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Gravitational Waves will let us see Inside Stars as Supernovae Happen

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century ago, has opened up new avenues of research for cosmologists and astrophysicists. Since that time, more detections have been made, all of which were said to be the result of black holes merging.

However, according to a team of astronomers from Glasgow and Arizona, astronomers need not limit themselves to detecting waves caused by massive gravitational mergers. According to a study they recently produced, the Advanced LIGO, GEO 600, and Virgo gravitational-wave detector network could also detect the gravitational waves created by supernova. In so doing, astronomers will able to see inside the hearts of collapsing stars for the first time.

The study, titled “Inferring the Core-Collapse Supernova Explosion Mechanism with Three-Dimensional Gravitational-Wave Simulations“, recently appeared online. Led by Jade Powell, who recently finished her PhD at the Institute for Gravitational Research at the University of Glasgow, the team argue that current gravitational wave experiments should be able to detect the waves created by Core Collapse Supernovae (CSNe).

Otherwise known as Type II supernovae, CCSNe are what happens when a massive star reaches the end of its lifespan and experiences rapid collapse. This triggers a massive explosion that blows off the outer layers of the star, leaving behind a remnant neutron star that may eventually become a black hole. In order for a star to undergo such collapse, it must be at least 8 times (but no more than 40 to 50 times) the mass of the Sun.

When these types of supernovae take place, it is believed that neutrinos produced in the core transfer gravitational energy released by core collapse to the cooler outer regions of the star. Dr. Powell and her colleagues believe that this gravitational energy could be detected using current and future instruments. As they explain in their study:

“Although no CCSNe have currently been detected by gravitational-wave detectors, previous studies indicate that an advanced detector network may be sensitive to these sources out to the Large Magellanic Cloud (LMC). A CCSN would be an ideal multi-messenger source for aLIGO and AdV, as neutrino and electromagnetic counterparts to the signal would be expected. The gravitational waves are emitted from deep inside the core of CCSNe, which may allow astrophysical parameters, such as the equation of state (EOS), to be measured from the reconstruction of the gravitational-wave signal.”

Dr. Powell and her also outline a procedure in their study that could be implemented using the Supernova model Evidence Extractor (SMEE). The team then conducted simulations using the latest three-dimensional models of gravitational-wave core collapse supernovae to determine if background noise could be eliminated and proper detection of CCSNe signals made.

As Dr. Powell explained to Universe Today via email:

“The Supernova Model Evidence Extractor (SMEE) is an algorithm that we use to determine how supernovae get the huge amount of energy they need to explode. It uses Bayesian statistics to distinguish between different possible explosion models. The first model we consider in the paper is that the explosion energy comes from the neutrinos emitted by the star. In the second model the explosion energy comes from rapid rotation and extremely strong magnetic fields.”

From this, the team concluded that in a three-detector network researchers could correctly determine the explosion mechanics for rapidly-rotating supernovae, depending on their distance. At a distance of 10 kiloparsecs (32,615 light-years) they would be able to detect signals of CCSNe with 100% accuracy, and signals at 2 kiloparsecs (6,523 light-years) with 95% accuracy.

In other words, if and when a supernova takes place in the local galaxy, the global network formed by the Advanced LIGO, Virgo and GEO 600 gravitational wave detectors would have an excellent chance of picking up on it. The detection of these signals would also allow for some groundbreaking science, enabling scientists to “see” inside of exploding stars for the first time. As Dr. Powell explained:

“The gravitational waves are emitted from deep inside the core of the star where no electromagnetic radiation can escape. This allows a gravitational wave detection to tell us information about the explosion mechanism that can not be determined with other methods. We may also be able to determine other parameters such as how rapidly the star is rotating.”

Illustration showing the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. Credit: LIGO/T. Pyle

Dr. Powell, having recently completed work on her PhD will also be taking up a postdoc position with the RC Centre of Excellence for Gravitational Wave Discovery (OzGrav), the gravitational wave program hosted by the University of Swinburne in Australia. In the meantime, she and her colleagues will be conducting targeted searchers for supernovae that occurred during the first and seconds advanced detector observing runs.

While there are no guarantees at this point that they will find the sought-after signals that would demonstrate that supernovae are detectable, the team has high hopes. And given the possibilities that this research holds for astrophysics and astronomy, they are hardly alone!

Further Reading: arXiv

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Cosmic Census Says There Could be 100 Million Black Holes in our Galaxy Alone

In January of 2016, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Supported by the National Science Foundation (NSF) and operated by Caltech and MIT, LIGO is dedicated to studying the waves predicted by Einstein’s Theory of General Relativity and caused by black hole mergers.

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 two additional 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!

Further Reading: UCI, MNRAS

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Stars Orbiting Supermassive Black Hole Show Einstein was Right Again!

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.

Their study, titled “Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center“, was recently published in the Astrophysical Journal. As is indicated in the course of it, the team applied new analysis techniques to existing observations that were made by European Southern Observatory’s (ESO) Very Large Telescope (VLT) and other telescopes over the course of the past 20 years.

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:

Further Reading: ESO, Astrophysical Journal

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Astronomers Measure the Mass of a White Dwarf, and Prove Einstein was Right… Again

It’s been over a century since Einstein firs proposed his Theory of General Relativity, his groundbreaking proposal for how gravity worked on large scales throughout the cosmos. And yet, after all that time, experiments are still being conducted that show that Einstein’s field equations were right on the money. And in some cases, old experiments are finding new uses, helping astronomers to unlock other astronomical mysteries.

