'Invisible' stars about 2,500 light-years away have been discovered

The Gaia satellite is a mission launched by the European Space Agency to map our galaxy. In order to do this, Gaia completely checks the sky over and over again but, whilst looking at space to fix the positions of the stars it is also able to detect thousands of transient objects (i.e., something in the sky that appears, disappears or changes) so that astronomers can quickly target other ground and space telescopes to study these objects in detail, which can be phenomenons such as supernova explosions or other stellar bursts.

In the summer of 2016, astronomers saw a star come to life 2,500 light-years away in the Cygnus constellation, as if preparing to explode in a colossal supernova. The next day, however, the star returned to normal. There was no explosion. The curious thing is that, after a few weeks, the strange phenomenon repeated itself: the star suddenly lit up, and then dimmed. This cycle was repeated five times in 500 days. What was this flash in the sky?

"This was very unusual behavior. Almost no supernova or other star does this,” said Lukasz Wyrzykowski of the University of Warsaw (Poland) and co-author of the work published by the magazine Astronomy & Astrophysics.

Some very strange behaviour

Follow-up observations with more than 50 telescopes around the world revealed that the source, since then called Gaia16aye, was behaving in a rather strange way.  However, it wasn’t because of anything the star was doing; but rather because of the gravity of an invisible object that deformed the fabric of space-time, magnifying the star’s light as it passed. Experts have discovered that this invisible object is a binary star 2,544 light-years away from Earth, which is so faint that we can’t see it, as if it were invisible. However, based on how the star’s gravity magnifies light, astronomers have been able to calculate the mass, distance, and orbit of this pair of red dwarf stars.

"If you have a single lens, caused by a single object, there will be a small constant increase in brightness and then there will be a gentle decrease as the lens passes in front of the distant source and then moves away," Wyrzykowski clarifies.

The anomalous brightness of the star suggested a binary object that produces what is known as gravitational microlensing; an effect predicted by Einstein, which takes place when the gravity of an object in the foreground causes space-time doubles, magnifying something behind it, similar to a field of cosmic magnifying glasses. These lenses amplified the light of the star every time it passed behind them, meaning that those stars were indeed invisible from the Earth.

"We received all this data, we analysed it and we managed to get a very accurate picture of this system, which would not normally be seen," the expert added.

Scientists have used the gravitational microlensing phenomenon to closely observe some of the oldest stars, galaxies and objects in the universe, but the effect can also reveal the properties of much closer and fainter objects and even more rare and mysterious cosmic phenomena, such as black holes, which are normally only detectable when they are spitting out nearby matter and ejecting jets of gaseous light. Wyrzykowski is optimistic and believes that this year we will have the first black holes.

Reference: Ł. Wyrzykowski et al. Full orbital solution for the binary system in the northern Galactic disc microlensing event Gaia16aye, Astronomy & Astrophysics (2019). DOI: 10.1051/0004-6361/201935097