Galactic Center Revealed!

NASA'a GREAT OBSERVATORY PROGRAM (and the ESA) 'SEE' into the CENTER of the MILKY WAY

Not too many years ago, astronomers despaired that they would ever get a clear, sharp, informative picture of the galactic center. The giant clouds of dense gas that dominate the center of the Milky Way seemed to be an impenetrable barrier. We cannot 'see' the inhabitants of the Galactic Center (GC) because either they emit only weak radiation in the visible portion of the electromagnetic spectrum; and/or the 'light' they do emit is effectively blocked by the dense giant dust clouds that are in the line of sight between Earth and the Galactic Center. Nonetheless, persistence paid off and a few years ago, three of the most extraordinary telescopes ever built began to acquire new data. On November 10, 2009, NASA released break-through images and our view of the Milky Way changed forever. Objects that are dramatic and important, that we cannot 'see', were made 'visible' by color coding infrared, X-ray, and gamma ray emissions onto the visible portion of the electromagnetic spectrum. For readers with a technical interest in photography, there is a marvelous article in Sky and Telescope that walks through the protocol by which these beautiful photographs are prepared, and also answer questions about the match between 'reality' and the color palette in NASA's photo lab.

The lead image above is one of the extraordinary photographs released by NASA's Great Observatories in honor of the International Year of Astronomy, 2009. The Four Great Observatories (in order of launch date/operational) are: the Hubble Space Telescope(s), the Compton Gamma Ray Observatory, the Chandra X-Ray observatory, and the Spitzer Space Telescope (infrared space observatory). Each telescope has unique capabilities and there is only modest overlap in regions of the electromagnetic spectrum that the instruments on each observatory investigate.

“The Hubble Space Telescope was initially funded at US $36 million in 1978, a low budget that mandated collaboration with the European Space Agency and the first planned launch date was in 1983. This ‘telescope’ is not a single instrument but an observatory with five important instruments. It was named after the famous astronomer Edwin Hubble who had discovered that the universe was expanding. Marshall Space Flight Center designed and built the telescope. By September 1986, budget for the HST Program had reached $1.175 billion, including cost overruns and delays for the spacecraft being built by Lockheed.”

“As of May, 2009, the costing to the United States for Hubble Space Telescopes, including all four Service Missions, was $9.6 billion. Expenses by the European Union are ~ €593 million. The Hubble Space Telescope was finally placed into orbit by the space shuttle Discovery on April 24, 1990. In low orbit at 347 miles (559 km) above the earth, the atmosphere is so thin that visual spectrum optics can retrieve images of previously unobtainable clarity.”

Hubble Space Telescope (Service Mission 4) / Advanced Camera for Surveys
Photo – NASA

“Advanced Camera for Surveys was Hubble’s workhorse until power failures reduced its capability. By 2006, only the ultraviolet camera channel was still operative. A blown fuse in 2007 mandated that the astronauts on Service Mission 4 would have to repair the ACS in space. They did so with total success an efficient bypass solution to restoring power was implemented. A side benefit is that power demand dropped by more than 2X and ‘noise’ level of the UV detectors was reduced”

Hubble Space Telescope SM 4 / Wide Field Camera

Hubble Space Telescope (Service Mission 4) / Wide Field Camera 3
Diagram – NASA/ ESA

“SM 4 Day One saw the installation of a new Wide Field (WF) Camera along with a new Science Instrument Command and Data Handling unit (SICDH). Wide Field Camera 3 was constructed at Goddard Space Flight Center and Ball Aerospace in the USA, with some components built by contractors in the UK. WF 3 is a bridge to the advanced infrared observations that will be carried out by Hubble’s successor, the James Web Space Telescope. The Wide Field Camera is Hubble’s only panoramic instrument; it can ‘see’ over a wide range of the electromagnetic spectrum. WFC 3 provides a 15-30X increase in capability over its predecessors, depending upon the specific region in the electromagnetic spectrum under investigation. The Wide Field Camera is enhanced by an interface with the Advanced Camera for Surveys (ACS), which increased the observation power of the Wide Field Cameras by 10X. A short circuit ended ACS operation in January 2007 and SM 4 installed a new ACS power supply and replaced four circuit boards.”

