Avalanche

An avalanche (also called a snowslide or snowslip) is a rapid flow of snow down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.

Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches. The remainder of this article refers to snow avalanches.

The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches. A popular myth is that avalanches can be triggered by loud noise or shouting, but the pressure from sound is orders of magnitude too small to trigger an avalanche.

Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds.

There is no universally accepted classification of avalanches—different classifications are useful for different purposes. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.

Explanation from: https://en.wikipedia.org/wiki/Avalanche

Hummingbird hawk-moth

The hummingbird hawk-moth (Macroglossum stellatarum) is a species of Sphingidae. Its long proboscis and its hovering behaviour, accompanied by an audible humming noise, make it look remarkably like a hummingbird while feeding on flowers. It should not be confused with the moths called hummingbird moths in North America, genus Hemaris, members of the same family and with similar appearance and behavior. The resemblance to hummingbirds is an example of convergent evolution. It flies during the day, especially in bright sunshine, but also at dusk, dawn, and even in the rain, which is unusual for even diurnal hawkmoths. Its visual abilities have been much studied, and it has been shown to have a relatively good ability to learn colours.

The hummingbird hawk-moth is distributed throughout the northern Old World from Portugal to Japan, but is resident only in warmer climates (southern Europe, North Africa, and points east). It is a strong flier, dispersing widely and can be found virtually anywhere in the hemisphere in the summer. However it rarely survives the winter in northern latitudes (e.g. north of the Alps in Europe, north of the Caucasus in Russia). They have been spotted in Newfoundland, Canada and Malaysia as well.

Moths in the Hemaris genus of the family Sphingidae are known as "hummingbird moths" in the US, and "bee moths" in Europe, which sometimes causes confusion between this species and the North American genus.

Image Credit & Copyright: Malcolm Randle
Explanation from: https://en.wikipedia.org/wiki/Hummingbird_hawk-moth

Interacting Galaxies Arp 87

Arp 87 is a stunning pair of interacting galaxies. Stars, gas, and dust flow from the large spiral galaxy, NGC 3808, forming an enveloping arm around its companion. The shapes of both galaxies have been distorted by their gravitational interaction. Arp 87 is located in the constellation of Leo, the Lion, approximately 300 million light-years away from Earth. Arp 87 appears in Arp's Atlas of Peculiar Galaxies. As also seen in similar interacting galaxies, the corkscrew shape of the tidal material suggests that some stars and gas drawn from the larger galaxy have been caught in the gravitational pull of the smaller one. This image was taken in February 2007 with Hubble's Wide Field Planetary Camera 2 detector.

Image Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Explanation from: https://www.spacetelescope.org/images/heic0717a/

Pillars of Creation in the Infrared

The NASA/ESA Hubble Space Telescope has revisited one of its most iconic and popular images: the Eagle Nebula’s Pillars of Creation.

This image shows the pillars as seen in infrared light, allowing it to pierce through obscuring dust and gas and unveil a more unfamiliar — but just as amazing — view of the pillars.

In this ethereal view the entire frame is peppered with bright stars and baby stars are revealed being formed within the pillars themselves. The ghostly outlines of the pillars seem much more delicate, and are silhouetted against an eerie blue haze.

Image Credit: NASA, ESA/Hubble and the Hubble Heritage Team
Explanation from: https://www.spacetelescope.org/images/heic1501b/

NASA’s IBEX Observations Pin Down Interstellar Magnetic Field

Far beyond the orbit of Neptune, the solar wind and the interstellar medium interact to create a region known as the inner heliosheath, bounded on the inside by the termination shock, and on the outside by the heliopause.

Immediately after its 2008 launch, NASA’s Interstellar Boundary Explorer, or IBEX, spotted a curiosity in a thin slice of space: More particles streamed in through a long, skinny swath in the sky than anywhere else. The origin of the so-called IBEX ribbon was unknown – but its very existence opened doors to observing what lies outside our solar system, the way drops of rain on a window tell you more about the weather outside.

This simulation shows the origin of ribbon particles

of different energies or speeds outside the heliopause

(labeled HP). The IBEX ribbon particles interact with

the interstellar magnetic field (labeled ISMF) and

travel inwards toward Earth, collectively giving

the impression of a ribbonspanning across the sky.

