According to a new study, the strongest material in the Universe is the "nuclear pasta" found inside neutron stars. Credit: NASA/Goddard Space Flight Center
Ever since they were first discovered in the 1930s, scientists have puzzled over the mystery that is neutron stars. These stars, which are the result of a supernova explosion, are the smallest and densest stars in the Universe. While they typically have a radius of about 10 km (6.2 mi) – about 1.437 x 10-5 times that of the Sun – they also average between 1.4 and 2.16 Solar masses.
At this density, which is the same as that of atomic nuclei, a single teaspoon of neutron star material would weigh about as much as 90 million metric tons (100 million US tons). And now, a team of scientists has conducted a study that indicates that the strongest known material in the Universe – what they refer to as “nuclear pasta” – exists deep inside the crust of neutron stars.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
For decades, scientists have theorized that beyond the edge of the Solar System, at a distance of up to 50,000 AU (0.79 ly) from the Sun, there lies a massive cloud of icy planetesimals known as the Oort Cloud. Named in honor of Dutch astronomer Jan Oort, this cloud is believed to be where long-term comets originate from. However, to date, no direct evidence has been provided to confirm the Oort Cloud’s existence.
This is due to the fact that the Oort Cloud is very difficult to observe, being rather far from the Sun and dispersed over a very large region of space. However, in a recent study, a team of astrophysicists from the University of Pennsylvania proposed a radical idea. Using maps of the Cosmic Microwave Background (CMB) created by the Planck mission and other telescopes, they believe that Oort Clouds around other stars can be detected.
The study – “Probing Oort clouds around Milky Way stars with CMB surveys“, which recently appeared online – was led by Eric J Baxter, a postdoctoral researcher from the Department of Physics and Astronomy at the University of Pennsylvania. He was joined by Pennsylvania professors Cullen H. Blake and Bhuvnesh Jain (Baxter’s primary mentor).
To recap, the Oort Cloud is a hypothetical region of space that is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun – though some estimates indicate it could reach as far as 100,000 to 200,000 AU (1.58 and 3.16 ly). Like the Kuiper Belt and the Scattered Disc, the Oort Cloud is a reservoir of trans-Neptunian objects, though it is over a thousands times more distant from our Sun as these other two.
This cloud is believed to have originated from a population of small, icy bodies within 50 AU of the Sun that were present when the Solar System was still young. Over time, it is theorized that orbital perturbations caused by the giant planets caused those objects that had highly-stable orbits to form the Kuiper Belt along the ecliptic plane, while those that had more eccentric and distant orbits formed the Oort Cloud.
According to Baxter and his colleagues, because the existence of the Oort Cloud played an important role in the formation of the Solar System, it is therefore logical to assume that other star systems have their own Oort Clouds – which they refer to as exo-Oort Clouds (EXOCs). As Dr. Baxter explained to Universe Today via email:
“One of the proposed mechanisms for the formation of the Oort cloud around our sun is that some of the objects in the protoplanetary disk of our solar system were ejected into very large, elliptical orbits by interactions with the giant planets. The orbits of these objects were then affected by nearby stars and galactic tides, causing them to depart from orbits restricted to the plane of the solar system, and to form the now-spherical Oort cloud. You could imagine that a similar process could occur around another star with giant planets, and we know that there are many stars out there that do have giant planets.”
As Baxter and his colleagues indicated in their study, detecting EXOCs is difficult, largely for the same reasons for why there is no direct evidence for the Solar System’s own Oort Cloud. For one, there is not a lot of material in the cloud, with estimates ranging from a few to twenty times the mass of the Earth. Second, these objects are very far away from our Sun, which means they do not reflect much light or have strong thermal emissions.
For this reason, Baxter and his team recommended using maps of the sky at the millimeter and submillimeter wavelengths to search for signs of Oort Clouds around other stars. Such maps already exist, thanks to missions like the Planck telescope which have mapped the Cosmic Microwave Background (CMB). As Baxter indicated:
“In our paper, we use maps of the sky at 545 GHz and 857 GHz that were generated from observations by the Planck satellite. Planck was pretty much designed *only* to map the CMB; the fact that we can use this telescope to study exo-Oort clouds and potentially processes connected to planet formation is pretty surprising!”
This is a rather revolutionary idea, as the detection of EXOCs was not part of the intended purpose of the Planck mission. By mapping the CMB, which is “relic radiation” left over from the Big Bang, astronomers have sought to learn more about how the Universe has evolved since the the early Universe – circa. 378,000 years after the Big Bang. However, their study does build on previous work led by Alan Stern (the principal investigator of the New Horizons mission).