Case in point: using the Hubble Space Telescope, NASA astronomers have repeated a century-old test of General Relativity to determine the mass of a white dwarf star. In the past, this test was used to determine how it deflects light from a background star. In this case, it was used to provide new insights into theories about the structure and composition of the burned-out remnants of a star.

White dwarfs are what become of a star after it has exited the Main Sequence of its lifespan after exhausting their nuclear fuel. This is followed by the star expelling most of its outer material, usually through a massive explosion (aka. a supernova). What is left behind is a small and extreme dense (second only to a neutron star) which exerts an incredible gravitational force.

Illustration revealing how the gravity of a white dwarf star warps space and bends the light of a distant star behind it. Credits: NASA, ESA, and A. Feild (STScI)

This attribute is what makes white dwarfs a good means for testing General Relativity. By measuring how much they deflect the light from a background star, astronomers are able to see the effect gravity has on the curvature of spacetime. This is precisely similar to what British astronomer Sir Arthur Eddington did in 1919, when he led an expedition to determine how much the Sun’s gravity deflected the light of a background star during a solar eclipse.

Known as gravitational microlensing, this same experiment was repeated by the NASA team. Using the Hubble Space Telescope, they observed Stein 2051B – a white dwarf located just 17 light-years from Earth – on seven different occasions during a two-year period. During this period, it passed in front of a background star located about 5000 light-years distant, which produced a visible deviation in the path of the star’s light.

The resulting deviation was incredibly small – only 2 milliarseconds from its actual position – and was only discernible thanks to the optical resolution of Hubble’s Wide Field Camera 3 (WFC3). Such a deviation would have been impossible to detect using instruments that predate Hubble. And more importantly, the results were consistent with what Einstein predicted a century ago.

As Kailash Sahu, an astronomer at the Space Telescope Science Institute (STScI) and the lead researcher on the project, explained in a NASA press release, this method is also an effective way to test a star’s mass. “This microlensing method is a very independent and direct way to determine the mass of a star,” he said. “It’s like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale.”

Animation showing the white dwarf star Stein 2051B as it passes in front of a distant background star. Credit: NASA

The deflection measurement yielded highly-accurate results concerning the mass of the white dwarf star – roughly 68 percent of the Sun’s mass (aka. 0.68 Solar masses) – which was also consistent with theoretical predictions. This is highly significant, in that it opens the door to a new and interesting method for determining the mass of distant stars that do not have companions.

In the past, astronomers have typically determined the mass of stars by observing binary pairs and calculating their orbital motions. Much in the same way that radial velocity measurements are used by astronomers to determine if a planet has a system of exoplanets, measuring the influence two stars have on each other is used to determine how much mass each possesses.

This was how astronomers determined the mass of the Sirius star system, which is located about 8.6 light years from Earth. This binary star system consists of a white supergiant (Sirius A) and a white dwarf companion (Sirius B) which orbit each other with a radial velocity of 5.5 km/s. These measurements helped astronomers determine that Sirius A has a mass of about 2.02 Solar masses while Sirius B weighs in at 0.978 Solar masses.

And while Stein 2051B has a companion (a bright red dwarf), astronomers cannot accurately measure its mass because the stars are too far apart – at least 8 billion km (5 billion mi). Hence, this method could be used in the future wherever companion stars are unavailable or too distant. The Hubble observations also helped the team to independently verify the theory that a white dwarf’s radius can be determined by its mass.

Artist’s impression of the binary pair made up by a white dwarf star in orbit around Sirius (a white supergiant). Credit: NASA, ESA and G. Bacon (STScI)

This theory was first proposed by Subrahmanyan Chandrasekhar in 1935, the Indian-American astronomer whose theoretical work on the evolution of stars (and black holes) earned him the Nobel Prize for Physics in 1983. They could also help astronomers to learn more about the internal composition of white dwarfs. But even with an instrument as sophisticated as the WFC3, obtaining these measurements was not without its share of difficulties.

As Jay Anderson, an astronomer with the STScI who led the analysis to precisely measure the positions of stars in the Hubble images, explained:

“Stein 2051B appears 400 times brighter than the distant background star. So measuring the extremely small deflection is like trying to see a firefly move next to a light bulb. The movement of the insect is very small, and the glow of the light bulb makes it difficult to see the insect moving.”

Dr. Sahu presented his team’s findings yesterday (June 7th) at the American Astronomical Society meeting in Austin, Texas. The team’s result will also appear in the journal Science on June 9th. And in the future, the researchers plan to use Hubble to conduct a similar microlensing study on Proxima Centauri, our solar system’s closest stellar neighbor and home to the closest exoplanet to Earth (Proxima b).

It is important to note that this is by no means the only modern experiment that has validated Einstein’s theories. In recent years, General Relativity has been confirmed through observations of rapidly spinning pulsars, 3D simulations of cosmic evolution, and (most importantly) the discovery of gravitational waves. Even in death, Einstein is still making valued contributions to astrophysics!

Further Reading: NASA

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Do Gravitational Waves Permanently Alter the Nature of Spacetime?

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century prior, opened new avenues of research for cosmologists and astrophysicists. It was also a watershed for researchers at Monash University, who played an important role in the discovery.

