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Jai Ganesh

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Registered: 2005-06-28

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Galactic Center

Galactic Center

Details

Our Solar System is just one of hundreds of billions of similar solar systems that form the swirling disk of our Milky Way Galaxy. We live in our Galaxy's suburbs and, like all our neighbors, we hurtle through space under the influence of the summed gravity of all the other stars, gas, dust, and mysterious dark matter that together form the Galaxy. In the very middle of this gravitational whirlpool is the strange region known as the “Galactic Center.”

Finding the Center

The centers of other large galaxies are generally easy to spot. Located inside of the swirling spiral arms, they tend to greatly outshine the rest of their galaxies, thanks to the very large number of stars that are packed in there.

Stars in the Galactic Center are so concentrated that they typically are only a few light weeks away from each other. In contrast, our local neighborhood of stars are separated from one another by light years. If we found ourselves on a planet near the Galactic Center, our nighttime sky would light up in a blazing display every night, filled with stars as bright as the planet Venus looks to us.

Galactic Center & The Sun

One might then conclude that the center of our Galaxy should be easy to spot. However, there’s a problem – it is essentially hidden from us in visible light. This is because we live out in the disk of the Milky Way – a disk that is filled with clouds of dust that form a kind of interstellar “smog.” The dust filters out the visible light from distant parts of the galaxy and so the center simply cannot be seen even when we are looking right at it.

Astronomers did not begin to figure out where the center was hiding until the early 20th century. Harlow Shapley made a pretty good deduction of its general location by looking at the way star clusters in the rest of the Galaxy were distributed. He concluded it was towards the constellation of Sagittarius, but it took a revolution in infrared telescopes before we finally could precisely see the region.

In infrared light the view through the galactic plane becomes stunningly different. Dust becomes virtually transparent at these longer wavelengths, and the “smog” fades away to reveal the bright Galactic Center. If we humans had infrared eyes, even the most ancient stargazing cultures would have known exactly where it was located.

About the Galactic Center

Our Galaxy’s center falls within the constellation of Sagittarius, which can best be seen at night during the month of August. It is about 27,000 light years away from us, which places us about halfway out in the Milky Way’s disk. A bulge of stars surrounds the center, blending with a straight bar-like glow of stars that connects the center region to the spiral arms of our Galaxy.

The appearance of the Galactic Center changes depending on how far into the infrared spectrum we push our observations. Pictures taken by 2MASS are in near-infrared wavelengths, and show how the dust has started to become transparent, allowing us to see more and more of the starlight. Moving to longer mid- and far-infrared wavelengths, taken with the Spitzer and Herschel space telescopes, the stars become faint, but the dust itself begins to glow.

At wavelengths of around 5 to 15 microns (mid infrared), this glow is largely produced by the fluorescence of dust grains containing loops of carbon atoms, glowing under the light of stars. From 20 to 50 microns (mid to far infrared), we are instead seeing the thermal blackbody glow from the warmer clouds of dust heated by the light of newborn stars. At longer wavelengths of 100 microns or more (far infrared), we begin to see the thermal glow of the very coldest dust, only a dozen or so degrees above absolute zero.

Far infrared light is the only good tool we have for observing these coldest clouds of dust in the inner Galaxy. Seeing the full distribution of dust, which is also intermixed with gas, helps us understand more about our Galaxy’s structure. For instance, astronomers studying the dust patterns at wavelengths of 70 to 350 microns identified an odd, cold dust ring that is about 300 light years across. Curiously, it is in a figure-8 shape, suggesting the ring is slightly twisted. These kinds of warps can be caused by gravitational interactions between galaxies as they pass one another or merge, hinting at interesting close encounters in the Milky Way’s recent history.

Additional Information

The Galactic Center is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. 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 Shaula, 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 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.

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