Coronal loop

The corona is the outer part of the solar atmosphere. Its name derives from the fact that, since it is extremely tenuous with respect to the lower atmosphere, it is visible in the optical band only during the solar eclipses as a faint crown (corona in Latin) around the black moon disk. When inspected through spectroscopy the corona reveals unexpected emission lines, which were first identified as due to a new element (coronium) but which were later ascertained to be due to high excitation states of iron. It became then clear that the corona is made of very high temperature gas, hotter than 1 MK(megakelvin). Almost all the gas is fully ionized there and thus interacts effectively with the ambient magnetic field. It is for this reason that the corona appears so inhomogeneous when observed in the X-ray band, in which plasma at million degrees emits most of its radiation. In particular, the plasma is confined inside magnetic flux tubes which are anchored on both sides to the underlying photosphere. When the confined plasma is heated more than the surroundings, its pressure and density increase. Since the tenuous plasma is optically thin, the intensity of its radiation is proportional to the square of the density, and the tube becomes much brighter than the surrounding ones and looks like a bright closed arch: a coronal loop.
Coronal Loops: Observations and Modeling of Confined Plasma

Credit: Fabio Reale

Coronal loop

The corona is the outer part of the solar atmosphere. Its name derives from the fact that, since it is extremely tenuous with respect to the lower atmosphere, it is visible in the optical band only during the solar eclipses as a faint crown (corona in Latin) around the black moon disk. When inspected through spectroscopy the corona reveals unexpected emission lines, which were first identified as due to a new element (coronium) but which were later ascertained to be due to high excitation states of iron. It became then clear that the corona is made of very high temperature gas, hotter than 1 MK(megakelvin). Almost all the gas is fully ionized there and thus interacts effectively with the ambient magnetic field. It is for this reason that the corona appears so inhomogeneous when observed in the X-ray band, in which plasma at million degrees emits most of its radiation. In particular, the plasma is confined inside magnetic flux tubes which are anchored on both sides to the underlying photosphere. When the confined plasma is heated more than the surroundings, its pressure and density increase. Since the tenuous plasma is optically thin, the intensity of its radiation is proportional to the square of the density, and the tube becomes much brighter than the surrounding ones and looks like a bright closed arch: a coronal loop.

Credit: Fabio Reale

Spiral galaxy ESO 137-001

This Hubble image shows ESO 137-001, a galaxy located in the southern constellation of Triangulum Australe (The Southern Triangle) — a delicate and beautiful spiral galaxy, but with a secret. The image not only captures the galaxy and its backdrop in stunning detail, but also something more dramatic — intense blue streaks streaming outwards from the galaxy, seen shining brightly in ultraviolet light.
These streaks are actually hot young stars, encased in wispy streams of gas that are being torn away from the galaxy by its surroundings as it moves through space. This violent galactic disrobing is due to a process known as ram pressure stripping — a drag force felt by an object moving through a fluid . The fluid in question here is superheated gas, which lurks at the centres of galaxy clusters.
This image combines NASA/ESA Hubble Space Telescope observations with data from the Chandra X-ray Observatory.

Credit: NASA, ESA, CXC

Spiral galaxy ESO 137-001

This Hubble image shows ESO 137-001, a galaxy located in the southern constellation of Triangulum Australe (The Southern Triangle) — a delicate and beautiful spiral galaxy, but with a secret. The image not only captures the galaxy and its backdrop in stunning detail, but also something more dramatic — intense blue streaks streaming outwards from the galaxy, seen shining brightly in ultraviolet light.

These streaks are actually hot young stars, encased in wispy streams of gas that are being torn away from the galaxy by its surroundings as it moves through space. This violent galactic disrobing is due to a process known as ram pressure stripping — a drag force felt by an object moving through a fluid . The fluid in question here is superheated gas, which lurks at the centres of galaxy clusters.

This image combines NASA/ESA Hubble Space Telescope observations with data from the Chandra X-ray Observatory.