And now, a little over a year later, a team of researchers from the Monash Center for Astrophysics has announced another potential revelation. Based on their ongoing studies of gravitational waves, the team recently proposed a theoretical concept known as ‘orphan memory’. If true, this concept could revolutionize the way we think about gravitational waves and spacetime.

Researchers from Monash Center for Astrophysics are part of what is known as the LIGO Scientific Collaboration (LSC) – a group of scientists dedicated to developing the hardware and software needed to study gravitational waves. In addition to creating a system for vetting detections, the team played a key role in data analysis – observing and interpreting the data that was gathered – and were also instrumental in the design of the LIGO mirrors.

Looking beyond what LIGO and other experiments (like the Virgo Interferometer) observed, the research team sought to address how these detectors capabilities could be extended further by finding the “memory” of gravitational waves. The study that describes this theory was recently published in the Physical Review Letters under the title “Detecting Gravitational Wave Memory without Parent Signals“.

According to their new theory, spacetime does not return to its normal state after a cataclysmic event generates gravitational waves that cause it to stretch out. Instead, it remains stretched, which they refer to as “orphan memory” – the word “orphan” alluding to the fact the “parent wave” is not directly detectable. While this effect has yet to be observed, it could open up some very interesting opportunities for gravitational wave research.

At present, detectors like LIGO and Virgo are only able to discern the presence of gravitational waves at certain frequencies. As such, researchers are only able to study waves generated by specific types of events and trace them back to their source. As Lucy McNeill, a researchers from the Monash Center for Astrophysics and the lead author on the paper, said in a recent University press statement:

“If there are exotic sources of gravitational waves out there, for example, from micro black holes, LIGO would not hear them because they are too high-frequency. But this study shows LIGO can be used to probe the universe for gravitational waves that were once thought to be invisible to it.”

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

As they indicate in their study, high-frequency gravitational-wave bursts (i.e. ones that are in or below the kilohertz  range) would produce orphan memory that the LIGO and Virgo detectors would be able to pick up. This would not only increase the bandwidth of these detectors exponentially, but open up the possibility of finding evidence of gravity wave bursts in previous searches that went unnoticed.

Dr Eric Thrane, a lecturer at the Monash School of Physics and Astronomy and another a member of the LSC team, was also one of the co-authors of the new study. As he stated, “These waves could open the way for studying physics currently inaccessible to our technology.”

But as they admit in their study, such sources might not even exist and more research is needed to confirm that “orphan memory” is in fact real. Nevertheless, they maintain that searching for high-frequency sources is a useful way to probe for new physics, and it just might reveal things we weren’t expecting to find.

“A dedicated gravitational-wave memory search is desirable. It will have enhanced sensitivity compared to current burst searches,” they state. “Further, a dedicated search can be used to determine whether a detection candidate is consistent with a memory burst by checking to see if the residuals (following signal subtraction) are consistent with Gaussian noise.”

Alas, such searches may have to wait upon the proposed successors to the Advanced LIGO experiment. These include the Einstein Telescope and Cosmic Explorer, two proposed third-generation gravitational wave detectors. Depending on what future surveys find, we may discover that spacetime not only stretches from the creation of gravitational waves, but also bears the “stretch marks” to prove it!

Further Reading: Physical Review Letters

 

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ESO Survey Shows Dark Matter to be Pretty “Smooth”

Dark Matter has been something of a mystery ever since it was first proposed. In addition to trying to find some direct evidence of its existence, scientists have also spent the past few decades developing theoretical models to explain how it works. In recent years, the popular conception has been that Dark Matter is “cold”, and distributed in clumps throughout the Universe, an observation supported by the Planck mission data.

However, a new study produced by an international team of researchers paints a different picture. Using data from the Kilo Degree Survey (KiDS), these researchers studied how the light coming from millions of distant galaxies was affected by the gravitational influence of matter on the largest of scales. What they found was that Dark Matter appears to more smoothly distributed throughout space than previously thought.

For the past five years, the KiDS survey has been using the VLT Survey Telescope (VST) – the largest telescope at the ESO’s La Silla Paranal Observatory in Chile – to survey 1500 square degrees of the southern night sky. This volume of space has been monitored in four bands (UV, IR, green and red) using weak gravitational lensing and photometric redshift measurements.

All-sky survey data from ESA's Planck space telescope. Credit: ESA
All-sky survey data from ESA’s Planck space telescope. Credit: ESA

Consistent with Einstein’s Theory of General Relativity, gravitational lensing involves studying how the gravitational field of a massive object will bend light. Meanwhile, redshift attempts to gauge the speed at which other galaxies are moving away from ours by measuring the extent to which their light is shifted towards the red end of the spectrum (i.e. its wavelength becomes longer the faster the source is moving away).

Gravitational lensing is especially useful when it comes to determining how the Universe came to be. Our current cosmological model, known as the Lambda Cold Dark Matter (Lambda CDM) model, states that Dark Energy is responsible for the late-time acceleration in the expansion of the Universe, and that Dark Matter is made up of massive particles that are responsible for cosmological structure formation.

