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Observational astronomers George Slipher from the University of Arizona and Edwin Hubble from Cal Tech attached spectrophotometers to telescopes and calculated the speeds of galaxies. They discovered that most galaxies are moving away from us. Hubble used the brightness of Henrietta Leavitt's Cepheid variable stars as a standard candle and calculated the distances of those galaxies.
Figure 1‑31. V.M. Slipher, Lowell Observatory astronomer from 1901 to 1954. Credit: Lowell Observatory (unknown photographer) Used here per CC BY-SA 4.0.
The Clark telescope in Arizona and the Hooker telescope in California revealed the expanding universe and led to the discovery of the Big Bang model. In the late 19th century, Lowell Observatory director Percival Lowell contracted Alvin Clark to build the 24-inch diameter Clark telescope (Figure 1‑32) in Flagstaff, Arizona. Percival Lowell first used the telescope to detect what he thought were canals on Mars. Although this supposed evidence of intelligent life on Mars was incorrect, the Clark Telescope is most famous for Vesto Slipher's observations (1910s) of galactic movement. Slipher (1875-1969) attached a spectrograph to the Clark telescope, and thus began the third revolution in astronomy. With his spectroscope, Slipher (Figure 1‑31) discovered that the light from most galaxies was shifted toward the red. A spectrograph divides the light into wavelengths like a prism, but with greater precision. Slipher placed photographic plates under the spectrograph, which revealed the shifted positions of hydrogen absorption lines. He used these shifts to calculate the redshift or blueshift of light and consequently the velocity of galaxies.
Figure 1‑32. Clark telescope used by Vesto Slipher in Flagstaff, Arizona - installed in 1896. The telescope rotates and tilts, and the dome rotates on automobile tires. Credit: Emery Littlefield. Used here per CC BY-SA 4.0.
In 1917, Cal Tech built the 100-inch diameter Hooker telescope (Figure 1‑33) on Mt. Wilson, which is north of Los Angeles. The diameter was larger than previous 60 inch telescopes and four times larger than the Clark telescope. Edwin Hubble and other Cal Tech astronomers could view stars and galaxies at 24 X greater magnification than any previous telescope. Hubble used the Hooker telescope to see much farther out into space and observe much larger redshifts than Slipher’s telescope.
Figure 1‑33. The 100 inch diameter Hooker telescope used by Edwin Hubble on Mt Wilson in southern California. Credit: Ken Spencer. Used here per CC BY-SA 3.0.
Slipher observed a blueshift in the nearby Andromeda Galaxy, which indicated that the galaxy was approaching Earth at 300 km/sec .[1] This speed was much faster than the speed of stars in the Milky Way. Slipher thought that this speed was unbelievable but stated that it was the only interpretation of the data. The reason that the Andromeda Galaxy is moving toward us is that it is part of our Local Group of galaxies, which are bound by gravity to each other. The Milky Way and the Andromeda Galaxy will crash in 4.5 billion years.
Slipher analyzed 25 nebulae and found that 21 out of 25 nebulae were moving away from us (redshifts). One galaxy in the Virgo cluster (Figure 1‑29) was moving away from us at 1,100 km/second.[2] The average velocity (500 km/sec) was about 25 times as large as typical star velocities. Slipher thus concluded that the spiral nebulae were not within the Milky Way but were distant galaxies, and he endorsed Kant’s island universe theory – the Milky Way is just one island universe in a sea of other island universes (galaxies).
“It has for a long time been suggested that the spiral nebulae are stellar systems (galaxies) seen at great distances. This is the so-called “island universe” theory, which regards our stellar system and the Milky Way as a great spiral nebula, which we see from within. This theory seems to gain favor in the present observations.” [3]
In 1917, Cal Tech built the 100-inch diameter Hooker telescope (Figure 1‑33) on Mt. Wilson, which is north of Los Angeles. The diameter was much larger than the previous 60-inch telescope on Mt Wilson and four times larger than the Clark telescope. Edwin Hubble and other Cal Tech astronomers could view stars and galaxies at 24 X greater magnification than any previous telescope. Hubble used the Hooker telescope to see much farther out into space and observe much larger redshifts than Slipher’s telescope.