Credit: NASA, ESA, CXC

Accretion Disks

Accretion flows are ubiquitous in astrophysics: they occur around protostars, accreting compact objects in binary systems, and supermassive black holes at the cores of galaxies. Much of professor James M. Stone's work has concerned studies of the local hydrodynamic and magnetohydrodynamics (MHD) processes that can lead to outward angular momentum transport in accretion disks. As computers become more powerful, previous studies of local patches of an accretion flow are being expanded into global studies that encompass the entire disk.

Accretion flows that cannot cool via emission of radiation become vertically thick and nearly spherical. Thus, they are intrinsically multidimensional. To study the structure and evolution of non-radiative accretion flows, 2D (axisymmetric) hydrodynamical simulations were performed using a non-uniform grid that spanned more than two decades in radius.

The most striking property of the flow is the large fluctuations produced by strong convection. Convective eddies transport a lot of mass both inwards and outwards, but the net mass accretion rate is very small and set by the properties of the flow near the inner boundary. A vanishingly small accretion rate may help to explain the deficit of high energy emission observed from accreting compact sources.

While understanding the properties of hydrodynamical accretion flows is important, it is generally agreed that angular momentum transport is in fact mediated by magnetic stresses. Thus, repeating the global simulations of non-radiative accretion flows with MHD calculations is vital.

Credit: James M. Stone

Plasmoids

A plasmoid is a coherent structure of plasma and magnetic fields. Plasmoids have been proposed to explain natural phenomena such as ball lightning, magnetic bubbles in the magnetosphere, and objects in cometary tails, in the solar wind, in the solar atmosphere, and in the heliospheric current sheet. Plasmoids produced in the laboratory include field-reversed configurations, spheromaks, and in dense plasma focuses.

The word plasmoid was coined in 1956 by Winston H. Bostick (1916-1991) to mean a “plasma-magnetic entity”. Bostick went on to apply his theory of plasmoids to astrophysics phenomena. 

Active regions on the solar surface are often the site of eruptions. These are associated with magnetic fields from the solar interior rising to the surface and gradually expanding into the Sun’s outer atmosphere, the corona, in a process known as magnetic flux emergence.

A group of scientists from the University of St Andrews developed 3D computer models of these phenomena, revealing that the emergence of magnetic flux naturally leads to the formation and expulsion of plasmoids that adopt a twisted tube configuration.

The formation of the plasmoids is due to the motion of plasma in the lower atmosphere of the Sun. These motions bring magnetic fieldlines closer together to reconnect and build a new magnetic flux system (i.e. the plasmoid). Whether the plasmoids are ‘failed’ or ‘successful’ (i.e. they erupt into space) depends on the level of interaction between the new emerging field and the old, pre-existing magnetic field in the solar corona.

Credit: Vasilis Archontis

The Submillimeter Array telescope unveils how small cosmic seeds grow into big stars

New images from the Smithsonian’s Submillimeter Array (SMA) telescope provide the most detailed view yet of stellar nurseries within the Snake Nebula. These images offer new insights into how cosmic seeds can grow into massive stars.
Stretching across almost 100 light-years of space, the Snake Nebula is located about 11,700 light-years from Earth in the direction of the constellation Ophiuchus. In images from NASA’s Spitzer Space Telescope, it appears as a sinuous dark tendril against the starry background. It was targeted because it shows the potential to form many massive stars (stars heavier than eight times our Sun).
Full Article

Image Credit: Spitzer/GLIMPSE/MIPS, Herschel/HiGal, Ke Wang (ESO)

The Submillimeter Array telescope unveils how small cosmic seeds grow into big stars

New images from the Smithsonian’s Submillimeter Array (SMA) telescope provide the most detailed view yet of stellar nurseries within the Snake Nebula. These images offer new insights into how cosmic seeds can grow into massive stars.

Stretching across almost 100 light-years of space, the Snake Nebula is located about 11,700 light-years from Earth in the direction of the constellation Ophiuchus. In images from NASA’s Spitzer Space Telescope, it appears as a sinuous dark tendril against the starry background. It was targeted because it shows the potential to form many massive stars (stars heavier than eight times our Sun).

