For decades, powerful cosmic beacons known as quasars have defied efforts to measure the physical structure of features thought to be driving the brilliant beams of light and other types of radiation quasars produce.

Now, astronomers using the Hubble Space Telescope – with a lot of help from stars in an intervening galaxy – have captured light from the disk of dust and gas feeding a supermassive black hole at the heart of a quasar. Clues embedded in the light have allowed them to estimate the size of the accretion disk and provide a crude map of the disk's temperature differences.

Quasars are large galaxies with cores that emit vast amounts of radiation as material approaches their central black holes and heat to enormous temperatures before vanishing. The supermassive black holes themselves are millions to billions of times more massive than the sun.

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The quasar in question is some 10.8 billion light-years away. The disk, the team calculates, extends up to seven light-days – or about 1,300 astronomical units – from the central black hole. One astronomical unit spans the distance between Earth and the sun.

The detection is a bit like spotting a single grain of moon dust from Earth, according to the team conducting the study.

Until now, most of what scientists have learned about the structure of quasars has come from measurements of their energy output. This information has been used to develop a theoretical understanding of quasar processes and structure, explains Christopher Kochanek, an astrophysicist at Ohio State University and a member of the team conducting the study.

The new approach is revolutionary, he says, because it provides a way of actually measuring the size of the black hole's accretion disk.

"It's a kind of funky, indirect size measurement. We still can't get a picture of it," Dr. Kochanek acknowledges. "But we can tell you how big it is."

The team of US and Spanish astronomers, led by Jose Muñoz of the University of Valencia in Spain, used a phenomenon known as gravitational microlensing to spot light from the accretion disk.

It's a weaker, smaller scale version of lensing that occurs when the enormous gravity from black holes, galaxies, or galaxy clusters bend light.

In this case, the intervening galaxy produced a lensed image of the quasar. But individual stars in the galaxy also acted as lenses – lenses that responded to light from small regions of the quasar.

How might an accretion disk reveal itself?

Material in an accretion disk heats as it approaches a black hole. Material on the disk's perimeter tends to be cooler, so its light is redder than light from the region of the disk closest to the black hole, where temperatures are higher and the light bluer.

And that's in effect what the team saw in images taken at different times, during which stars in the intervening galaxy moved across the beam from the quasar, sampling different regions of the disk.

From the data the team was able to estimate the disk's radius, some 4 to 7 light-days across.