All-sky data obtained by the ESA’s Planck mission, showing the different wavelenghts. Credit: ESA
In 1991, along with John Stocke (of the University of Colorado, Boulder) and Paul Weissmann (from NASA’s Jet Propulsion Laboratory), Stern conducted a study titled “An IRAS search for extra-solar Oort clouds“. In this study, they suggested using data from the Infrared Astronomical Satellite (IRAS) for the purpose of searching for EXOCs. However, whereas this study focused on certain wavelengths and 17 star systems, Baxter and his team relied on data for tens of thousands of systems and at a wider range of wavelengths.
“Furthermore, the Gaia satellite has recently mapped out very accurately the positions and distances of stars in our galaxy,” Baxter added. “This makes choosing targets for exo-Oort cloud searches relatively straightforward. We used a combination of Gaia and Planck data in our analysis.”
To test their theory, Baxter and is team constructed a series of models for the thermal emission of exo-Oort clouds. “These models suggested that detecting exo-Oort clouds around nearby stars (or at least putting limits on their properties) was feasible given existing telescopes and observations,” he said. “In particular, the models suggested that data from the Planck satellite could potentially come close to detecting an exo-Oort cloud like our own around a nearby star.”
The relative sizes of the inner Solar System, Kuiper Belt and the Oort Cloud. (Credit: NASA, William Crochot)
In addition, Baxter and his team also detected a hint of a signal around some of the stars that they considered in their study – specifically in the Vega and Formalhaut systems. Using this data, they were able to place constraints on the possible existence of EXOCs at a distance of 10,000 to 100,000 AUs from these stars, which roughly coincides with the distance between our Sun and the Oort Cloud.
However, additional surveys will be needed before the existence any of EXOCs can be confirmed. These surveys will likely involve the James Webb Space Telescope, which is scheduled to launch in 2021. In the meantime, this study has some rather significant implications for astronomers, and not just because it involves the use of existing CMB maps for extra-solar studies. As Baxter put it:
“Just detecting an exo-Oort cloud would be really interesting, since as I mentioned above, we don’t have any direct evidence for the existence of our own Oort cloud. If you did get a detection of an exo-Oort cloud, it could in principle provide insights into processes connected to planet formation and the evolution of protoplanetary disks. For instance, imagine that we only detected exo-Oort clouds around stars that have giant planets. That would provide pretty convincing evidence that the formation of an Oort cloud is connected to giant planets, as suggested by popular theories of the formation of our own Oort cloud.”
As our knowledge of the Universe expands, scientists become increasingly interested in what our Solar System has in common with other star systems. This, in turn, helps us to learn more about the formation and evolution of our own system. It also provides possible hints as to how the Universe changed over time, and maybe even where life could be found someday.
Illustration of a possible scenario for the powerful blast seen 170 years ago from the star system Eta Carinae. Credit: HubbleSite.com
Eta Carinae, a double star system located 7,500 light years away in the constellation Carina, has a combined luminosity of more than 5 million Suns – making it one of the brightest stars in the Milky Way galaxy. But 170 years ago, between 1837 and 1858, this star erupted in what appeared to be a massive supernova, temporarily making it the second brightest star in the sky.
Strangely, this blast was not enough to obliterate the star system, which left astronomers wondering what could account for the massive eruption. Thanks to new data, which was the result of some “forensic astronomy” (where leftover light from the explosion was examined after it reflected off of interstellar dust) a team of astronomers now think they have an explanation for what happened.
In their first study, the team indicates how they studied the “light echoes” produced by the explosion, which were reflected off of interstellar dust and are just now visible from Earth. From this, they observed that the eruption resulted in material expanding at speeds that were up to 20 times faster than with any previously-observed supernova.
In the second study, the team studied the evolution of the echo’s light curve, which revealed that it experienced spikes before 1845, then plateaued until 1858 before steadily declining over the next decade. Basically, the observed velocities and light curve were consistent with the blast wave of a supernova explosion rather than the relatively slow and gentle winds expected from massive stars before they die.
The light echoes were first detected in images obtained in 2003 by telescopes at the Cerro Tololo Inter-American Observatory in Chile. For the sake of their study, the team consulted spectroscopic data from the Magellan telescopes at the Las Campanas Observatory and the Gemini South Observatory, both located in Chile. This allowed the team to measure the light and determine the ejecta’s expansion speeds – more than 32 million km/h (20 million mph).
Based on this data, the team hypothesized that the eruption may have been triggered by a prolonged battle between three stars, which destroyed one star and left the other two in a binary system. This battle may have culminated with a violent explosion when Eta Carinae devoured one of its two companions, sending more than 10 Solar masses into space. This ejected mass created the gigantic bipolar nebula (aka. “the Homunculus Nebula”) which is seen today.
As Smith explained in a recent HubbleSite press release:
“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived. The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds.”
In this scenario, Eta Carinae started out as a trinary system, with two massive stars orbiting close to each other and the third orbiting further away. When the most massive of the binary neared the end of its life, it began to expand and then transfer much of its material onto its slightly smaller companion. This caused the smaller star to accumulate just enough energy to cause it to eject its outer layers, but not enough to completely annihilate it.
