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How You Can Weigh Black Holes

Messier 81 (also known as NGC 3031 or Bode’s Galaxy) is a barred spiral galaxy in the constellation Ursa Major 11.4 million light years away. The pitch angle of its spiral arms is 13.4°, which correlates with the black hole at its centre being 67.6 million times more massive than our Sun. Credit: © Ken Crawford http://www.imagingdeepsky.com/Galaxies/
M81/M81.htm

Messier 81 (also known as NGC 3031 or Bode’s Galaxy) is a barred spiral galaxy in the constellation Ursa Major 11.4 million light years away. The pitch angle of its spiral arms is 13.4°, which correlates with the black hole at its centre being 67.6 million times more massive than our Sun. Credit: © Ken Crawford http://www.imagingdeepsky.com/Galaxies/ M81/M81.htm

By Ben Davis

The largest invisible monsters in our universe are hidden at the centres of galaxies, and we can predict how massive they are by the shape of spiral arms in their host galaxies. Here’s how you can take part in a global “citizen science” census of black holes.

The full text of this article can be purchased from Informit.

Nearly a century ago, Sir James Jeans and Edwin Hubble noted that spiral galaxies with large central bulges possess tightly-wound spiral arms, while spiral galaxies with small bulges display wide-open spiral arms. Their studies have defined the classification scheme of galaxies famously represented in the “tuning fork” diagram (Fig. 1).

For nearly 100 years, astronomers have been classifying spiral galaxies by eye and denoting their “Hubble type”, with the tightness of winding of spiral arms denoted by a lowercase letter (e.g. “Sa” for galaxies with large bulges and tightly-wound spiral arms). This is a qualitative description of the winding rate of spiral galaxies, and is a bit like labelling automobiles into a few descriptive categories according to their fuel economy: efficient, mediocre and gas-guzzler. It is a helpful description, but before you invest thousands of dollars in a vehicle you might want to know the specific measurement of a vehicle’s average fuel economy in some quantifiable unit like kilometres per litre.

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