Using a slight variation on this technique known as cosmic sheer, the research team studied light from distant galaxies to determine how it is warped by the presence of the largest structures in the Universe (such as superclusters and filaments). As Dr. Hendrik Hildebrandt – an astronomer from the Argelander Institute for Astronomy (AIfA) and the lead author of the paper – told Universe Today via email:

“Usually one thinks of one big mass like a galaxy cluster that causes this light deflection. But there is also matter all throughout the Universe. The light from distant galaxies gets continuously deflected by this so-called large-scale structure. This results in galaxies that are close on the sky to be “pointing” in the same direction. It’s a tiny effect but it can be measured with statistical methods from large samples of galaxies.When we have measured how strongly galaxies are “pointing” in the same direction we can infer from this the statistical properties of the large-scale structure, e.g. the mean matter density and how strongly the matter is clumped/clustered.”

Beautiful image of sprites at La Silla Observatory, captured by ESO Photo Ambassador Petr Horálek. Sprites are extremely rare atmosphere phemomena caused by irregularities in the ionosphere, high above storm clouds, at altitudes of about 80 kilometres. Typically seen as groups of red-orange flashes, they are triggered by positive cloud-to-ground lightning, which is rarer and more powerful than its negative counterpart, as the lightning discharge originates from the upper regions of the cloud, further from the ground. In a short burst, the sprite extends rapidly downwards, creating dangling red tendrils before disappearing. The sprite pictured here was most likely over 500 kilometres away (compare with a satellite image showing the storm over Argentina), spanned a height of up to 80 kilometres and lasted only a fraction of a second. Links: Midsummer Night Brings Sprites — Rare phenomenon caught on camera at La Silla Red Sprites at La Silla Observatory Sprites at Paranal Observatory
A rare phenomena known as “sprites” being seen above the La Silla Observatory in Chile,  Credit: ESO/Petr Horálek

Using this technique, the research team conducted an analysis of 450 square degrees of KiDS data, which corresponds to about 1% of the entire sky. Within this volume of space, the observed how the light coming from about 15 million galaxies interacted with all the matter that lies between them and Earth.

Combining the extremely sharp images obtained by VST with advanced computer software, the team was able to carry out one of the most precise measurements ever made of cosmic shear. Interestingly enough, the results were not consistent with those produced by the ESA’s Planck mission, which has been the most comprehensive mapper of the Universe to date.

The Planck mission has provided some wonderfully detailed and accurate information about the Cosmic Microwave Background (CMB). This has helped astronomers to map the early Universe, as well as develop theories of how matter was distributed during this period. As Hildebrandt explained:

“Planck measures many cosmological parameters with exquisite precision from the temperature fluctuations of the cosmic microwave background, i.e. physical processes that happened 400,000 years after the Big Bang. Two of those parameters are the mean matter density of the Universe and a measure of how strongly this matter is clumped. With cosmic shear, we also measure these two parameters but a much later cosmic times (a few billion years ago or ~10 billion years after the Big Bang), i.e. in our more recent past.”

This model assumes the cosmological principle. The LCDM universe is homogeneous and isotropic. Time dilation and redshift z are attributed to a Doppler-like shift in electromagnetic radiation as it travels across expanding space. This model assumes a nearly "flat" spatial geometry. Light traveling in this expanding model moves along null geodesics. Light waves are 'stretched' by the expansion of space as a function of time. The expansion is accelerating due to a vacuum energy or dark energy inherent in empty space. Approximately 73% of the energy density of the present universe is estimated to be dark energy. In addition, a dark matter component is currently estimated to constitute about 23% of the mass-energy density of the universe. The 5% remainder comprises all the matter and energy observed as subatomic particles, chemical elements and electromagnetic radiation; the material of which gas, dust, rocks, planets, stars, galaxies, etc., are made. This model includes a single originating big bang event, or initial singularity, which constitutes an abrupt appearance of expanding space containing radiation. This event was immediately followed by an exponential expansion of space (inflation).
The LCDM cosmological model assumes the existence of Dark Matter and Dark Energy, and that both played an active role in the formation of the Universe. Credit: Wikipedia Commons/Alex Mittelmann

However, Hildebrandt and his team found values for these parameters that were significantly lower than those found by Planck. Basically, their cosmic shear results suggest that there is less matter in the Universe and that it is less clustered than what the Planck results predicted. These results are likely to have an impact on cosmological studies and theoretical physics in the coming years.

As it stands, Dark Matter remains undetectable using standard methods. Like black holes, its existence can only be inferred from the observable gravitational effects it has on visible matter. In this case, its presence and fundamental nature are measured by how it has affected the evolution of the Universe over the past 13.8 billion years. But since the results appear to be conflicting, astronomers may now have to reconsider some of their previously held notions.

“There are several options: because we do not understand the dominant ingredients of the Universe (dark matter and dark energy) we can play with the properties of both,” said Hildebrandt. “For example, different forms of dark energy (more complex than the simplest possibility, which is Einstein’s “cosmological constant”) could explain our measurements. Another exciting possibility is that this is a sign that the laws of gravity on the scale of the Universe are different from General Relativity. All we can say for now is that something appears to be not quite right!”

Further Reading: ESO, arXiv

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How Strong is the Force of Gravity on Earth?

Gravity is a pretty awesome fundamental force. If it wasn’t for the Earth’s comfortable 1 g, which causes objects to fall towards the Earth at a speed of 9.8 m/s², we’d all float off into space. And without it, all us terrestrial species would slowly wither and die as our muscles degenerated, our bones became brittle and weak, and our organs ceased to function properly.