Full Article

Image Credit: Spitzer/GLIMPSE/MIPS, Herschel/HiGal, Ke Wang (ESO)

 The Bolshoi Simulation

What if you could fly through the universe and see dark matter? While the technology for taking such a flight remains under development, the technology for visualizing such a flight has taken a grand leap forward with the completion of the Bolshoi Cosmological Simulation.
After 6 million CPU hours, the world’s seventh fastest supercomputer output many scientific novelties including the above flight simulation. Starting from the relatively smooth dark matter distribution of the early universe discerned from the microwave background and other large sky data sets, the Bolshoi tracked the universe’s evolution to the present epoch shown above, given the standard concordance cosmology. The bright spots in the simulation above are all knots of normally invisible dark matter, many of which contain normal galaxies. Long filaments and clusters of galaxies, all gravitationally dominated by dark matter, become evident.
Statistical comparison between the Bolshoi and current real sky maps of actual galaxies show good agreement. Although the Bolshoi simulation bolsters the existence of dark matter, many questions about our universe remain, including the composition of dark matter, the nature of dark energy, and how the first generation of stars and galaxies formed.
For more information about the Bolshoi Cosmological Simulation, click here.

Credit:  A. Klypin (NMSU), J. Primack (UCSC) et al., Chris Henze (NASA Ames), NASA’s Pleiades Supercomputer

The Bolshoi Simulation

What if you could fly through the universe and see dark matter? While the technology for taking such a flight remains under development, the technology for visualizing such a flight has taken a grand leap forward with the completion of the Bolshoi Cosmological Simulation.

After 6 million CPU hours, the world’s seventh fastest supercomputer output many scientific novelties including the above flight simulation. Starting from the relatively smooth dark matter distribution of the early universe discerned from the microwave background and other large sky data sets, the Bolshoi tracked the universe’s evolution to the present epoch shown above, given the standard concordance cosmology. The bright spots in the simulation above are all knots of normally invisible dark matter, many of which contain normal galaxies. Long filaments and clusters of galaxies, all gravitationally dominated by dark matter, become evident.

Statistical comparison between the Bolshoi and current real sky maps of actual galaxies show good agreement. Although the Bolshoi simulation bolsters the existence of dark matter, many questions about our universe remain, including the composition of dark matter, the nature of dark energy, and how the first generation of stars and galaxies formed.

  • For more information about the Bolshoi Cosmological Simulation, click here.

Credit: A. Klypin (NMSU), J. Primack (UCSC) et al., Chris Henze (NASA Ames), NASA’s Pleiades Supercomputer

What is a gamma-ray burst?

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes. The initial burst is usually followed by a longer-lived “afterglow” emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).
Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova or hypernova as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process - this may be due to the merger of binaryneutron stars. The cause of the precursor burst observed in some of these short events may be due to the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.
Gamma-ray bursts are thought to be highly focused explosions, with most of the explosion energy collimated into a narrow jet traveling at speeds exceeding 99.995% of the speed of light. The approximate angular width of the jet (that is, the degree of spread of the beam) can be estimated directly by observing the achromatic “jet breaks” in afterglow light curves: a time after which the slowly decaying afterglow begins to fade rapidly as the jet slows and can no longer beam its radiation as effectively

Image credit: NASA/Swift/Cruz deWilde

What is a gamma-ray burst?

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes. The initial burst is usually followed by a longer-lived “afterglow” emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).

Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova or hypernova as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process - this may be due to the merger of binaryneutron stars. The cause of the precursor burst observed in some of these short events may be due to the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.

Gamma-ray bursts are thought to be highly focused explosions, with most of the explosion energy collimated into a narrow jet traveling at speeds exceeding 99.995% of the speed of light. The approximate angular width of the jet (that is, the degree of spread of the beam) can be estimated directly by observing the achromatic “jet breaks” in afterglow light curves: a time after which the slowly decaying afterglow begins to fade rapidly as the jet slows and can no longer beam its radiation as effectively

Image credit: NASA/Swift/Cruz deWilde

Pulsar kick

A pulsar kick is the phenomenon that a neutron star often does not move with the velocity of its progenitor star (the origin star of a supernova explosion), but rather with a substantially greater speed. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes.
It is generally accepted today that the average pulsar kick ranges from 200–500 km/s. However, some pulsars have a much greater velocity. For example, the hypervelocity star B1508+55 has been reported to have a speed of 1100 km/s and a trajectory leading it out of the galaxy.
An extremely convincing example of a pulsar kick can be seen in the Guitar Nebula, where the bow shock - generated by the pulsar B2224+65, is moving relative to the supernova remnant nebula has been observed and confirms a velocity of 800 km/s. The Guitar Nebula is a stellar corpse that is tearing through interstellar gas and creating a guitar-shaped wake of hot hydrogen.

