Radiation Pressure

A collection of astronomical images

[rad-press]

Crescent Earth viewed from Rosetta on November 2009

Image of the Earth acquired with the OSIRIS narrow-angle camera from a distance of 633 000 km on 12 November 2009 at 13:28 CET.

The resolution is 12 km/pixel. The image is part of a sequence of images taken every hour through one full rotation (24 hours).

Three images with an orange, green, and blue filter were combined to create this one. The illuminated crescent is centered roughly around the South Pole (South at the bottom of the image). The outline of Antarctica is visible under the clouds that form the striking south-polar vortex. Pack ice in front of the coastline with its strong spectacular reflection is the cause for the very bright spots on the image.

[rad-press]

Unraveling the history of galaxies

An image from the research paper.

A team of international scientists has shown for the first time that galaxies can change their structure over the course of their lifetime.

By observing the sky as it is today, and peering back in time using the Hubble and Herschel telescopes, the team have shown that a large proportion of galaxies have undergone a major ‘metamorphosis’ since they were initially formed after the Big Bang.

By providing the first direct evidence of the extent of this transformation, the team hope to shed light on the processes that caused these dramatic changes, and therefore gain a greater understanding of the appearance and properties of the Universe as we know it today.

In their study, which has been published in the Monthly Notices of the Royal Astronomical Society¸ the researchers observed around 10,000 galaxies currently present in the Universe using a survey of the sky created by the Herschel ATLAS and GAMA projects.

The researchers then classified the galaxies into the two main types: flat, rotating, disc-shaped galaxies (much like our own galaxy, the Milky Way); and large, spherical galaxies with a swarm of disordered stars.

Using the Hubble and Herschel telescopes, the researchers then looked further out into the Universe, and thus further back in time, to observe the galaxies that formed shortly after the Big Bang.

The researchers showed that 83 per cent of all the stars formed since the Big Bang were initially located in a disc-shaped galaxy.

However, only 49 per cent of stars that exist in the Universe today are located in these disc-shaped galaxies—the remainder are located in spherical-shaped galaxies.

The results suggest a massive transformation in which disc-shaped galaxies became spherical-shaped galaxies.

A popular theory is that the this transformation was caused by many cosmic catastrophes, in which two disk-dominated galaxies, straying too close to each other, were forced by gravity to merge into a single galaxy, with the merger destroying the disks and producing a huge pileup of stars. An opposing theory is that the transformation was a more gentle process, with stars formed in a disk gradually moving to the centre of a disk and producing a central pile-up of stars.

Lead author of the study Professor Steve Eales, from Cardiff University’s School of Physics and Astronomy, said: “Many people have claimed before that this metamorphosis has occurred, but by combining Herschel and Hubble, we have for the first time been able to accurately measure the extent of this transformation.

“Galaxies are the basic building blocks of the Universe, so this metamorphosis really does represent one of the most significant changes in its appearance and properties in the last 8 billion years.”

[rad-press]

AB7 Nebula

This unique image shows AB7, one of the highest excitation nebulae in the Magellanic Clouds (MCs), two satellite galaxies of our own Milky Way. AB7 is a binary star, consisting of one WR-star — highly evolved massive star - and a mid-age massive companion of spectral type O. These exceptional stars have very strong stellar winds: they continuously eject energetic particles — like the "solar wind" from the Sun — but some 10 to 1,000 million times more intensely than our star! These powerful winds exert an enormous pressure on the surrounding interstellar material and forcefully shape those clouds into "bubbles", well visible in the photos by their blue colour. AB7 is particularly remarkable: the associated huge nebula and HeII region indicate that this star is one of the, if not the, hottest WR-star known so far, with a surface temperature in excess of 120,000 degrees! Just outside this nebula, a small network of green filaments is visible — they are the remains of a supernova explosion.

[rad-press]

LEDA 074886, a Rare "Emerald-Cut" Galaxy

Different views of LEDA 074886.

