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If you are looking forward to the August 21 solar eclipse as much as we are, you’ll want to check out a new series on CuriosityStream called “Eclipse Across America.”
The four-part series takes an inside preview at this highly anticipated event. NASA experts, an international group of astronomers and seasoned eclipse chasers reveal secret viewing spots and tips on how to safely view what could be for millions of people a once-in-a-lifetime opportunity to see one of the most spectacular sights in nature: a total solar eclipse.
Our friend and Astronomy Cast co-host Dr. Pamela Gay, who is also the Director of Technology at the Astronomical Society of the Pacific, is part of the all-star “astronomy cast” (pun intended) of experts, and she talked with us about her part in the show.
“I get to talk about the really cool parts of an eclipse, where you can experience totality and actually feel the temperature change,” she said. Using state-of-the-art special effects, Pamela gets to show off what the eclipse will likely look like at spots around her location in the St. Louis, MO area. “There’s the fickleness of it, where some buildings will see totality and another won’t,” she said. “Of course, the closer you get to the centerline the longer totality will be, and there are some beautiful locations where you can see it.”
A special effects image from CuriosityStream showing how the solar eclipse could look in St. Louis, Missouri. Credit: CuriosityStream.
The really fun part of this particular eclipse is that it takes a path across the entire continental U.S. I asked Pamela if, given the rather divisive political and social climate these days in this country, could this total solar eclipse become a possible unifying moment?
“I think that during the event there will be a lot of overwhelming experiences that people sometimes have of being in a crowd – such as will there be enough bathrooms,” she said. “But afterwards, I think it will be this shared ‘do you remember when’ moment that we’ll have for the rest of our lives. We all have certain moments that we remember and currently here in the US a lot of those memories are ‘where were you when 911 happened? Or where were you when we started bombing Iraq.’ This will be ‘where you when the sun was eclipsed,’ and that is so much better than ‘where were you when the towers fell down.’”
If you already have a CuriosityStream account, you can watch the series starting with Episode One here. If you don’t, you can take advantage of a 7-day free trial in order to watch this series, and all the other great science offerings available, such as Stephen Hawking’s Universe, Brian Cox’s Wonders of Life, and other topics from astronomy observing tips to info about various missions to theoretical physics. Check it out. If you’re interested in continuing after your free trial, the ad-free streaming service costs $2.99, $5.99 and $11.99 per month for standard definition, high definition, and ultra high definition 4K respectively.
At the center of our galaxy, roughly 26,000 light years from Earth, lies the Supermassive Black Hole (SMBH) known as Sagittarius A*. Measuring 44 million km across, this object is roughly 4 million times as massive as our Sun and exerts a tremendous gravitational pull. Since astronomers cannot detect black holes directly, its existence has been determined largely from the effect it has on the small group of stars orbiting it.
In this respect, scientists have found that observing Sagittarius A* is an effective way of testing the physics of gravity. For instance, in the course of observing these stars, a team of German and Czech astronomers noted subtle effects caused by the black hole’s gravity. In so doing, they were able to yet again confirm some of the predictions made by Einstein’s famous Theory of General Relativity.
Artist’s impression of part of S2s orbit around the supermassive black hole at the center of the Milky Way. Credit: ESO/M. Parsa/L. Calçada
From this, they measured the orbits of the stars that orbit Sagittarius A* to test predictions made by classical Newtonian physics (i.e. Universal Gravitation), as well as predictions based on general relativity. What they found was that one of the stars (S2) showed deviations in its orbit which were defied the former, but were consistent with the latter.
This star, which has 15 times the mass of our Sun, follows an elliptical orbit around the SMBH, completing a single orbit in about 15.6 years. At its closest, it gets to within 17 light hours of the black hole, which is the equivalent of 120 times the distance between the Sun and the Earth (120 AU). Essentially, the research team noted that S2 had the most elliptical orbit of any star orbiting the Supermassive Black Hole.
They also noted a slight change in its orbit – a few percent in the shape and about one-sixth of a degree in orientation. This could only be explained as being due to the relativistic effects caused by Sagittarius A* intense gravity, which cause a precession in its orbit. What this means is, the elliptical loop of S2’s orbit rotates around the SMBH over time, with its perihelion point aimed in different directions.
Interestingly enough, this is similar to the effect that was observed in Mercury’s orbit – aka. the “perihelion precession of Mercury” – during the late 19th century. This observation challenged classical Newtonian mechanics and led scientists to conclude that Newton’s theory of gravity was incomplete. It is also what prompted Einstein to develop his theory of General Relativity, which offered a satisfactory explanation for the issue.
