Thanks to the extreme magnification produced by gravitational lensing, two different groups of astronomers were able to observe two individual stars that lived in the very early universe. It was possible to observe the two objects because both of them underwent changes that made them twinkle. The papers are published in Nature Astronomy here and here.
The light of the nearest object left its star 7.8 billion years ago, and its light was magnified by a foreground galaxy cluster known as MACS J0416.1-2403. The cluster is so dense that it warps space-time, acting like a lens. The phenomenon is known as strong gravitational lensing. The team actually detected two transient events, but given the way gravitational lenses distort light, the researchers think it was emitted at two different times by the same object.
The transient events are called HFF14Spo-NW (northwest) and HFF14Spo-SE (southeast), and the object has been nicknamed Spock. The researchers at first thought these were two supernovae explosions, but Spock's sudden increase in luminosity doesn’t match with known supernova models.
For this reason, researchers are looking into alternatives ideas. Among the suggested models, there's the possibility it's a blue variable star or a massive supergiant on its way to becoming a supernova. It also might be a more regular nova, a star that’s absorbing material and experiencing sudden surface explosions. There is another explanation too, which is related to the other study.
The furthest star is dubbed MACS J1149 Lensed Star 1 (LS1) and its light was emitted 9.3 billion years ago. Due to the expansion of the universe, the star is now over 14.4 billion light-years away. The star is in a faraway galaxy that was magnified by the massive galaxy cluster MACS J1149, which is also far away but in the foreground for us.
LS1 was seen twinkling by astronomers. The twinkling was caused by another lensing phenomenon, called microlensing. Something very dense in the lensing cluster passed in front of this distant star, boosting its brightness by a staggering amount. LS1 was magnified more than 2,000 times. This might also be the case for Spock.
Regardless if both stars' twinkling is the fruit of microlensing, the fact that we can study microlensing in these massive clusters has important consequences. Microlensing is used to study dark matter distribution in galaxies, providing us with intriguing insights on the life and invisible structures in galaxies far and very far from our own.