The Compton Gamma Ray Observatory (CGRO) followed the Hubble Space Telescope in the Great Observatories Program at NASA and was named after Dr. Arthur Holly Compton, who won a Nobel Prize for his work with gamma ray physics. Launched on April5, 1991 after 14 years of development, CGRO operated until June 4, 2000. At 37,000 lbs, CGRO was the record setting astrophysics payload of its time. It was deployed in a low earth orbit of 450 km (280m) so as to avoid the Van Allen Radiation Belts.

The Compton Gamma Ray Observatory was a success, its four instruments made important discoveries and contributed significant data that will be analyzed for years to come. Two all sky surveys at different gamma ray wavelengths were completed in which many new gamma ray emission sources were discovered. Gamma ray burst events (GRBs) were studied intensively and it was learned that most of these originate in very distant galaxies and are therefore extremely energetic. Short and long GRBs profiles were clarified and the first soft gamma ray repeater objects identified by the CGRO. Earth based gamma ray emissions were identified in thunder clouds.

Using data gathered by the Compton Gamma Ray Observatory in the 1990s, this extraordinary montage makes visible photons whose energy is more than 40 x 106X visible light. Running horizontally through the center of the image is the diffuse gamma-ray glow from the galactic plane. The brightest spots are pulsars – rapidly spinning, magnetized neutron stars. Above and blow the galactic plane are quasars, distant very young galaxies that have massive black holes in their centers.

This pair of photos illustrates the important discoveries that await astronomers who study the gamma ray spectrum. When GRB990123 was photographed in 1999 by Hubble's Imaging Spectrograph STScl, it was the most powerful explosion in the universe on record. For a brief moment, GRB990123 gave forth radiation equal to that from 100 billion stars. When this photograph was taken, the explosion had faded to one four millionth of its original intensity. GRB990123 is 2/3 of the way to the end of the visible universe. It is quite blue, which indicates intense star formation activity, and record setting gamma ray explosions occur when pairs of neutron stars and/ or black holes merge, or hypernova explode. Hubble's Imaging Spectrograph STScl will be able to record the most extreme gamma ray events occurring at the Galactic Center.

Proposed in 1976, then completely redesigned in 1992 to a much smaller satellite with fewer telescopes, the Chandra X-ray Observatory is the third of NASA's Great Observatories and a partnership with Harvard University. and it was launched on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the mass limit for white dwarf stars to become neutron stars. It also might have indirectly been named for Dr. Sivasubramanian Chandrasegarampillai, the fictional designer of HAL 9000 in the Space Odyssey series. "Chandra" also means "moon" or "luminous" in Sanskrit. See Source #8.

The Space Shuttle Columbia delivered Chandra to a low Earth orbit. Then, the inertial upper stage rocket boosted Chandra up to a higher altitude where a built-in propulsion system took the X-ray telescope to its final orbit. This elliptical orbit takes the spacecraft to an altitude more than a third of the distance to the moon at its greatest distance from Earth, before returning to a lowest altitude of 16,000 kilometers (9,942 mi). Chandra takes ~64 hrs, 18' to complete an orbit.

Earth's atmosphere absorbs most X-ray emissions and so placement of Chandra in a very high altitude orbit of 133,000 km (82,646 mi) was essential. Orbital characteristics and the high angular resolution of Chandra telescope's mirrors combine to give Chandra X-ray 'eyes' 100X more sensitivity than earlier generation X-ray telescopes. Chandra spacecraft spends ~85% of its orbit above the bands of charged particles that surround the Earth (Van Allen Belts), which allows for uninterrupted observations as long as 55 hours. Chandra has been one of NASA's total success stories. Initial lifetime estimate was 5 years, but latest analysis has revised that figure upward and Chandra may still be sending down extraordinary data in 2022. Chandra has greatly advanced our understanding of: a) largest star explosions (supernovae), b) the massive black hole at the center of the Milky Way (discussed below in this article), and c) X-ray emissions given off by many star types. Chandra has discovered mid size black holes and several years ago, a high school project using Chandra data discovered a neutron star inside a supernovae remnant.