The companion star would have then grown to become about 100 times the mass of our Sun and extremely bright. The other star, now weighing only 30 Solar masses, would have been stripped of its hydrogen layers, exposing its hot helium core – which represent an advanced stage of evolution in the lives of massive stars. As Armin Rest – a researcher from the STSI, The John Hopkins University and a co-author on the paper – explained:
“From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes. So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.”
The Homunculus Nebula, surrounding Eta Carinae. Credit: ESO, IDA, Danish 1.5 m, R. Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron
This transfer of mass would have altered the gravitational balance of the system, causing the helium-core star to move farther away from its now-massive companion and eventually travel so far that it would interact with the outermost third star. This would cause the third star to move towards the massive star and eventually merge with it, producing an outflow of material.
Initially, the merger caused ejecta that expanded relatively slowly, but as the two stars finally joined together, they produced an explosive event that blasted material off 100 times faster. This material caught up to the slow ejecta, pushing it forward and heating the material until it glowed. This glowing material was the main light source that was viewed by astronomers 170 years ago.
In the end, the smaller helium-core star settled into an elliptical orbit around around its massive counterpart, passing through the star’s outer layers every 5.5 years and generating X-ray shock waves. According to Smith, while this explanation cannot account for everything observed in Eta Carinae, it does explain both the brightening and the fact that the star remains:
“The reason why we suggest that members of a crazy triple system interact with each other is because this is the best explanation for how the present-day companion quickly lost its outer layers before its more massive sibling.”
These studies have provided new clues as to the mystery of how Eta Carinae appeared to explode in a massive supernova, but left behind a massive star and nebula. In addition, a better understanding of the physics behind the Eta Carinae explosion could help astronomers to learn more about the complicated interactions that govern binary and multiple star systems – which are critical to our understanding of the evolution and death of massive stars.
Hubble image of a luminous red galaxy (LRG) gravitationally distorting the light from a much more distant blue galaxy, a technique known as gravitational lensing. The shape of the galaxy doing the lensing created an almost circular image. An oblong galaxy would create more of an Einstein Ring effect. Credit: ESA/Hubble & NASA
When looking to study the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.
This technique has allowed for the study of individual stars in distant galaxies. In a recent study, an international team of astronomers used a galaxy cluster to study the farthest individual star ever seen in the Universe. Although it normally to faint to observe, the presence of a foreground galaxy cluster allowed the team to study the star in order to test a theory about dark matter.
For the sake of their study, Prof. Kelly and his associates used the galaxy cluster known as MACS J1149+2223 as their lens. Located about 5 billion light-years from Earth, this galaxy cluster sits between the Solar System and the galaxy that contains Icarus. By combining Hubble’s resolution and sensitivity with the strength of this gravitational lens, the team was able to see and study Icarus, a blue giant.
Icarus, named after the Greek mythological figure who flew too close to the Sun, has had a rather interesting history. At a distance of roughly 9 billion light-years from Earth, the star appears to us as it did when the Universe was just 4.4 billion years old. In April of 2016, the star temporarily brightened to 2,000 times its normal luminosity thanks to the gravitational amplification of a star in MACS J1149+2223.
As Prof. Kelly explained in a recent UCLA press release, this temporarily allowed Icarus to become visible for the first time to astronomers:
“You can see individual galaxies out there, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions.”
Kelly and a team of astronomers had been using Hubble and MACS J1149+2223 to magnify and monitor a supernova in the distant spiral galaxy at the time when they spotted the new point of light not far away. Given the position of the new source, they determined that it should be much more highly magnified than the supernova. What’s more, previous studies of this galaxy had not shown the light source, indicating that it was being lensed.
Icarus, the farthest individual star ever seen, shown at left. Panels at right show the view in 2011, without Icarus visible, compared with the star’s brightening in 2016. Credit: NASA, ESA and Patrick Kelly/University of Minnesota
As Tommaso Treu, a professor of physics and astronomy in the UCLA College and a co-author of the study, indicated:
“The star is so compact that it acts as a pinhole and provides a very sharp beam of light. The beam shines through the foreground cluster of galaxies, acting as a cosmic magnifying glass… Finding more such events is very important to make progress in our understanding of the fundamental composition of the universe.
In this case, the star’s light provided a unique opportunity to test a theory about the invisible mass (aka. “dark matter”) that permeates the Universe. Basically, the team used the pinpoint light source provided by the background star to probe the intervening galaxy cluster and see if it contained huge numbers of primordial black holes, which are considered to be a potential candidate for dark matter.