So one can say without exaggerations that gravity is not only a fact of life here on Earth, but a prerequisite for it. However, since human beings seem intent on getting off this rock – escaping the “surly bonds of Earth”, as it were – understanding Earth’s gravity and what it takes to escape it is necessary. So just how strong is Earth’s gravity?

Definition:

To break it down, gravity is a natural phenomena in which all things that possess mass are brought towards one another – i.e. asteroids, planets, stars, galaxies, super clusters, etc. The more mass an object has, the more gravity it will exert on objects around it. The gravitational force of an object is also dependent on distance – i.e. the amount it exerts on an object decreases with increased distance.

Artist's impression of the effect Earth's gravity has on spacetime. Credit: NASA
Artist’s impression of the effect Earth’s gravity has on spacetime. Credit: NASA

Gravity is also one of the four fundamental forces which govern all interactions in nature (along with weak nuclear force, strong nuclear force, and electromagnetism). Of these forces, gravity is the weakest, being approximately 1038 times weaker than the strong nuclear force, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak nuclear force.

As a consequence, gravity has a negligible influence on matter at the smallest of scales (i.e. subatomic particles). However, at the macroscopic level – that of planets, stars, galaxies, etc. – gravity is the dominant force affecting the interactions of matter. It causes the formation, shape and trajectory of astronomical bodies, and governs astronomical behavior. It also played a major role in the evolution of the early Universe.

It was responsible for matter clumping together to form clouds of gas that underwent gravitational collapse, forming the first stars – which were then drawn together to form the first galaxies. And within individual star systems, it caused dust and gas to coalesce to form the planets. It also governs the orbits of the planets around stars,  of moons around planets, the rotation of stars around their galaxy’s center, and the merging of galaxies.

Universal Gravitation and Relativity:

Since energy and mass are equivalent, all forms of energy, including light, also cause gravitation and are under the influence of it. This is consistent with Einstein’s General Theory of Relativity, which remains the best means of describing gravity’s behavior. According to this theory, gravity is not a force, but a consequence of the curvature of spacetime caused by the uneven distribution of mass/energy.

Artist's impression of the frame-dragging effect in which space and time are dragged around a massive body. Credit: einstein.stanford.edu
Artist’s impression of the frame-dragging effect in which space and time are dragged around a massive body. Credit: einstein.stanford.edu

The most extreme example of this curvature of spacetime is a black hole, from which nothing can escape. Black holes are usually the product of a supermassive star that has gone supernova, leaving behind a white dwarf remnant that has so much mass, it’s escape velocity is greater than the speed of light. An increase in gravity also results in gravitational time dilation, where the passage of time occurs more slowly.

For most applications though, gravity is best explained by Newton’s Law of Universal Gravitation, which states that gravity exists as an attraction between two bodies. The strength of this attraction can calculated mathematically, where the attractive force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Earth’s Gravity:

On Earth, gravity gives weight to physical objects and causes the ocean tides. The force of Earth’s gravity is the result of the planets mass and density – 5.97237 × 1024 kg (1.31668×1025 lbs) and 5.514 g/cm3, respectively. This results in Earth having a gravitational strength of 9.8 m/s² close to the surface (also known as 1 g), which naturally decreases the farther away one is from the surface.

In addition, the force of gravity on Earth actually changes depending on where you’re standing on it. The first reason is because the Earth is rotating. This means that the gravity of Earth at the equator is 9.789 m/s2, while the force of gravity at the poles is 9.832 m/s2. In other words, you weigh more at the poles than you do at the equator because of this centripetal force, but only slightly more.

The International Space Station (ISS), seen here with Earth as a backdrop. Credit: NASA
The International Space Station (ISS), seen here from an undocked crew mission with Earth as a backdrop. Credit: NASA

Finally, the force of gravity can change depending on what’s under the Earth beneath you. Higher concentrations of mass, like high-density rocks or minerals can change the force of gravity that you feel. But of course, this amount is too slight to be noticeable. NASA missions have mapped the Earth’s gravity field with incredible accuracy, showing variations in its strength, depending on location.

Gravity also decreases with altitude, since you’re further away from the Earth’s center. The decrease in force from climbing to the top of a mountain is pretty minimal (0.28% less gravity at the top of Mount Everest), but if you’re high enough to reach the International Space Station (ISS), you would experience 90% of the force of gravity you’d feel on the surface.

However, since the station is in a state of free fall (and also in the vacuum of space) objects and astronauts aboard the ISS are capable of floating around. Basically, since everything aboard the station is falling at the same rate towards the Earth, those aboard the ISS have the feeling of being weightless – even though they still weight about 90% of what they would on Earth’s surface.

Earth’s gravity is also responsible for our planet having an “escape velocity” of 11.186 km/s (or 6.951 mi/s). Essentially, this means that a rocket needs to achieve this speed before it can hope to break free of Earth’s gravity and reach space. And with most rocket launches, the majority of their thrust is dedicated to this task alone.

Because of the difference between Earth’s gravity and the gravitational force on other bodies – like the Moon (1.62 m/s²; 0.1654 g) and Mars (3.711 m/s²; 0.376 g) – scientists are uncertain what the effects would be to astronauts who went on long-term missions to these bodies.