The Spitzer X-ray Telescope was launched on a Delta II rocket from Cape Canaveral Air Force Station (Florida) on August 25, 2003. This $USD 800 million satellite telescope had a projected lifetime of 5 years which was the time estimated for the liquid helium essential to cool the telescope's mirrors to have evaporated. The primary telescope lasted a longer, ending useful operation on May 15, 2009. However, the two shortest wavelength mirrors do not need the cryogen and Spitzer continues with the Warm Mission. Placing Spitzer in orbit around the sun and far from earth allowed for the use of passive heating and greatly reduced the liquid helium payload.

Major contributions from the Spitzer X-ray instruments include: a) first direct radiation capture from an exoplanet; b) discovery of the youngest star known; c) detection of 'light' from the very early universe, galactic objects 'only' 100 million years old; c) discovery of the Double Helix Nebula whose twisted shaped is generated by massive magnetic fields at the Galactic Center that are 300 light years distant from the Double Helix Nebula; d) detection of a collision between two exoplanets in orbit around a young star; and e) discovery of the large, tenuous Phoebe Ring around Saturn. Spitzer undertook the GLIMPSE, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire that produced an archive of 444,000 images; and MIPSGAL which surveyed 278° of the galactic disk at longer wavelengths and produced a portrait of the Milky Way by stitching together 800,000 IR snapshots.

Spitzer undertaken two of the most wide ranging and impressive galactic surveys which comment again on the extraordinary capacity of this observatory. GLIMPSE, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire, produced an archive of 444,000 images. MIPSGAL surveyed 278° of the galactic disk at longer wavelengths and produced a portrait of the Milky Way by stitching together 800,000 IR snapshots.

The top panoramic image in this trio of photos covers 8 degrees of the Milky Way. The red dust clouds are lit up by nearby star formation and reveal that large organic molecules are present. The IRAC camera collected the data used to make this four wavelength composite: 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). Black are dust clouds that are so impenetrable that even Spitzer's incredible X-ray instruments cannot penetrate. White arcs are huge incubators where stars are born. Blue dots are older stars. Boston University has developed a computer program that can search for star clusters that are almost impossible to 'see' even after image data has been processed and colorized.

Integral is a fabulously successful Gamma Ray Astrophysics Laboratory managed by the European Space Agency in partnership with NASA and the Russian Space Agency. Integral is not part of the Great Observatories Program but it has contributed invaluable data to our understanding of the Galactic Center. Launched from Baikonur spaceport in Kazakhstan in 2002 aboard a Proton-DM2 rocket, Integral's orbit has a period of 72 hours with high eccentricity. Although perigee is close to the Earth at 10,000 km and is within the magnetosphere radiation belt, most of each orbit is spent outside this region so that effective scientific observations are maximized. Apogee at 153,000 km is in the northern hemisphere in order to increase contact time with ground stations and mission control at ESOC in Darmstadt, Germany.

Integral's predicted lifetime of 2.2 years has been exceeded and it is expected to be operational for another six years. Interestingly, the body and instrument structures are made almost entirely from composites and Integral has become another test bed for these controversial materials.

Integral is equipped to detect the most energetic radiation that might be produced by distant objects in the universe. It carries the most sensitive gamma ray instruments ever built. Integral observes in both the soft and hard X-ray spectrum from 3 to 35 keV, and one of the four instruments provides all sky coverage. Integral has detected the mysterious 'iron quasar' and a number of important distant, violent burst objects that emit record setting intensities of gamma ray radiation.

The top panorama in this group caputres a swath through the GC that is 760 light years long. The four smaller photos across the lower panel, left to right, are : a) the star forming region featuring 'owl eyes and the only photo in this montage not depicting an area at, or near, the GC; b) five massive, young stars known as the Quintuplet Cluster below which is the Pistol Nebula, which was ejected by the central Pistol Star; c) long, narrow filaments at the base of the Arches Filaments with bright star forming regions to their right; and d) the Galactic Center which marks the location of a supermassive black hole around which rotates a ring of dust and gas known as the circumnuclear disk. Each of these regions and structures is discussed in more detail below.

