter of 60 million kilometres. For comparison, Earth is 150 million kilometres from the Sun, and Mercury is 46 million kilometres from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination.[13]

In 2017, direct radio images were taken of Sagittarius A* and M87* by the Event Horizon Telescope.[14][15] The Event Horizon Telescope uses interferometry to combine images taken from widely spaced observatories at different places on Earth in order to gain a higher picture resolution. It is hoped the measurements will test Einstein's theory of relativity more rigorously than has previously been done. If discrepancies between the theory of relativity and observations are found, scientists may have identified physical circumstances under which the theory breaks down.[16]

In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft[17] revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from −280 °F (−173.3 °C) to 17,500 °F (9,700 °C),[18] to flow into an orbit around Sagittarius A*, keeping black hole emissions low.[19]

## History

Karl Jansky, considered a father of radio astronomy, discovered in August 1931 that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way.[20] The radio source later became known as Sagittarius A. His observations did not extend quite as far south as we now know to be the Galactic Center.[21] Observations by Jack Piddington and Harry Minnett using the CSIRO radio telescope at Potts Hill Reservoir, in Sydney discovered a discrete and bright "Sagittarius-Scorpius" radio source,[22] which after further observation with the 80-foot CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.[23]

Later observations showed that Sagittarius A actually consists of several overlapping sub-components; a bright and very compact component Sgr A* was discovered on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory.[24][25] The name Sgr A* was coined by Brown in a 1982 paper because the radio source was "exciting", and excited states of atoms are denoted with asterisks.[26][27]

Detection of an unusually bright X-ray flare from Sgr A*[28]

Since the 1980s it has been evident that the central component of Sgr A* is likely a black hole. In 1994, infrared and submillimetre spectroscopy studies by a Berkeley team involving Nobel Laureate Charles H. Townes and future Nobelist Reinhard Genzel showed that the mass of Sgr A* was tightly concentrated and of the order 3 million Suns.[29]

On October 16, 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* throughout a period of ten years. According to the team's analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole.[30] The observations of S2 used near-infrared (NIR) interferometry (in the K-band, i.e. 2.2 μm) because of reduced interstellar extinction in this band. SiO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 (and other nearby stars) easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images.

Dusty cloud G2 passes the supermassive black hole at the center of the Milky Way[31]

The VLBI radio observations of Sagittarius A* could also be aligned centrally with the NIR images, so the focus of S2's elliptical orbit was found to coincide with the position of Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 4.1±0.6 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU).[32] Later observations of the star S14 showed the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU) or about 6.7 billion kilometres.[33] S175 passed within a similar distance.[34] For comparison, the Schwarzschild radius is 0.08 AU. They also determined the distance from Earth to the Galactic Center (the rotational center of the Milky Way), which is important in calibrating astronomical distance scales, as (8.0±0.6)×103 parsecs. In November 2004 a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting 3 light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimated the object's mass at 4.31±0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009.[35] Reinhard Genzel, team leader of the research, said the study has delivered "what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."[36]

On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sgr A*, according to astronomers.[28]

On 13 May 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object.[37]

Supernova remnant ejecta producing planet-forming material

## Central black hole

NuSTAR has captured these first, focused views of the supermassive black hole at the heart of the Milky Way in high-energy X-rays
A computer simulation of how the central black hole might appear to the Event Horizon Telescope

In a paper published on October 31, 2018, the discovery of conclusive evidence that Sagittarius A* is a black hole was announced. Using the GRAVITY interferometer and the four telescopes of the Very Large Telescope (VLT) to create a virtual telescope 130 metres in diameter, astronomers detected clumps of gas moving at about 30% of the speed of light. Emission from highly energetic electrons very close to the black hole was visible as three prominent bright flares. These exactly match theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.[38][39]

In July 2018, it was reported that S2 orbiting Sgr A* had been recorded at 7,650 km/s, or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about 120 AU (approximately 1,400 Schwarzschild radii) from Sgr A*. At that close distance to the black hole, Einstein's theory of general relativity (GR) predicts that S2 would show a discernible gravitational redshift in addition to the usual velocity redshift; the gravitational redshift was detected, in agreement with the GR prediction within the 10 percent measurement precision.[40][41]

Assuming that general relativity is still a valid description of gravity near the event horizon, the Sagittarius A* radio emissions are not centered on the black hole, but arise from a bright spot in the region around the black hole, close to the event horizon, possibly in the accretion disc, or a relativistic jet of material ejected from the disc.[12] If the apparent position of Sagittarius A* were exactly centered on the black hole, it would be possible to see it magnified beyond its size, because of gravitational lensing of the black hole. According to general relativity, this would result in a ring-like structure, which has a diameter about 5.2 times the black hole's Schwarzschild radius. For a black hole of around 4 million solar masses, this corresponds to a size of approximately 52 μas, which is consistent with the observed overall size of about 50 μas.[12]

