Eärendel is found in old Norse mythology as a star created by Thor out of one of the toes of Aurvandill. Eärendel was later adopted by Tolkien to refer to the morning star. This reference to an early star is appropriate to talk about Eärendel, the farthest star ever observed.
EärendelThe first stars are expected to form when the universe is between 50 and 100 million years old. Although this may sound as a big number, in reality the universe was still very young at this point. If we compare the universe today (13700 million years old) with a person of 80 years of age, the first 100 million years in the universe would be similar to the first six months of the 80 year old person. That is, still a baby universe. These first stars were expected to be very massive (up to 1000 times the mass of the Sun) and luminous, and composed of basically two elements, Hydrogen and Helium (with traces of Lithium). Given the large size of these stars, they burn very rapidly (like a big fire) and do not live very long (compared with smaller stars like the Sun). After several millions of years these stars die. The death of stars represents one of the most important events for life, since it is then when elements such as Carbon, Oxygen, Iron etc are formed. Eärendel is not one of these first stars, but it could be a star formed from the ashes of these first stars after mixing with more Hydrogen. Eärendel is so far that the light we see from it now started its journey when the universe was still an infant (in Cosmological terms). If we compare again the age of the universe today with an 80 year old person, the light we see today from Eärendel was created when the universe was 5 years old, a universe coming out of its toddler years. The light from Eärendel has been traveling for almost 13000 million years before reaching our telescope (the Hubble Space Telescope to be more precise).
Eärendel and Gravitational Lensing
In its journey toward us, the light from Eärendel has crossed several structures. One such structure is the galaxy cluster WHL0137–08, at approximately 1/4 the distance between us and Eärendel. This cluster is a large collection of galaxies, gas and dark matter, and is so massive that it can bend the space around it. This bending of space makes the light traveling through the cluster to bend as well. The effect is known as gravitational lensing, and is similar to the bending of light when it crosses a dense transparent medium, such as a lens made of glass.
This effect was predicted by Einstein and has been observed many times around very massive objects, like galaxy clusters. The gravitational lensing effect can significantly amplify the light of distant objects, making them detectable with current telescopes. This is exactly what is happening with Eärendel. Without he gravitational lensing effect, we could not have observed Eärendel, but thanks to the amplification from the galaxy cluster, we observe Eärendel thousands of times brighter than what we would have observed without this effect. Other stars at smaller (but still incredibly large) distances have been observed in the past thanks to this effect as well. In the past we have discussed the case of Icarus and more recently Godzilla. All these stars have in common the fact that we are seeing them thanks to the extra magnification provided by gravitational lensing that effectively transforms a relatively small telescope like Hubble into a much larger telescope with a mirror size typically 30 to 70 times larger. Telescopes of this size are impossible to build with current technology, even less if they have to operate from space, but gravitational lensing makes it possible to experience having such a gigantic telescope.
Stars like Eärendel, Icarus and Godzilla are extremely bright and rare but are offering unique opportunities to study the evolution of stars in the earlier epochs of the universe. Future telescopes like JWST working in conjunction with gravitational lenses will push the limits even further and will discover stars even more distant than Eärendel, reaching perhaps the first stars mentioned at the beginning of this post.