In the November 2009 annotated photo montage of the Galactic Center, pink represents low energy X-rays and blue indicates high energy X-rays. Regions of hundreds of tiny dots reveal black holes. Some of these very small dots are emission loci from material just outside the event horizon of the supermassive GC black hole. Other groups of tiny dots are emission loci from other exotic objects. Sagittarius A* is the most energetic object in the Milky Way and is believed to be congruent (= equivalent) with the supermassive black hole at the Galactic Center.

The challenge to identify who lives at the center of our galaxy is formidable. Should you ever visit, here are some of the entities you'll likely encounter.

n 1971, astronomers Lynden-ll and Rees predicted that a massive, compact radiation source existed at the center of the Milky Way. Their prediction was verified in 1974 when strong radio emission that emanated from the galactic center was detected by Balick and Brown and the first hard X-ray emissions from the GC were detected by the XRT Telescope in Spacelab 2 and reported in 1987. The first (soft) X-rays from the Galactic Center were detected in 1999 by Chandra. Data collected 1994-2005 clarified that gas near the black hole brightens and then quickly fades in the X-ray spectrum, presumably in response to X-ray pules emanating from just outside the black hole. When gas spirals inward towards the black hole, it heats up to millions of degrees and then emits X-rays. Sgr A* appears to be in a quiet, resting phase now because although it contains ~ 4X million Sun's mass, the energy radiating from its immediate surroundings is billions of time weaker than that from comparable black holes in other galaxies.

The giant stars in these clusters belong to binary systems, and a few stand out because of their striking characteristics . They become powerful, 'point' X-ray sources when winds blowing off their surface collide with winds from an orbiting companion star. The Infrared and X-ray point sources (mostly stars) increase in density in inverse proportion to their distance from the GC. Six important structures in the nuclear region of the Galactic Center give rise to complex radiation outputs because they interact with each other. These components are: a) SgrA*, the supermassive black hole with a mass of at last 3.5 x 106 Sun; b) the surrounding cluster of evolved and young stars; c) ionized gas streamers, some of which form a three armed spiral centered on SgA* that is known as SgrA West; d) a dusty molecular ring surrounding Sgr A West; e) diffuse hot gas; and f) a powerful supernovae remnant known as Sgr A East.

“The IR and X-ray point source population increases in spatial density approximately as 1/R2, where R is the distance from the GC (Serabyn & Morris 1996; Muno et al. 2003a). The first imaging observations of the GC in hard X-rays were performed by the XRT Telescope on Spacelab 2 in the range 2.5–20 keV with an angular resolution of 3′. A source located within 1′.1 of SgrA* was detected (Skinner et al. 1987). 1E 1740.7-2942 is a black hole candidate and micro-quasar, KS 1741-293 and A 1742-294 are neutron star Low-Mass X-Ray Binary (LMBX) burster systems, SLX 1744-299/300 are in fact two LMXBs that cannot be resolved as such by INTEGRAL due to their very close proximity, and 1E 1743.1-2843 is an X-ray source whose nature is still uncertain." - INTEGRAL detection of hard X-ray emission from the Galactic nuclear region, G. Belanger, A. Godwurm, P. Goldoni, J. Paul, R. Terrier, M. Falanga et al, 7 Nov 2003.

Contour maps of X-ray emissions at the Galactic Center using data collected by the IBIS and ISGRI instruments on the ESA INTEGRAL X-Ray Observatory and published in 2003. The two maps represent two different energy ranges. For further detail see Source #11, and the longer and more detailed version of this article published at ahrtp.com.

Milky Way’s centre seen in a 2◦x2◦ field by the IBIS/ISGRI instrument in the energy ranges 20–40keV (top) and 40–100keV (bottom). Each image pixel size is equivalent to about 5 arcmin. Ten contour levels mark iso-significance linearly from about 4 to 15.

