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Sagittarius A

October 27, 2014

We're all suburbanites, since our Solar System is located in the "suburbs" of our Milky Way Galaxy. We're in a region where the average distance between stars is a few light years. The nearest star to us, Proxima Centauri, is 4.24 light years away, which is like acre zoning. The distance between stars near the Galactic Center, is only a light week, so the galactic center is like the city to our suburb.

Figure caption
Our Solar System is in the middle of this boxed region. It's a region that's intermediate between the crush of the urban core, and the desolation of the rural rim. In this case, the word, "desolation," can be considered to be a pun, since "sol" is the Latin word for Sun. (Fermi National Laboratory image, simplified.)

Our knowledge of the structure of our galaxy was limited to the analogy with the nearby Andromeda Galaxy until late in the 20th century. Just after World War II, pioneering radio observations of the Doppler shift of the microwave hydrogen line by Dutch astronomers, Hendrik van de Hulst, Jan Oort and C.A. Muller, elucidated the spiral structure of the Milky Way. As a student, van de Hulst realized that the microwave signature of neutral hydrogen at 1420.40575177 MHz (21.10611405413 cm wavelength) could be detected and act as a means to view through the cosmic dust of the galactic plane.

One interesting radio feature of the Milky Way is an intense radio source at its center. In fact, this radio source, named Sagittarius A since it's in the direction of the constellation, Sagittarius, was the first extraterrestrial radio source detected. This discovery, was made by accident, as are many scientific discoveries, in 1932 by Karl Jansky, a physicist at Bell Laboratories.

Replica of the Jansky radio telescope
This replica of Jansky's radio telescope is found at the National Radio Astronomy Observatory, Green Bank, West Virginia. Photograph via Wikimedia Commons.

Jansky's specific task was to quantify all sources of natural radio interference, such as lightning storms. For these observations, he built a highly-directional antenna to listen at 20.5 MHz (about 14.6 meters wavelength).[2-3] An antenna must have a size comparable to the signal wavelength, so Jansky's antenna was about 100 feet (30.5 meters) long and 20 feet (6 meters) high, and it was designed to rotate. Jansky found a radio source whose period was a sidereal day, which confirmed its extraterrestrial origin.

Jansky's discovery was front page news in the May 5, 1933, New York Times, and it reached the technical literature in the 1933 Proceedings of the IRE (a founding organization of the IEEE).[4] To quote from the IRE paper's abstract,
"Electromagnetic waves of an unknown origin were detected during a series of experiments on atmospherics at high frequencies... the waves come from some source outside the solar system."[4]

Jansky discovery data trace
The discovery data for Sagittarius A from Jansky's radio telescope. Presuming that the copyright was renewed for the IRE journals, Jansky's paper will not be in the public domain until 2028. It is Wikipedia's position that works consisting entirely of information that is common property and containing no original authorship are in the public domain. (See Wikimedia Commons for one version of these data.)[4]

Jansky wanted to do a more precise, follow-up study, but his telephone company employer wasn't interested. The next serious work was done several years later by electrical engineer and amateur radio operator, Grote Reber. From the late 1930s to the end of World War II, Reber was the world's only radio astronomer. Most agree that Jansky, who died in 1950 at age 44, would have won the Nobel Prize had he lived a little longer. Two other Bell Labs physicists, Arno Penzias and Robert W. Wilson, shared the 1978 Nobel Prize in Physics for their discovery of the cosmic microwave background radiation.

Jansky's instrument had an angular resolution of ± 7.5 degrees in one dimension, and ± 30 degrees in the other, so no details of the galactic center could be seen.[4] Larger telescopes and interferometer techniques in much later years showed that the galactic center has three discrete radio emitters. The most intense of these, Sagittarius A* (abbreviated Sgr A*), is thought to be a supermassive black hole sited at the center of the galaxy. It is thought that all spiral galaxies, like our own, and all elliptical galaxies, have supermassive black holes at their centers.

Star map of Sagittarius A by Roberto MuraA busy place.

The galactic center is at a far corner of the Sagittarius constellation, whose boundaries are shown.

(Map by Roberto Mura, cropped, via Wikimedia Commons.)

Figure captioninfrared image of Sagittarius-A* at 300 microarcsecond resolution, produced using adaptive optics at the ESO Very Large Telescope array.