Recent lower resolution observations revealed that the radio source of Sagittarius A* is symmetrical.[42] Simulations of alternative theories of gravity depict results that may be difficult to distinguish from GR.[43] However, a 2018 paper predicts an image of Sagittarius A* that is in agreement with recent observations; in particular, it explains the small angular size and the symmetrical morphology of the source.[44]

The mass of Sagittarius A* has been estimated in two different ways:

1. Two groups—in Germany and the U.S.—monitored the orbits of individual stars very near to the black hole and used Kepler's laws to infer the enclosed mass. The German group found a mass of 4.31±0.38 million solar masses,[35] whereas the American group found 4.1±0.6 million solar masses.[33] Given that this mass is confined inside a 44-million-kilometre-diameter sphere, this yields a density ten times higher than previous estimates.
2. More recently, measurement of the proper motions of a sample of several thousand stars within approximately one parsec from the black hole, combined with a statistical technique, has yielded both an estimate of the black hole's mass at 3.6+0.2
−0.4
×106
M, plus a distributed mass in the central parsec amounting to (1±0.5)×106 M.[45] The latter is thought to be composed of stars and stellar remnants.
Magnetar found very close to the supermassive black hole, Sagittarius A*, at the center of the Milky Way galaxy

The comparatively small mass of this supermassive black hole, along with the low luminosity of the radio and infrared emission lines, imply that the Milky Way is not a Seyfert galaxy.[10]

Ultimately, what is seen is not the black hole itself, but observations that are consistent only if there is a black hole present near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.[38] The black hole itself is thought to emit only Hawking radiation at a negligible temperature, on the order of 10−14 kelvin.

The European Space Agency's gamma-ray observatory INTEGRAL observed gamma rays interacting with the nearby giant molecular cloud Sagittarius B2, causing X-ray emission from the cloud. The total luminosity from this outburst (L≈1,5×1039 erg/s) is estimated to be a million times stronger than the current output from Sgr A* and is comparable with a typical active galactic nucleus.[46][47] In 2011 this conclusion was supported by Japanese astronomers observing the Milky Way's center with the Suzaku satellite.[48]

In July 2019, astronomers reported finding a star, S5-HVS1, traveling 1,755 km/s (3.93 million mph). The star is in the Grus (or Crane) constellation in the southern sky, and about 29,000 light-years from Earth, and may have been propelled out of the Milky Way galaxy after interacting with Sagittarius A*, the supermassive black hole at the center of the galaxy.[49][50]

## Orbiting stars

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way's center[51]

There are a number of stars in close orbit around Sagittarius A*, which are collectively known as "S stars" in various catalogues. These stars are observed primarily in K band infrared wavelengths, as interstellar dust drastically limits visibility in visible wavelengths. This is a rapidly changing field—in 2011, the orbits of the most prominent stars then known were plotted in the diagram at right, showing a comparison between their orbits and various orbits in the solar system. Since then, S62 and then S4714 have been found to approach even more closely than those stars.

The high velocities and close approaches to the supermassive black hole makes these stars useful to establish limits on the physical dimensions of Sagittarius A*, as well as to observe general-relativity associated effects like periapse shift of their orbits. An active watch is maintained for the possibility of stars approaching the event horizon close enough to be disrupted, but none of these stars are expected to suffer that fate. The observed distribution of the planes of the orbits of the S stars limits the spin of Sagittarius A* to less than 10% of its theoretical maximum value.[52]

As of 2020, S4714 is the current record holder of closest approach to Sagittarius A*, at about 12.6 AU (1.88 billion km), almost as close as Saturn gets to the Sun, traveling at about 8% of the speed of light. These figures given are approximate, the formal uncertainties being 12.6±9.3 AU and 23,928±8,840 km/s. Its orbital period is 12 years, but an extreme eccentricity of 0.985 gives it the close approach and high velocity.[53]

An excerpt from a table of this cluster (see Sagittarius A* cluster), featuring the most prominent members. In the below table, id1 is the star's name in the Gillessen catalog and id2 in the catalog of the University of California, Los Angeles. a, e, i, Ω and ω are standard orbital elements, with a measured in arcseconds. Tp is the epoch of pericenter passage, P is the orbital period in years and Kmag is the K-band apparent magnitude of the star. q and v are the pericenter distance in AU and pericenter speed in percent of the speed of light,[54] and Δ indicates the standard deviation of the associated quantities.