Now, a new study uses IBEX data and simulations of the interstellar boundary – which lies at the very edge of the giant magnetic bubble surrounding our solar system called the heliosphere – to better describe space in our galactic neighborhood. The paper, published February 8, 2016, in The Astrophysical Journal Letters, precisely determines the strength and direction of the magnetic field outside the heliosphere. Such information gives us a peek into the magnetic forces that dominate the galaxy beyond, teaching us more about our home in space.

The new paper is based on one particular theory of the origin of the IBEX ribbon, in which the particles streaming in from the ribbon are actually solar material reflected back at us after a long journey to the edges of the sun’s magnetic boundaries. A giant bubble, known as the heliosphere, exists around the sun and is filled with what’s called solar wind, the sun’s constant outflow of ionized gas, known as plasma. When these particles reach the edges of the heliosphere, their motion becomes more complicated.

“The theory says that some solar wind protons are sent flying back towards the sun as neutral atoms after a complex series of charge exchanges, creating the IBEX ribbon,” said Eric Zirnstein, a space scientist at the Southwest Research Institute in San Antonio, Texas, and lead author on the study. “Simulations and IBEX observations pinpoint this process – which takes anywhere from three to six years on average – as the most likely origin of the IBEX ribbon.”

Outside the heliosphere lies the interstellar medium, with plasma that has different speed, density, and temperature than solar wind plasma, as well as neutral gases. These materials interact at the heliosphere’s edge to create a region known as the inner heliosheath, bounded on the inside by the termination shock – which is more than twice as far from us as the orbit of Pluto – and on the outside by the heliopause, the boundary between the solar wind and the comparatively dense interstellar medium.

Some solar wind protons that flow out from the sun to this boundary region will gain an electron, making them neutral and allowing them to cross the heliopause. Once in the interstellar medium, they can lose that electron again, making them gyrate around the interstellar magnetic field. If those particles pick up another electron at the right place and time, they can be fired back into the heliosphere, travel all the way back toward Earth, and collide with IBEX’s detector. The particles carry information about all that interaction with the interstellar magnetic field, and as they hit the detector they can give us unprecedented insight into the characteristics of that region of space.

“Only Voyager 1 has ever made direct observations of the interstellar magnetic field, and those are close to the heliopause, where it’s distorted,” said Zirnstein. “But this analysis provides a nice determination of its strength and direction farther out.”

The directions of different ribbon particles shooting back toward Earth are determined by the characteristics of the interstellar magnetic field. For instance, simulations show that the most energetic particles come from a different region of space than the least energetic particles, which gives clues as to how the interstellar magnetic field interacts with the heliosphere.

For the recent study, such observations were used to seed simulations of the ribbon’s origin. Not only do these simulations correctly predict the locations of neutral ribbon particles at different energies, but the deduced interstellar magnetic field agrees with Voyager 1 measurements, the deflection of interstellar neutral gases, and observations of distant polarized starlight.

However, some early simulations of the interstellar magnetic field don’t quite line up. Those pre-IBEX estimates were based largely on two data points – the distances at which Voyagers 1 and 2 crossed the termination shock.

“Voyager 1 crossed the termination shock at 94 astronomical units, or AU, from the sun, and Voyager 2 at 84 AU,” said Zirnstein. One AU is equal to about 93 million miles, the average distance between Earth and the sun. “That difference of almost 930 million miles was mostly explained by a strong, very tilted interstellar magnetic field pushing on the heliosphere.”

But that difference may be accounted for by considering a stronger influence from the solar cycle, which can lead to changes in the strength of the solar wind and thus change the distance to the termination shock in the directions of Voyager 1 and 2. The two Voyager spacecraft made their measurements almost three years apart, giving plenty of time for the variable solar wind to change the distance of the termination shock.

“Scientists in the field are developing more sophisticated models of the time-dependent solar wind,” said Zirnstein.

The simulations generally jibe well with the Voyager data.

The IBEX ribbon is a relatively narrow strip of particles flying in towards the sun from outside the heliosphere. A new study corroborates the idea that particles from outside the heliosphere that form the IBEX ribbon actually originate at the sun – and reveals information about the distant interstellar magnetic field.

