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Exploring the Galactic Center Exploring the V Galactic Center Goinyk/Shutterstock.com olodymyr A case study of how USRA answered a question posed by James Webb in 1966. On 14 January 1966, NASA (5) C r e Administrator James Webb (1906-1992) wrote a letter d i Should we change the t : N to the prominent Harvard physicist, Professor Norman A orientation of some of S F. Ramsey Jr. (1915-2011), asking him to establish an A our NASA Centers? advisory group that would: (6) What steps should Review the resources at our NASA field centers, and James Webb be taken in scientific such other institutions as would be appropriate, staffing, both inside and against the requirements of the next generation of outside NASA, over the next few years to assure that space projects and advise NASA on a number of key we have the proper people at the proper places to do problems, such as: the job? (1) How can we organize these major projects so that (7) How can we obtain the competent scientists to the most competent scientists and engineers can take the key roles in these major projects? 1 participate? Ramsey assembled his advisory group, and they (2) How can academic personnel participate and at worked through the spring and summer on their report, the same time continue in strong academic roles? which they delivered to the Administrator on 15 August 1966. Their first recommendation was that the NASA (3) What mechanism should be used to determine Administrator appoint a General Advisory Committee the scientific investigations which should be to bring to bear “maximum competence” on “the conducted? formulation and execution of long-term programs of NASA.”2 (4) How does a scientist continue his career development during the six to eight years it requires This recommendation, and many of the others in the to develop an ABL [Automated Biological Laboratory] report, were not what NASA was looking for, and so or a large astronomical facility? the Administrator turned to the National Academy of Sciences to find answers for at least some of the Infrared radiation gets Cr ed i t: A questions posed to Ramsey. The result of the extended through the dust, because m e r i deliberations between the Academy and the university its wavelengths are c a n 3 I n community was the formation of USRA. longer… s t i t u t e o f P At about the time that James Webb approached the h It was in August of 1966. y s i c president of the National Academy of Sciences with I was up at Mt. Wilson. s questions on how best to involve university students It was a beautiful night Eric Becklin and faculty in NASA’s research programs, a graduate on the small 24-inch student at Caltech discovered the location of the center telescope. And, as we were of the Milky Way Galaxy. The student was Eric Becklin, looking with the infrared detector, we were seeing and his thesis advisor was Professor Gerry Neugebauer more and more stars. (1932-2014), who was one of the pioneers of infrared astronomy. This is the signal in the infrared, and each star gives you more signal, and we were building up, as we Becklin and Neugebauer had used an infrared were getting closer to the center, more and more photometer with the 24-inch telescope on Mt. Wilson stars. And we were actually seeing through the dust, to scan the small region of the sky in the Sagittarius for the first time, and then came to a peak, and then constellation that was thought to be the dynamical back down again, and I knew, immediately, that that center of the rotating Milky Way Galaxy. Becklin later was the center of our Milky Way, and that I was the told the story of his discovery: first person to actually see the stars in the very core of our galaxy.4 Back then, people weren’t even sure where the center was. There was The photons recorded by Becklin some vague understanding. had a wavelength of 2.2 There was a radio source microns (µ), also written called “Sagittarius A,” 2.2 micrometers a very strong radio (µm), which is 2.2 source, but there x 10-6 meters. was even debate This wavelength whether that is in the “near was really the infrared” center or not. part of the electromagnetic There is so spectrum, i.e., much dust near to the between us visible range, and the galactic which runs from center, it is about 0.38 microns completely opaque. (violet light) to 0.75 A strip-chart recording of a right-ascension scan You do not see the of the galactic center region at 2.2 µ. (From Fig. 2 microns (red light). The stars in the galactic of Becklin and Neugebauer, 1968, p. 147.) “mid infrared” part of the center. The most powerful electromagnetic spectrum telescopes cannot see it. corresponds to photons