id1id2aΔaeΔei (°)ΔiΩ (°)ΔΩω (°)ΔωTp (yr)ΔTpP (yr)ΔPKmagq (AU)Δqv (%c)Δv
S1S0-10.59500.02400.55600.0180119.140.21342.040.32122.301.402001.8000.150166.05.814.702160.76.70.550.03
S2S0-20.12510.00010.88430.0001133.910.05228.070.0466.250.042018.3790.00116.10.013.95118.40.22.560.00
S8S0-40.40470.00140.80310.007574.370.30315.430.19346.700.411983.6400.24092.90.414.50651.722.51.070.01
S12S0-190.29870.00180.88830.001733.560.49230.101.80317.901.501995.5900.04058.90.215.50272.92.01.690.01
S13S0-200.26410.00160.42500.002324.700.4874.501.70245.202.402004.8600.04049.00.115.801242.02.40.690.01
S14S0-160.28630.00360.97610.0037100.590.87226.380.64334.590.872000.1200.06055.30.515.7056.03.83.830.06
S620.09050.00010.97600.002072.764.58122.610.5742.620.402003.3300.0109.90.016.1016.41.57.030.04
S47140.1020.0120.9850.011127.70.28129.280.63357.250.082017.290.0212.00.317.712.69.38.03

## Discovery of G2 gas cloud on an accretion course

First noticed as something unusual in images of the center of the Milky Way in 2002,[55] the gas cloud G2, which has a mass about three times that of Earth, was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012.[56] Predictions of its orbit suggested it would make its closest approach to the black hole (a perinigricon) in early 2014, when the cloud was at a distance of just over 3,000 times the radius of the event horizon (or ≈260 AU, 36 light-hours) from the black hole. G2 has been observed to be disrupting since 2009,[56] and was predicted by some to be completely destroyed by the encounter, which could have led to a significant brightening of X-ray and other emission from the black hole. Other astronomers suggested the gas cloud could be hiding a dim star, or a binary star merger product, which would hold it together against the tidal forces of Sgr A*, allowing the ensemble to pass by without any effect.[57] In addition to the tidal effects on the cloud itself, it was proposed in May 2013[58] that, prior to its perinigricon, G2 might experience multiple close encounters with members of the black-hole and neutron-star populations thought to orbit near the Galactic Center, offering some insight to the region surrounding the supermassive black hole at the center of the Milky Way.[59]

The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass[60] and is only detectable because it is so close to Earth. It was thought that the passage of G2 in 2013 might offer astronomers the chance to learn much more about how material accretes onto supermassive black holes. Several astronomical facilities observed this closest approach, with observations confirmed with Chandra, XMM, VLA, INTEGRAL, Swift, Fermi and requested at VLT and Keck.[61]

Simulations of the passage were made before it happened by groups at ESO[62] and Lawrence Livermore National Laboratory (LLNL).[63]

As the cloud approached the black hole, Dr. Daryl Haggard said "It's exciting to have something that feels more like an experiment", and hoped that the interaction would produce effects that would provide new information and insights.[64]

Nothing was observed during and after the closest approach of the cloud to the black hole, which was described as a lack of "fireworks" and a "flop".[65] Astronomers from the UCLA Galactic Center Group published observations obtained on March 19 and 20, 2014, concluding that G2 was still intact (in contrast to predictions for a simple gas cloud hypothesis) and that the cloud was likely to have a central star.[66]

An analysis published on July 21, 2014, based on observations by the ESO's Very Large Telescope in Chile, concluded alternatively that the cloud, rather than being isolated, might be a dense clump within a continuous but thinner stream of matter, and would act as a constant breeze on the disk of matter orbiting the black hole, rather than sudden gusts that would have caused high brightness as they hit, as originally expected. Supporting this hypothesis, G1, a cloud that passed near the black hole 13 years ago, had an orbit almost identical to G2, consistent with both clouds, and a gas tail thought to be trailing G2, all being denser clumps within a large single gas stream.[65][67]

Professor Andrea Ghez et al. suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star.[57][68]

Artist impression of the accretion of gas cloud G2 onto Sgr A*. Credit: ESO[69]
This simulation shows a gas cloud, discovered in 2011, as it passes close to the supermassive black hole at the center of the Milky Way
This video sequence shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the center of the Milky Way.