"The new findings can be used to better understand how our space environment interacts with the interstellar environment beyond the heliopause,” said Eric Christian, IBEX program scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in this study. “In turn, understanding that interaction could help explain the mystery of what causes the IBEX ribbon once and for all.”

Image Credit: NASA/IBEX/Adler Planetarium, SwRI/Zirnstein
Explanation from: https://www.nasa.gov/feature/goddard/2016/nasa-s-ibex-observations-pin-down-interstellar-magnetic-field

Double Tornado under a Supercell in Colorado

Simla, Colorado, USA

June 4, 2015

Image Credit & Copyright: Kelly DeLay

High Resolution Global View of Jupiter's moon Io

Io, the most volcanic body in the Solar System is seen in the highest resolution obtained to date by NASA's Galileo spacecraft. The smallest features that can be discerned are 2.5 kilometers in size. There are rugged mountains several kilometers high, layered materials forming plateaus, and many irregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no landforms resembling impact craters, as the volcanism covers the surface with new deposits much more rapidly than the flux of comets and asteroids can create large impact craters. The picture is centered on the side of Io that always faces away from Jupiter; north is to the top.

Color images acquired on September 7, 1996 have been merged with higher resolution images acquired on November 6, 1996 by the Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft. The color is composed of data taken, at a range of 487,000 kilometers, in the near-infrared, green, and violet filters and has been enhanced to emphasize the extraordinary variations in color and brightness that characterize Io's face. The high resolution images were obtained at ranges which varied from 245,719 kilometers to 403,100 kilometers.

Image Credit: NASA/JPL/University of Arizona
Explanation from: http://photojournal.jpl.nasa.gov/catalog/PIA00583

Sun's reflection on Pacific Ocean seen from the International Space Station

ISS, Orbit of the Earth

June 2014

Image Credit: NASA/ESA

Zhangjiajie National Forest Park

• One pillar is 1080-meter-high (Southern Sky Column)

The Zhangjiajie National Forest Park is a unique national forest park located in Zhangjiajie City in northern Hunan Province in the People's Republic of China. It is one of several national parks within the Wulingyuan Scenic Area.

History

In 1982 it was recognized as China's first national forest park with an area of 4,810 ha (11,900 acres). Zhangjiajie National Forest Park is part of a much larger 397.5 km2 (153.5 sq mi) Wulingyuan Scenic Area. In 1992, Wulingyuan was officially recognized as a UNESCO World Heritage Site. It was then approved by the Ministry of Land and Resources as Zhangjiajie Sandstone Peak Forest National Geopark (3,600 square kilometres (1,400 sq mi)) in 2001. In 2004, Zhangjiajie Geopark was listed as a UNESCO Global Geopark.

The most notable geographic features of the park are the pillar-like formations that are seen throughout the park. Although resembling karst terrain, this area is not underlain by limestones and is not the product of chemical dissolution, which is characteristic of limestone karst. They are the result of many years of physical, rather than chemical, erosion. Much of the weathering which forms these pillars are the result of expanding ice in the winter and the plants which grow on them. The weather is moist year round, and as a result, the foliage is very dense. The weathered material is carried away primarily by streams. These formations are a distinct hallmark of Chinese landscape, and can be found in many ancient Chinese paintings.

One of the park's quartz-sandstone pillars, the 1,080-metre (3,540 ft) Southern Sky Column, had been officially renamed "Avatar Hallelujah Mountain" in honor of the eponymous film in January 2010. According to park officials, photographs from Zhangjiajie inspired the floating Hallelujah Mountains seen in the film. The film's director and production designers said that they drew inspiration for the floating rocks from mountains from around the world, including those in the Hunan province.

Image Credit: chensiyuan via wikipedia.org
Explanation from: https://en.wikipedia.org/wiki/Zhangjiajie_National_Forest_Park

A Deeper Look at Centaurus A

The strange galaxy Centaurus A is pictured in a new image from the European Southern Observatory. With a total exposure time of more than 50 hours this is probably the deepest view of this peculiar and spectacular object ever created. The image was produced by the Wide Field Imager of the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.