These black holes are believed to have formed during the birth of the Universe and have masses tens of times larger than the Sun. However, the results of this test showed that light fluctuations from the background star, which had been monitored by Hubble for thirteen years, disfavor this theory. If dark matter were indeed made up of tiny black holes, the light coming from Icarus would have looked much different.
Since it was discovered in 2016 using the gravitational lensing method, Icarus has provided a new way for astronomers to observe and study individual stars in distant galaxies. In so doing, astronomers are able to get a rare and detailed look at individual stars in the early Universe and see how they (and not just galaxies and clusters) evolved over time.
When the James Webb Space Telescope (JWST) is deployed in 2020, astronomers expect to get an even better look and learn so much more about this mysterious period in cosmic history.
This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852, also known as Boyajian's Star or Tabby's Star. Credit: NASA/JPL-Caltech
In September of 2015, the star KIC 8462852 (aka. Tabby’s Star) captured the world’s attention when it was found to be experiencing a mysterious drop in brightness. In the years since then, multiple studies have been conducted that have tried to offer a natural explanation for this behavior – and even an unnatural one (i.e. the “alien megastructure” theory). At the same time, multiple observatories have been tracking the star regularly for further dimming.
Well, it seems that Tabby’s Star is at it again! On Friday, March 16th, Tabetha Boyajian (the astronomer who was responsible for discovering the star’s variations in flux) and her colleagues reported that the star was dimming yet again. As they indicated recently their blog – Where’s the Flux? – the star experienced its greatest dip since it was observed by the Kepler mission in 2013.
The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. The locations of the observed stars above and below the disk, which are used to test the perturbation scenario, are indicated. Credit: T. Mueller/C. Laporte/NASA/JPL-Caletch
Despite thousands of years of research and observation, there is much that astronomers still don’t know about the Milky Way Galaxy. At present, astronomers estimate that it spans 100,000 to 180,000 light-years and consists of 100 to 400 billion stars. In addition, for decades, there have been unresolved questions about how the structure of our galaxy evolved over the course of billions of years.
For example, astronomers have long suspected that galactic halo came from – giant structures of stars that orbit above and below the flat disk of the Milky Way – were formed from debris left behind by smaller galaxies that merged with the Milky Way. But according to a new study by an international team of astronomers, it appears that these stars may have originated within the Milky Way but were then kicked out.
The study recently appeared in the journal Nature under the title “Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk“. The study was led by Margia Bergmann, a researcher from the Max Planck Institute for Astronomy, and included members from the Australian National University, the California Institute of Technology, and multiple universities.
Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
For the sake of their study, the team relied on data from the W.M. Keck Observatory to determine the chemical abundance patterns from 14 stars located in the galactic halo. These stars were located in two different halo structures – the Triangulum-Andromeda (Tri-And) and the A13 stellar overdensities – which are bout 14,000 light years above and below the Milky Way disc.
As Bergemann explained in a Keck Observatory press release:
“The analysis of chemical abundances is a very powerful test, which allows, in a way similar to the DNA matching, to identify the parent population of the star. Different parent populations, such as the Milky Way disk or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately link them to their parent populations.”
The team also obtained spectra from one additional using the European Southern Observatory’s Very Large Telescope (VLT) in Chile. By comparing the chemical compositions of these stars with the ones found in other cosmic structures, the scientists noticed that the chemical compositions were almost identical. Not only were they similar within and between the groups being studies, they closely matched the abundance patterns of stars found within the Milky Way’s outer disk.
Computer model of the Milky Way and its smaller neighbor, the Sagittarius dwarf galaxy. Credit: Tollerud, Purcell and Bullock/UC Irvine
From this, they concluded that these stellar population in the Galactic Halo were formed in the Milky Way, but then relocated to locations above and below the Galactic Disk. This phenomena is known as “galactic eviction”, where structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk. This process causes oscillations that eject stars from the disk, in whichever the dwarf galaxy is moving.
“The oscillations can be compared to sound waves in a musical instrument,” added Bergemann. “We call this ‘ringing’ in the Milky Way galaxy ‘galactoseismology,’ which has been predicted theoretically decades ago. We now have the clearest evidence for these oscillations in our galaxy’s disk obtained so far!”
These observations were made possible thanks to the High-Resolution Echelle Spectrometer (HiRES) on the Keck Telescope. As Judy Cohen, the Kate Van Nuys Page Professor of Astronomy at Caltech and a co-author on the study, explained:
“The high throughput and high spectral resolution of HIRES were crucial to the success of the observations of the stars in the outer part of the Milky Way. Another key factor was the smooth operation of Keck Observatory; good pointing and smooth operation allows one to get spectra of more stars in only a few nights of observation. The spectra in this study were obtained in only one night of Keck time, which shows how valuable even a single night can be.”
