What made this discovery possible?
Diego: The first time we saw this star was in April of 2016. We had been studying a famous supernova, the first supernova to be lensed multiple times by a galaxy cluster. In one of our observations we saw this dot that suddenly went up in flux.
After we saw that, were very excited; we didn't know what it was. As we continued to observe it, the flux kept increasing. It took us a few weeks to realize it probably had something to do with lensing.
We decided that the dot must be a star located in the lens around a galaxy cluster, close to the critical curve where magnification can be extremely large and any object aligned with the lens can be thousands of times brighter. As the star gets closer and closer to the region of maximum magnification, the flux goes higher. It was the first time we had seen anything like that.
Later on we found it was even more complex. After some time, we would have expected the star to dim and eventually disappear. Although the brightness went down, the star stayed more or less with the same flux. It only fit with one of our predictions which was that, in addition to being perfectly aligned to the cluster, it aligned with yet another star in the cluster, which would act as a microlens.
When people talk about gravitational lenses they often make an analogy that they are like a natural telescope. In this case, it would be like a natural microscope because you have two perfectly aligned lenses: The big lens (the galaxy cluster) and the smaller lens, perfectly aligned with it, a star or maybe a black hole, so these two lenses are perfectly aligned.
It was pure luck that, not only did the star align with these two lenses, but the star was already extremely bright, and we were using the Hubble Space Telescope, which is one of the few telescopes powerful enough to have seen it.
What have we learned from this detection, and what sorts of questions does it open up?
Vega: The star was a type of hot, luminous, massive star known as a blue supergiant. The star fit in with the models we had made of that class of star, which have already been observed many times throughout the universe.
Diego: With this star, at least for me, the most interesting part is this small lens. We know that the big lens is a galaxy cluster, but we don’t know what the small lens is. We only know what it can’t be. One of the candidates we ruled out are primordial black holes, a candidate for dark matter.
If primordial black holes were a significant fraction of dark matter and were producing these microlenses, we wouldn't have seen this star. We would have seen something different. So if these primordial black holes exist, they cannot be very numerous, and they cannot explain dark matter. That was an interesting consequence of this work as well.
What would the ability to detect more stars using this technique mean for the field of cosmology? What sorts of questions would it allow to be answered?
Diego: We’re still looking at this object because, if we are right, there will be more than one of these small lenses. At some point, another small lens is going to align with this star so we should see more images. This would tell us something more about what these small lenses are, whether they’re black holes, stars, or neutron stars.
Vega: With this star we will see more down the road. And, if we’re lucky, we’ll see even more events like this. Using this technique, we’re going to try to look for the very first generation of stars to have formed. If we’re lucky, one of these first stars will align with these types of lenses and be magnified, which is the only way we would be able to see them. Over the next five years, there’s a 10 percent chance we will see one of these stars crossing one of these lenses. That will be the farthest you can see because before that there were no stars.
Diego: We didn't learn much from this star, but if we see the first stars we will learn a lot because we know nothing about them. Currently, there are all kinds of models but nobody knows which one is right. There’s a lot of debate about things like their metallicity, how long they live, and how many existed. Having one of these observations, if it happens to be bright enough, we might be able to get some kind of spectrum that will help us refine our models and put more constraints on the first generation of stars.