Centaurus A, also known as NGC 5128, is a peculiar massive elliptical galaxy with a supermassive black hole at its heart. It lies about 12 million light-years away in the southern constellation of Centaurus (The Centaur) and has the distinction of being the most prominent radio galaxy in the sky. Astronomers think that the bright nucleus, strong radio emission and jet features of Centaurus A are produced by a central black hole with a mass of about 100 million times that of the Sun. Matter from the dense central parts of the galaxy releases vast amounts of energy as it falls towards the black hole.

This Wide Field Imager (WFI) picture allows us to appreciate the galaxy’s elliptical nature, which shows up as the elongated shape of the fainter outer parts. The glow that fills much of the picture comes from hundreds of billions of cooler and older stars. Unlike most elliptical galaxies, however, Centaurus A’s smooth shape is disturbed by a broad and patchy band of dark material that obscures the galaxy’s centre.

The dark band harbours large amounts of gas, dust and young stars. Bright young star clusters appear at the upper-right and lower-left edges of the band along with the red glow of star-forming clouds of hydrogen, whilst some isolated dust clouds are silhouetted against the stellar background. These features, and the prominent radio emission, are strong evidence that Centaurus A is the result of a merger between two galaxies. The dusty band is probably the mangled remains of a spiral galaxy in the process of being ripped apart by the gravitational pull of the giant elliptical galaxy.

The new set of images from WFI include long exposures through red, green and blue filters as well as filters specially designed to isolate the light from glowing hydrogen and oxygen. The latter help us to spot the known optical jet features around Centaurus A, which were barely visible in a previous image from the Wide Field Imager).

Extending from the galaxy to the upper left corner of the image are two groups of reddish filaments, which are roughly lined up with the huge jets that are prominent in radio images. Both sets of filaments are stellar nurseries, containing hot young stars. Above the left side of the dusty band, we find the inner filaments, lying about 30 000 light-years away of the nucleus. Further out, around 65 000 light-years away from the galaxy’s nucleus and close to the upper left corner of the image, the outer filaments are visible. There is also possibly a very much fainter trace of a counter jet extending to the lower right.

Centaurus A has been extensively studied at wavelengths ranging from radio all the way to gamma-rays. In particular, radio and X-ray observations have been crucial for studying the interaction between the energetic output of the central supermassive black hole and its surroundings. Studies of Centaurus A with ALMA are just beginning.

Many of the observations of Centaurus A used to make this image were taken to see whether it was possible to use ground-based surveys to detect and study variable stars in galaxies like Centaurus A outside the local group. More than 200 new variable stars in Centaurus A were discovered.

Image Credit: ESO
Explanation from: https://www.eso.org/public/news/eso1221/

Horsetail Fall in Yosemite National Park

Horsetail Fall, located in Yosemite National Park in California, is a seasonal waterfall that flows in the winter and early spring. The fall occurs on the east side of El Capitan. If Horsetail Fall is flowing in February and the weather conditions are just right, the setting sun illuminates the waterfall, making it glow orange and red. This natural phenomenon is often referred to as the "Firefall", a name that pays homage to the manmade Firefall that once took place in Yosemite.

This waterfall descends in two streams side by side, the eastern one being the larger but both quite small. The eastern one drops 1,540 feet (470 m), and the western one 1,570 feet (480 m), the highest fully airborne waterfall in Yosemite that runs at some point every year. The waters then gather and descend another 490 feet (150 m) on steep slabs, so the total height of these waterfalls is 2,030 ft (620 m) to 2,070 ft (630 m).

The fall is best seen and photographed from a small clearing close to the picnic area on the north road leading out of Yosemite Valley east of El Capitan.

Image Credit & Copyright: shank0205 via instagram.com
Explanation from: https://en.wikipedia.org/wiki/Horsetail_Fall_(Yosemite)

Blue Glaucus

Glaucus atlanticus (common names include the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small, blue sea slug, a pelagic aeolid nudibranch, a shell-less gastropod mollusk in the family Glaucidae.

These sea slugs are pelagic: they float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents. Glaucus atlanticus is camouflaged: the blue side of their body faces upwards, blending in with the blue of the water. The silver/grey side of the sea slugs faces downwards, blending in with the silvery surface of the sea.

Glaucus atlanticus feeds on other pelagic creatures, including the venomous cnidarian, the Portuguese man o' war. This sea slug stores stinging nematocysts from the cnidarian within its own tissues as defense against predation. Humans handling the slug may receive a very painful and potentially dangerous sting.