Should the results of their study be confirmed, this will be the first time that the effects of general relativity have been precisely calculated using the stars that orbit a Supermassive Black Hole. Marzieh Parsa – a PhD student at the University of Cologne, Germany and lead author of the paper – was understandably excited with these results. As she stated in an ESO press statement:
“The Galactic Center really is the best laboratory to study the motion of stars in a relativistic environment. I was amazed how well we could apply the methods we developed with simulated stars to the high-precision data for the innermost high-velocity stars close to the supermassive black hole.“
This study was made possible thanks to the high-accuracy of the VLT’s instruments; in particular, the adaptive optics on the NACO camera and the SINFONI near-infrared spectrometer. These instruments were vital in tracking the star’s close approach and retreat from the black hole, which allowed for the team to precisely determine the shape of its orbit and thusly determine the relativistic effects on the star.
In addition to the more precise information about S2’s orbit, the team’s analysis also provided new and more accurate estimates of Sagittarius A* mass, as well as its distance from Earth. This could open up new avenues of research for this and other Supermassive Black Holes, as well as additional experiments that could help scientists to learn more about the physics of gravity.
The central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO’s Very Large Telescope. Credit: ESO/MPE/S. Gillessen et al.
The results also provided a preview of the measurements and tests that will be taking place next year. In 2018, the star S2 will be making a very close approach to Sagittarius A*. Scientists from around the world will be using this opportunity to test the GRAVITY instrument, a second-generation instrument that was recently installed on the Very Large Telescope Interferometer (VLTI).
Developed by an international consortium led by the Max Planck Institute for Extraterrestrial Physics, this instrument has been conducting observations of the Galactic Center since 2016. In 2018, it will be used to measure the orbit of S2 with even greater precision, which is expected to be most revealing. At this time, astrophysicists will be seeking to make additional measurements of the SMBH’s general relativistic effects.
Beyond that, they also hope to detect additional deviations in the star’s orbit that could hint at the existence of new physics! With the right tools trained on the right place, and at the right time, scientists just might find that even Einstein’s theories of gravity were not entirely complete. But in the meantime, it looks like the late and great theoretical physicist was right again!
And be sure to check out this video of the recent study, courtesy of the ESO:
According to a new study by a team of astronomers from the Center of Cosmology at the University of California Irvine, such mergers are far more common than we thought. After conducting a survey of the cosmos intended to calculate and categorize black holes, the UCI team determined that there could be as many as 100 million black holes in the galaxy, a finding which has significant implications for the study of gravitational waves.
The study which details their findings, titled “Counting Black Holes: The Cosmic Stellar Remnant Population and Implications for LIGO“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Oliver D. Elbert, a postdoc student with the department of Physics and Astronomy at UC Irvine, the team conducted an analysis of gravitational wave signals that have been detected by LIGO.
LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu
Their study began roughly a year and a half ago, shortly after LIGO announced the first detection of gravitational waves. These waves were created by the merger of two distant black holes, each of which was equivalent in mass to about 30 Suns. As James Bullock, a professor of physics and astronomy at UC Irvine and a co-author on the paper, explained in a UCI press release:
“Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein’s general theory of relativity. But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, ‘How common are black holes of this size, and how often do they merge?’”
Traditionally, astronomers have been of the opinion that black holes would typically be about the same mass as our Sun. As such, they sought to interpret the multiple gravitational wave detections made by LIGO in terms of what is known about galaxy formation. Beyond this, they also sought to create a framework for predicting future black hole mergers.
From this, they concluded that the Milky Way Galaxy would be home to up to 100 million black holes, 10 millions of which would have an estimated mass of about 30 Solar masses – i.e. similar to those that merged and created the first gravitational waves detected by LIGO in 2016. Meanwhile, dwarf galaxies – like the Draco Dwarf, which orbits at a distance of about 250,000 ly from the center of our galaxy – would host about 100 black holes.
They further determined that today, most low-mass black holes (~10 Solar masses) reside within galaxies of 1 trillion Solar masses (massive galaxies) while massive black holes (~50 Solar masses) reside within galaxies that have about 10 billion Solar masses (i.e. dwarf galaxies). After considering the relationship between galaxy mass and stellar metallicity, they interpreted a galaxy’s black hole count as a function of its stellar mass.
In addition, they also sought to determine how often black holes occur in pairs, how often they merge and how long this would take. Their analysis indicated that only a tiny fraction of black holes would need to be involved in mergers to accommodate what LIGO observed. It also offered predictions that showed how even larger black holes could be merging within the next decade.
As Manoj Kaplinghat, also a UCI professor of physics and astronomy and the second co-author on the study, explained:
“We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw. Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem… If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years.”
