Edwin Hubble (1889-1953) was an American astronomer born in Missouri, like Harlow Shapley. He was educated at the University of Chicago and, as a Rhodes Scholar, Oxford University in England. Most of his significant scientific contributions were made at Mt. Wilson Observatory in the San Gabriel Mountains above the Los Angeles basin. He is best remembered for three things:
(1) He settled the great debate by discovering Cepheid variables in nearby spiral nebulae. The distances to these Cepheids (and, hence the nebulae) convincingly established that spiral nebulae are external galaxies.
(2) He classified galaxies into four major types (two types of spirals, elliptical, and irregular).
(3) He discovered that the spectra of all external galaxies, with the exception of the very nearest, are red-shifted. Furthermore, the greater the distance, the greater the red-shift. This implies that that the universe is expanding. The slope of the graph plotting distance to galaxies vs. their red-shift computed velocities is known as H, the Hubble constant. The reciprocal of the Hubble constant can be interpreted as the maximum age of the universe.
· In 1925 Hubble published a paper in which he announced his discovery of Cepheid variables in two nearby spiral nebulae, M33 and M31. Using Shapley’s distance calibration curve, he estimated these two nebulae to be about one million light years away. At this distance the only conclusion to draw was that these spirals must be external galaxies similar to the Milky Way.
· Hubble’s results convinced most astronomers that the spirals were external galaxies. However, three bits of data still needed to be explained: (1) van Maanen’s rotation data, (2) the zone of avoidance, and (3) the extremely luminous nova in M31 observed in 1885.
· By 1935 Hubble and others had examined van Maanen’s data and concluded that no rotation was visible within experimental error. Two papers published in 1935, one by Hubble and another by van Maanen, effectively ended the argument. Hubble gave his conclusions and van Maanen more or less agreed. Thus, rapid rotation ceased to be an issue.
· In the decade following Hubble’s 1925 paper it became increasingly obvious that interstellar space is filled with obscuring matter. In 1930, for example, Robert J. Trumpler of Lick observatory published convincing evidence that clusters in the plane of the Milky Way were dimmed by the presence of dust and gas. These data led most astronomers to believe that the zone of avoidance arose, not because no spiral nebulae were present in this region, but because they were blocked from view by obscuring matter.
· The presence of obscuring matter also effectively reduced the size of the Milky Way. Shapley had assumed that obscuring matter was insignificant, so the dimness of the globular clusters was due entirely to distance. However, with obscuring matter factored in, part of the reduction in brightness can be attributed to obscuring matter. Thus, the globular clusters were closer than Shapley estimated, making the galaxy smaller in the process. We now estimate the Milky Way to be approximately 100,000 light years in diameter, about 1/3 of the value supported by Shapley and about 3 times the value supported by Curtis in the “”great debate.
·
What about the bright 1885 nova seen in M31? The findings of nuclear physics from the
1930s onward were incorporated into our understanding of stellar energy
production and evolution. Using these
models, astronomers were able to show convincingly that, at the end of their
lives, massive stars do explode with such violence that their luminosities can,
indeed, exceed the combined luminosities of billions of ordinary stars. Such objects are known as supernovae,
and they are commonly observed in external galaxies.
·
To summarize the great debate: Both Shapley and Curtis
were partly right and partly wrong.
Shapley’s placement of the sun far from the galactic center is now
accepted as correct. However, his
dimensions for the Milky Way were too large by a factor of three. On the other hand, we now accept as correct
Curtis’ view that the spiral nebulae are external galaxies. But Curtis was also off by a factor of three
concerning the size of the Milky Way in the opposite direction form
Shapley. Curtis’ number was three times
too small!