360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO. Credit: ESO/S. Brunier
These findings are very exciting for two reasons. On the one hand, it demonstrates that halo stars likely originated in the Galactic think disk – a younger part of the Milky Way. On the other hand, it demonstrates that the Milky Way’s disk and its dynamics are much more complex than previously thought. As Allyson Sheffield of LaGuardia Community College/CUNY, and a co-author on the paper, said:
“We showed that it may be fairly common for groups of stars in the disk to be relocated to more distant realms within the Milky Way – having been ‘kicked out’ by an invading satellite galaxy. Similar chemical patterns may also be found in other galaxies, indicating a potential galactic universality of this dynamic process.”
As a next step, the astronomers plan to analyze the spectra of additional stars in the Tri-And and A13 overdensities, as well as stars in other stellar structures further away from the disk. They also plan to determine masses and ages of these stars so they can constrain the time limits of when this galactic eviction took place.
In the end, it appears that another long-held assumption on galactic evolution has been updated. Combined with ongoing efforts to probe the nuclei of galaxies – to see how their Supermassive Black Holes and star formation are related – we appear to be getting closer to understanding just how our Universe evolved over time.
A recent survey has discovered the first stars of the Milky Way. Credit: Gabriel Pérez, SMM (IAC)
According to modern cosmological models, the Universe began in a cataclysm event known as the Big Bang. This took place roughly 13.8 billion years ago, and was followed by a period of expansion and cooling. During that time, the first hydrogen atoms formed as protons and electrons combined and the fundamental forces of physics were born. Then, about 100 million years after the Big Bang, that the first stars and galaxies began to form.
The formation of the first stars was also what allowed for the creation of heavier elements, and therefore the formation of planets and all life as we know it. However, until now, how and when this process took place has been largely theoretical since astronomers did not know where the oldest stars in our galaxy were to be found. But thanks to a new study by a team of Spanish astronomers, we may have just found the oldest star in the Milky Way!
Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
This star is located roughly 7,500 light years from the Sun, and was found in the halo of the Milky Way along the line of sight to the Lynx constellation. Known as J0815+4729, this star is still in its main sequence and has a low mass, (around 0.7 Solar Masses), though the research team estimates that it has a surface temperature that is about 400 degrees hotter – 6,215 K (5942 °C; 10,727 °F) compared to 5778 K (5505 °C; 9940 °F).
For the sake of their study, the team was looking for a star that showed signs of being metal-poor, which would indicate that it has been in its main sequence for a very long time. The team first selected J0815+4729 from the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey (SDSS-III/BOSS) and then conducted follow-up spectroscopic investigations to determine its composition (and hence its age).
Consistent with what modern theory predicts, the star was found in the Galactic halo – the extended component of our galaxy that reaches beyond the galactic disk (the visible portion). It is in this region that the oldest and most metal-poor stars are believed to be found in galaxies, hence why the team was confident that a star dating back to the early Universe would be found here.
The William Herschel Telescope, part of the Isaac Newton group of telescopes, located on Canary Island. Credit: ing.iac.es
As Jonay González Hernández – a professor from the University of La Laguna, a member of the IAC and a co-author on the paper – explained in an IAC press release:
“Theory predicts that these stars could use material from the first supernovae, whose progenitors were the first massive stars in the galaxy, around 300 million years after the Big Bang. In spite of its age, and its distance away from us, we can still observe it.”
Spectra obtained by both the ISIS and OSIRIS instruments confirmed that the star was poor in metals, indicating that J0815+4729 has only one-millionth of the calcium and iron that the Sun contains. In addition, the team also noticed that the star has a higher carbon content than our Sun, accounting for almost 15% percent of its solar abundance (i.e. the relative abundance of its elements).
In short, J0815+4729 may be the most iron-poor and carbon-rich star currently known to astronomers. Moreover, finding it was rather difficult since the star is both weak in luminosity and was buried within a massive amount of SDSS/BOSS archival data. As Carlos Allende Prieto, another IAC researcher and a co-author on the paper, indicated:
“This star was tucked away in the database of the BOSS project, among a million stellar spectra which we have analysed, requiring a considerable observational and computational effort. It requires high-resolution spectroscopy on large telescopes to detect the chemical elements in the star, which can help us to understand the first supernovae and their progenitors.”
In the near future, the team predicts that next-generation spectrographs could allow for further research that would reveal more about the star’s chemical abundances. Such instruments include the HORS high-resolution spectrograph, which is presently in a trial phase on the Gran Telescopio Canarias (GTC).
“Detecting lithium gives us crucial information related to Big Bang nucleosynthesis,” said Rafael Rebolo, the director of the IAC and a coauthor of the paper. “We are working on a spectrograph of high-resolution and wide spectral range in order to measure the detailed chemical composition of stars with unique properties such as J0815+4719.”
These future studies are sure to be a boon for astronomers and cosmologists. In addition to being a chance to study stars that formed when the Universe was still in its infancy, they could provide new insight into the early stages of the universe, the formation of the first stars, and the properties of the first supernovae. In other words, they would put us a step closer to know how the Universe as we know it formed and evolved.