In other words, our galaxy could be teeming with black holes, and mergers could be happening in a regular basis (relative to cosmological timescales). As such, we can expect that many more gravity wave detections will be possible in the coming years. This should come as no surprise, seeing as how LIGO has made twoadditional detections since the winter of 2016.
With many more expected to come, astronomers will have many opportunities to study black holes mergers, not to mention the physics that drive them!
In February of 2017, a team of European astronomers announced the discovery of a seven-planet system orbiting the nearby star TRAPPIST-1. Aside from the fact that all seven planets were rocky, there was the added bonus of three of them orbiting within TRAPPIST-1’s habitable zone. As such, multiple studies have been conducted that have sought to determine whether or not any planets in the system could be habitable.
When it comes to habitability studies, one of the key factors to consider is the age of the star system. Basically, young stars have a tendency to flare up and release harmful bursts of radiation while planets that orbit older stars have been subject to radiation for longer periods of time. Thanks to a new study by a pair of astronomers, it is now known that the TRAPPIST-1 system is twice as old as the Solar System.
The study, which will be published in The Astrophysical Journal under the title “On The Age Of The TRAPPIST-1 System“, was led by Adam Burgasser, an astronomer at the University of California San Diego (UCSD). He was joined by Eric Mamajek, the deputy program scientist for NASA’s Exoplanet Exploration Program (EEP) at the Jet Propulsion Laboratory.
Together, they consulted data on TRAPPIST-1s kinematics (i.e. the speed at which it orbits the center of the galaxy), its age, magnetic activity, density, absorption lines, surface gravity, metallicity, and the rate at which it experiences stellar flares. From all this, they determined that TRAPPIST-1 is quite old, somewhere between 5.4 and 9.8 billion years of age. This is up to twice as old as our own Solar System, which formed some 4.5 billion years ago.
These results contradict previously-held estimates, which were that the TRAPPIST-1 system was about 500 millions yeas old. This was based on the fact that it would have taken this long for a low-mass star like TRAPPIST-1 (which has roughly 8% the mass of our Sun) to contract to its minimum size. But with an upper age limit that is just under 10 billion years, this star system could be almost as old as the Universe itself!
As Dr. Burgasser explained in a recent NASA press statement:
“Our results really help constrain the evolution of the TRAPPIST-1 system, because the system has to have persisted for billions of years. This means the planets had to evolve together, otherwise the system would have fallen apart long ago.”
The implications of this could be very significant as far as habitability studies are concerned. For one, older stars experience less in the way of flareups compared to younger ones. From their study, Burgasser and Mamajek confirmed that TRAPPIST-1 is relatively quiet compared to other ultra-cool dwarf stars. However, since the planets around TRAPPIST-1 orbit so close to their star, they have been exposed to billions of years of radiation at this point.
An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl
As such, it is possible that most of the planets which orbit TRAPPIST-1 – expect for the outermost two, g and h – would probably have had their atmospheres stripped away – similar to what happened to Mars billions of years ago when it lost its protective magnetic field. This is certainly consistent with many recent studies, which concluded that TRAPPIST-1’s solar activity would not be conducive to life on any of its planets.
Whereas some of these studies addressed TRAPPIST-1s level of stellar flare, others examined the role magnetic fields would play. In the end, they concluded that TRAPPIST-1 was too variable, and that its own magnetic field would likely be connected to the fields of its planets, allowing particles from the star to flow directly onto the planets atmospheres (thus allowing them to be more easily stripped away).
However, the results were not entirely bad news. Since the TRAPPIST-1 planets have estimated densities that are lower than that of Earth, it is possible that they have large amounts of volatile elements (i.e. water, carbon dioxide, ammonia, methane, etc). These could have led to the formation of thick atmospheres that protected the surfaces from a lot of harmful radiation and redistributed heat across the tidally-locked planets.
Then again, a thick atmosphere could also have an effect akin to Venus, creating a runaway greenhouse effect that would have resulted in incredibly thick atmospheres and extremely hot surfaces. Under the circumstances, then, any life that emerged on these planets would have had to be extremely hardy in order to survive for billions of years.
Artist’s impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.
Another positive thing to consider is TRAPPIST-1’s constant brightness and temperature, which are also typical of M-class (red dwarf) stars. Stars like our Sun have an estimated lifespan of 10 billion years (which it is almost halfway through) and grow steadily brighter and hotter with time. Red dwarfs, on the other hand, are believed to exist for as much as 10 trillion years – far longer than the Universe has existed – and do not change much in intensity.