While studies have shown that long-duration missions in microgravity (i.e. on the ISS) have a detrimental effect on astronaut health (including loss of bone density, muscle degeneration, damage to organs and to eyesight) no studies have been conducted regarding the effects of lower-gravity environments. But given the multiple proposals made to return to the Moon, and NASA’s proposed “Journey to Mars“, that information should be forthcoming!

As terrestrial beings, we humans are both blessed and cursed by the force of Earth’s gravity. On the one hand, it makes getting into space rather difficult and expensive. On the other, it ensures our health, since our species is the product of billions of years of species evolution that took place in a 1 g environment.

If we ever hope to become a truly space-faring and interplanetary species, we better figure out how we’re going to deal with microgravity and lower-gravity. Otherwise, none of us are likely to get off-world for very long!

We have written many articles about the Earth for Universe Today. Here’s Where Does Gravity Come From?, Who Discovered Gravity?, Why is the Earth Round?, Why Doesn’t the Sun Steal the Moon?, Could We Make Artificial Gravity?, and The “Potsdam Gravity Potato” Shows Variations in Earth’s Gravity.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth, and Episode 318: Escape Velocity.

Sources:

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“Spotters Guide” for Detecting Black Hole Collisions

A supermassive black hole has been found in an unusual spot: an isolated region of space where only small, dim galaxies reside. Image credit: NASA/JPL-Caltech

When it comes to the many mysteries of the Universe, a special category is reserved for black holes. Since they are invisible to the naked eye, they remain visibly undetected, and scientists are forced to rely on “seeing” the effects their intense gravity has on nearby stars and gas clouds in order to study them.

That may be about to change, thanks to a team from Cardiff University. Here, researchers have achieved a breakthrough that could help scientists discover hundreds of black holes throughout the Universe.

Led by Dr. Mark Hannam from the School of Physics and Astronomy, the researchers have built a theoretical model which aims to predict all potential gravitational-wave signals that might be found by scientists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors.

These detectors, which act like microphones, are designed to search out remnants of black hole collisions. When they are switched on, the Cardiff team hope their research will act as a sort of “spotters guide” and help scientists pick up the faint ripples of collisions – known as gravitational waves – that took place millions of years ago.

X-ray/radio composite image of two supermassive black holes spiral towards each other near the center of a galaxy cluster named Abell 400. Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL
X-ray/radio composite image of two supermassive black holes spiraling towards each other near the center of Abell 400 galaxy cluster. Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL

Made up of postdoctoral researchers, PhD students, and collaborators from universities in Europe and the United States, the Cardiff team will work with scientists across the world as they attempt to unravel the origins of the Universe.

“The rapid spinning of black holes will cause the orbits to wobble, just like the last wobbles of a spinning top before it falls over,” Hannam said. “These wobbles can make the black holes trace out wild paths around each other, leading to extremely complicated gravitational-wave signals. Our model aims to predict this behavior and help scientists find the signals in the detector data.”

Already, the new model has been programmed into the computer codes that LIGO scientists all over the world are preparing to use to search for black-hole mergers when the detectors switch on.

Dr Hannam added: “Sometimes the orbits of these spinning black holes look completely tangled up, like a ball of string. But if you imagine whirling around with the black holes, then it all looks much clearer, and we can write down equations to describe what is happening. It’s like watching a kid on a high-speed spinning amusement park ride, apparently waving their hands around. From the side lines, it’s impossible to tell what they’re doing. But if you sit next to them, they might be sitting perfectly still, just giving you the thumbs up.”

Researchers crunched Einstein's theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. Image Credit: Henze, NASA
Researchers crunched Einstein’s theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. Credit: Henze, NASA

But of course, there’s still work to do: “So far we’ve only included these precession effects while the black holes spiral towards each other,” said Dr. Hannam. “We still need to work our exactly what the spins do when the black holes collide.”

For that they need to perform large computer simulations to solve Einstein’s equations for the moments before and after the collision. They’ll also need to produce many simulations to capture enough combinations of black-hole masses and spin directions to understand the overall behavior of these complicated systems.

In addition, time is somewhat limited for the Cardiff team. Once the detectors are switched on, it will only be a matter of time before the first gravitational wave-detections are made. The calculations that Dr. Hannam and his colleagues are producing will have to ready in time if they hope to make the most of them.

But Dr. Hannam is optimistic. “For years we were stumped on how to untangle the black-hole motion,” he said. “Now that we’ve solved that, we know what to do next.”

Further Reading: News Center – Cardiff U

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Where Have All the Pulsars Gone? The Mystery at the Center of Our Galaxy

The galactic center is a happening place, with lots of gas, dust, stars, and surprising binary stars orbiting a supermassive black hole about three million times the size of our sun. With so many stars, astronomers estimate that there should be hundreds of dead ones. But to date, scientists have found only a single young pulsar at the galactic center where there should be as many as 50.

The question thus arises: where are all those rapidly spinning, dense stellar corpses known as pulsars? Joseph Bramante of Notre Dame University and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters.

Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Basically, they disappeared into the fabric of space and time by becoming so massive that they punched a hole right through it.

Dark matter, as you may know, is the theoretical mass that astrophysicists believe fills roughly a quarter of our universe. Alas, it is invisible and undetectable by conventional means, making its presence known only in how its gravitational pull interacts with other stellar objects.

One of the more popular candidates for dark matter is Weakly Interacting Massive Particles, otherwise known as WIMPs. Underground detectors are currently hunting for WIMPs and debate has raged over whether gamma rays streaming from the galactic center come from WIMPs annihilating one another.