The ExTrA telescopes are sited at ESO’s La Silla Observatory in Chile. They will be used to search for and study Earth-sized planets orbiting nearby red dwarf stars. Credit: ESO/Emmanuela Rimbaud
Ever since the Kepler space telescope began discovering thousands of exoplanets in our galaxy, astronomers have been eagerly awaiting the day when next-generation missions are deployed. These include the much-anticipated James Webb Space Telescope, which is scheduled to take to space in 2019, but also the many ground-based observatories that are currently being constructed.
One of these is the Exoplanets in Transits and their Atmospheres (ExTrA) project, which is the latest addition to the ESO’s La Silla Observatory in Chile. Using the Transit Method, this facility will rely on three 60-centimeter (23.6 in) telescopes to search for Earth-sized exoplanets around M-type (red dwarf) stars in the Milky Way Galaxy. This week, the facility began by collecting its first light.
The Transit Method (aka. Transit Photometry) consists of monitoring stars for periodic dips in brightness. These dips are caused by planets passing in front of the star (aka. transiting) relative to the observer. In the past, detecting planets around M-type stars using this method has been challenging since red dwarfs are the smallest and dimmest class of star in the known Universe and emit the majority of their light in the near-infrared band.
Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
However, these stars have also proven to be treasure trove when it comes to rocky, Earth-like exoplanets. In recent years, rocky planets have been discovered around star’s like Proxima Centauri and Ross 128, while TRAPPIST-1 had a system of seven rocky planets. In addition, there have been studies that have indicated that potentially-habitable, rocky planets could be very common around red dwarf stars.
Unlike other facilities, the ExTrA project is well-suited to conduct surveys for planets around red dwrfs because of its location on the outskirts of the Atacama Desert in Chile. As Xavier Bonfils, the project’s lead researcher, explained:
“La Silla was selected as the home of the telescopes because of the site’s excellent atmospheric conditions. The kind of light we are observing – near-infrared – is very easily absorbed by Earth’s atmosphere, so we required the driest and darkest conditions possible. La Silla is a perfect match to our specifications.”
In addition, the ExTrA facility will rely on a novel approach that involves combining optical photometry with spectroscopic information. This consists of its three telescopes collecting light from a target star and four companion stars for comparison. This light is then fed through optical fibers into a multi-object spectrograph in order to analyze it in many different wavelengths.
The ExTrA telescopes are sited at ESO’s La Silla Observatory in Chile. Credit: ESO/Petr Horálek
This approach increases the level of achievable precision and helps mitigate the disruptive effect of Earth’s atmosphere, as well as the potential for error introduced by instruments and detectors. Beyond the goal of simply finding planets transiting in front of their red dwarf stars, the ExTrA telescopes will also study the planets it finds in order to determine their compositions and their atmospheres.
In short, it will help determine whether or not these planets could truly be habitable. As Jose-Manuel Almenara, a member of the ExTrA team, explained:
“With ExTrA, we can also address some fundamental questions about planets in our galaxy. We hope to explore how common these planets are, the behaviour of multi-planet systems, and the sorts of environments that lead to their formation,”
The potential to search for extra-solar planets around red dwarf stars is an immense opportunity for astronomers. Not only are they the most common star in the Universe, accounting for 70% of stars in our galaxy alone, they are also very long-lived. Whereas stars like our Sun have a lifespan of about 10 billion years, red dwarfs are capable of remaining in their main sequence phase for up to 10 trillion years.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
For these reasons, there are those who think that M-type stars are our best bet for finding habitable planets in the long run. At the same time, there are unresolved questions about whether or not planets that orbit red dwarf stars can stay habitable for long, owing to their variability and tendency to flare up. But with ExTrA and other next-generation instruments entering into service, astronomers may be able to address these burning questions.
“With the next generation of telescopes, such as ESO’s Extremely Large Telescope, we may be able to study the atmospheres of exoplanets found by ExTra to try to assess the viability of these worlds to support life as we know it. The study of exoplanets is bringing what was once science fiction into the world of science fact.”
ExTrA is a French project funded by the European Research Council and the French Agence National de la Recherche and its telescopes will be operated remotely from Grenoble, France. Also, be sure to enjoy this video of the ExTrA going online, courtesy of the ESOcast:
The two brightest stars of the Centaurus constellation - (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker
What springs to mind when we think of the most commonly-known stars in the night sky? Chances are, it would be stars like Sirius, Vega, Deneb, Rigel, Betelgeuse, Polaris, and Arcturus – all of which derive their names from Arabic, Greek or Latin origins. Much like the constellations, these names have been passed down from one astronomical tradition to another and were eventually adopted by the International Astronomical Union (IAU).