Given the amount of time it took for complex life to have emerged on Earth (over 4.5 billion years), this longevity and consistency could make red dwarf star systems the best long-term bet for habitability. Such was the conclusion of one recent study, which was conducted by Prof. Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA). And as Mamajek explained:
“Stars much more massive than the Sun consume their fuel quickly, brightening over millions of years and exploding as supernovae. But TRAPPIST-1 is like a slow-burning candle that will shine for about 900 times longer than the current age of the universe.”
NASA has also expressed excitement over these findings. “These new results provide useful context for future observations of the TRAPPIST-1 planets, which could give us great insight into how planetary atmospheres form and evolve, and persist or not,” said Tiffany Kataria, an exoplanet scientist at JPL. At the moment, habitability studies of TRAPPIST-1 and other nearby star systems are confined to indirect methods.
However, in the near future, next-generation missions like the James Webb Space Telescope are expected to reveal additional information – such as whether or not these planets have atmospheres and what their compositions are. Future observations with the Hubble Space Telescope and the Spitzer Space Telescope are also expected to improve our understanding of these planets and possible conditions on their surface.
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 54!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these objects is the globular cluster known as Messier 54. Located in the direction of the Sagittarius constellation, this cluster was once thought to be part of the Milky Way, located about 50,000 light years from Earth, In recent decades, astronomers have come to realize that it is actually part of the Sagittarius Dwarf Galaxy, located some 87,000 light-years away.
What You Are Looking At:
Running away from us at a speed of 142 kilometers per second, this compact globe of stars could be as wide as 150 light years in diameter and as far away as 87,400 light years. Wait… Hold the press… Almost 90 thousand light years? Yeah. Messier 54 isn’t part of our own Milky Way Galaxy!
In 1994 astronomers made a rather shocking discovery… this tough to resolve globular was actually part of the Sagittarius Dwarf Elliptical Galaxy. As Michael H. Siegal (et al) said in their study:
“As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H]~0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.”
Inside its compact depths lurk at least 82 known variable stars – 55 of which are the RR Lyrae type. But astronomers using the Hubble Space telescope have have also discovered there are two semi-regular red variables with periods of 77 and 101 days. Kevin Charles Schlaufman and Kenneth John Mighell of the National Optical Astronomy Observatory explained in their study:
“Most of our candidate variable stars are found on the PC1 images of the cluster center – a region where no variables have been reported by previous ground-based studies of variables in M54. These observations cannot be done from the ground, even with AO as there are far too many stars per resolution element in ground-based observations.”
The globular cluster Messier 54. Credit: NASA
But what other kinds of unusual stars could be discovered inside such distant cosmic stellar evolutionary laboratory? Try a phenomena known as blue hook stars! As Alfred Rosenberg (et al) said in their study:
“We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called “blue hook” stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, Omega Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around Teff = 27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.”
But with the stars packaged together so tightly, even more has been bound to occur inside of Messier 54. As Tim Adams (et al) indicated in their study:
“We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the colour gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables.”
Messier 54, as imaged by the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile. Credit: ESO
But the discoveries haven’t ended yet…. Because 2009 studies have revealed evidence for an intermediate mass black hole inside Messier 54 – the first known to have ever been discovered in a globular cluster.
“We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ± 0.7 km s-1, decreasing to 16.4 ± 0.4 km s-1 at 2farcs5 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M sun.” says R. Ibata (et al), “However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.”
History of Observation:
On July 24, 1778 when Charles Messier first laid eyes on this faint fuzzy, he had no clue that he was about to discover the very first extra-galactic globular cluster. In his notes he writes: “Very faint nebula, discovered in Sagittarius; its center is brilliant and it contains no star, seen with an achromatic telescope of 3.5 feet. Its position has been determined from Zeta Sagittarii, of 3rd magnitude.”
Years later Sir William Herschel would also study M54, and in his private notes he writes: “A round, resolvable nebula. Very bright in the middle and the brightness diminishing gradually, about 2 1/2′ or 3′ in diameter. 240 shews too pretty large stars in the faint part of the nebulosity, but I rather suppose them to have no connection with the nebula. I believe it to be no other than a miniature cluster of very compressed stars.”
Countless other observations would follow as the M54 became cataloged by other astronomers and each would in turn describe it only as having a much brighter core and some resolution around the edges. Have fun trying to crack this one!
Locating Messier 54:
M54 isn’t hard to find… Just skip down to Zeta Sagittarii, the southwestern-most star of Sagittarius “teapot” and hop a half degree south and a finger width (1.5 degrees) west. The problem is seeing it! In small optics, such as binoculars or a finder scope, it will appear almost stellar because of its small size. However, if you just look for what appears like a larger, dim star that won’t quite come into perfect focus, then you’ve found it.