with longer wavelengths, and the “far infrared” DEVelopments IN Space Research 2 HIGHLIGHTS OF USRA’S ACHIEVEMENTS, Part 2 Credit: Erin Senoz/USRA Most of the Infrared spectrum is absorbed by the atmospheric gasses so the wavelength is best observed from space, Gamma or by a flying observatory. Rays, Near-Infrared Mid-Infrared Far-Infrared X-Rays and Ultraviolet Wavelength Range Wavelength Range Wavelength Range Light are (microns) (microns) (microns) blocked by (0.7-1) to 5 5 to (25-40) (25-40) to (200-350) the upper Temperature Range Temperature Range Temperature Range atmosphere (degrees Kelvin) (degrees Kelvin) (degrees Kelvin) and are best 740 to (3,000 - 5,200) (92.5-140) to 740 (10.6-18.5) to (92.5-140) observed from What We See What We See What We See space. and NASA image courtesyInset of IPAC Cooler red stars Planets, comets and asteroids Emission from cold dust Red giants Dust warmed by starlight Central regions of galaxies Dust is transparent Protoplanetary disks Very cold molecular clouds Visible Light is observable from Earth Short-wavelength radio waves with some atmospheric distortion. are observable from Earth. with still longer wavelengths. Near infrared radiation coming from 10 microns. In 1975, Becklin and The accompanying table shows a distant source in the universe is Neugebauer published a paper5 the infrared spectral regions, not affected by intervening dust. As in which they used an infrared wavelength ranges, temperatures can be seen from an all-sky mosaic photometer with the 200 inch of the sources of the radiation, of the Milky Way shown below, this Hale telescope at the Palomar and what can be seen in the three is not true of the visible light, which Observatory. In the article, they ranges. is blocked by dust. showed maps of the central 1 minute of arc of the Milky Way Earth’s atmosphere absorbs Eric Becklin had located the Galaxy with resolutions of a few photons in much of the infrared center of our galaxy, and he and seconds of arc6 . In this central 1 wavelengths. As shown in the Neugebauer at Caltech, as well minute of arc, they identified 19 accompanying figure, however, the as other researchers elsewhere, sources of infrared radiation at atmosphere is partially transparent began to map the central region 2.2 microns and 9 sources at 10 to the wavelength that Becklin used of the Galaxy in wavelengths that microns. at the Mt. Wilson Observatory, i.e., corresponded to atmospheric at 2.2 microns. “windows,” e.g., 2.2 microns and Becklin and his colleagues were beginning to see the structures at the center of the Milky Way Galaxy, but they knew that to determine the nature of the sources of radiation they would need to make high- resolution observations at longer infrared wavelengths. Unfortunately, the Earth’s atmosphere is mostly opaque Kuiper Airborne Observatory at these wavelengths. The solution to this problem was to make observations from the Earth’s stratosphere, above 99% of the atmospheric water vapor, which causes the blockage of far infrared radiation. By 1975, NASA had developed a means of doing that with the Gerard P. Kuiper Airborne Observatory (KAO). The KAO was a modified Lockheed C-141 military cargo plane, outfitted with a 36-inch reflecting telescope in the front part of the plane. A retractable door above the telescope was closed for landings and take offs and opened only when the plane reached the desired altitude for observation, normally between 41,000 and 45,000 ft. Becklin and his colleagues at Caltech soon were able to schedule flights on the KAO, and in 1975 they viewed the Galactic center at a resolution of about 1 minute (60 seconds) of arc. The Caltech team measured the flux of infrared radiation in three wavelength bands simultaneously, 30, 50, and 100-microns, from a single field of view. From this data they mapped the far infrared surface brightness and the color temperature7 Credit Serge Brunier of The World at Night. Credit Serge Brunier of The World Figure 2 in Becklin et al., 1982, p. 137. The cross denotes the centroid of the galactic center emissions at 34 microns as derived from ground-based measurements. An all-sky mosaic of the Milky Way recorded from Earth-based observatories over several months of 2008 and 2009. (Courtesy of the European Southern Observatory (ESO), Serge Brunier and Frederick Tapissier.) The image is oriented so that the plane of the Milky Way is horizontal, and the bulge of the Galactic Center is in the center. group of researchers from Cornell University on KAO flights in 1995.11 They found a “minispiral” structure surrounded by a circumnuclear ring of dust and gas at the center of the Galaxy.
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