Hubble will see a supernova in 2016. Here's how we know
Tuesday, November 24, 2015, 12:47 PM - Spotting an exploding star is usually a matter of sheer luck, but astronomers now know where and when to point the Hubble Space Telescope in 2016 to see a supernova just as it happens. Here's how.
This story was updated on June 2, 2016.
Knowing exactly when a star will go supernova is not an easy task. First, there's more than one way a star can go supernova. Second, we often can't even see these stars before they explode, since, until that point, they're just some non-descript resident of a distant galaxy that we can't even pick out from its neighbours. Third, we still don't know quite enough about distant stars - even ones in our own Milky Way galaxy - to tell what's really happening with them as they reach the end of their lifespan. At best, astronomers can predict that specific stars in our galaxy will very likely go supernova, sometime in the next million years or so.
There's one case that's different from the rest, however, and it surely has astronomers excited for the first few months of 2016.
In November of 2014, astronomers using the Hubble Space Telescope spotted a distant supernova, which they called Refsdal, that took place in a cluster of galaxies that goes by the name MACS J1149+2223. What's remarkable about this is that they didn't just see this supernova once. They saw it four times at once!
A spiral galaxy in galaxy cluster MACS J1149+2223 produces an Einstein Cross (inset) from the light of a supernova. Credits: NASA, ESA, S. Rodney (John Hopkins University, USA) and the FrontierSN team; T. Treu (University of California Los Angeles, USA), P. Kelly (University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)
This arrangement of the supernova's light is known as an Einstein Cross. It looks like that because somewhere between us and the exploding star is a lens that's being formed due to the immense gravitational forces acting on spacetime in that region of the universe. With that lens in the way, the light from the supernova has to travel around it on different paths, and it shows up to us as four images of the same explosion.
While astronomers have seen many examples of this gravitational lensing effect with different stars, galaxies and other objects, this was the very first time they'd ever seen an Einstein Cross from a supernova.
This discovery was very exciting in its own right, however that's not the end of the story, or the last time we will be able to see this supernova.
As it happens, the team of astronomers poring over this field of view found that the much larger gravitational lens produced by the massive galaxy cluster is showing us multiple images of the galaxy that hosts this supernova. That's not all. By carefully examining these images, the team determined that each separate image of the galaxy is showing us a different point in time.
How is that possible? Due to the actual position of the galaxy relative to the cluster's gravitation lens, or due to the shape of the gravitational lens, or both, some paths around the lens are longer than others - even for light. Therefore, the light forming each separate image takes a different amount of time to make it around the lens and arrive here at Earth.
In the view of this region of space shown below, three images of this same galaxy are circled.
Three images of the same galaxy, produced by the gravitational lens of the galaxy cluster surrounding it, are circled. Credits: NASA, ESA, S. Rodney (John Hopkins University, USA) and the FrontierSN team; T. Treu (University of California Los Angeles, USA), P. Kelly (University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)
Based on their results, the astronomers say that the light forming the top image takes the least amount of time of the three to reach Earth. Apparently, if Hubble had been aimed in this direction in 1995, that would have been the first spotting of SN Refsdal.
The bottom image is where Refsdal formed the Einstein Cross in November of 2014. The light that forms this image takes a longer path around the cluster's gravitational lens than the light forming the top image. Thus, the light emitted by the supernova also took longer, and it showed up later.
The image circled in the center of the view is the one of interest now. As the supernova fades in the bottom image, the astronomers are predicting that it will flare to life in this one.
When will this happen? By their reckoning, the astronomers say it will happen sometime between January and April of 2016, and they plan on having Hubble pointed that way to see it.
Note: A previous version of this story indicated that SN Refsdal would be showing up as a full Einstein cross in the latest observations. This is, unfortunately, not true. The Einstein cross from Refsdal was an isolated event, due to one small part of the galaxy cluster's gravitational lens affecting only that one spiral arm of that one specific image of Refsdal's host galaxy. According to Patrick Kelly, from University of California Berkeley, the single instance spotted in December is the only reappearance of Refsdal that is expected. Results from the full observation of this reappearance are due out sometime soon.
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