In smaller telescopes, you’ll get no resolution on this class III globular cluster because it is so dense. Large aperture doesn’t fare much better either, with only some individual stars making their appearance at the outer perimeters. Because of magnitude and size, Messier 54 is better suited to dark sky conditions.
The location of Messier 54 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
And here are the quick facts on this Messier Object to help you get started:
Object Name: Messier 54 Alternative Designations: M54, NGC 6715 Object Type: Class III Extragalactic Globular Cluster Constellation: Sagittarius Right Ascension: 18 : 55.1 (h:m) Declination: -30 : 29 (deg:m) Distance: 87.4 (kly) Visual Brightness: 7.6 (mag) Apparent Dimension: 12.0 (arc min)
Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.
The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.
The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.
Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth (left side). Credit: Wikipedia
For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.
Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.
Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.
Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.
Tidally-locked, rocky planets are common around low-mass, M-type (red dwarf) stars, due to their close orbits. Credit: M. Weiss/CfA
As he explained in a University of Washington press statement:
“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”
From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.
In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.
For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.
Artist’s impression of a “Earth-like” planet orbiting a nearby red dwarf star. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.
In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.
Prof. Barnes paper was accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy. The research was funded by a NASA grant through the Virtual Planetary Laboratory.
The rotation rate of the Sun's core has been accurately measured for the first time. The Sun, which has been remarkably stable for the past 4.6 billion years, is held together by the almost perfect equilibrium between the force of gravity, which tends to cause it to collapse, and the pressure of the thermonuclear reactions in its core. Now, researchers working together with a team at the Laboratoire Lagrange (CNRS/Observatoire de la Côte d'Azur/Université Nice Sophia Antipolis) have determined that the Sun's core makes a complete rotation once per week.
Using the GOLF instrument, orbiting around the Sun on board the SOHO space observatory, to measure solar oscillations, they developed a novel approach that enabled them to unambiguously detect gravity oscillation modes within our star. This work, which will certainly stimulate a new era of research into the physics of the solar core, is published in the journal Astronomy & Astrophysics.
The Sun, which has been remarkably stable for the past 4.6 billion years, is held together by the almost perfect equilibrium between the force of gravity, which tends to cause it to collapse, and the pressure of the thermonuclear reactions in its core. The GOLF instrument, orbiting around the Sun on board the SOHO space observatory, measures solar oscillations, which carry information about the physical properties of its different layers. Every ten seconds, GOLF, which has been orbiting our star for over twenty years, records an integrated signal of oscillations of the solar surface. Various teams analyze this flow of data with the aim of identifying the many oscillation modes exhibited by the Sun.
Now, researchers from the Laboratoire Lagrange (CNRS/Observatoire de la Côte d'Azur/Université Nice Sophia Antipolis), the Institut d'Astrophysique Spatiale (CNRS/Université Paris-Sud), the Laboratoire Astrophysique, Interprétation, Modélisation (CNRS/Université Paris Diderot/CEA), the Laboratoire d'Astrophysique de Bordeaux (CNRS/Université de Bordeaux), the Instituto de Astrofísica de Canarias, and UCLA (University of California, Los Angeles) have successfully detected the Sun's gravity modes.
These are similar to waves in which gravity is the restoring force, such as waves on the surface of the sea, although in the Sun they can only exist in its very deepest layers. Since these oscillations are particularly difficult to observe, the researchers used the GOLF data in a novel way, by making use of a differential parameter of the acoustic oscillation modes, which are observable at the surface. This parameter measures the round trip time of acoustic waves traveling through the center of the Sun. The researchers detected the impact of gravity modes on them, thus demonstrating their existence.
The first result of this detection is that the researchers were able to accurately measure the mean rotation rate of the Sun's thermonuclear core, about which little was previously known. The core makes a complete rotation in one week, which is 3.8 times faster than the outer and intermediate layers. This work should stimulate much research in solar physics, making it possible to further refine models of the Sun's birth, evolution, structure and chemical composition. In particular, the gravity modes indicate that there is a region at the boundary of the thermonuclear core where the speed varies enormously, which is not predicted by the standard model of the Sun. It will also stimulate discussion about the nature of a possible magnetic field in the Sun's center.
Most of the nuclear reactions that drive the nucleosynthesis of the elements in our universe occur in very extreme stellar plasma conditions. This intense environment found in the deep interiors of stars has made it nearly impossible for scientists to perform nuclear measurements in these conditions -- until now.
In a unique cross-disciplinary collaboration between the fields of plasma physics, nuclear astrophysics and laser fusion, a team of researchers, including scientists from Lawrence Livermore National Laboratory (LLNL), Ohio University, the Massachusetts Institute of Technology (MIT) and Los Alamos National Laboratory (LANL), describe experiments performed in conditions like those of stellar interiors. The team's findings were published today by Nature Physics.