In general, any particle and its antimatter partner will annihilate each other in a flurry of energy. But WIMPs don’t have an antimatter counterpart. Instead, they’re thought to be their own antiparticles, meaning that one WIMP can annihilate another.

But over the last few years, physicists have considered another class of dark matter called asymmetric dark matter. Unlike WIMPs, this type of dark matter does have an antimatter counterpart.

Numerical simulation of the density of matter when the universe was one billion years old. Cosmic Infrared Background ExpeRIment (CIBER) Credit: Caltech/Jamie Bock
 Cosmic Infrared Background ExpeRIment (CIBER) simulation of the density of matter when the universe was one billion years old, as produced by large-scale structures from dark matter. Credit: Caltech/Jamie Bock

Asymmetric dark matter appeals to physicists because it’s intrinsically linked to the imbalance of matter and antimatter. Basically, there’s a lot more matter in the universe than antimatter – which is good considering anything less than an imbalance would lead to our annihilation. Likewise, according to the theory, there’s much more dark matter than anti-dark-matter.

Physicists think that in the beginning, the Big Bang should’ve created as much matter as antimatter, but something altered this balance. No one’s sure what this mechanism was, but it might have triggered an imbalance in dark matter as well – hence it is “asymmetric”.

Dark matter is concentrated at the galactic center, and if it’s asymmetric, then it could collect at the center of pulsars, pulled in by their extremely strong gravity. Eventually, the pulsar would accumulate so much mass from dark matter that it would collapse into a black hole.

The idea that dark matter can cause pulsars to implode isn’t new.  But the new research is the first to apply this possibility to the missing-pulsar problem.

If the hypothesis is correct, then pulsars around the galactic center could only get so old before grabbing so much dark matter that they turn into black holes. Because the density of dark matter drops the farther you go from the center, the researchers predict that the maximum age of pulsars will increase with distance from the center. Observing this distinct pattern would be strong evidence that dark matter is not only causing pulsars to implode, but also that it’s asymmetric.

“The most exciting part about this is just from looking at pulsars, you can perhaps say what dark matter is made of,” Bramante said. Measuring this pattern would also help physicists narrow down the mass of the dark matter particle.

Artist's illustration of a pulsar that was found to be an ultraluminous X-ray source. Credit: NASA, Caltech-JPL
Artist’s illustration of a pulsar that was found to be an ultraluminous X-ray source.
Credit: NASA, Caltech-JPL

But as Bramante admits, it won’t be easy to detect this signature. Astronomers will need to collect much more data about the galactic center’s pulsars by searching for radio signals, he claims. The hope is that as astronomers explore the galactic center with a wider range of radio frequencies, they will uncover more pulsars.

But of course, the idea that dark matter is behind the missing pulsar problem is still highly speculative, and the likelihood of it is being called into question.

“I think it’s unlikely—or at least it is too early to say anything definitive,” said Zurek, who was one of the first to revive the notion of asymmetric dark matter in 2009. The tricky part is being able to know for sure that any measurable pattern in the pulsar population is due to dark-matter-induced collapse and not something else.

Even if astronomers find this pulsar signature, it’s still far from being definitive evidence for asymmetric dark matter. As Kathryn Zurek of the Lawrence Berkeley National Laboratory explained: “Realistically, when dark matter is detected, we are going to need multiple, complementary probes to begin to be convinced that we have a handle on the theory of dark matter.”

And asymmetric dark matter may not have anything to do with the missing pulsar problem at all. The problem is relatively new, so astronomers may find more plausible, conventional explanations.

“I’d say give them some time and maybe they come up with some competing explanation that’s more fleshed out,” Bramante said.

Nevertheless, the idea is worth pursuing, says Haibo Yu of the University of California, Riverside. If anything, this analysis is a good example of how scientists can understand dark matter by exploring how it may influence astrophysical objects. “This tells us there are ways to explore dark matter that we’ve never thought of before,” he said. “We should have an open mind to see all possible effects that dark matter can have.”

There’s one other way to determine if dark matter can cause pulsars to implode: To catch them in the act. No one knows what a collapsing pulsar might look like. It might even blow up.

“While the idea of an explosion is really fun to think about, what would be even cooler is if it didn’t explode when it collapsed,” Bramante said. A pulsar emits a powerful beam of radiation, and as it spins, it appears to blink like a lighthouse with a frequency as high as several hundred times per second. As it implodes into a black hole, its gravity gets stronger, increasingly warping the surrounding space and time.

Studying this scenario would be a great way to test Einstein’s theory of general relativity, Bramante says. According to theory, the pulse rate would get slower and slower until the time between pulses becomes infinitely long. At that point, the pulses would stop entirely and the pulsar would be no more.

Further Reading: APS Physics, WIRED

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Remembering the “World War I Eclipse”

The paths of total solar eclipses care not for political borders or conflicts, often crossing over war-torn lands.

Such was the case a century ago this week on August 21st, 1914 when a total solar eclipse crossed over Eastern Europe shortly after the outbreak of World War I.

Known as the “War to End All Wars,” — which, of course, it didn’t — World War I would introduce humanity to the horrors of modern warfare, including the introduction of armored tanks, aerial bombing and poison gas. And then there was the terror of trench warfare, with Allied and Central Powers slugging it out for years with little gain.