(ESO image by Stefan Gillessen, Reinhard Genzel, and Frank Eisenhauer, via Via Wikimedia Commons. See this press release for details.)[5-7]

Astronomers are still trying to see more deeply into the galactic center. A team of astronomers from the University of California (Santa Barbara, California), the Lebedev Physical Institute of the Russian Academy of Sciences (Moscow, Russia), the Max Planck Institute for Radio Astronomy (Bonn, Germany), and the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), have imaged the galactic center by combining signals from the Very Long Baseline Array and the Green Bank Telescope to create a radio interferometer at a 1.3 cm wavelength and a baseline of 3,000 kilometers.[8-10] One object of these studies is to determine whether the flow of gas is towards, or away from, the supermassive black hole.[10]

The motivation for the study was data obtained with a radio interferometer with a huge baseline of up to 350,000 km formed by the Russian RadioAstron spacecraft, launched in July, 2011, and various ground-based radio telescopes. Some definite substructure was evident in that data, so the ten identical antennas of the Very Long Baseline Array, distributed across the United States, and the 100-meter Green Bank Telescope in Green Bank, West Virginia were used for confirmatory observations. The new data showed the substructure, although the images were extremely faint.[10]

The data show that the emission region is just twenty times the diameter of the presumed event horizon of the black hole. The next follow-up would use the proposed Event Horizon Telescope, a combination of individual telescopes into an interferometer operating at 230-450 GHz.[8-9] Says study coauthor, Carl Gwinn of the University of California, Santa Barbara,
"We're also interested in looking at shorter wavelengths... We might possibly be able to make a simple image of how matter falls into a black hole or is ejected from it. It would be very exciting to produce such an image."[10]

Composite of Sagittarius A* radio imagesComposite of Sagittarius A* radio images from the NRAO Very Large Array (green), BIMA (red) and the NASA Spitzer Space Telescope (blue).

(NRAO/AUI image, via the University of California, Santa Barbara.)


  1. Hugo van Woerden and Richard G. Strom, "The Beginnings of Radio Astronomy on The Netherlands," Journal of Astronomical History and Heritage, vol. 9, no. 1 (2006), pp. 3-20 (PDF File).
  2. C. M. Jansky, Jr., "My Brother Karl Jansky and His Discovery of Radio Waves from Beyond the Earth." March 23, 1956 (Reprinted in Cosmic Search, vol. 1, no. 4).
  3. "Karl Jansky and the Discovery of Cosmic Radio Waves," National Radio Astronomy Observatory, May 16, 2008.
  4. K.G. Jansky, "Electrical Disturbances Apparently of Extraterrestrial Origin," Proceedings of the Institute of Radio Engineers, vol. 21, no. 10 (October, 1933), pp. 1387-1398, DOI: 10.1109/JRPROC.1933.227458.
  5. S. Gillessen, F. Eisenhauer, S. Trippe, T. Alexander, R. Genzel, F. Martins, and T. Ott, "Monitoring stellar orbits around the Massive Black Hole in the Galactic Center," The Astrophysical Journal, vol. 692, no. 2 (February 20, 2009), pp. 1075ff., doi:10.1088/0004-637X/692/2/1075.
  6. S. Gillessen, F. Eisenhauer, S. Trippe, T. Alexander, R. Genzel, F. Martins, and T. Ott, "Monitoring stellar orbits around the Massive Black Hole in the Galactic Center," arXiv, October 28, 2008.
  7. Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole, European Southern Observatory Press Release No, eso0846, December 10, 2008.
  8. C.R. Gwinn, Y.Y. Kovalev, M.D. Johnson, and V.A. Soglasnov, "Discovery of Substructure in the Scatter-Broadened Image of Sgr A*," The Astrophysical Journal Letters, vol. 794, no. 1 (October 10, 2014), L14, doi:10.1088/2041-8205/794/1/L14.
  9. C.R. Gwinn, Y.Y. Kovalev, M.D. Johnson, and V.A. Soglasnov, "Discovery of Substructure in the Scatter-Broadened Image of Sgr A*," arXiv, September 11, 2014.
  10. Julie Cohen, "Inside the Milky Way - UCSB astrophysicist uses data gathered by a Russian spacecraft to bring science one step closer to figuring out the mysteries of our galaxy’s core," University of California, Santa Barbara, Press Release, October 13, 2014.

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