In these signal-significance maps of the central two degrees of the Galaxy where ten contour levels mark iso-significance linearly from about 4 up to 15, we can see what appear to be six distinct sources: . . . . , and a source coincident with the radio position of SgrA*. Of these sources, 1E 1740.7-2942 is a black hole candidate and micro-quasar, KS 1741-293 and A 1742-294 are neutron star Low-Mass X-Ray Binary (LMBX) burster systems, SLX 1744-299/300 are two LMXBs that cannot be resolved as such by INTEGRAL due to their very close proximity, and 1E 1743.1-2843 is an X-ray source whose nature is still uncertain. In the 20–40 keV band, contours of the central source clearly peak at the position of SgrA* with the maximum at a significance level of 8.7 but are elongated towards GRS 1741.9-2853. This suggests some contribution to the emission from this transient neutron star LMXB burster system recently observed to have returned to an active state (Muno et al. 2003b), but could also be due to an uncorrected background structure. The central source is also marginally visible in the 40–100 keV band at a level of 4.7, but without any contribution from the direction of GRS 1741.9-2853." - INTEGRAL detection of hard X-ray emission from the Galactic nuclear region, G. Belanger, A. Godwurm, P. Goldoni, J. Paul, R. Terrier, M. Falanga et al, 7 Nov 2003.

1E 1743.1-2843 at no less than 20kpc distance from the GC, is an X-ray source that has been studied intensively and continues to defy attempts to describe it with specificity. Early studies of 1E 1743.1-2843 were an ESA mission, with NASA contributions, that utilized the XMM-Newton satellite-telescope. Hard and soft X-ray emissions, Black Body temperatures and steep power law indicated that 1E 1743.1-2843 is a neutron star or black hole that is absorbing material from its companion, a violent process that creates X ray emissions. (Don't you wish that 1E 1743.1-2843 had an easy to remember name? !) It was discovered when the Galactic Center was first studied at the X-ray region of the spectrum by the Einstein Observatory. 1E 1743.1-2843 has an extreme columnar density for this region of the sky which indicates a distance similar or greater than the GC. LMXBs in which neutron stars draw off matter from a low mass companion (< 1X Sun) show Type 1 X-ray bursts that are produced by thermonuclear flashes when the material from the "small" companion star impacts the surface of the neutron star. Observational data are also compatible with a black hole in a low/hard radiation state. Yet a third possibility, is an extragalactic radiation source that we on Earth observe through the Galactic Plane. The majority opinion, however, is that 1E 1743.1-2843 lies within 20' of the GC. "1E 1743.1-2843 lies on the periphery of SNR G0.33+0.04 where the SNR emission is the brightest at 90 cm. 1E 1743.1-2843 lies very close, again in projection, to a giant molecular cloud core GCM+0.25+0.01 (h 46m 10.1s, 42 48.4), which appears to be located at the GC region and to contain embedded low-mass star formation. If 1E 1743.1-2843 is located in the GC region, and behind this cloud, this could explain the high absorption of its soft X-ray flux."

As there are no records of Type 1 X-ray bursts from 1E 1743.1-2843 during a 20 year observation history, a precise description remains elusive. The overall X-ray luminosity (1036-1037 erg s-1) is typical for objects that emit X-ray bursts, perhaps due to the stable burning of hydrogen and helium in stars functioning close to the Eddington Limit. However, this profile requires that 1E 1743.1-2843 be more than several tens of kiloparsecs distant from Earth. Very compact, super dense stellar objects have high magnetic fields on their surface which can suppress Type-1 X-ray bursts. The absence of periodic X-ray pulses and/or eclipses, and the presence of a soft X-ray spectrum favor a Low Mass X-ray Binary star (LMXB) as the larger star in the binary system of 1E 1743.1-2843.

Hubble data has determined that the massive stars pour radiation and wind into large areas of dense, warm gas (lower left photos in photo montage) that are found throughout the galactic center. Large arc like structures are formed of which. the 'Sickle' is the most prominent: see photo montage

Reading right to left, the third inset in the bottom row of photos in the infrared portrait of the galactic center taken by the Spitzer Telescope (above photos) is the first visualization of the long, stringy formations at the base of a structure known as the Arches Filaments. These filaments are about 10 light years long and less than one light year wide. The fourth inset in this lower row of photos shows some of the brightest star regions in the infra-red map of Milky Way, and we surmise that star formation activity is intense.

Four million years ago within 100 light years of the GC, the Quintuplet Cluster formed and is now slowly dissapating. It is home to the Pistol Star which is the brightest known star in the Milky Way.

