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

Did LIGO really see massive black holes?

gravitational-waves-18
GW being emitted by a pair of black holes

The LIGO (and now Virgo) experiment has opened a new window to explore one of the most mysterious objects in nature, black holes (BH). When two black holes merge, they create a cataclysmic event that sends waves through the fabric of space itself and can travel cosmic distances. This is similar to an earthquake shaking Earth. These waves are known as gravitational waves (GW) and until 2015 they were just pure speculation as no experiment was ever able to detect them. Despite the tremendous amount of energy released when two BH merge (a binary BH merger), these waves, or ripples in space-time, are incredibly difficult to observe. The distortion  that a binary BH merger  in a nearby galaxy induces in space-time is minuscule when it reaches Earth. So minuscule that LIGO need to measure tiny shifts in the relative position between two mirrors which are several orders of magnituude smaller than the size of the smallest atom. This is an incredible achievement.  LIGO’s first detections of GW have brought a few surprises though. And they started with a bang!

The first event detected by LIGO in 2015 was interpreted as a heavier than expected binary BH merging in a  closer than expected galaxy. Similar events have been observed since raising several questions. Are these events more common than previously thought? Why have not we see them farther away if they are stringer than expected?  The mass of the individual black holes forming the binary BH were inferred to be approximately 30 solar masses each. Note that I say inferred because these masses could not be measured directly. What LIGO can measure with relative high precision is what is known as the observed chirp mass.  The intrinsic chirp mass is some combination of the two masses of the binary black hole. If both masses are similar, the chirp mass is similar to those masses as well. If the two masses of the binary BH are very different, the chirp mass will be a value in between these two masses (but closer to the mass of the lightest component). The observed chirp mass is related with the intrinsic chirp mass by the factor (1+z) where z is the redshift of the binary BH. The redshift is a measure of the distance so more distant objects have a larger redshift (our redshift is zero).  In other words, what LIGO can measure with good precision is Mo=Mc*(1+z) where Mc is the intrinsic chirp mass. Mo determines the frequency at which the GW is oscillating, a number that LIGO can estimate quite well. For that very first event, LIGO found that Mo had to be approximately 30 solar masses and that the distance was relatively small, that is z was close to zero. Hence, the intrinsic chirp mass (and mass of the individual BH before they merged)  had to be also close to 30 solar masses. This came as a surprise since many predictions made years earlier anticipated that such high values for Mo should be very rare. In fact, what was expected was to find values for Mo between 7 and 15 solar masses. This was in part motivated by observations of X-ray binary stars in our Galaxy, for which  it is possible to estimate the mass of the BH. An X-ray binary is a pair of closely orbiting  objects where one is a star and the other one is either a neutron star or BH. In this article we consider only the BH case. Roughly speaking, by measuring the amount of light emitting by the gas (from the star) spiraling towards the BH one can measure the mass of the BH.  In our Galaxy, the mass of about a dozen BH has been measured using this technique. The results show that the BH masses are between ~7 and ~14 solar masses. So far, no BH with a mass higher than 20 solar masses has been found in our Galaxy raising another, even more fundamental, question. Is our Galaxy special or is there something else we are missing regarding the BH masses and distances inferred by LIGO?