Taxonomy

This species looks similar to, and is closely related to, Glaucus marginatus, which is now understood to be not one species, but a cryptic species complex of four separate species which live in the Indian Ocean and Pacific Ocean.

Description

At maturity Glaucus atlanticus can be up to 3 centimetres (1.2 in) in length. It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its head. It has a tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.

The radula of this species bears serrated teeth.

Buoyancy and coloration

With the aid of a gas-filled sac in its stomach, G. atlanticus floats at the surface. Due to the location of the gas sac, this species floats upside down. The upper surface is actually the foot (the underside in other slugs and snail), and this has either a blue or blue-white coloration. The true dorsal surface (carried downwards in G. atlanticus) is completely silver-grey. This coloration is an example of countershading, which helps protect it from predators that might attack from below and from above. The blue coloration is also thought to reflect harmful UV sunlight.

Life history and behavior

G. atlanticus preys on other larger pelagic organisms. The sea slugs can move toward prey or mates by using their cerata to make slow swimming movements. They are known to prey on the dangerously venomous Portuguese man o' war Physalia physalis; the by-the-wind-sailor Velella velella; the blue button Porpita porpita; and the violet snail, Janthina janthina. Occasionally, individuals will attack and eat other individuals in captivity.

G. atlanticus is able to feed on Physalia physalis due to its immunity to the venomous nematocysts. The slug consumes the entire organism and appears to select and store the most venomous nematocysts for its own use. The nematocysts are collected in specialized sacs (cnidosacs) at the tip of the animal's cerata, the thin feather-like "fingers" on its body. Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.

Like almost all heterobranchs, Glaucus is a hermaphrodite, having both male and female reproductive organs. Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing.After mating, both animals produce egg strings.

Sting

The Glaucus atlanticus is able to swallow the venomous nematocysts from siphonophores such as the Portuguese man o' war, and store them in the extremities of its finger-like cerata. Picking up the animal can result in a painful sting, with symptoms similar to those caused by the Portuguese man o' war.

Image Credit & Copyright: Sylke Rohrlach
Explanation from: https://en.wikipedia.org/wiki/Glaucus_atlanticus

Wolf–Rayet star WR 31a in Carina

Sparkling at the centre of this beautiful NASA/ESA Hubble Space Telescope image is a Wolf–Rayet star known as WR 31a, located about 30 000 light-years away in the constellation of Carina (The Keel).

The distinctive blue bubble appearing to encircle WR 31a, and its uncatalogued stellar sidekick, is a Wolf–Rayet nebula — an interstellar cloud of dust, hydrogen, helium and other gases. Created when speedy stellar windsinteract with the outer layers of hydrogen ejected by Wolf–Rayet stars, these nebulae are frequently ring-shaped or spherical. The bubble — estimated to have formed around 20 000 years ago — is expanding at a rate of around 220 000 kilometres per hour!

Unfortunately, the lifecycle of a Wolf–Rayet star is only a few hundred thousand years — the blink of an eye in cosmic terms. Despite beginning life with a mass at least 20 times that of the Sun, Wolf–Rayet stars typically lose half their mass in less than 100 000 years. And WR 31a is no exception to this case. It will, therefore, eventually end its life as a spectacular supernova, and the stellar material expelled from its explosion will later nourish a new generation of stars and planets.

Image Credit: ESA/Hubble & NASA, Judy Schmid
Explanation from: http://www.spacetelescope.org/images/potw1608a/

Alnitak + Alnilam + Mintaka = Orion's Belt

Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,500 light-years away, born of Orion's well-studied interstellar clouds. In fact, clouds of gas and dust adrift in this region have intriguing and some surprisingly familiar shapes, including the dark Horsehead Nebula and Flame Nebula near Alnitak at the lower left. The famous Orion Nebula itself lies off the bottom of this star field that covers an impressive 4.4x3.5 degrees on the sky. The color picture was composited from digitized black and white photographic plates recorded through red and blue astronomical filters, with a computer synthesized green channel. The plates were taken using the Samuel Oschin Telescope, a wide-field survey instrument at Palomar Observatory, between 1987 and 1991.