• Galactic Center GeV Excess
• List of nearest black holes

## Notes

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53. ^ Peißker, Florian; Eckart, Andreas; Zajaček, Michal; Basel, Ali; Parsa, Marzieh (August 2020). "S62 and S4711: Indications of a Population of Faint Fast-moving Stars inside the S2 Orbit—S4711 on a 7.6 yr Orbit around Sgr A*". The Astrophysical Journal. 889 (50): 5. arXiv:2008.04764. Bibcode:2020ApJ...899...50P. doi:10.3847/1538-4357/ab9c1c. S2CID 221095771.
54. ^ Næss, S. (October 4, 2019). "Galactic center S-star orbital parameters".
55. ^ Matson, John (2012-10-22). "Gas Guzzler: Cloud Could Soon Meet Its Demise in Milky Way's Black Hole". Scientific American. Retrieved 2012-10-30.
56. ^ a b Gillessen, S.; Genzel; Fritz; Quataert; Alig; Burkert; Cuadra; Eisenhauer; Pfuhl; Dodds-Eden; Gammie; Ott (5 January 2012). "A gas cloud on its way towards the supermassive black hole at the Galactic Centre". Nature. 481 (7379): 51–54. arXiv:1112.3264. Bibcode:2012Natur.481...51G. doi:10.1038/nature10652. PMID 22170607. S2CID 4410915.
57. ^ a b Witzel, G.; Ghez, A. M.; Morris, M. R.; Sitarski, B. N.; Boehle, A.; Naoz, S.; Campbell, R.; Becklin, E. E.; G. Canalizo; Chappell, S.; Do, T.; Lu, J. R.; Matthews, K.; Meyer, L.; Stockton, A.; Wizinowich, P.; Yelda, S. (1 January 2014). "Detection of Galactic Center Source G2 at 3.8 μm during Periapse Passage". Astrophysical Journal Letters. 796 (1): L8. arXiv:1410.1884. Bibcode:2014ApJ...796L...8W. doi:10.1088/2041-8205/796/1/L8. S2CID 36797915.
58. ^ Bartos, Imre; Haiman, Zoltán; Kocsis, Bence; Márka, Szabolcs (May 2013). "Gas Cloud G2 Can Illuminate the Black Hole Population Near the Galactic Center". Physical Review Letters. 110 (22): 221102 (5 pages). arXiv:1302.3220. Bibcode:2013PhRvL.110v1102B. doi:10.1103/PhysRevLett.110.221102. PMID 23767710. S2CID 12284209.
59. ^ de la Fuente Marcos, R.; de la Fuente Marcos, C. (August 2013). "Colliding with G2 near the Galactic Centre: a geometrical approach". Monthly Notices of the Royal Astronomical Society: Letters. 435 (1): L19–L23. arXiv:1306.4921. Bibcode:2013MNRAS.435L..19D. doi:10.1093/mnrasl/slt085. S2CID 119287777.
60. ^ Morris, Mark (4 January 2012). "Astrophysics: The Final Plunge". Nature. 481 (7379): 32–33. Bibcode:2012Natur.481...32M. doi:10.1038/nature10767. PMID 22170611. S2CID 664513.
61. ^ Gillessen. "Wiki Page of Proposed Observations of G2 Passage". Retrieved 30 October 2012.
62. ^ "A Black Hole's Dinner is Fast Approaching". ESO. 2011-12-14. Retrieved 2015-02-27.
63. ^ Robert H Hirschfeld (2012-10-22). "Milky Way's black hole getting ready for snack". [www.Llnl.gov Lawrence Livermore National Laboratory]. Retrieved 2015-02-27.
64. ^ space.com, Doomed Space Cloud Nears Milky Way's Black Hole as Scientists Watch, 28 April 2014 "Cosmic encounter that might reveal new secrets on how such supermassive black holes evolve"; "We get to watch it unfolding in a human lifetime, which is very unusual and very exciting"
65. ^ a b Cowen, Ron (2014). "Why galactic black hole fireworks were a flop : Nature News & Comment". Nature. doi:10.1038/nature.2014.15591. S2CID 124346286. Retrieved 2015-02-27.
66. ^ A. M. Ghez; G . Witzel; B. Sitarski; L. Meyer; S. Yelda; A. Boehle; E. E. Becklin; R. Campbell; G. Canalizo; T. Do; J. R. Lu; K. Matthews; M. R. Morris; A. Stockton (2 May 2014). "Detection of Galactic Center Source G2 at 3.8 micron during Periapse Passage Around the Central Black Hole". The Astronomer's Telegram. 6110 (6110): 1. Bibcode:2014ATel.6110....1G. Retrieved May 3, 2014.
67. ^ Pfuhl, Oliver; Gillessen, Stefan; Eisenhauer, Frank; Genzel, Reinhard; Plewa, Philipp M.; Thomas Ott; Ballone, Alessandro; Schartmann, Marc; Burkert, Andreas (2015). "The Galactic Center Cloud G2 and its Gas Streamer". The Astrophysical Journal. 798 (2): 111. arXiv:1407.4354. Bibcode:2015ApJ...798..111P. doi:10.1088/0004-637x/798/2/111. ISSN 0004-637X. S2CID 118440030.
68. ^ "How G2 survived the black hole at our Milky Way's heart - EarthSky.org". 4 November 2014.
69. ^ "Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way". ESO. Retrieved 2015-02-27.

