Hubble sharpens measurement of distance to ancient cluster

Boffins do give a fig about trig after all

Hubble anniversary image

Boffins have combined NASA’s aging Hubble Space Telescope and some good old-fashioned trigonometry to measure the distance to a cluster of stars that were formed shortly after the big bang.

The researchers pointed Hubble’s Wide Field Camera 3 at a globular stellar cluster known as NGC 6397, which had previously been the subject of a 2004 study into the age of the Milky Way, once every six months over a period of two years.

By measuring the tiny apparent shifts in an object’s position due to a change in the point of view of the observer (in this instance, the motion of the earth around the sun), scientists were able to use trigonometric parallax to pin down the exact distance to the cluster.

The team tracked 40 stars in the NGC 6397 cluster and combined the results to improve precision.

One of the authors of the research, Stefano Casertano of the Space Telescope Science Institute (STScI), said: “Because we are looking at a bunch of stars, we can get a better measurement than simply looking at individual Cepheid variable stars.”

Cepheid variables are pulsating stars and used as markers to calculate the expansion rate of the universe.

The results put the distance of NGC 6397 at 7,800 light years, with a three per cent margin of error.

This is quite an improvement over the 10 to 20 per cent margin of uncertainty associated with methods used up until now, where the colour and luminosity of stars is compared to theoretical models and neighbouring stars.

Since distances to globular clusters are used as a reference in the study of stellar populations, improved accuracy will give scientists more confidence in the application of updated models to distant stars.

The wobbles in position were equivalent to 1/100th of a pixel on the camera (with a precision of 1/3000th of a pixel), and scientists hope to combine the current results with those from the European Space Agency’s Gaia observatory to get the margin of error down to as low as one per cent.

“Getting to one per cent accuracy will nail this distance measurement forever,” said Tom Brown of the STScI and leader of the Hubble study.

Moon conspiracy theorists will be overjoyed to read that even without Gaia’s input, the current measurements are “equivalent to measuring the size of an automobile on the moon to a precision of one inch.” Sadly, imaging the moon with Hubble doesn’t work quite like that. ®

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