The experiments are the first thermonuclear measurements of nuclear reaction cross-sections -- a quantity that describes the probability that reactants will undergo a fusion reaction -- in high-energy-density plasma conditions that are equivalent to the burning cores of giant stars, i.e., 10-40 times more massive than the sun. These extreme plasma conditions boast hydrogen-isotope densities compressed by a factor of a thousand to near that of solid lead and temperatures heated to approximately 50 million Kelvin. These are the conditions in stars that lead to supernovae, the most massive explosions in the universe.
"Ordinarily, these kinds of nuclear astrophysics experiments are performed on accelerator experiments in the laboratory, which become particularly challenging at the low energies often relevant for nucleosynthesis," said LLNL physicist Dan Casey, the lead author on the paper. "As the reaction cross-sections fall rapidly with decreasing reactant energy, bound electron screening corrections become significant, and terrestrial and cosmic background sources become a major experimental challenge."
The work was conducted at LLNL's National Ignition Facility (NIF), the only experimental tool in the world capable of creating temperatures and pressures like those found in the cores of stars and giant planets. Using the indirect drive approach, NIF was used to drive a gas-filled capsule implosion, heating capsules to extraordinary temperatures and compressing them to high densities where fusion reactions can occur.
"One of the most important findings is that we reproduced prior measurements made on accelerators in radically different conditions," Casey said. "This really establishes a new tool in the nuclear astrophysics field for studying various processes and reactions that may be difficult to access any other way."
"Perhaps most importantly, this work lays groundwork for potential experimental tests of phenomena that can only be found in the extreme plasma conditions of stellar interiors. One example is of plasma electron screening, a process that is important in nucleosynthesis but has not been observed experimentally," Casey added.
Now that the team has established a technique to perform these measurements, related teams like that led by Maria Gatu Johnson at MIT are looking to explore other nuclear reactions and ways to attempt to measure the impact of plasma electrons on the nuclear reactions.
Casey was joined by co-authors Daniel Sayre, Vladimir Smalyuk, Robert Tipton, Jesse Pino, Gary Grim, Bruce Remington, Dave Dearborn, Laura (Robin) Benedetti, Robert Hatarik, Nobuhiko Izumi, James McNaney, Tammy Ma, Steve MacLaren, Jay Salmonson, Shahab Khan, Arthur Pak, Laura Berzak Hopkins, Sebastien LePape, Brian Spears, Nathan Meezan, Laurent Divol, Charles Yeamans, Joseph Caggiano, Dennis McNabb, Dean Holunga, Marina Chiarappa-Zucca, Tom Kohut and Thomas Parham from LLNL, Carl Brune from Ohio University, Johan Frenje and Maria Gatu Johnson from MIT and George Kyrala from LANL.
A partial lunar eclipse took place on August 7/8, 2017, the second of two lunar eclipses in 2017. The moon was only slightly covered by the Earth's umbral shadow at maximum eclipse. The partial eclipse lasted for one hour and 55 minutes.
Most of Asia, Africa, Europe and Australia was treated to a spectacular partial lunar eclipse. The phenomenon occurs when the moon moves through the outer part of the Earth's shadow, blocking part of the sunlight from reaching the moon and causing it to appear larger than normal.
"The interesting thing about lunar eclipse timings is that anywhere on Earth from where the moon is visible during eclipse, the time will be the same. This is in contrast to a solar eclipse in which the timings of the contacts change as the location changes on Earth," said N Rathnashree, director of the Nehru Planetarium in Delhi, India.
The moon inside the umbral shadow was a subtle red, but hard to see in contrast to the much brighter moon in the outer penumbral shadow.
The solar eclipse of August 21, 2017 occurs fourteen days later, in the same eclipse season. It will be the first total solar eclipse visible in the contiguous United States since the solar eclipse of February 26, 1979.
Only a small part of the universe consists of visible matter. By far the largest part is invisible and consists of dark matter and dark energy. Very little is known about dark energy, but there are many theories and experiments on the existence of dark matter designed to find these as yet unknown particles. Scientists at Johannes Gutenberg University Mainz (JGU) have now come up with a new theory on how dark matter may have been formed shortly after the origin of the universe. This new model proposes an alternative to the WIMP paradigm that is the subject of various experiments in current research.