## References

Sagittarius A*.jpg
Sagittarius A*. This image was taken with NASA's Chandra X-Ray Observatory. Ellipses indicate light echoes. Full-field is 12.5 arcmin across. original source
Black hole - Messier 87 crop max res.jpg
Author/Creator: Event Horizon Telescope, uploader cropped and converted TIF to JPG, Licence: CC BY 4.0
The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. In coordinated press conferences across the globe, EHT researchers revealed that they succeeded, unveiling the first direct visual evidence of the supermassive black hole in the centre of Messier 87 and its shadow.
In this image of M87* taken on 11 April 2017 (a representative example of the images collected in a global 2017 EHT campaign), the shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across. While this may sound large, this ring is only about 40 microarcseconds across — equivalent to measuring the length of a credit card on the surface of the Moon.
Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm in the 2017 campaign. Each telescope of the EHT produced enormous amounts of data – roughly 350 terabytes per day – which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.
This image is the average of three different imaging methods after convolving each with a circular Gaussian kernel to give matched resolutions. The image is shown in units of brightness temperature, ${\displaystyle {T}_{\rm {b}}=S{\lambda }^{2}/2{k}_{\rm {B}}{\rm {\Omega }}}$, where S is the flux density, λ is the observing wavelength, ${\displaystyle {k}_{\rm {B}}}$ is the Boltzmann constant, and Ω is the solid angle of the resolution element.
Crab Nebula.jpg
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.

The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
Stylised atom with three Bohr model orbits and stylised nucleus.svg
Author/Creator:

SVG by Indolences.

Recoloring and ironing out some glitches done by Rainer Klute., Licence: CC-BY-SA-3.0
Stylised atom. Blue dots are electrons, red dots are protons and black dots are neutrons.
He1523a.jpg
Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
• About 13.2 billion years old
• Approximately 7500 light years far from Earth
• Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
RocketSunIcon.svg
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
Earth-moon.jpg
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
Solar system.jpg
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
• the Mercury image was taken by Mariner 10,
• the Venus image by Magellan,
• the Earth and Moon images by Galileo,
• the Mars image by Mars Global Surveyor,
• the Jupiter image by Cassini, and
• the Saturn, Uranus and Neptune images by Voyager.
• Pluto is not shown as it is no longer a planet. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
• Sun (to photosphere) 1,392,684 km
• Mercury 4,879.4 km
• Venus 12,103.7 km
• Earth 12,756.28 km
• Moon 3,476.2 km
• Mars 6,804.9 km
• Jupiter 142,984 km
• Saturn 120,536 km
• Uranus 51,118 km
• Neptune 49,528 km
Cloudlets swarm around our local supermassive black hole.tif
Author/Creator: ALMA (ESO/NAOJ/NRAO)/ J. R. Goicoechea (Instituto de Física Fundamental, CSIC, Spain), Licence: CC BY 4.0
This image from the Atacama Large Millimeter/submillimeter Array (ALMA) shows the area surrounding Sagittarius A*, the supermassive black hole that lurks at the centre of the Milky Way — highlighted here with a small circle. New research has revealed exciting evidence of interstellar gas and dust orbiting the black hole at high speeds.

The molecular-hydrogen-rich gas clouds which have been identified are known as molecular cloudlets, and they have never before been unambiguously detected. This image actually shows the distribution of molecules including carbon monoxide, the cloudlets’ second most abundant molecular component. The cloudlets lie 26 000 light-years away from us, orbiting fast and relatively close to the black hole, at a distance of about one light year. ALMA’s high resolution allowed scientists to detect the cloudlets, which are the products of pre-existing massive clouds rotating around the centre of our galaxy. These clouds were tidally disrupted into dense fragments and a lower density, short-lived component. The latter was identified thanks to the signs left by the passage of the synchrotron radiation emitted by Sagittarius A* through diffuse gas between the cloudlets.

Although clouds of molecular gas have the potential to form new stars, these cloudlets are unlikely to create stellar newborns. They have a comparatively small mass of around 60 times that of the Sun, and exist close to the huge, turbulent, punishing gravitational forces exerted by Sagittarius A*.

While the stars orbiting Sagittarius A* have been systematically observed, these dense molecular cloudlets have not been detected so close to the centre of our galaxy before.
A simulation of how a gas cloud that has been observed approaching the supermassive black hole at the centre of the galaxy.jpg
Author/Creator: ESO/MPE/Marc Schartmann, Licence: CC BY 4.0
This view shows a simulation of how a gas cloud that has been observed approaching the supermassive black hole at the centre of the galaxy may break apart over the next few years. This is the first time ever that the approach of such a doomed cloud to a supermassive black hole has been observed and it is expected to break up completely during 2013. The remains of the gas cloud are shown in red and yellow, with the cloud's orbit marked in red. The stars orbiting the black hole are also shown along with blue lines marking their orbits. This view simulates the expected positions of the stars and gas cloud in the year 2021.
Simulation of gas cloud being ripped apart by the black hole at the centre of the Milky Way.ogv
Author/Creator: ESO/S. Gillessen/MPE/Marc Schartmann/L. Calçada, Licence: CC BY 4.0
This simulation shows a gas cloud, discovered in 2011, as it passes close to the supermassive black hole at the centre of the galaxy.
Magnetar-SGR1745-2900-20150515.jpg
Magnetar Near Supermassive Black Hole Delivers Surprises

SGRA Magnetar

May 13, 2015

In 2013, astronomers announced they had discovered a magnetar exceptionally close to the supermassive black hole at the center of the Milky Way using a suite of space-borne telescopes including NASA’s Chandra X-ray Observatory.