Image Credit: Davide De Martin & the ESA/ESO/NASA
Explanation from: http://apod.nasa.gov/apod/ap051013.html

Moon over Antelao

Late afternoon at San Vito di Cadore, Italy the moon shines over Monte Antelao. The snow-covered dolomite ridge of the mountain and the Earth’s only natural satellite bear a striking resemblance to one another, contrasting against the bright blue of the afternoon sky.

San Vito di Cadore, Veneto, Italy

January 31, 2015

Image Credit & Copyright: Marcella Giulia
Explanation by: Royal Observatory Greenwich

Globular Cluster Messier 13

This image, taken by the Advanced Camera for Surveys on the Hubble Space Telescope, shows the core of the great globular cluster Messier 13 and provides an extraordinarily clear view of the hundreds of thousands of stars in the cluster, one of the brightest and best known in the sky. Just 25 000 light-years away and about 145 light-years in diameter, Messier 13 has drawn the eye since its discovery by Edmund Halley, the noted British astronomer, in 1714. The cluster lies in the constellation of Hercules and is so bright that under the right conditions it is even visible to the unaided eye. As Halley wrote: “This is but a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent.” Messier 13 was the target of a symbolic Arecibo radio telescope message that was sent in 1974, communicating humanity’s existence to possible extraterrestrial intelligences. However, more recent studies suggest that planets are very rare in the dense environments of globular clusters.

Messier 13 has also appeared in literature. In his 1959 novel, The Sirens of Titan, Kurt Vonnegut wrote “Every passing hour brings the Solar System forty-three thousand miles closer to Globular Cluster M13 in Hercules — and still there are some misfits who insist that there is no such thing as progress.” The step from Halley’s early telescopic view to this Hubble image indicates some measure of the progress in astronomy in the last three hundred years.

This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys on the Hubble Space Telescope. Data through a blue filter (F435W) are coloured blue, data through a red filter (F625W) are coloured green and near-infrared data (through the F814W filter) are coloured red. The exposure times are 1480 s, 380 s and 567 s respectively and the field of view is about 2.5 arcminutes across.

Image Credit: ESA/Hubble and NASA
Explanation from: https://www.spacetelescope.org/images/potw1011a/

Terra and Sol seen from the International Space Station

ISS, Orbit of the Earth

July 2014

Image Credit: NASA/ESA

Aurora and Sunrise seen from the International Space Station

ISS, Orbit of the Earth

February 2016

Video Credit: ESA/NASA/Tim Peake

Orion: Head to Toe

Cradled in cosmic dust and glowing hydrogen, stellar nurseries in Orion the Hunter lie at the edge of a giant molecular cloud some 1,500 light-years away. Spanning nearly 25 degrees, this breath-taking vista stretches across the well-known constellation from head to toe (left to right). The Great Orion Nebula, the closest large star forming region, is right of center. To its left are the Horsehead Nebula, M78, and Orion's belt stars. Sliding your cursor over the picture will also find red giant Betelgeuse at the hunter's shoulder, bright blue Rigel at his foot, and the glowing Lambda Orionis (Meissa) nebula at the far left, near Orion's head. Of course, the Orion Nebula and bright stars are easy to see with the unaided eye, but dust clouds and emission from the extensive interstellar gas in this nebula-rich complex, are too faint and much harder to record. In this mosaic of broadband telescopic images, additional image data acquired with a narrow hydrogen alpha filter was used to bring out the pervasive tendrils of energized atomic hydrogen gas and the arc of the giant Barnard's Loop.

Image Credit & Copyright: Rogelio Bernal Andreo
Explanation from: http://apod.nasa.gov/apod/ap101023.html

Noctilucent Clouds over Sunderland

Taken on 7 July 2014 from Seaburn Beach, Sunderland in the North of England. The photographer had seen nightly displays of noctilucent clouds for up to seven days prior to taking this shot – his first attempt at astrophotography - of the most vibrant display he witnessed. Noctilucent clouds are the highest clouds in the Earth’s atmosphere and form above 200,000 ft.

