Gravitational lens shows star shape

Einstein's technique was used for the first time as a cosmic "slat" for the shape of a celestial body

Abell 2218 © NASA / HST
Read out

Even fifty years after his death, Einstein's findings still serve as important pacemakers for new discoveries. An international team of astronomers has now for the first time used the phenomenon of gravitational lenses postulated by Einstein in 1936 to determine the shape of stars.

Most stars appear from the earth as punctiform light sources in the sky and this makes it difficult to determine their exact shape. Even advances in optical interferometry have so far only allowed a few stars to be measured. But the knowledge of the star forms is important to test, among other theoretical models.

Sky body "bends" beam of light

For the first time, an astronomy team led by NJ Rattenbury of the Jodrell Bank Observatory has used gravitational lensing for this survey. The method is based on the diffraction of light rays by gravity: If a ray of light from a distant star passes a massive celestial body in the foreground close enough, it is easily deflected from its orbit. As a result, the image of the star viewed from Earth appears changed.

If the foreground object - the gravitational lens - is punctiform and directly in line with earth and star, the image of the star appears ring-shaped, the so-called "Einstein Ring". In most cases, however, the change is more complex. In gravitational microlensing, the gravity of the foreground object is not enough to represent a resolvable image of the background star, but the distorted star image is slightly brighter than the non-lens-enhanced light source. In some cases, this cosmic "magnifying lens" works so much that the brightness of the background source is increased by a thousandfold.

So far, astronomers have used microlensing technology to search for dark matter around our Milky Way and other galaxies, as well as to discover extrasolar planets. Rattenbury and his colleagues were able to determine the shape of a star for the first time using this method. Using a microlens constellation known as MOA-33, they combined the observations of five telescopes on Earth with those of the Hubble Space Telescope to fundamentally analyze the event. display

Double star as a lens

The "lens" of MOA-33 was a binary star system whose distortion, thanks to its special geometry and the constellation of background stars and earth, allowed conclusions to be drawn about the shape of the background star. Normally, astronomers use a spherical star as their source of light, but in this case, they wanted to know more about it and tested some parameters that revealed more about the shape.

Based on these data, astronomers estimate that the background star is slightly elongated, with a polar to equatorial radius of 1.02. However, given a fault tolerance of 0.02 downwards and 0.04 upwards, they can not completely exclude a spherical shape. But despite these uncertainties, the researchers have shown for the first time that Einstein's cosmic lenses can, in principle, also be used to determine the shape of stars that were previously too far away, using conventional methods such as interferometry to measure.

However, the measurement by means of microlenses relies on very specific and therefore rare constellations. The astronomers estimate that only about 0.1 percent of all microlens events have the required properties. However, each year only about 1, 000 microlenses are observed at all but in the future, thanks to better technologies, this number could increase significantly.

(Journal Astronomy and Astrophysics, 31.05.2005 - NPO)