But what about the astronomical traditions of Earth’s many, many other cultures? Don’t the names they applied to heavens also deserve mention? According to the IAU, they do indeed! After a recent meeting by the Working Group on Star Names (WGSN), the IAU formally adopted 86 new names for stars drawn largely from the Australian Aboriginal, Chinese, Coptic, Hindu, Mayan, Polynesian, and South African peoples.
The WGSN is an international group of astronomers tasked with cataloging and standardizing the star names used by the international astronomical community. This job entails establishing IAU guidelines for the proposals and adoption of names, searching through international historical and literary sources for star names, adopting names of unique historical and cultural value, and maintaining and disseminating the official IAU star catalog.
Star map painting by Senior Wardaman Elder Bill Yidumduma Harney, featuring the Milky Way, the Moon, and ancestor spirits. Credit: IAU
Last year, the WGSN approved the names for 227 stars. With these latest additions, the catalog now contains the names of 313 stars. Unlike standard star catalogs containing millions or billions of stars designated using strings of letters and numbers, the IAU star catalog consists of bright stars with proper names derived from historical and cultural sources. As Eric Mamajek, chair and organizer of the WGSN, indicated in an IAU press release:
“The IAU Working Group on Star Names is researching traditional star names from cultures around the world and adopting unique names and spellings to avoid confusion in astronomical catalogues and star atlases. These names help ensure that intangible astronomical heritage from skywatchers around the world, and across the centuries, are preserved for use in an era of exoplanetary systems.”
A total of eleven Chinese star names were incorporated into the catalog, three of which are derived from the “lunar mansions” of traditional Chinese astronomy. This refers to vertical strips of the sky that act as markers for the progress of the Moon across the sky in the course of a year. In this sense, they provide a basis for the lunar calendar in the same way that the zodiac worked for Western calendars.
Two names were derived from the ancient Hindu lunar mansions as well. These stars are Revati and Bharani, which designate Zeta Piscium and 41 Arietis, respectively. In addition to being a lunar mansion, Revati was also the daughter of King Kakudmi in Hindu mythology and the consort of the God Balarama – the elder brother of Krishna. On the other hand, Bharani is the name for the second lunar mansion in Hindu astronomy and is ruled by Shurka (Venus).
Position of the Hindu Nakshatra Mandala, the “lunar mansions” in Hindu mythology. Credit: Wikipedia Commons/ Kishorekumar 62
Beyond the astronomical traditions of India and China, there are also two names adopted from the Khoikhoi people of South Africa and the people of Tahiti – Xamidimura and Pipirima. These names were approved for Mu¹ and Mu² Scorpii, the stars that make up a binary system located in the constellation of Scorpius. Xamidimura is derived from the Khoikhoi name for the star xami di mura – literally “eyes of the lion.”
Pipirima refers to the inseparable twins from Tahitian mythology, a boy and a girl who ran away from their parents and became stars in the night sky. Then you have the Yucatec Mayan name Chamukuy, a small bird that now designates the star Theta-2 Tauri, which is located in the Hyades star cluster in Taurus.
Four Aboriginal Australian star names were also added to the catalog, including the Wardaman names Larawag, Ginan, and Wurren and the Boorong name Unurgunite. These names now designate Epsilon Scorpii, Epsilon Crucis, Zeta Pheonicis, and Sigma Canis Majoris, respectively. Given that Aboriginal Australians have traditions that go back as far as 65,000 years, these names are some of the oldest in existence.
The brightest star to receive a new name was Alsephina, which was given to the star previously designated as Delta Velorum. The name stems from the Arabic name al-safinah (“the ship”), which refers to the ancient Greek constellation Argo Navis (the ship of the Argonauts). This name goes back to the 10th-century Arabic translation of the Almagest, which was compiled by Ptolemy in the 2nd century CE.
Artist’s concept of exoplanets orbiting a red dwarf star. Credit: NASA/JPL-Caltech
The new catalog also includes Barnard’s Star, a name that has been in common usage for about a century but was never an official designation. This red dwarf star, which is less than six light-years from Earth, is named after the astronomer who discovered it – Edward Emerson Barnard – in 1916. It now joins Alsafi (Sigma Draconis), Achird (Eta Cassiopeiae), and Tabit (Pi-3 Orionis) as being one of four nearby stars whose proper names were approved in 2017.
One of the hallmarks of modern astronomy is how naming conventions are moving away from traditional Western and Classical sources and broadening to become more worldly. In addition to being a more inclusive, multicultural approach, it reflects the spirit of modern astronomical research and space exploration, which is characterized by international cooperation.
Someday, assuming our progeny ever go forth and begin to colonize distant star systems, we can expect that the stars and planets they come to know will have names that reflect the diverse astronomical traditions of Earth’s many cultures.
