A masing new ruler

26 January 2000

Observations of astronomical water masers have provided us with a new measure of distance in space.

Maser emission was detected by a team using the US Very Long Baseline
Array (VLBA), from material rotating around a black hole in the centre of galaxy NGC 4258. With the rotation disk very nearly edge on, both the maser’s angular velocity and acceleration
could be measured. Then, using geometry, its distance from the galaxy could be calculated.

Uncertainties in the value of the Hubble constant, and thus the age of the universe, are
mainly caused by errors in measuring large distances. “Standard candles” – Cepheid variable stars and type 1 supernova remnants – used for distance measurements, sometimes give
wildly differing results. Errors occur because of “blending”, where the stars appear to be brighter than they actually are because their image is mixed with that of nearby stars.
The spectacular universe is revealed by the Hubble Space Telescope: an interaction between 2 spiral galaxies NGC 2207 and IC 2163. (NASA/ESA.)Finding one’s place in the world –
Edwin Hubble at the Mount Wilson telescope. (Space Telescope Science Institute.)

XMM is launched

On 10 December 1999, an Ariane 5 rocket launched the European Space Agency’s X-ray multi-mirror satellite (XMM). XMM is Europe’s largest scientific satellite. It is 10 m tall and has
120 m2 of mirrors, which are coated in a 0.5 µm thickness of gold. The telescope will be the most sensitive in the world. It is set to revolutionize the study of X-ray sources
including black holes, exploding stars and gamma-ray bursters (CERN Courier July 1999 p13)

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