Pulsar kick

A pulsar kick is the phenomenon that a neutron star often does not move with the velocity of its progenitor star (the origin star of a supernova explosion), but rather with a substantially greater speed. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes.

It is generally accepted today that the average pulsar kick ranges from 200–500 km/s. However, some pulsars have a much greater velocity. For example, the hypervelocity star B1508+55 has been reported to have a speed of 1100 km/s and a trajectory leading it out of the galaxy.

An extremely convincing example of a pulsar kick can be seen in the Guitar Nebula, where the bow shock - generated by the pulsar B2224+65, is moving relative to the supernova remnant nebula has been observed and confirms a velocity of 800 km/s. The Guitar Nebula is a stellar corpse that is tearing through interstellar gas and creating a guitar-shaped wake of hot hydrogen.

Binary star

A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star or secondary.

Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightness during eclipses and transits, photometric binaries.

The first GIF shows an artist’s impression of an eclipsing binary star system. As the two stars orbit each other they pass in front of one another and their combined brightness, seen from a distance, decreases. 

Algol, known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, (2nd GIF) although Algol is actually a three-star system (Beta Persei A, B, and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B.

The second animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase.

Image credit: ESO/CHARA

Supernova explosion (artist’s impression) 

One of the most massive cosmic explosive events in the universe is a supernova. A supernova is the violent death of a luminous supergiant star. These blindingly bright star bursts occur at the end of a star’s lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy.
If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star’s envelope into interstellar space. Astronomers originally classified supernovae into two “types”, I and II. Type I had no hydrogen emission lines in their spectra whereas Type II exhibited hydrogen emission lines. Later it was realized that there were in fact three quite distinct Type I supernovae, now labelled Type Ia, Type Ib and Type Ic.
Supernovae play a fundamental role in a great cosmic recycling program. We believe that almost all of the elements in the Universe that are heavier than hydrogen and helium are created either in the centres of stars during their lifetimes or in the supernova explosions that mark the demise of larger stars.


Image Credit: freeara

Supernova explosion (artist’s impression)

One of the most massive cosmic explosive events in the universe is a supernova. A supernova is the violent death of a luminous supergiant star. These blindingly bright star bursts occur at the end of a star’s lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy.

If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star’s envelope into interstellar space. Astronomers originally classified supernovae into two “types”, I and II. Type I had no hydrogen emission lines in their spectra whereas Type II exhibited hydrogen emission lines. Later it was realized that there were in fact three quite distinct Type I supernovae, now labelled Type Ia, Type Ib and Type Ic.

Supernovae play a fundamental role in a great cosmic recycling program. We believe that almost all of the elements in the Universe that are heavier than hydrogen and helium are created either in the centres of stars during their lifetimes or in the supernova explosions that mark the demise of larger stars.


Image Credit: freeara


XZ Tauri is a binary system approximately 450 light-years away in the constellation Taurus. The system is composed of two T Tauri stars orbiting each other about 6 billion kilometers apart (roughly the same distance as Pluto is from the Sun). The system made news in 2000 when a superflare was observed in the system.

Image credit: NASA, ESA and J. Schmidt

XZ Tauri is a binary system approximately 450 light-years away in the constellation Taurus. The system is composed of two T Tauri stars orbiting each other about 6 billion kilometers apart (roughly the same distance as Pluto is from the Sun). The system made news in 2000 when a superflare was observed in the system.