Magnetars are dense, collapsed stars (called “neutron stars”) that possess enormously powerful magnetic fields. At a distance that could be as small as 0.3 light years (or about 2 trillion miles) from the 4-million-solar mass black hole in the center of our Milky Way galaxy, the magnetar is by far the closest neutron star to a supermassive black hole ever discovered and is likely in its gravitational grip.

Since its discovery two years ago when it gave off a burst of X-rays, astronomers have been actively monitoring the magnetar, dubbed SGR 1745-2900, with Chandra and the European Space Agency’s XMM-Newton. The main image of the graphic shows the region around the Milky Way’s black hole in X-rays from Chandra (red, green, and blue are the low, medium, and high-energy X-rays respectively). The inset contains Chandra’s close-up look at the area right around the black hole, showing a combined image obtained between 2005 and 2008 (left) when the magnetar was not detected, during a quiescent period, and an observation in 2013 (right) when it was caught as a bright point source during the X-ray outburst that led to its discovery.

A new study uses long-term monitoring observations to reveal that the amount of X-rays from SGR 1745-2900 is dropping more slowly than other previously observed magnetars, and its surface is hotter than expected.

The team first considered whether “starquakes” are able to explain this unusual behavior. When neutron stars, including magnetars, form, they can develop a tough crust on the outside of the condensed star. Occasionally, this outer crust will crack, similar to how the Earth’s surface can fracture during an earthquake. Although starquakes can explain the change in brightness and cooling seen in many magnetars, the authors found that this mechanism by itself was unable to explain the slow drop in X-ray brightness and the hot crustal temperature.. Fading in X-ray brightness and surface cooling occur too quickly in the starquake model.

The researchers suggest that bombardment of the surface of the magnetar by charged particles trapped in twisted bundles of magnetic fields above the surface may provide the additional heating of the magnetar’s surface, and account for the slow decline in X-rays. These twisted bundles of magnetic fields can be generated when the neutron star forms.

The researchers do not think that the magnetar’s unusual behavior is caused by its proximity to a supermassive black hole, as the distance is still too great for strong interactions via magnetic fields or gravity.

Astronomers will continue to study SGR 1745-2900 to glean more clues about what is happening with this magnetar as it orbits our galaxy’s supermassive black hole.

These results appear in Monthly Notices of the Royal Astronomical Society in a paper led by the PhD student Francesco Coti Zelati (Universita’ dell’ Insubria, University of Amsterdam, INAF-OAB), within a large international collaboration.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for the agency’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
Dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way.jpg
Author/Creator: ESO/A. Eckart, Licence: CC BY 4.0
This composite image shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the centre of the Milky Way. These new observations with ESO’s VLT have shown that the cloud appears to have survived its close encounter with the black hole and remains a compact object that is not significantly extended. In this image the position of the cloud in the years 2006, 2010, 2012 and February and September 2014 are shown, from left to right. The blobs have been colourised to show the motion of the cloud, red indicated that the object is receding and blue approaching. The cross marks the position of the supermassive black hole.
15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
March 19, 2015

RELEASE 15-044 NASA’s SOFIA Finds Missing Link Between Supernovae and Planet Formation

SOFIA data on a supernova http://www.nasa.gov/sites/default/files/thumbnails/image/15-044a.jpg SOFIA data reveal warm dust (white) surviving inside a supernova remnant. The SNR Sgr A East cloud is traced in X-rays (blue). Radio emission (red) shows expanding shock waves colliding with surrounding interstellar clouds (green).

Using NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), an international scientific team discovered that supernovae are capable of producing a substantial amount of the material from which planets like Earth can form.

These findings are published in the March 19 online issue of Science magazine.

"Our observations reveal a particular cloud produced by a supernova explosion 10,000 years ago contains enough dust to make 7,000 Earths," said Ryan Lau of Cornell University in Ithaca, New York.

The research team, headed by Lau, used SOFIA's airborne telescope and the Faint Object InfraRed Camera for the SOFIA Telescope, FORCAST, to take detailed infrared images of an interstellar dust cloud known as Supernova Remnant Sagittarius A East, or SNR Sgr A East.

Supernova remnant dust as seen by SOFIA http://www.nasa.gov/sites/default/files/thumbnails/image/15-044b.jpg Supernova remnant dust detected by SOFIA (yellow) survives away from the hottest X-ray gas (purple). The red ellipse outlines the supernova shock wave. The inset shows a magnified image of the dust (orange) and gas emission (cyan).

The team used SOFIA data to estimate the total mass of dust in the cloud from the intensity of its emission. The investigation required measurements at long infrared wavelengths in order to peer through intervening interstellar clouds and detect the radiation emitted by the supernova dust.

Astronomers already had evidence that a supernova’s outward-moving shock wave can produce significant amounts of dust. Until now, a key question was whether the new soot- and sand-like dust particles would survive the subsequent inward “rebound” shock wave generated when the first, outward-moving shock wave collides with surrounding interstellar gas and dust.