A team of Brazilian scientists recently observed a binary star system consisting of a white dwarf and a brown dwarf companion. Credit: FAPESP
Eclipsing binary star systems are relatively common in our Universe. To the casual observer, these systems look like a single star, but are actually composed of two stars orbiting closely together. The study of these systems offers astronomers an opportunity to directly measure the fundamental properties (i.e. the masses and radii) of these systems respective stellar components.
Recently, a team of Brazilian astronomers observed a rare sight in the Milky Way – an eclipsing binary composed of a white dwarf and a low-mass brown dwarf. Even more unusual was the fact that the white dwarf’s life cycle appeared to have been prematurely cut short by its brown dwarf companion, which caused its early death by slowly siphoning off material and “starving” it to death.
The Observatorio del Roque de los Muchachos, located on the island of La Palma. Credit: IAC
For the sake of their study, the team conducted observations of a binary star system between 2005 and 2013 using the Pico dos Dias Observatory in Brazil. This data was then combined with information from the William Herschel Telescope, which is located in the Observatorio del Roque de los Muchachos on the island of La Palma. This system, known as of HS 2231+2441, consists of a white dwarf star and a brown dwarf companion.
White dwarfs, which are the final stage of intermediate or low-mass stars, are essentially what is left after a star has exhausted its hydrogen and helium fuel and blown off its outer layers. A brown dwarf, on the other hand, is a substellar object that has a mass which places it between that of a star and a planet. Finding a binary system consisting of both objects together in the same system is something astronomers don’t see everyday.
As Leonardo Andrade de Almeida explained in a FAPESP press release, “This type of low-mass binary is relatively rare. Only a few dozen have been observed to date.”
This particular binary pair consists of a white dwarf that is between twenty to thirty percent the Sun’s mass – 28,500 K (28,227 °C; 50,840 °F) – while the brown dwarf is roughly 34-36 times that of Jupiter. This makes HS 2231+2441 the least massive eclipsing binary system studied to date.
This artist’s impression shows an eclipsing binary star system. Credit: ESO/L. Calçada.
In the past, the primary (the white dwarf) was a normal star that evolved faster than its companion since it was more massive. Once it exhausted its hydrogen fuel, its formed a helium-burning core. At this point, the star was on its way to becoming a red giant, which is what happens when Sun-like stars exit their main sequence phase. This would have been characterized by a massive expansion, with its diameter exceeding 150 million km (93.2 million mi).
At this point, Almeida and his colleagues concluded that it began interacting gravitationally with its secondary (the brown dwarf). Meanwhile, the brown dwarf began to be attracted and engulfed by the primary’s atmosphere (i.e. its envelop), which caused it it lose orbital angular momentum. Eventually, the powerful force of attraction exceeded the gravitational force keeping the envelop anchored to its star.
Once this happened, the primary star’s outer layers began to be stripped away, exposing its helium core and sending massive amounts of matter to the brown dwarf. Because of this loss of mass, the remnant effectively died, becoming a white dwarf. The brown dwarf then began orbiting its white dwarf primary with a short orbital period of just three hours. As Almeida explained:
“This transfer of mass from the more massive star, the primary object, to its companion, which is the secondary object, was extremely violent and unstable, and it lasted a short time… The secondary object, which is now a brown dwarf, must also have acquired some matter when it shared its envelope with the primary object, but not enough to become a new star.”
Artist’s impression of a brown dwarf orbiting a white dwarf star. Credit: ESO
This situation is similar to what astronomers noticed this past summer while studying the binary star system known as WD 1202-024. Here too, a brown dwarf companion was discovered orbiting a white dwarf primary. What’s more, the team responsible for the discovery indicated that the brown dwarf was likely pulled closer to the white dwarf once it entered its Red Giant Branch (RGB) phase.
At this point, the brown dwarf stripped the primary of its atmosphere, exposing the white dwarf remnant core. Similarly, the interaction of the primary with a brown dwarf companion caused premature stellar death. The fact that two such discoveries have happened within a short period of time is quite fortuitous. Considering the age of the Universe (which is roughly 13.8 billion years old), dead objects can only be formed in binary systems.
In the Milky Way alone, about 50% of low-mass stars exist as part of a binary system while high mass stars exist almost exclusively in binary pairs. In these cases, roughly three-quarters will interact in some way with a companion – exchanging mass, accelerating their rotations, and eventually en merging.
As Almeida indicated, the study of this binary system and those like it could seriously help astronomers understand how hot, compact objects like white dwarfs are formed. “Binary systems offer a direct way of measuring the main parameter of a star, which is its mass,” he said. “That’s why binary systems are crucial to our understanding of the life cycle of stars.”
It has only been in recent years that low-mass white dwarf stars were discovered. Finding binary systems where they coexist with brown dwarfs – essentially, failed stars – is another rarity. But with every new discovery, the opportunities to study the range of possibilities in our Universe increases.