Second from the left in the lower row of smaller photos (montage above) is the extremely luminous Quintuplet stars, five massive stars that have are buried in thick dust clouds. The Hubble NICMOS telescope has provided the clearest views yet taken of the Quintuplet Cluster which is 25,000 light years from Earth, and less than 100 light years from the GC.

This monster star cluster has a mass equivalent of 10,000 Suns and is 10X larger than the typical young star cluster scattered throughout the Milky Way. Now 4 million years old, the Quintuplet will soon come to a violent end, ripped apart in a few million years by the huge gravitational tidal forces at the core of the Milky Way. Astronomers expect to see supernovae in the Quintuplet before too many more years pass. Hidden behind thick black dust clouds in the constellation Sagittarius, if the Quintuplet Cluster could be seen from Earth it would be as bright as 3rd magnitude star to the naked eye,and 1/6th the full moon's diameter.

Within the Quintuplet Cluster is the brightest star of all in the Milky Way, the Pistol Star which is a massive luminous blue variable. A star at the center of the Pistol Nebula was first postulated with some confidence in 1990 and early photographs revealed a pistol-like shape. The Pistol Star is blue variable giant that is 120-200X mass Sun and 1.7 x 10x6 the Sun's luminosity. If not for the impenetrable dust between Earth and the Galactic Center, the Pistol Star would be visible to the naked eye.

Data from two of Hubble's cameras was combined to create this photograph that showed an image of the Pistol Star that is hidden behind a vast quantity of thick dust . The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) with infrared vision can penetrate the thick dust clouds. The Pistol Star has blown off two expanding shells of gas that are 'false' colored in this photo as magenta. The two dust shells have a total mass several times that of our Sun, and the largest shell has a radius of 2 light years that would reach from the Sun to the star nearest Earth. The two novae that created these two dust shells are estimated to have occurred 4,000 and 6,000 years ago, respectively. The mass loss to the Pistol Star was considerable, its original mass might have been 200 X Sun.

Milky Way GC – Pistol Star Nebula / NICMOS , Hubble / NASA
Photo – Wmahan / Wikimedia

The Pistol Nebula is the most massive (L)uminous (B)lue (V)ariable (N)ebula known in the sense of its ejected material and it completely surrounds the Pistol Star. The mass of the Pistol Nebula is higher than in any other LBVN. Spectra are asymmetric as line intensity is strongest at the northern and western edges of the Pistol Nebula. Data allows for estimation of the velocity of ejected material when the Pistol Nebula first formed: 60 – 95 km/s. The terminal velocity of the stellar wind near the Pistol Star is ~95km/s. As the present expansion velocity of the nebula is 60km/s, it has been slowed down by interaction with the surrounding medium, with more de-acceleration on the far side of the expanding shell. This analysis of stellar winds and expansion allowed for age estimates to be made for when the Pistol Star exploded (novae) to create the two dust shells.

The present velocity of gas expansion in the outer shell of the nebula is 60 km/s. The velocity structure of the ionized gas in the nebula as revealed by spectroscopy resembles that of planetary nebula. The Pistol Nebula is primarily ionized by nearby very hot stars, and it physically interacts with the strong winds of these stars. There are two direction spikes of emission lines that extend north and south from the Pistol Star. The northern line terminates in a bright emission knot known as Northern Knot II. Two newly identified spectral emission north of the Pistol Star are likely the hottest known stars in the GC with temperatures > 50,000 K. The GC ambient magnetic field likely helps shape the Pistol Nebula which is elongated along the magnetic field which tells us that the ambient magnetic field is strong enough to constrain the nebula expansion in a direction perpendicular to the field. However the overall shape of the Pistol Nebula is rectangular with long sides parallel to the magnetic field of the Galactic Center, boundaries are heavily influenced by NTFs else the Nebula would be much more oval in shape.