Image credit: NASA, ESA and J. Schmidt

Metallicity

In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Because stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term “metal” to describe all other elements collectively. Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be “metal-rich” in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of “metal”; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Earth-like chemistry therefore has little or no relevance in stellar interiors.
The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Image credit: NASA, ESA, and H. Richer (University of British Columbia)

Metallicity

In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Because stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term “metal” to describe all other elements collectively. Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be “metal-rich” in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of “metal”; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Earth-like chemistry therefore has little or no relevance in stellar interiors.

The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Image credit: NASA, ESA, and H. Richer (University of British Columbia)

Future exploration of the outer solar system

Exploration of the giant planets of our solar system over the past few decades has revealed four unique, complex and dynamic worlds. Jupiter, Saturn, Uranus and Neptune have deep fluid interiors, gaseous atmospheres and extended magnetospheres, which serve as natural planetary-scale laboratories for the fundamental physical and chemical processes at work throughout our galaxy.
Shrinking the planetary radii to occupy the same scale allows scientists to compare the planetary ring and satellite systems for the four giant planets.
Their bulk compositions and internal structures provide signatures of the conditions within our solar nebula during the epoch of planet formation. Each harbours a complex system of planetary rings and a diverse collection of satellite environments, some with deep hidden oceans that may be of astrobiological importance. And although our understanding of these systems remains in its infancy, the four giants serve as templates for the interpretation of exoplanetary systems being discovered throughout our galaxy. The scope for new discoveries in this vast region beyond Mars is enormous, and there is no shortage of exciting mission concepts.
View larger version of the image

Credit: Leigh Fletcher

Future exploration of the outer solar system

Exploration of the giant planets of our solar system over the past few decades has revealed four unique, complex and dynamic worlds. Jupiter, Saturn, Uranus and Neptune have deep fluid interiors, gaseous atmospheres and extended magnetospheres, which serve as natural planetary-scale laboratories for the fundamental physical and chemical processes at work throughout our galaxy.

Shrinking the planetary radii to occupy the same scale allows scientists to compare the planetary ring and satellite systems for the four giant planets.

Their bulk compositions and internal structures provide signatures of the conditions within our solar nebula during the epoch of planet formation. Each harbours a complex system of planetary rings and a diverse collection of satellite environments, some with deep hidden oceans that may be of astrobiological importance. And although our understanding of these systems remains in its infancy, the four giants serve as templates for the interpretation of exoplanetary systems being discovered throughout our galaxy. The scope for new discoveries in this vast region beyond Mars is enormous, and there is no shortage of exciting mission concepts.

Credit: Leigh Fletcher

Tour of Cat’s Eye Nebula

This composite of data from NASA’s Chandra X-ray Observatory and the Hubble Space Telescope is another look for NGC 6543, better known as the Cat’s Eye nebula. This famous object is a so-called planetary nebula that represents a phase of stellar evolution that the Sun should experience several billion years from now. When a star like the Sun begins to run out of fuel, it becomes what is known as a red giant. In this phase, a star sheds some of its outer layers. A fast wind streaming away from the hot core rams into the ejected atmosphere, pushing it outward, and creating the graceful filamentary structures seen with optical telescopes. In the case of the Cat’s Eye, material shed by the star is flying away at a speed of about 4 million miles per hour. The hot core left behind will eventually collapse to form a dense white dwarf star.

Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI

The transit light curve

The transit light curve gives an astronomer a wealth of information about the transiting planet as well as the star. It is only for transiting exoplanets that astronomers have been able to get direct estimates of the exoplanet mass and radius. With these parameters at hand astronomers are able to set the most fundamental constraints on models which reveal the physical nature of the exoplanet, such as its average density and surface gravity. As mentioned above the transit events do not just give information about the exoplanet, but quite often also information about the star. With telescopes capable of high precision photometry, transit curve anomalies can say something about the activity of the star. An example of this is when an exoplanet crosses star spots (Fig. 2) [source]. This can be seen in the light curve as a small increase in flux due to the light of a cooler part of the star being blocked out.

With a very high precision light curve with a high Signal to Noise (S/N), the light curve can also be used to infer the presence of other planets in the system. Perturbations in the timing of exoplanet transits may be used to infer the presence of satellites or additional planetary companions [source,source].

More information

Credit: Paul Anthony Wilson