Eärendel star

Eärendel is found in Norse mythology and Tolkien’s books (morning star).

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.

Eärendel is the small dot marked with an arrow. The arc where Eärendel is found is being magnified by a galaxy cluster.

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.

Link to research article in Nature and Press releases

A highy magnified star at redshift 6.2

Press release NASA

Press release ESA

Godzilla, a Monster Star

Godzilla is probably a star similar Eta Carinae, but much bigger and brighter.

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.

Godzilla. King of the Stars during an “episode” of intense activity

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

Seeing through Dark Matter with gravitational waves

We covered the topic of dark matter before in this post (Dark Matter under the microscope). Dark matter remains one of the bigegst mysteries of Science. One of the candidates for dark matter are Primordial Black Holes or PBH. PBH are black holes that formed during the first instants of the universe. Like dark matter, PBH do not emit light and interact with the rest of the universe basically only through gravity. The LIGO experiment has been detecting a surprisingly high number of massive black holes. The origin of these black holes is uncertain but one of the possibilities is that they could be PBH. We also discussed LIGO detections in this earlier post (Did LIGO really see massive black holes?) . In order to explain the current observations by LIGO, only a fraction of the dark matter needs to be in the form of PBH. In particular, a fraction as small as 1% of the total dark matter would be sufficient to explain the unusually elevated rate of black hole mergers with masses above 20 solar masses.

In a new work we discuss a novel method to explore the possibility that PBH constitute part of the dark matter. Our latest paper (see link at the end of this post) studies for the first time the interference produced when gravitational waves cross a portion of the sky populated with a realistic distribution of stellar bodies (stars, neutron stars or black holes) or microlenses. Earlier work have considered only the simple, but unrealistic, case of isolated microlenses and at most assuming that they are located near a larger lens (galaxy or cluster) but always on the side with positive parity (a tecnicallity that describes one of the two possible configurations for a lensed image). Our work goes further than these simple exmaples by studying the combined effect produced by a realustic population of microlenses and also considers the unexplored regime of macroimages with negative parity (they constitute roughly half  the images produced in the string lensing regime). The figure accompanying this post shows an example of a single microlens embeded in a macrolens and on the side of the lens plane with negative parity. The numbers in orange represent relative time delays (in milliseconds) between the different microimages (the numbers in white indicate the magnification of each microimage and the grey scale shows the magnification in the lens plane with the critical curves shown as two white circular regions. The inset in the bottom-right shows the corresponding magnification in the source plane with the position of two sources, one white and one yellow). At LIGO frequencies (approx 100-500 Hz), a time delay between 1/500 seconds or 1/100 seconds (that is or 2 or 10 milliseconds  respectively)  can produce constructive or destructive interference in the incoming gravitational wave at the detector. For the example in the figure, the microlens has a mass of 100 solar masses. These type of masses where known before to be capable of producing such interference but what our work show is that the mass can still be significantly smaller (a few solar masses) provided several microlenses can work together to produce time delays of order several milliseconds. This cooperative behaviour takes place naturally when one is observing gravitational waves that are being lensed by large factors (of order 100 or more) since in this case, two microlenses which are relatively distant from each other in the lens plane, can overlap their regions of high magnification (known as caustics) in the source plane, if the magnification from the macromodel (galaxy or cluster) is sufficiently large (in a fashion similar to how a magnifying glass works that can bring photons that are separated by some relatively large distance to come together at the focal point of the magnifying glass). Our study shows that interference of a gravitational wave with itself due to microlenses is not only possible, but unavoidable if the magnification from the macromodel is sufficiently large.

This result opens the door to constrain the abundance of PBH. If PBH are as abundant as 1% of the total dark matter, the interference signal observed in detected gravitational waves here on Earth would be significantly different. Next in the list is to study by how much we can constrain this abundance as a function of the mass function of the PBH. Stay tunned …

Preprint to the science article