Dark matter is present throughout the universe, forming galaxies and the largest known structures in the cosmos. It makes up around 23 percent of our universe, whereas the particles visible to us that make up the stars, planets, and even life on Earth represent only about four percent of it. The current assumption is that dark matter is a cosmological relic that has essentially remained stable since its creation. "We have called this assumption into question, showing that at the beginning of the universe dark matter may have been unstable," explained Dr. Michael Baker from the Theoretical High Energy Physics (THEP) group at the JGU Institute of Physics. This instability also indicates the existence of a new mechanism that explains the observed quantity of dark matter in the cosmos.
The stability of dark matter is usually explained by a symmetry principle. However, in their paper, Dr. Michael Baker and Professor Joachim Kopp demonstrate that the universe may have gone through a phase during which this symmetry was broken. This would mean that it is possible for the hypothetical dark matter particle to decay. During the electroweak phase transition, the symmetry that stabilizes dark matter would have been reestablished, enabling it to continue to exist in the universe to the present day.
With their new theory, Baker and Kopp have introduced a new principle into the debate about the nature of dark matter that offers an alternative to the widely accepted WIMP theory. Up to now, WIMPs, or weakly interacting massive particles, have been regarded as the most likely components of dark matter, and experiments involving heavily shielded underground detectors have been carried out to look for them. "The absence of any convincing signals caused us to start looking for alternatives to the WIMP paradigm," said Kopp.
The two physicists claim that the new mechanism they propose may be connected with the apparent imbalance between matter and antimatter in the cosmos and could leave an imprint which would be detected in future experiments on gravitational waves. In their paper published in the scientific journal Physical Review Letters, Baker and Kopp also indicate the prospects of finding proof of their new principle at CERN's LHC particle accelerator and other experimental facilities.
A new analysis of data from ESO’s Very Large Telescope and other telescopes suggests that the orbits of stars around the supermassive black hole at the center of the Milky Way may show the subtle effects predicted by Einstein’s general theory of relativity. There are hints that the orbit of the star S2 is deviating slightly from the path calculated using classical physics. This tantalizing result is a prelude to much more precise measurements and tests of relativity that will be made using the GRAVITY instrument as star S2 passes very close to the black hole in 2018.
At the center of the Milky Way, 26 000 light-years from Earth, lies the closest supermassive black hole, which has a mass four million times that of the Sun. This monster is surrounded by a small group of stars orbiting at high speed in the black hole’s very strong gravitational field. It is a perfect environment in which to test gravitational physics, and particularly Einstein’s general theory of relativity.
A team of German and Czech astronomers have now applied new analysis techniques to existing observations of the stars orbiting the black hole, accumulated using ESO’s Very Large Telescope (VLT) in Chile and others over the last twenty years. They compare the measured star orbits to predictions made using classical Newtonian gravity as well as predictions from general relativity.
The team found suggestions of a small change in the motion of one of the stars, known as S2, that is consistent with the predictions of general relativity. The change due to relativistic effects amounts to only a few percent in the shape of the orbit, as well as only about one sixth of a degree in the orientation of the orbit. If confirmed, this would be the first time that a measurement of the strength of the general relativistic effects has been achieved for stars orbiting a supermassive black hole.
Marzieh Parsa, PhD student at the University of Cologne, Germany and lead author of the paper, is delighted: "The Galactic Center really is the best laboratory to study the motion of stars in a relativistic environment. I was amazed how well we could apply the methods we developed with simulated stars to the high-precision data for the innermost high-velocity stars close to the supermassive black hole."
The high accuracy of the positional measurements, made possible by the VLT’s near-infrared adaptive optics instruments, was essential for the study. These were vital not only during the star’s close approach to the black hole, but particularly during the time when S2 was further away from the black hole. The latter data allowed an accurate determination of the shape of the orbit.
"During the course of our analysis we realized that to determine relativistic effects for S2 one definitely needs to know the full orbit to very high precision," comments Andreas Eckart, team leader at the University of Cologne.
As well as more precise information about the orbit of the star S2, the new analysis also gives the mass of the black hole and its distance from Earth to a higher degree of accuracy.
Co-author Vladimir Karas from the Academy of Sciences in Prague, the Czech Republic, is excited about the future: "This opens up an avenue for more theory and experiments in this sector of science."
This analysis is a prelude to an exciting period for observations of the Galactic Center by astronomers around the world. During 2018 the star S2 will make a very close approach to the supermassive black hole. This time the GRAVITY instrument, developed by a large international consortium led by the Max-Planck-Institut für extraterrestrische Physik in Garching, Germany, and installed on the VLT Interferometer, will be available to help measure the orbit much more precisely than is currently possible. Not only is GRAVITY, which is already making high-precision measurements of the Galactic Center, expected to reveal the general relativistic effects very clearly, but also it will allow astronomers to look for deviations from general relativity that might reveal new physics.
