Big stars are rare but make themselves obvious since they are very luminous. In our Galaxy, there are some massive and superluminous stars. For example Eta Cariane, shown in the illustration above, is a very massive star and among the most luminous in our Galaxy. The explosion-like appearance of Eta Carinae is the result of dramatic episodes in the past, where the star chokes on its own massive energy production and ejects large amounts of matter at great speeds from the outer layers in the star. One such eruption took place in the mid 19th century and is known as the Great Eruption. The eruption did not destroy the star which is still producing large amounts of energy. Other eruptions had taken place in the past. Typically every 300 years Eta Carinae has one of this epileptic episodes. During the last Great Eruption, Eta Carinae become one of the brightest stars in the sky for several years. Even though it is still one of the most luminous stars in the Milky Way, due to its large distance to us, nowadays it appears much fainter than it was in the mid 19th century. It is expected that Eta Carinae will go through another violent episode like the Great Eruption, maybe the last one, hopefully not with a gamma ray burst associated to it and pointing to us, which would jeopardize life here on Earth. But that is another bed-night story…
Today we talk about another star like Eta Carinae, but which is much, much, much farther away. We name this star Godzilla because it truly is a Monster Star. At the moment of writing this post, Godzilla is in fact the farthest star ever observed by humans (that has been published in arxiv). This record will not last long, as in a couple of weeks we will announce the discovery of another star that is even further away (look for a press release by NASA/ESA on March 31st). But is fair to say that Godzilla is the most luminous star we have ever observed, and this record may hold for quite a long time.
Godzilla has been observed with several telescopes, including the Hubble Space Telescope (HST). This star is in the famous Sunburst galaxy, at redshift 2.37, or in layman terms, this galaxy is almost at half the distance (comovil) to the edge of the observable universe (defined by the cosmic microwave background). Alternatively, the light from Godzilla took 10900 millions of years to reach us and the universe was 20% of its age when the light we see today left the surface of Godzilla. Since massive stars are like rock stars (live fast and die young), Godzilla is long dead, but we still see its light since it takes very long to reach us (10900 millions of years as mentioned above).
Godzilla is a unique star for several reasons. It’s luminosity is brighter but still comparable to that of Eta Carinae during the Great Eruption. That is, the light we are receiving now from Godzilla was emitted when Godzilla had an eruption, and temporarily increased its luminosity by a factor ~100 during a period of several years or decades. Since the universe is expanding, if a distant event has a duration of 1 year, when we observe it the duration is increased by a factor (1+z), where z is the redshift (z=2.37 for Godzilla). Hence, if the eruption in Godzilla lasts 10 years in Godzilla’s time, for us the same event will last almost 34 years. This effect is called time dilation, and you can experience it by yourself the next time you go to the airport. When you see one of the moving ways at the airport, walk (or better run) in the direction opposite to where the moving way moves. It will take you longer to reach the other end than if you cover the same distance walking (or running) outside the moving way. For light traveling through an expanding universe, the effect is somewhat similar since as the light travels (at the speed of light) the distance between Godzilla and the telescope increases.
The fact that Godzilla is so luminous is unique in its own way, but that alone would not make Godzilla the special star it is. We estimate there must be millions of stars like Godzilla in the universe but at the distances of Godzilla, these stars would still be too faint to be detected, even with powerful telescopes such as HST. In order to see them, one would need much larger telescopes. Building such enormous telescopes is beyond the reach of current technology but nature can be playful some times, and has offered us a way to emulate gigantic telescopes and tease our curiosity.
A giant natural telescope
Godzilla was discovered thanks to a natural gigantic telescope that happens to be perfectly aligned with our solar system. Pointing a telescope like Hubble towards this gigantic telescope allows us to see what lies behind with incredible resolution. The natural telescope is a gravitational lens, which works thanks to the bending of space predicted by Albert Einstein around massive object. This massive object is a big galaxy cluster with a mass many trillions of times the mass of the Sun. The galaxy cluster can amplify the light of objects which are placed in particular positions behind the cluster, the same way a regular magnifying glass magnifies more the objects which are closer to the central part of the lens. Godzilla happens to be placed in one of this very special locations and we estimate the magnification from the cluster is approximately a factor x3000. When observing with the Hubble telescope, the combined effect of Hubble plus the cluster is similar to having a telescope ~50 times larger, i.e a space telescope with a diameter of 120 meters! The largest telescope on Earth has a diameter of approximately 10 meters, and the largest telescope in space is the new JWST with a diameter of 6.5 meters. The next generation largest telescope planned for the next decade is the ELT with a diameter of 40 meters. The opportunities offered by these natural telescopes, or gravitational lenses, will not be matched by our technology for many decades. Hence they can offer a glimpse of portions of the distant universe with unprecedented detail. Godzilla happens to be in one of these very special positions so we can look through a pinhole of the vast cosmos where nature has placed one of this natural telescopes for us to take a pick.
Godzilla is just the first of many examples of Monster Stars with temporary eruptions that will be discovered through a similar technique. Other stars have been discovered in the past thanks to similar lucky alignments of natural telescopes (see for instance Icarus, the first of such stars) but Godzilla is an even more rare star due to its current activity. Exploiting the possibility offered by natural telescopes, we will soon discover more stars like Godzilla and start to study them in their different phases in order to understand how they evolve and eventually die (sometimes with a very energetic phenomena like a SN).
Link to the research article: https://arxiv.org/abs/2203.08158