Eclipse
The path of the total solar eclipse of August 21st, 1914 laid out across modern day Europe. Credit: Google Maps/Fred Espenak/NASA/GSFC.

But ironically, the same early 20th century science that was hard at work producing mustard gas and a better machine gun was also pushing back the bounds of astronomy. Einstein’s Annus Mirabilis or “miracle year” occurred less than a decade earlier on 1905. And just a decade later in 1924, Edwin Hubble would expand our universe a million-fold with the revelation that “spiral nebulae” were in fact, island universes or galaxies in their own right.

Indeed, it’s tough to imagine that many of these discoveries are less than a century in our past. It was against this backdrop that the total solar eclipse of August 21st, 1914 crossed the eastern European front embroiled in conflict.

Solar eclipses have graced the field of battle before. An annular solar eclipse occurred during the Battle of Isandlwana in 1879 during the Zulu Wars, and a total solar eclipse in 585 B.C. during the Battle of Thales actually stopped the fighting between the Lydians and the Medes.

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A photograph of an “eclipse camp” in the Crimea in 1914. Credit: University of Cambridge DSpace.

But unfortunately, no celestial spectacle, however grand, would save Europe from the conflagration war. In fact, several British eclipse expeditions were already en route to parts of Russia, the Baltic, and Crimea when the war broke out less than two months prior to the eclipse with the assassination of Archduke Ferdinand on June 28th, 1914. Teams arrived to a Russia already mobilized for war, and Britain followed suit on August 4th, 1914 and entered the war when Germany invaded Belgium.

You can see an ominous depiction of the path of totality from a newspaper of the day, provided from the collection of Michael Zeiler:

1914_August_22_TSE_The_Graphic_1
An illustration of the 1914 total solar eclipse “scorching” a war-ravaged Europe. Credit: From the collection of Michael Zeiler. Used with permission.

Note that the graphic depicts a Europe aflame and adds in the foreboding description of Omen faustum, inferring that the eclipse might be an “auspicious omen…” eclipses have never shaken their superstitious trappings in the eyes of man, which persists even with today’s fears of a “Blood Moon.”

A race was also afoot against the wartime backdrop to get an expedition to a solar eclipse to prove or disprove Einstein’s newly minted theory of general relativity. One testable prediction of this theory is that gravity bends light, and astronomers soon realized that the best time to catch this in action would be to measure the position of a star near the limb of the Sun — the most massive light bending object in our solar system — during a total solar eclipse. The advent of World War I would scrub attempts to observe this effect during the 1914 and 1916 eclipses over Europe.

An expedition led by astronomer Arthur Eddington to observe an eclipse from the island of Principe off of the western coast of Africa in 1919 declared success in observing this tiny deflection, measuring in less than two seconds of arc. And it was thus that a British expedition vindicated a German physicist in the aftermath of the most destructive war up to that date.

The total solar eclipse of August 21st 1914 was a member of saros cycle 124, and was eclipse number 49 of 73 in that particular series. Eclipses in the same saros come back around to nearly the same circumstances once every triple saros period of 3 times 18 years and 11.3 days, or about every 54+ years, and there was an eclipse with similar circumstances slightly east of the 1914 eclipse in 1968 — the last total eclipse of saros 124 — and a partial eclipse from the same saros will occur again on October 25th, 2022.

All historical evidence we’ve been able to track down suggests that observers that did make it into the path of totality were clouded out at show time, or at very least, no images of the August 21st 1914 eclipse exist today. Can any astute reader prove us wrong? We’d love to see some images of this historical eclipse unearthed!

Starry Night
A simulation of the total solar eclipse of August 21st 1914 as seen from Latvia. Created using Starry Night Education software.

And, as with all things eclipse related, the biggest question is always: when’s the next one? Well, we’ve got another of total lunar eclipse coming right up on October 8th, 2014, again favoring North America. The next total solar eclipse occurs on March 20th, 2015 but is only visible along a path covering the Faroe and Svalbard Islands, with a path crossing the Norwegian Sea.

But, by happy coincidence, we’re also only now three years out this week from the total solar eclipse of August 21st, 2017 that spans the contiguous “Lower 48” of the United States. The shadow of the Moon will race from the northwest and make landfall off of the Pacific coast of Oregon before reaching a maximum duration for totality at 2 minutes and 40 seconds across Missouri, southern Illinois and Kentucky and will then head towards the southeastern U.S. to depart land off of the coast of South Carolina. Millions will witness this event, and it will be the first total solar eclipse for many. A total solar eclipse hasn’t crossed the contiguous United States since 1979, so you could say that we’re “due”!

Credit
The path of the 2017 total solar eclipse across the United States. Credit: Eclipse-Maps.

Already, towns in Kentucky to Nebraska have laid plans to host this event. The eclipse occurs towards the afternoon for residents of the eastern U.S., which typically sees afternoon thunderstorms popping up in the sultry August summer heat. Eclipse cartographer Michael Zeiler states that the best strategy for eclipse chasers three years hence is to “go west, young man…”

It’s fascinating to ponder tales of eclipses past, present, and future and the role that they play in human history… where will you be on August 21st, 2017?

–      Check out Michael Zeiler’s  new site, GreatAmericanEclipse.com

–      Eclipses pop up in science fiction on occasion as well… check out our history spanning eclipse tale Exeligmos.