Galactic Center

The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, part of which is a very compact radio source arising from a bright spot in the region around the black hole, near the event horizon. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Lambda Scorpii, south to the Pipe Nebula. There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf–Rayet stars from star formation in the region around 1 million years ago. The core stars are a small part within the much wider central region, called the galactic bulge.

Discovery

Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft (low-energy) X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard (high-energy) X-ray, infrared, submillimetre, and radio wavelengths. Immanuel Kant stated in Universal Natural History and Theory of the Heavens (1755) that a large star was at the center of the Milky Way galaxy, and that Sirius might be the star. Harlow Shapley stated in 1918 that the halo of globular clusters surrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the dark molecular clouds in the area blocked the view for optical astronomy. In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime blackout conditions in nearby Los Angeles, to conduct a search for the center with the 100-inch (250 cm) Hooker Telescope. He found that near the star Alnasl (Gamma Sagittarii), there is a one-degree-wide void in the interstellar dust lanes, which provides a relatively clear view of the swarms of stars around the nucleus of the Milky Way galaxy. This gap has been known as Baade's Window ever since. At Dover Heights in Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at the CSIRO, led by Joseph Lade Pawsey, used sea interferometry to discover some of the first interstellar and intergalactic radio sources, including Taurus A, Virgo A and Centaurus A. By 1954 they had built an 80-foot (24 m) fixed dish antenna and used it to make a detailed study of an extended, extremely powerful belt of radio emission that was detected in Sagittarius. They named an intense point-source near the center of this belt Sagittarius A, and realised that it was located at the very center of the Galaxy, despite being some 32 degrees south-west of the conjectured Galactic Center of the time. In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero coordinate point for the system of galactic latitude and longitude. In the equatorial coordinate system the location is: RA 17h 45m 40.04s, Dec −29° 00′ 28.1″ (J2000 epoch). In July 2022, astronomers reported the discovery of massive amounts of prebiotic molecules, including some associated with RNA, in the Galactic Center of the Milky Way galaxy.

Distance to the Galactic Center

The exact distance between the Solar System and the Galactic Center is not certain, although estimates since 2000 have remained within the range 24–28.4 kilolight-years (7.4–8.7 kiloparsecs). The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:

7.4±0.2(stat) ± 0.2(syst) or 7.4±0.3 kpc (≈24±1 kly) 7.62±0.32 kpc (≈24.8±1 kly) 7.7±0.7 kpc (≈25.1±2.3 kly) 7.94 or 8.0±0.5 kpc (≈26±1.6 kly) 7.98±0.15(stat) ± 0.20(syst) or 8.0±0.25 kpc (≈26±0.8 kly) 8.33±0.35 kpc (≈27±1.1 kly) 8.0±0.3 kpc (≈25.96±0.98 kly) 8.7±0.5 kpc (≈28.4±1.6 kly) 8.122±0.031 kpc (≈26.49±0.1 kly) 8.178±0.013(stat) ± 0.022(syst) kpc (≈26.67±0.1 kly) An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by numerous effects, which include an ambiguous reddening law, a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction, and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center. The nature of the Milky Way's bar, which extends across the Galactic Center, is also actively debated, with estimates for its half-length and orientation spanning between 1–5 kpc (short or a long bar) and 10–50°. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. The bar is delineated by red-clump stars (see also red giant); however, RR Lyrae variables do not trace a prominent Galactic bar. The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.

Supermassive black hole

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