Image Credit & Copyright: Matt Robinson
Explanation by: Royal Observatory Greenwich

Island and Clouds near French Polynesia seen from the International Space Station

ISS, Orbit of the Earth

June 2012

Image Credit: NASA/ESA

The NGC 1097 Galaxy

In this image, the larger-scale structure of the galaxy is barely visible: its comparatively dim spiral arms, which surround its heart in a loose embrace, reach out beyond the edges of this frame.

This face-on galaxy, lying 45 million light-years away from Earth in the southern constellation of Fornax (The Furnace), is particularly attractive for astronomers. NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. The area immediately around the black hole shines powerfully with radiation coming from the material falling in.

The distinctive ring around the black hole is bursting with new star formation due to an inflow of material toward the central bar of the galaxy. These star-forming regions are glowing brightly thanks to emission from clouds of ionised hydrogen. The ring is around 5000 light-years across, although the spiral arms of the galaxy extend tens of thousands of light-years beyond it.

NGC 1097 is also pretty exciting for supernova hunters. The galaxy experienced three supernovae (the violent deaths of high-mass stars) in the 11-year span between 1992 and 2003. This is definitely a galaxy worth checking on a regular basis.

However, what it is really exciting about NGC 1097 is that it is not wandering alone through space. It has two small galaxy companions, which dance “the dance of stars and the dance of space” like the gracious dancer of the famous poem The Dancer by Khalil Gibran.

The satellite galaxies are NGC 1097A, an elliptical galaxy orbiting 42 000 light-years from the centre of NGC 1097 and a small dwarf galaxy named NGC 1097B. Both galaxies are located out beyond the frames of this image and they cannot be seen. Astronomers have indications that NGC 1097 and NGC 1097A have interacted in the past.

This picture was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.

Image Credit: ESA/Hubble & NASA, E. Sturdivant
Explanation from: https://www.spacetelescope.org/images/potw1252a/

Cyclone Winston Threatens Fiji

Powerful Tropical Cyclone Winston continued to intensify as it neared Fiji and NASA-NOAA's Suomi NPP satellite captured an image of the strengthening storm with a clear eye. Warnings are posted in Fiji as the storm is expected to make landfall there as a major hurricane.

Warnings were in effect in Fiji on Feb. 19 as Winston moved westward in the Southern Pacific Ocean and continued to intensify. In Fiji, a Hurricane Warning remains in effect for Vanuabalavu, Yacata, Mago, Cicia, Tuvuca, Nayau, Koro, Gau, Vanuavatu, Taveuni, Qamea, Laucala, Ovalau, Wakaya and southern Vanua Levu. A Storm Warning remained in effect for Lakeba, Oneata, Moce, Komo, Namuka, Ogea, Moala and rest of Vanua Levu, eastern half of Viti Levu. A Gale Warning was effect for the rest of Fiji. Meanwhile, all warnings in Tonga have been dropped as the storm moved away.

On Feb. 19 at 01:15 UTC (Feb. 18 at 8:15 p.m. EST) the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard NASA-NOAA's Suomi NPP satellite captured a visible image of Tropical Cyclone Winston in the South Pacific Ocean. The VIIRS image showed that Winston had a 15 nautical mile wide eye circled by a thick band of thunderstorms.

At 1500 UTC (10 a.m. EST) on Feb. 19 Tropical cyclone Winston's increased to 125 knots (143.8 mph/231.5 kph). It was centered near 17.3 degrees south latitude and 177.5 degrees west longitude, about 239 nautical miles (275 miles/442.6 km) east of Suva, Fiji. Winston was moving to the west at 15 knots (17.2 mph/27.7 kph). Winston was generating very rough seas with maximum significant wave height to 38 feet (11.5 meters).

Winston is moving west and the Joint Typhoon Warning Center forecast calls for Winston to strengthen to 130 knots (149.6 mph/240.8 kph) before making landfall on the east coast of Fiji as a major Category 4 hurricane on the Saffir-Simpson Wind Scale.

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane's sustained wind speed. This scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage.

According to the Saffir-Simpson Wind Scale for Category 4 hurricane, "catastrophic damage will occur. Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

The storm will then veer sharply south and weaken quickly as it moves over an area with increasing vertical wind shear and cooler sea surface temperatures.

Image Credit: NASA Goddard Rapid Response/NOAA
Explanation from: http://www.nasa.gov/feature/goddard/2016/winston-southwestern-pacific-ocean