At present, the Pistol star is being photo-ionized by nearby hot stars in the Quintuplet Cluster, and the ejected mass is greater by 2X than the most massive ejecta from any other LBV. The Pistol Nebula was ejected in the presence of a strong magnetic field generated at the Galactic Center, and the powerful solar winds from nearby stars. The side of the Pistol Nebula nearest to us is approaching Earth even as it is ionized by hot stars in the Quintuplet Cluster which has also de-accelerated highest red-shifted gas in the Pistol Nebula. This profile of the Pistol Star relies upon an important paper first published in the Astrophysical Journal in 1999. In 13 well written pages, eight researchers reported a breakthrough in understanding the structure and history of the Pistol Star, how it gave rise to the Pistol Nebula and what features of the immediate environment at the GC continue to heavily influence the Pistol Nebula.

Discovered by the Spitzer Space Telescope, the Double Helix Nebula is ~300 light years from the Galactic Center and takes its name from a resemblance to the double helix geometry of the DNA molecule. Likely magnetic torsion twisted the original nebula into the shape of two connected spirals.

The visible segment of the Double Helix Nebula is about 80 light years long. Models for its origin and shape propose magnetic fields at the GC that are 1,000X stronger than those generated by our Sun and driven by the massive disc of gas orbiting Sagittarius A*.

The very bright central spot, which the single brightest spot in the entire photo mosaic, indicates Sagittarius A*, the supermassive black hole that exists in the center of our galaxy. The extreme infra-red radiation emission of the central dust hiding Sagittarius A* is is caused by heated from a compact cluster of very hot stars that are likely in the first stages of their life cycles. Sagittarius A* is the most energetic object in the entire Milky Way galaxy. The circumnuclear disk, which is the rotating ring of gas and dust that surrounds Sagittarius A*, can also be seen in the photo.

The European Southern Observatory recently concluded an extraordinary 16 year observation program of the GC using a novel approach to pin down Sagittarius A*. The NACO instrument 'sees' in the near infrared and it followed 28 stars closest to the Galactic Center. While most of these stars swarm around the GC like angry bees, the most distant six stars from the GC orbit it in a disc. Mass and distance for Sagittarius A* were estimated from these data.

A recent, important discovery by Chandra and XMM-Newton is that Sgr A* gives off powerful X-ray flares during which time the soft X-ray luminosity can rapidly increase 50X – 180X over a period of time up to 3 hours. Very recently, the (V)ery (L)arge (T)elescope and NACO imaging instrument and the Keck Telescope determined that Sgr A* is also the source of infrared flares. This activity suggests that an important population of nonthermal electrons exists near the black hole.

If hard X-rays can be observed at the GC, that would further clarify the relative role of accretion and ejection in the Sgr A* system, and there is a candidate for such an object. IGR J17456-2901, which was discovered by the European Space Agency Integral satellite telescope, is within 0'.9” of the GC and nearly coincident with Sgr A*. 20 to 100KeV luminosity has been measured and IGR J17456-2901 is the first report of hard X-ray emission that is likely emanating from within 10' of the GC, perhaps from the black hole itself.

The white dot at center of this image is the exact postion of the very strong radio source that led to the discovery of the massive black hole at the center of the MilkyWay that is surrounded by a diffuse cloud of hot X-ray gas. This image spans 10 light years at the GC. As various data make clear in this article, the name Sgr A* is now considered synonomous with the massive black hole at the GC. Radio and X-ray emission is generated by material falling into the black hole, whose mass is now estimated to be at least 4 million Suns.

Hidden behind massive dust clouds in Sagittarius, the Galactic Center is now disclosing its secrets thanks to the Integral space observatory of the ESA, and NASA's Great Observatory Program. Hubble, Chandra and Spitzer and Integral can 'see' in the infrared, gamma ray and X-ray regions of the electromagnetic spectrum. Until these regions of the spectrum became readily available, humankind was 75% blind when looking into the Galactic Center or outward to the far reaches of the universe. Surrounding Sagittarius A* at close distances are very young stars and their associated large nebula and record setting energy output. When the extreme GC environment allows, these young giant, hyperactive stars organize themselves into star clusters. The magnetic field at the GC is extremely strong, and has a profound influence on nearby nebular geometry and orientation. The picture of the GC that is now emerging is important because it believed applicable to many large (spiral) galaxies throughout the universe. Galactic structure may be restricted to a relatively small number of models, God imposed considerable order on the universe before Sunday brunch.

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