"The dust survived the later onslaught of shock waves from the supernova explosion, and is now flowing into the interstellar medium where it can become part of the 'seed material' for new stars and planets," Lau explained.

These results also reveal the possibility that the vast amount of dust observed in distant young galaxies may have been made by supernova explosions of early massive stars, as no other known mechanism could have produced nearly as much dust.

"This discovery is a special feather in the cap for SOFIA, demonstrating how observations made within our own Milky Way galaxy can bear directly on our understanding of the evolution of galaxies billions of light years away," said Pamela Marcum, a SOFIA project scientist at Ames Research Center in Moffett Field, California.

SOFIA is a heavily modified Boeing 747 Special Performance jetliner that carries a telescope with an effective diameter of 100 inches (2.5 meters) at altitudes of 39,000 to 45,000 feet (12 to 14 km). SOFIA is a joint project of NASA and the German Aerospace Center. The aircraft observatory is based at NASA's Armstrong Flight Research Center facility in Palmdale, California. The agency’s Ames Research Center in Moffett Field, California, is home to the SOFIA Science Center, which is managed by NASA in cooperation with the Universities Space Research Association in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart.

http://www.dsi.uni-stuttgart.de/index.en.html
X-RayFlare-BlackHole-MilkyWay-20140105.jpg
Astronomers have observed the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way galaxy. This event, detected by NASA’s Chandra X-ray Observatory, raises questions about the behavior of this giant black hole and its surrounding environment.

The supermassive black hole at the center of our galaxy, called Sagittarius A*, or Sgr A*, is estimated to contain about 4.5 million times the mass of our sun.

Astronomers made the unexpected discovery while using Chandra to observe how Sgr A* would react to a nearby cloud of gas known as G2.

“Unfortunately, the G2 gas cloud didn’t produce the fireworks we were hoping for when it got close to Sgr A*,” said lead researcher Daryl Haggard of Amherst College in Massachusetts. “However, nature often surprises us and we saw something else that was really exciting.”

On Sept. 14, 2013, Haggard and her team detected an X-ray flare from Sgr A* 400 times brighter than its usual, quiet state. This “megaflare” was nearly three times brighter than the previous brightest X-ray flare from Sgr A* in early 2012. After Sgr A* settled down, Chandra observed another enormous X-ray flare 200 times brighter than usual on Oct. 20, 2014.

Astronomers estimate that G2 was closest to the black hole in the spring of 2014, 15 billion miles away. The Chandra flare observed in September 2013 was about a hundred times closer to the black hole, making the event unlikely related to G2.

The researchers have two main theories about what caused Sgr A* to erupt in this extreme way. The first is that an asteroid came too close to the supermassive black hole and was torn apart by gravity. The debris from such a tidal disruption became very hot and produced X-rays before disappearing forever across the black hole's point of no return, or event horizon.

“If an asteroid was torn apart, it would go around the black hole for a couple of hours – like water circling an open drain – before falling in,” said co-author Fred Baganoff of the Massachusetts Institute of Technology in Cambridge, Massachusetts. “That’s just how long we saw the brightest X-ray flare last, so that is an intriguing clue for us to consider.”

If this theory holds up, it means astronomers may have found evidence for the largest asteroid to produce an observed X-ray flare after being torn apart by Sgr A*.

A second theory is that the magnetic field lines within the gas flowing towards Sgr A* could be tightly packed and become tangled. These field lines may occasionally reconfigure themselves and produce a bright outburst of X-rays. These types of magnetic flares are seen on the sun, and the Sgr A* flares have similar patterns of intensity.

“The bottom line is the jury is still out on what’s causing these giant flares from Sgr A*,” said co-author Gabriele Ponti of the Max Planck Institute for Astrophysics in Garching, Germany. “Such rare and extreme events give us a unique chance to use a mere trickle of infalling matter to understand the physics of one of the most bizarre objects in our galaxy.”

In addition to the giant flares, the G2 observing campaign with Chandra also collected more data on a magnetar: a neutron star with a strong magnetic field, located close to Sgr A*. This magnetar is undergoing a long X-ray outburst, and the Chandra data are allowing astronomers to better understand this unusual object.

These results were presented at the 225th meeting of the American Astronomical Society being held in Seattle. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
March 19, 2015

RELEASE 15-044 NASA’s SOFIA Finds Missing Link Between Supernovae and Planet Formation

SOFIA data on a supernova http://www.nasa.gov/sites/default/files/thumbnails/image/15-044a.jpg SOFIA data reveal warm dust (white) surviving inside a supernova remnant. The SNR Sgr A East cloud is traced in X-rays (blue). Radio emission (red) shows expanding shock waves colliding with surrounding interstellar clouds (green).

Using NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), an international scientific team discovered that supernovae are capable of producing a substantial amount of the material from which planets like Earth can form.

These findings are published in the March 19 online issue of Science magazine.

