What Really Happens When the Stars Align

Illustration of gravitational lensing. A distant star emits light, which is bent toward Earth by the gravity of a massive object in between, in this case a red dwarf star. The effect is a magnification of the light from the source. Mark A. Garlick / space-art.co.uk

2,600 rare microlensing events predicted to occur by the end of the century, potentially revealing elusive exoplanets for astronomers to study

  • Microlensing allows astronomers to take precise measurements of stars and other celestial objects.
  • NYUAD researchers predict more than 2,600 instances of microlensing will occur by the end of this century.
  • These predictions will help astronomers focus their instruments on precise locations in the sky at the correct times to gain a deeper understanding of stars and their orbiting planets.

Gravitational lensing is a phenomenon where light from a distant source, such as a star or galaxy, is bent around a massive object sitting between the light source and Earth. The effect was predicted by Albert Einstein in his theory of general relativity, but it wasn't until the late 20th century that images of gravitational lensing were captured by astronomers.

Beyond serving as evidence for Einstein’s theory, gravitational lensing is useful for several reasons. Typically, the mass of a distant star can only be determined if it is in orbit with another star — this is called a binary system. But a particular class of gravitational lensing, called microlensing due to the relatively low mass of the lens object and the subtle effect it has on the source, allows astronomers to make “precise mass measurements” of lone stars that wouldn’t be possible using other methods, said Daniel Bramich, research associate in the physics program at NYU Abu Dhabi.  What’s more, this technique also allows researchers to study objects — such as planets — that emit little or no light of their own.

Bramich and his colleague Martin Nielsen, postdoctoral associate at NYUAD's Center for Space Science, have recently authored papers that predict more than 2,600 instances of microlensing that will occur by the end of this century. These predictions will allow researchers to focus their instruments on precise locations at the correct times to gain a deeper understanding of thousands of objects in the sky. One paper has already been published in Astronomy & Astrophysics; two are forthcoming in Acta Astronomica.

Microlensing happens quickly relative to the cosmic time scale, lasting for hours or days.

This ephemeral nature means that preparation and timing are critical if astronomers hope to observe them. And once they happen, they are gone forever, which makes Bramich and Nielsen's almanac particularly valuable for astronomers.

Microlensing events are intrinsically rare occurrences that depend on chance stellar alignments …. Predicting when and where they will occur is highly advantageous for the collection of data.

Daniel Bramich, NYUAD research associate

To make their predictions, Bramich and Nielsen crunched data provided by Gaia, a space observatory that analyzes the three-dimensional position and motion of stars in the Milky Way. Hundreds of scientists were waiting for the data to be released, as Gaia has prompted a "massive paradigm shift in the quality and quantity of data and how far we can look in our own galaxy" since the spacecraft has been launched, Bramich said.

The rapid succession of three research papers that used the information provided by Gaia is a testament to Bramich and Nielsen’s approach to data analysis and their ability to predict a huge amount of microlensing events: they foretold 2,601 occurrences that will happen this century, while other groups that were using the same information predicted only a handful.

For the data analysis, the researchers looked to the High Performance Computing group at NYUAD and their supercomputer, Dalma. “Without the support of the High Performance Computing group, we would have been quite a bit slower" analyzing the data, Bramich said.

Moving forward, Nielsen noted that he and Bramich are collaborating with researchers at the Space Telescope Science Institute in Baltimore to carry out follow up observations of some of the predicted microlensing events using the Hubble Space Telescope. By measuring the precise mass of a star, these observations may even help them identify and gain an understanding of planets that are orbiting distant stars, whether they are “habitable worlds, or Jupiter-sized planets,” Nielsen said.

Beyond the study of exoplanets, Nielsen explained that this work can have an impact on pure physics.

“Stars, in a sense, act as distant laboratories where scientists can observe physics under extreme conditions that cannot be replicated on Earth,” he said. “The mass of a star is absolutely fundamental for understanding the physics that we observe."