What Lies at the Center of the Milky Way?

Milky Way GC at three wavelengths

Milky Way GC / infrared, gamma ray, X-ray
Photo Montage – Hubble, Spitzer, Chandra / NASA

This photo caused tremendous excitement when released by NASA on November 10, 2009. The Galactic Center we cannot ’see’ with our eyes has been made visible as never before by the genius of the digital image, technician-artists at NASA. Pink and blue represent low and high energy X-rays, respectively. Regions of hundreds of tiny dots reveal the uncountable numbers of black holes that live just outside the event horizon of the supermassive black hole (Sgr A*) at the Galactic Center (GC). Sagittarius A* is the most energetic object in the Milky Way and one of the major astronomy discoveries of recent years.

Sagittarius star cloud
Milky Way GC / Sagittarius star cloud
Photo – Hubble, NASA

Energy Beasties 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.

Milky Way GC / Sagittarius A*
Milky Way GC / Sagittarius A* awakes 300 mya
Photo – NASA / CXC / MIT / Frederick K. Baganoff et al

Verification of the massive black hole at the GC began in 1974 when very strong radio emission from its location was confirmed. The first hard X-ray emissions from the GC were detected in 1987; then Chandra found soft X-ray emissions in 1999. It is now understood that gas near the black hole brightens and then quickly fades in the X-ray spectrum, presumably in response to X-ray pulses 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* now appears to be in a quiet, resting phase. Although it contains ~ 4X million Sun’s mass, the energy radiating from its immediate surroundings is billions of times weaker than that from comparable black holes in other galaxies.

Milky Way GC - Arches_Quintuplet
Milky Way GC – Arches_Quintuplet / Chandra
Photo – NASA / Chandra, NASA

The giant stars become powerful ‘point’ X-ray sources when winds blowing off their surface collide with winds from an orbiting companion star. Six important structures in the nuclear region of the Galactic Center give rise to complex radiation outputs: a) SgrA*, the supermassive black hole with a mass of at least 3.5 x 10 million 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.

Milky Way GC – black hole swarm
Milky Way GC – X-ray binaries (neutron stars, black hole swarm)
Photo – NASA / CXC / UCLA / M.Muno et al.

X-Ray Binary – 1E 1743.1-2843

At no less than 20kpc distance from the GC, there is a complicated 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 done by an ESA mission with NASA contributions, which 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. However, 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 above).

Cygnus X-1 Black Hole
Cygnus X-1, black hole drawing material off its companion star.
Image – NASA-ESA / Hubble Space Telescope

Arched Filaments / Arched Cluster

Milky Way GC at infrared
Milky Way GC / infrared 0.8mu – Spitzer
Photo Montage ­ NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech)

Reading right to left, the third photo in the infrared portrait of the Galactic Center taken by the Spitzer Telescope 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 infrared map of the Milky Way, and likely star formation activity here is intense.

Quintuplet Cluster

To the immediate right of the owl-eye stellar nursery is an image of the extremely luminous Quintuplet stars, five massive stars that are buried in thick dust clouds. The Quintuplet Cluster is home to the Pistol Star which is the brightest known star in the Milky Way.

Milky Way GC – Star Clusters
Milky Way GC – Star Clusters / Quintuplet, Arches
Photos – Don Figer (STScI et al) / NASA

The Hubble NICMOS telescope has provided the clearest views yet taken of the Quintuplet Cluster which is 25,000 light years from Earth. This monster star cluster has a mass equivalent of 10,000 Suns and is 10X larger than the typical young star clusters scattered throughout the Milky Way.

Milky Way GC – Quintuplet Cluster
Milky Way GC – Quintuplet Star Cluster
Photo – Hubble, NASA

Now 4 million years old, the Quintuplet will 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. The Quintuplet Cluster is hidden behind thick black dust clouds in the constellation Sagittarius, but if it could be seen from Earth it would be as bright as a third magnitude star to the naked eye, and 1/6th the full moon’s diameter.

Pistol Star / Pistol Nebula

Within the Quintuplet Cluster is the brightest star of all in the Milky Way, the Pistol Star. A star at the center of the Pistol Nebula was first postulated with some confidence in 1990. Early photographs revealed a pistol-like shape. The Pistol Star is a blue variable giant that is 120-200X mass of the 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 also be visible to the naked eye.

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

Data from two of Hubble’s cameras was combined to create this photograph, which showed an image of the Pistol Star hidden behind a vast quantity of thick dust. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which also has 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. The largest shell has a radius of two light years, which is the distance from the Sun to the star nearest our Solar System. 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 200X Sun.

The Pistol Nebula is the most massive (L)uminous (B)lue (V)ariable (N)ebula. Its ejected material completely surrounds the Pistol Star. 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 overall shape of the Pistol Nebula is rectangular with long sides parallel to the magnetic field of the Galactic Center. The Pistol Nebula was ejected in the presence of a strong magnetic field generated at the Galactic Center and powerful solar winds from nearby stars. The side of the Pistol Nebula nearest to us is approaching Earth. Two newly identified spectral emissions north of the Pistol Star are likely the hottest known stars in the GC with temperatures > 50,000 K.

Double Helix 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.

Milky Way GC – Double Helix Nebula
Milky Way GC – Double Helix Nebula / Spitzer / NASA
Photo – M. Morris (UCLA) / NASA/JPL-Caltech / Audriusa, Wikimedia

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 are driven by the massive disc of gas orbiting Sagittarius A*.

Sagittarius A*

Sagittarius A* is 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 caused by heating 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 this photo.

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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.

Sagittarius A* as imaged by NASA’s Chandra Allah Observatory
Data processing: NASA, Chandra / Serendipodous, Wikipedia

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 increase 50X – 180X over a period of up to 3 hours. Very recently, the (V)ery (L)arge (T)elescope, 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.

Sagittarius A* - evolution
Origin and Evolution of Sagittarius A*
Diagram – Chandra / NASA

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. There is a candidate for such an object. Discovered by the European Space Agency Integral satellite telescope, IGR J17456-2901 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.

Milky Way GC – Sagittarius A*
Milky Way – Galactic Center / Sagittarius A* / X-ray image
Photo – G.Garmire (PSU), F. Baganoff (MIT), et al., CXO, NASA

Sagittarius A* is now considered synonymous with the massive black hole at the Galactic Center. 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. 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 with 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 is 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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