"A Setback for the Extragalactic Distance Scale?"
News Notes
The News and Notes section of October's edition of Sky and Telescope had an interesting article about how the newly fixed Hubble Space Telescope measures t 20520l1114u he distance the universe is expanding. "Nearly a decade of arduous observing, data processing, and analysis have led the participants in the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale to their recent, headline-making conclusion: the Hubble constant (H0), which quantifies the universe's present-day expansion rate, is 70 kilometers per second per megaparsec, plus or minus 10 percent."(p. 20) Yet on June 1st, the scientists with the Key Project and James Herrnstein of the National Radio Astronomy Observatory explained two different methods of calculating the distances to the nearest galaxies.
James Herrnstein explained that he and an international team of co.-investigators used radio-bright clouds of water vapor called masers to measure the distance to M106, which is a spiral galaxy. Their calculations concluded that the galaxy was 23.5 million light-years away, plus or minus a light year. Then he pointed out that the Hubble Space Telescope was lined up with the same galaxy and calculated that it was 28.5 million light-years. This prompted that one or both of their calculations were incorrect. When this occurs cosmologists are brought in to find the discrepancies.
They determined that Herrnstein used ".widely separated radio telescopes to measure the positions and velocities of water masers that lie within a light-year of M106's mildly active nucleus. The beacons appear to lie in a narrow, warped disk that surrounds a supermassive black hole. The Doppler shifts of some of the masers were measured on sight lines tangential to the disk, and the proper motions of others were measured as they shifted across the sky by minuscule amounts."(p.20) Herrnstein inferred M106's distance by equating these physical and angular velocities.
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