"Our observations reveal a particular cloud produced by a supernova explosion 10,000 years ago contains enough dust to make 7,000 Earths," said Ryan Lau of Cornell University in Ithaca, New York.

The research team, headed by Lau, used SOFIA's airborne telescope and the Faint Object InfraRed Camera for the SOFIA Telescope, FORCAST, to take detailed infrared images of an interstellar dust cloud known as Supernova Remnant Sagittarius A East, or SNR Sgr A East.

Supernova remnant dust as seen by SOFIA http://www.nasa.gov/sites/default/files/thumbnails/image/15-044b.jpg Supernova remnant dust detected by SOFIA (yellow) survives away from the hottest X-ray gas (purple). The red ellipse outlines the supernova shock wave. The inset shows a magnified image of the dust (orange) and gas emission (cyan).

The team used SOFIA data to estimate the total mass of dust in the cloud from the intensity of its emission. The investigation required measurements at long infrared wavelengths in order to peer through intervening interstellar clouds and detect the radiation emitted by the supernova dust.

Astronomers already had evidence that a supernova’s outward-moving shock wave can produce significant amounts of dust. Until now, a key question was whether the new soot- and sand-like dust particles would survive the subsequent inward “rebound” shock wave generated when the first, outward-moving shock wave collides with surrounding interstellar gas and dust.

"The dust survived the later onslaught of shock waves from the supernova explosion, and is now flowing into the interstellar medium where it can become part of the 'seed material' for new stars and planets," Lau explained.

These results also reveal the possibility that the vast amount of dust observed in distant young galaxies may have been made by supernova explosions of early massive stars, as no other known mechanism could have produced nearly as much dust.

"This discovery is a special feather in the cap for SOFIA, demonstrating how observations made within our own Milky Way galaxy can bear directly on our understanding of the evolution of galaxies billions of light years away," said Pamela Marcum, a SOFIA project scientist at Ames Research Center in Moffett Field, California.

SOFIA is a heavily modified Boeing 747 Special Performance jetliner that carries a telescope with an effective diameter of 100 inches (2.5 meters) at altitudes of 39,000 to 45,000 feet (12 to 14 km). SOFIA is a joint project of NASA and the German Aerospace Center. The aircraft observatory is based at NASA's Armstrong Flight Research Center facility in Palmdale, California. The agency’s Ames Research Center in Moffett Field, California, is home to the SOFIA Science Center, which is managed by NASA in cooperation with the Universities Space Research Association in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart.

http://www.dsi.uni-stuttgart.de/index.en.html
Galactic centre orbits.svg
Author/Creator: Cmglee, Licence: CC BY-SA 3.0
Inferred well-determined orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre based on data from "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month" by Eisenhauer et al, The Astrophysical Journal, 628:246-259, 2005. Note: For the comparison image, the scale is assumed to be 7940 AU/arcsecond (1" in radians × 7.94 kpc in AU) or 7.94 AU/pixel in SVG. At this scale, the distance between the Sun and Proxima Centauri (its nearest star) is 33.8 times (268 000 AU ÷ 7940 AU/arcsecond) the height of the chart.
Artist’s impression of the Milky Way.jpg
This detailed artist’s impression shows the structure of the Milky Way, including the location of the spiral arms and other components such as the bulge. This version of the image has been updated to include the most recent mapping of the shape of the central bulge deduced from survey data from ESO’s VISTA telescope at the Paranal Observatory.
Sagittarius A* black hole simulation.png
Author/Creator: Event Horizon Telescope project, Licence: CC BY-SA 4.0
A simulation of the central massive Sagittarius A* black hole by the European Space Observatory
Pointing X-ray Eyes at our Resident Supermassive Black Hole.jpg
NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light. The background image, taken in infrared light, shows the location of our Milky Way's humongous black hole, called Sagittarius A*, or Sgr A* for short.

In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. The time series at right shows a flare caught by NuSTAR over an observing period of two days in July; the middle panel shows the peak of the flare, when the black hole was consuming and heating matter to temperatures up to 180 million degrees Fahrenheit (100 million degrees Celsius).

The main image is composed of light seen at four different X-ray energies. Blue light represents energies of 10 to 30 kiloelectron volts (keV); green is 7 to 10 keV; and red is 3 to 7 keV. The time series shows light with energies of 3 to 30 keV.

The background image of the central region of our Milky Way was taken at shorter infrared wavelengths by NASA's Spitzer Space Telescope.
The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way.webm
Author/Creator: ESO/A. Eckart, Licence: CC BY 4.0
This video sequence shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the centre of the Milky Way.

These new observations with ESO’s VLT have shown that the cloud appears to have survived its close encounter with the black hole and remains a compact object that is not significantly extended.

In this sequence observations of the cloud during the period from 2006 and 2014 are shown. The final two images are from February and September 2014, before and after the object passed closest to the black hole. The cross marks the position of the supermassive black hole.