According to a saying you can’t teach an old dog new tricks. Clearly that’s not true for space telescopes because NASA announced that Hubble, in orbit since 1990, will now be able to accurately measure the distance of distant stars up to 10,000 light-years, ten times farther than was possible so far.
This result for the Hubble Space Telescope has been achieved by using a new technique called spatial scanning, developed by Nobel laureate Adam Riess together with Stefano Casertano of the Space Telescope Science Institute (STScI) in Baltimore, which greatly improves the accuracy in angular measurements. It’s been combined with a method called astronomical parallax which is very old instead.
The parallax is a trigonometric technique that is the most reliable method for measuring astronomical distances. The diameter of the Earth’s orbit is used as the base of a triangle whose apex is formed by a star. Measuring the angles of this triangle is relatively easy and at that point it’s possible to measure the length of its sides to obtain the distance of the star.
For distant stars within 100 light years from Earth, this technique provides accurate measurements. However, when the distance becomes greater, the angles formed by the diameter of Earth’s orbit become smaller and smaller. The consequence is that the measurements become progressively more approximate.
The new spatial scanning technique uses as a reference a particular class of stars called Cepheid variables. They have a very regular cycle of expansion and contraction and consequently there’s a close correlation between their period of variability and absolute luminosity. This makes them the perfect indicators in the cosmic distances, even at extragalactic level.
The first experiment was performed by measuring the distance of a Cepheid about 7,500 light years from Earth. Two different measurements were made six months apart using the Wide Field Camera 3 of the Hubble Space Telescope and a third one was performed after six more months. The complex calculations take into account the small variations in the position of the star and the brightness of the Cepheid eventually providing an accurate measure of its distance.
This new technique not only gives new life to the venerable Hubble Space Telescope, which recently turned 24 years old, but opens new frontiers to space observations. Adam Riess won the Nobel Prize for physics because, together with Saul Perlmutter and Brian P. Schmidt, proved that the universe’s expansion is accelerating. Now Riess intends to apply the new distance measurement technique to make better estimates of the universe expansion rate. This should help him and his colleagues to discover the mysteries of dark energy, considered responsible for this phenomenon .
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