This is the question we address in our latest work. Owing to the degeneracy with the redshift described above, would it be possible that the intrinsic chirp mass was smaller if the redshift was higher? If the redshift is, let’s say z=1, instead of z~0 then, the intrinsic chirp mass could be a factor of two times smaller than the value inferred by LIGO (while keeping the observed chirp mass constant) bringing it into agreement with the BH masses observed in our Galaxy. There is one caveat though.  If the GW was originated in a galaxy far away at redshift z=1, instead of in a galaxy nearby (z~0), the intensity of the GW would have been much smaller than what LIGO observed. The intensity of the GW  is (to first order) the quantity that is used by LIGO to determine the distance. The observed intensity determined then that the inferred distance had to be relatively small. A mere few hundred Mpc instead of several thousand Mpc which would be the distance for a galaxy at redshftz~1 so one would conclude that the GW originated in a nearby galaxy and consequently, the intrinsic chirp mass had to be high. But this is the funny thing. Nature has interesting ways of playing with us. One of these ways is gravitational lensing thanks to which, an object that is far away may appear to us as if it were much closer (that is, it can amplify the intensity mentioned above). Note that I used the expression infer again when referring to the distance estimation by LIGO. This estimation is made under the assumption that gravitational lensing is not intervening. This is normally a good assumption since, after all, only a very small fraction of distant objects get (significantly) affected  by gravitational lensing. To be more precise, 1 in approximately 1000 or 10000 objects at redshifts larger than z=1 are substantially magnified by the gravitational lensing effect. Hence, is it still possible that a significant fraction of the LIGO events are distant lower mass events that are being magnified by gravitational lensing?  In our work we find that lensing can just make the trick. At large distances, the volume of the universe that is reaching us now (and by this I mean the volume where the light or GW we see now originated) is much  larger  than the corresponding volume at much smaller distances. To visualize this, imagine the volume of a shell of radius R. This volume goes like the square of the radius. So a large shell with a radius 10 times larger than a smaller shell will have 100 times the volume (if they both have the same thickness).  By precise calculations of the gravitational lensing effect over distant gravitational waves we prove that the massive and nearby events found by LIGO can in fact be interpreted as normal but more distant events with masses comparable to the ones found in our Galaxy. This solves the puzzle mentioned at the beginning of this article. Is our Galaxy special? And if it is not, where are the masses that LIGO claims is finding in nearby galaxies? The answer is that those masses would be the same in our Galaxy and in other galaxies. What is wrong is the interpretation of the observation since the amplification due to lensing has been ignored (this story is very similar to the puzzling first bright galaxies detected by Herschel that turned out to be all gravitationally lensed distant galaxies) .

So why has not anybody realized this earlier? That is a good question and the answer is not because people have not thought about this before. For our model to work, there is one little thing that sets our study apart from other similar attempts. As we mentioned earlier, at z~1, only one in a few thousand events could be magnified substantially by gravitational lensing. On the other hand, by observing more distant objects one is observing a larger volume, so one is observing more events. The gain in volume with respect to nearby distances is in the range of two orders of magnitude (more precisely about 1.5 orders of magnitude between z=0.1 and z=1 for a shell of thickness dz=0.1). This gain in volume is not enough to compensate the small probability of lensing at z~1 (1/1000 or less). A significant rate of lensed  events (enough to explain the rate of observed events)  can be obtained ONLY IF (and this is the little thing)  one increases the rate of intrinsic mergers at z=1 with respect to the rate at z=0. Such evolution in the intrinsic rate is expected and has been considered in the past. Our study shows that in order for the lensing mechanism to work and be able to explain the LIGO observations (with the troubling masses), the rate at z=1 needs to be more extreme than previously considered. This is not necessarily a problem since we simply don’t know what this rate is and also there are models that predict such rapid evolution in the intrinsic rate of events between z=1 and z=0 but, surprisingly, this type of strong evolution models were not considered in the past so the role played by lensing  was not recognized.

So then. Are we right? Are we wrong? Time will tell. After all, only one (if at all) of the many interpretations proposed to explain the LIGO massive events will be the correct one. An important aspect of any model is that it needs to be testable and this one is. If lensing is the culprit, at high magnifications one would expect a pair of images with similar magnifications and with a small time delay between them (hours to days depending on the lens mass, lens distance and relative source-lens-observer position).  LIGO detections don’t come in pairs (at least no such detections have been reported yet). If the time delay is several hours or days, it is possible that one of the two lensed events falls below the detection threshold of LIGO since the visibility (determined in part by the geometric factor in LIGO, a technicality whose explanation is beyond the scope of this article) may have changed substantially.  For simplicity, we can say that an event that is directly overhead the detector results in a significantly stronger signal-to-noise ratio than the same event near the horizon. Since Earth rotates once every 24 hours, a position in the sky (like the Sun for instance) can move from the zenith to the horizon in six hours. Hence, two identical GW originating in the same spot in the sky may have significantly different signal-to-noise if they are separated by approximately six hours. There is however a limit for how many times you may get the unlucky configuration that permits to hide one of the two images. Eventually two events should be observed that have virtually the same observed chirp mass and a distance estimate that is consistent with the uncertainties introduced by the geometric factor. The ratio of signal-to-noise between the two events should be compatible with the angle rotated by Earth during the time separation between the two events. Finally, the inferred location in the sky (derived from the time difference between detections in different observatories) should be also consistent with being the same for both events. Data mining of the LIGO data may unveil some of these missing events in the near future and confirm the lensing nature of the massive LIGO events.

Link to the publication

You can download the paper with our study in this link