It is known that the sun's corona -- the outermost layer of the sun's atmosphere -- is roughly 100 times hotter than its photosphere -- the sun's visible layer. The reason for this mysterious heating of the solar coronal plasma, however, is not yet entirely understood. A research team in India has developed a set of numerical computations to shed light on this phenomenon, and present this week in Physics of Plasmas, from AIP Publishing, analysis examining the role of chaotic magnetic fields in potential heating mechanisms.
Operating under the idea that chaotically tangled magnetic field lines exist throughout astrophysical plasmas, the team used high-performance computer simulation to gain an understanding of these chaotic field lines. Specifically, they investigated conditions that create ribbons of intense electric current, known as current sheets.
The current sheets, believed to be produced in the coronal plasma, are potential sites for magnetic reconnections, which provide a mechanism for extreme heating of the corona. Moreover, within the current sheets, the electric field peaks up and accelerates charged particles.
"We want to go one step forward to explain the spontaneous generation of these current sheets," said Sanjay Kumar, a member of the research team.
The research method focused on allowing an incompressible, thermally homogeneous magnetofluid with infinite electrical conductivity to relax via viscous dissipation, toward a characterized final state. The computations were made consistent with well-accepted magnetostatic theory and resulted in spontaneous current sheet development, making them relevant for the study of particle acceleration in astrophysical plasmas.
Using Vikram-100, the 100TF High Performance Computing facility at the Physical Research Laboratory, the researchers simulated the viscous relaxation and verified accurate flux-freezing, a conservative behavior a reliable simulation must demonstrate. The team plotted the maximal intensities of volume current densities for specific trends of increasing magnetic field chaos, which provided a measure of the production of current sheets. Additionally, the maximal magnitudes of volume current density were found to scale with the numerical resolution used in the computer simulation, which showed the expected scaling of current sheet development.
The simple fact that the maximum value of volume current density was increased with increasing magnetic field line chaos, called "chaoticity," suggests a direct proportionality between the intensity of the current sheet and chaoticity.
In the three cases studied, the researchers found the formation of two different sets of current sheets. One set was arranged along the y-axis, while the second formed in a different location and at a time later than the first. From their analysis of this occurrence, the team determined that a favorable evolution bring non-parallel magnetic field lines into close proximity and intensify current sheets.
These simulations provide new and novel insight regarding the influence of chaotic magnetic field lines on the spontaneous development of current sheets, and hence potential places of particle acceleration.
"This is the first time we have explained the role of chaotic field line in generating these spontaneous current sheets," Kumar said, referring to the scientific community as a whole.
Scientists have discovered why heavyweight galaxies living in a dense crowd of galaxies tend to spin more slowly than their lighter neighbors. “Contrary to earlier thinking, the spin rate of the galaxy is determined by its mass, rather than how crowded its neighborhood is,” says study first author Associate Professor Sarah Brough of UNSW Sydney and the ARC Centre of Excellence for All-sky Astrophysics, CAASTRO.
The finding, based on a detailed study of more than 300 galaxies, is published in The Astrophysical Journal.
To measure how fast their galaxies rotated, the researchers used an instrument called the Sydney-AAO Multi-object Integral field spectrograph (SAMI) on the 4-meter Anglo-Australian Telescope in eastern Australia.
SAMI ‘dissects’ galaxies, obtaining optical spectra from 61 points across the face of each galaxy, 13 galaxies at a time.
“We want to know which factors really drive how galaxies evolve,” says team member Dr Matt Owers of the Australian Astronomical Observatory and Macquarie University. “In this case, we’ve sorted out nature versus nurture.”
The new finding runs counter to previous studies, made with smaller samples of galaxies, which concluded that a galaxy’s spin rate is determined by the other galaxies in its neighborhood.
Associate Professor Brough says this earlier conclusion was spurious. “Once you take into account the strong association with mass, there’s no link between a galaxy’s spin rate and its environment,” she says.
The research team was drawn from the Australian Astronomical Observatory; UNSW Sydney; the universities of Sydney, Melbourne, Queensland and Oxford; The Australian National University, Macquarie University, Swinburne University of Technology; Yonsei University in South Korea and the California Institute of Technology.
The ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) is a collaboration between The University of Sydney, The Australian National University, The University of Melbourne, Swinburne University of Technology, The University of Queensland, The University of Western Australia and Curtin University, the last two participating together as the International Centre for Radio Astronomy Research (ICRAR).
CAASTRO is funded under the Australian Research Council (ARC) Centre of Excellence program, with additional funding from the seven participating universities and from the NSW State Government's Science Leveraging Fund.
