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This chapter of the video
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introduces Vera Rubin and her measurements of the orbital speeds of stars in Andromeda.
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explains that Rubin expected the orbital speeds of the outer stars
to decrease the farther the stars were from
the centre of Andromeda (like the orbital speeds of planets in the Solar System).
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shows that, instead, the orbital speeds remained constant with
distance and were higher than expected.
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explains that, over time, Rubin's observations led physicists to rethink
the composition of the entire universe. |
Historical Background
Vera Rubin is an astronomer from Washington, D.C. Between 1967 and 1969 she, with assistance from her colleague Kent Ford, used two telescopes (at Kitt Peak and Lowell Observatory, both in the U.S.A.) to observe the orbital speeds of stars within the Andromeda galaxy.
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Although other astronomers had measured the orbital speeds of stars in various galaxies before, what made
Rubin's observations unique was the technology she used. Ford had recently built a highly sensitive spectrometer able to collect data from the faint outer regions of galaxies. This allowed Rubin to observe phenomena previously inaccessible to astronomers. The orbital speeds of the planets in the Solar System decrease the farther they are from the Sun and so Rubin expected to see a similar decrease in the orbital speeds of stars in Andromeda. Instead, the observed orbital speeds were as shown in Figure 6.
In the outer region of the galaxy, the orbital speeds were constant at around
225 km/s as far out as Rubin could measure. She also made observations in the inner region, which we do not include in the video. Here, she saw the orbital speeds increase linearly. We did not include this data as the pattern it exhibits is what we would expect in the absence of dark matter. Thus, the inner data does not provide compelling evidence for dark matter.
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Earlier Evidence for Dark Matter
Although Rubin's observations in the 1960s led to dark matter entering mainstream physics,tentative evidence for dark matter existed much earlier. In 1933, Swiss-born astronomer Fritz Zwicky examined the speeds of individual galaxies within the Coma Cluster of galaxies and found that they were so high they exceeded the cluster's escape velocity. This meant that the cluster should have been unstable and falling apart, when clearly it was not. Zwicky concluded that there must be a vast amount of unseen mass within the cluster holdingit together via gravity. However, Zwicky's data contained large uncertainties and other physicists were skeptical.
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