Galactic Center

• 1. Discovery And Observation History

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  Immanuel Kant wrote in 1755 that a large star sat at the center of the Milky Way galaxy. He suggested Sirius might be that central star, though he lacked proof. Harlow Shapley stated in 1918 that globular clusters surrounded the Milky Way seemed centered on Sagittarius. Dark molecular clouds blocked optical views of this region for decades. Walter Baade used wartime blackout conditions near Los Angeles to search for the center in the early 1940s. He found a one-degree-wide void in interstellar dust lanes near Alnasl. This gap became known as Baade's Window and allowed clear views of swarms around the nucleus. Joseph Lade Pawsey led a team from the Division of Radiophysics at CSIRO in Sydney. They built an fixed dish antenna by 1954 to study radio emission in Sagittarius. The team named an intense point-source Sagittarius A and realized it lay at the very center of the Galaxy. In 1958 the International Astronomical Union adopted Sagittarius A as the true zero coordinate point for galactic latitude and longitude.

• 2. Distance And Structural Geometry

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  Estimates since 2000 place the distance between the Solar System and the Galactic Center within a specific range. Geometric-based methods and standard candles yield distances ranging from roughly 7.6 to 8.3 kiloparsecs. Variable stars like RR Lyrae variables complicate accurate determination due to ambiguous reddening laws. Interstellar extinction biases sampling toward the near side of the Galactic bulge. The Milky Way features a bar extending across the Galactic Center with debated half-lengths spanning 1 to 5 kiloparsecs. Orientation estimates vary between 10 and 50 degrees relative to our line of sight. Red-clump stars delineate this bar structure while RR Lyrae variables do not trace it prominently. Some authors advocate that two distinct bars exist, one nested inside another. A ring called the 5-kpc ring surrounds the bar and contains most molecular hydrogen. This ring holds the majority of star formation activity in the entire galaxy. Viewed from the Andromeda Galaxy, this feature would appear as the brightest part of the Milky Way.

• 3. Sagittarius A* Black Hole Physics

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  The complex astronomical radio source Sagittarius A appears almost exactly at the Galactic Center. It contains an intense compact radio source named Sagittarius A*. This coincides with a supermassive black hole at the center of the Milky Way. Accretion of gas onto the black hole releases energy to power the radio source. A study in 2008 measured the diameter of Sagittarius A to be 44 million kilometers. This size is slightly less than the distance from Mercury to the Sun at closest approach. Scientists at the Max Planck Institute for Extraterrestrial Physics confirmed the existence of a supermassive black hole on the order of 4.3 million solar masses. Later studies estimated a mass of 3.7 million or 4.1 million solar masses. On the 5th of January 2015 NASA reported observing an X-ray flare 400 times brighter than usual. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole. Alternatively magnetic field lines within flowing gas could have become entangled near Sagittarius A.

• 4. Fermi Bubbles And Plasma Structures

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  In November 2010 two large elliptical lobe structures of energetic plasma were detected astride the Milky Way galaxy's core. These bubbles emit gamma- and X-rays and extend up to about 25,000 light years above and below the Galactic Center. The discovery team led by D. Finkbeiner worked around the galaxy's diffuse gamma-ray fog. They received the 2014 Bruno Rossi Prize for discovering this large unanticipated Galactic structure called the Fermi bubbles. The bubbles are connected via energy transport to the galactic core by columnar structures termed chimneys. In 2020 the lobes were seen in visible light for the first time. Optical measurements confirmed their presence and shape. By 2022 detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole. Prior observations had been hampered by the galaxy's diffuse gamma-ray fog which obscured these massive structures.

• 5. Stellar Population Paradoxes

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  The central cubic parsec around Sagittarius A* contains around 10 million stars. Most of them are old red giant stars yet the region is also rich in massive stars. More than 100 OB and Wolf, Rayet stars have been identified there so far. They seem to have all formed in a single star formation event a few million years ago. Experts expected tidal forces from the central black hole to prevent such formation. This paradox of youth applies even more strongly to stars on very tight orbits like S2 and S0-102. Current evidence favors star formation within a massive compact gas accretion disk around the central black-hole. Most of these 100 young massive stars appear concentrated within one or two disks rather than randomly distributed. Star formation does not seem to be occurring currently at the Galactic Center though the Circumnuclear Disk remains favorable. Work presented in 2002 by Antony Stark and Chris Martin mapped gas density revealing an accumulating ring with mass several million times that of the Sun. They predict an episode of starburst in approximately 200 million years when many stars form rapidly. Theoretical models predicted old stars should show a steeply-rising density near the black hole known as a Bahcall-Wolf cusp. Instead it was discovered in 2009 that the density peaks roughly 0.5 parsec from Sgr A* then falls inward.