Figure 1‑34. Cepheid variable star in Andromeda Galaxy (above). Credit: NASA
Slipher did not determine the distances to galaxies. Hubble used Henrietta Leavitt’s Cepheid variable stars (Figure 1‑34) to determine distances. The Cepheid variable stars have a period-luminosity relationship. This meant that Hubble could calculate their intrinsic brightness based on their pulsation rate. By comparing the observed brightness to their intrinsic brightness, Hubble used them as a “standard candle” to determine the distances to galaxies.
Figure 1‑35. Velocity vs. redshift of “nearby” galaxies. Credit Brews Ohare. Used here per CC BY-SA 3.0
The slope of the line in Figure 1‑35 is the Hubble constant (Ho), which is the ratio between galactic velocity (km/s) and distance (Megaparsecs) (Equation 1). The current estimate of the Hubble constant varies from 68 to 72 km/s /Mpc.
Although the relationship between galactic velocity and distance is now called Hubble's Law, Lemaitre was the first to realize that Hubble’s data implied this relationship, which was the basis of Lemaitre's eventual conception of the Big Bang model of the universe. Lemaitre published this relationship in an obscure French journal, but he did not argue when the credit was given to Hubble. Hubble had previously welcomed Lemaitre on a visit to the Hooker telescope. This type of data sharing and respect by observational and theoretical astronomers is a good example of how scientists should work together. Observational astronomers often do not have the mathematical background to derive theoretical relationships from data. On the other hand, theoretical astronomers must respect the work involved in collecting data and give credit, whenever possible, to observational astronomers. This type of collaboration is essential to the advancement of science.
There was a mistake in the initial estimate of the galactic distance/velocity relationship. Scientists did not realize that there were two types of Cepheid variable stars, and that the stars observed in distant galaxies were not the same type of Cepheid variable stars that Henrietta Leavitt observed in our galaxy. Thus, the initial estimate of the Hubble constant was in the range of 600 km/sec/Mpc. Lemaitre calculated the age of the universe as 1 to 2 billion years because of the erroneously high Hubble constant (expansion rate); however, radiometric dating had already shown that Earth is in the range of 4 billion years old. This discrepancy discredited Lemaitre’s Big Bang model. By the 1950s, scientists realized that there were two types of Cepheid variable stars and estimated the Hubble constant in the range of 50 to 90 km/s / Mpc. Improved measurements have narrowed the range to 68 to 72 km/s / Mpc. One measurement technique indicates that Ho = 68 km/s / Mpc, and another indicates that Ho = 72 km/s / Mpc. The refined estimate of the Hubble constant (Ho) is the basis for the calculated age of the universe being 13.7 billion years. With either measurement, the calculated age of the universe is slightly older than the Milky Way, which is 13 billion years old based on fraction of metals in stars and associated chemical evolution by stellar nucleosynthesis. Another indication of the age of the Milky Way is that the oldest globular clusters in the Milky Way are approximately 12 billion years old, based on the maximum sizes of stars remaining in the clusters. Thus, the age of the universe based on redshift now agrees with the estimated ages of the objects in the galaxy.
References
References
[1] Slipher, Vesto. Spectrographic Observations of Nebulae, American Astronomical Society. Report of the 17th Meeting. 22-24, Accessed at < http://www.roe.ac.uk/~jap/slipher/slipher_1915.pdf >
[2] Slipher, Vesto. Nebulae. Proceedings of the American Philosophical Society. 56 (1917), 405, Accessed at < http://www.roe.ac.uk/~jap/slipher/slipher_1917.pdf >
[3] Slipher, Spectrographic, 409.
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Vesto Slipher attached this spectrograph to the Clark telescope in order to measure redshift. Credit: Brewbooks. Used here per CC SA 2.0
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