A star exploded only once, but its light may soon allow us to witness the same event again.
On September 19, Hubble completed its 200,000th orbit around Earth. For this symbolic milestone, the telescope was observing the galaxy cluster MACS J0417. Astronomers expect Athena, a supernova discovered by the James Webb Space Telescope in 2025, to reappear in this direction.
Diagram of the gravitational lensing phenomenon.
In red, two different paths, one shorter and the other longer, carrying light from the same distant object or event.
Credit: NASA/ESA/L. Calçada
A supernova is the explosion of a star. Here, it will obviously not explode a second time. Its light simply takes several paths before reaching us. These different routes are caused by the enormous mass of the galaxy cluster located between the supernova and Earth.
To understand this phenomenon, we need to look at gravity’s effect on light. A strong concentration of matter distorts the space around it and bends the path of light rays. The MACS J0417 cluster thus acts as a natural lens, magnifying and distorting much more distant objects located behind it.
The different paths are not the same length and do not pass through exactly the same regions. Light from a single explosion can therefore arrive, traveling along several different paths, on different dates for the same observer. The result is surprising: astronomers can see multiple appearances of the same supernova, separated by several months or even several years.
Athena is expected to reappear between now and the beginning of March 2027, according to forecasts reported by NASA. Hubble regularly returns to observe the area in order to detect this new flash. Its ability to retake images of the same location over time is particularly well suited to this monitoring.
The exact timing of this reappearance will provide much more than a beautiful image. The delay depends on the distribution of matter in the cluster crossed by the light. By comparing observations with calculations, astronomers will be able to better map the mass of MACS J0417, including matter that does not emit light directly. This measurement may also help study the expansion rate of the Universe.
After more than 36 years in orbit, Hubble therefore remains in use for this type of work. The telescope has already traveled more than 8 billion kilometers around Earth and carried out more than 1.7 million observations.