During the early 1920s Hubble detected 15 stars in the small, irregular cloudlike object NGC 6822 that varied in luminosity, and he suspected that they might include Cepheids. After considerable effort, he determined that 11 of them were in fact Cepheid variables, with properties indistinguishable from those of normal Cepheids in the Milky Way Galaxy and in the Magellanic Clouds. Their periods ranged from 12 to 64 days, and they were all very faint, much fainter than their Magellanic counterparts. Nevertheless, they fit a P-L relation of the same nature as had been discovered by Leavitt.

Hubble then boldly assumed that the P-L relation was universal and derived an estimate for the distance to NGC 6822, using Shapley’s most recent (1925) version of the calibration of the relation. This calibration was wrong, as is now known, because of the confusion at that time over the nature of Cepheids. Shapley’s calibration included certain Cepheids in globular clusters that subsequent investigators found to have their own fainter P-L relation. (Such Cepheids have been designated Type II Cepheids to distinguish them from the normal variety, which are referred to as Type I.) Thus, Hubble’s distance for NGC 6822 was too small: he calculated a distance of only 700,000 light-years. Today it is recognized that the actual distance is closer to 2,000,000 light-years. In any case, this vast distance—even though underestimated—was large enough to convince Hubble that NGC 6822 must be a remote, separate galaxy, much too far away to be included even in Shapley’s version of the Milky Way Galaxy system. Technically, then, this faint nebula can be considered the first recognized external galaxy. The Magellanic Clouds continued to be regarded simply as appendages to the Milky Way Galaxy, and the other bright nebulae, M31 and M33, were still being studied at the Mount Wilson Observatory. Although Hubble announced his discovery of Cepheids in M31 at a meeting in 1924, he did not complete his research and publish the results for this conspicuous spiral galaxy until five years later.

While the Cepheids made it possible to determine the distance and nature of NGC 6822, some of its other features corroborated the conclusion that it was a separate, distant galaxy. Hubble discovered within it five diffuse nebulae, which are glowing gaseous clouds composed mostly of ionized hydrogen, designated H II regions. (H stands for hydrogen and II indicates that most of it is ionized; H I, by contrast, signifies neutral hydrogen.) He found that these five H II regions had spectra like those of gas clouds in the Milky Way Galaxy system—e.g., the Orion Nebula and Eta Carinae. Calculating their diameters, Hubble ascertained that the sizes of the diffuse nebulae were normal, similar to those of local examples of giant H II regions.

Five other diffuse objects discerned by Hubble were definitely not gaseous nebulae. He compared them with globular clusters (both in the Milky Way Galaxy and in the Magellanic Clouds) and concluded that they were too small and faint to be normal globular clusters. Convinced that they were most likely distant galaxies seen through NGC 6822, he dismissed them from further consideration. Modern studies suggest that Hubble was too hasty. Though probably not true giant globular clusters, these objects are in all likelihood star clusters in the system, fainter, smaller in population, and probably somewhat younger than normal globular clusters.

The Dutch astronomer Jacobus Cornelius Kapteyn showed in the early 20th century that statistical techniques could be used to determine the stellar luminosity function for the solar neighbourhood. (The luminosity function is a curve that shows how many stars there are in a given volume for each different stellar luminosity.) Eager to test the nature of NGC 6822, Hubble counted stars in the galaxy to various brightness limits and found a luminosity function for its brightest stars. When he compared it with Kapteyn’s, the agreement was excellent—another indication that the Cepheids had given about the right distance and that the basic properties of galaxies were fairly uniform. Step by step, Hubble and his contemporaries piled up evidence for the fundamental assumption that has since guided the astronomy of the extragalactic universe, the uniformity of nature. By its bold application, astronomers have moved from a limiting one-galaxy universe to an immense vastness of space populated by billions of galaxies, all grander in size and design than the Milky Way Galaxy system was once thought to be.

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The distance to the Andromeda Nebula

In 1929 Hubble published his epochal paper on M31, the great Andromeda Nebula. Based on 350 photographic plates taken at Mount Wilson, his study provided evidence that M31 is a giant stellar system like the Milky Way Galaxy.

Because M31 is much larger than the field of view of the 152- and 254-cm (60- and 100-inch) telescopes at Mount Wilson, Hubble concentrated on four regions, centred on the nucleus and at various distances along the major axis. The total area studied amounted to less than half the galaxy’s size, and the other unexplored regions remained largely unknown for 50 years. (Modern comprehensive optical studies of M31 have been conducted only since about 1980.)

Hubble pointed out an important and puzzling feature of the resolvability of M31. Its central regions, including the nucleus and diffuse nuclear bulge, were not well resolved into stars, one reason that the true nature of M31 had previously been elusive. However, the outer parts along the spiral arms in particular were resolved into swarms of faint stars, seen superimposed over a structured background of light. Current understanding of this fact is that spiral galaxies typically have central bulges made up exclusively of very old stars, the brightest of which are too faint to be visible on Hubble’s plates. Not until 1944 did the German-born astronomer Walter Baade finally resolve the bulge of M31. Using red-sensitive plates and very long exposures, he managed to detect the brightest red giants of this old population. Out in the arms there exist many young, bright, hot blue stars, and these are easily resolved. The brightest are so luminous that they can be seen even with moderate-sized telescopes.

The most important of Hubble’s discoveries was that of M31’s population of Cepheid variables. Forty of the 50 variables detected turned out to be ordinary Cepheids with periods ranging from 10 to 48 days. A clear relation was found between their periods and luminosities, and the slope of the relation agreed with those for the Magellanic Clouds and NGC 6822. Hubble’s comparison indicated that M31 must be 8.5 times more distant than the Small Magellanic Cloud (SMC), which would imply a distance of two million light-years if the modern SMC distance was used (the 1929 value employed by Hubble was about two times too small). Clearly, M31 must be a distant, large galaxy.

Other features announced in Hubble’s paper were M31’s population of bright, irregular, slowly varying variables. One of the irregulars was exceedingly bright; it is among the most luminous stars in the galaxy and is a prototype of a class of high-luminosity stars now called Hubble-Sandage variables, which are found in many giant galaxies. Eighty-five novae, all behaving very much like those in the Milky Way Galaxy, were also analyzed. Hubble estimated that the true occurrence rate of novae in M31 must be about 30 per year, a figure that was later confirmed by the American astronomer Halton C. Arp in a systematic search.

Hubble found numerous star clusters in M31, especially globular clusters, 140 of which he eventually cataloged. He clinched the argument that M31 was a galaxy similar to the Milky Way Galaxy by calculating its mass and mass density. Using the velocities that had been measured for the inner parts of M31 by spectrographic work, he calculated (on the basis of the distance derived from the Cepheids) that M31’s mass must be about 3.5 billion times that of the Sun. Today astronomers have much better data, which indicate that the galaxy’s true total mass must be at least 100 times greater than Hubble’s value, but even that value clearly showed that M31 is an immense system of stars. Furthermore, Hubble’s estimates of star densities demonstrated that the stars in the outer arm areas of M31 are spread out with about the same density as in the Milky Way Galaxy system in the vicinity of the Sun.