An international team of astronomers—from Australia, Germany, Switzerland, and Finland—discovered in 2012 a rare, rectangular-shaped galaxy (LEDA 074886) that has a striking resemblance to an emerald-cut diamond. While using the Subaru Prime Focus Camera (Suprime-Cam) to look for globular clusters of stars swarming around NGC 1407, a bright, giant galaxy in the Constellation Eridanus and 70 million light years from Earth, the researchers discovered an unusually shaped dwarf galaxy toward the edge of their image. Professor Alister Graham (Swinburne University of Technology, Australia), lead author of the paper describing the research, said, "It's one of those things that just makes you smile because it shouldn't exist, or rather, you don't expect it to exist." Its discovery allows astronomers to obtain useful information for modeling other galaxies.

Most galaxies in the universe around us exist in one of three forms: ellipsoidal, disk-like (usually in the shape of a flattened circular disk hosting a spiral pattern of stars), or irregular. Dwarf galaxies, probably the most common galaxies in the Universe, are small and have low intrinsic brightness (i.e., luminosity). One of the reasons that LEDA 074886 was hard to find is its dwarf-like status; it has 50 times less stars than our own Milky Way Galaxy, and its distance from Earth is equivalent to that spanned by 700 Milky Way galaxies placed end-to-end. The combined advantages of Subaru's large 8.2m primary mirror and its camera at prime focus gave the researchers such a wide field of view that they could observe objects beyond their intended targets and make the surprising discovery of the emerald-shaped dwarf galaxy. Additional information gleaned from the use of green, red, and infrared filters along with the good image quality seeing in the observation enabled the researchers to see and measure a stellar disk embedded within the rectangular-shaped galaxy. The blue color of the inner disk suggested a younger average age for this stellar population.

The astronomers suspect that the emerald-cut galaxy may resemble an inflated disk seen side-on, like a short cylinder. Research co-author Professor Duncan Forbes (Swinburne University of Technology, Australia) explained, "One possibility is that the galaxy may have formed out of the collision of two spiral galaxies. While the pre-existing stars from the initial galaxies were strewn to large orbits creating the emerald-cut shape, the gas sank to the mid-plane where it condensed to form new stars and the disk that we have observed."

A false-color image of LEDA 074886, taken with Suprime-Cam at the Subaru Telescope. The central contrast has been adjusted to reveal the inner disk/bar-like component. For reference, the major-axis of the boxy outer red annulus spans 3.2–3.8 kpc, while the outer-edge of the outer-most blue annulus has a major-axis of 5.2 kpc.

[rad-press]

The Traverse Gravimeter Experiment (November 1972)

The Traverse Gravimeter Experiment (S-199), with cover removed, which was used by the Apollo 17 crewmen at the Taurus-Littrow landing site. The purposes of this experiment were to make a high accuracy relative survey of the lunar gravitational field in the lunar landing area and to make an Earth-moon gravity tie. Specific experiment objectives related to these purposes were to:

measure the value of gravity, relative to the value at a lunar base station, at selected known locations along the lunar traverse;

measure the value of gravity at a known point on the lunar surface (base station) relative to the value of gravity at a known point on Earth.

[rad-press]

Celestial firework marks nearest galaxy collision

Colour image of the galaxy collision, made by combining the CTIO H-alpha image with red and blue images. The H-alpha image was taken with a “narrow-band” filter sensitive to the spectacular burst of star formation arising in the ring around the central elongated galaxy. The image measures approximately 4 arcminutes across, and all were taken on the Cerro-Tololo InterAmerican Observatory (CTIO) 4-m telescope in Chile. 

A spectacular galaxy collision has been discovered lurking behind the Milky Way. The closest such system ever found, the discovery was announced by a team of astronomers led by Prof. Quentin Parker at the University of Hong-Kong and Prof. Albert Zijlstra at the University of Manchester. The scientists publish their results in Monthly Notices of the Royal Astronomical Society. The galaxy is 30 million light years away, which means that it is relatively close by. It has been dubbed “Kathryn’s Wheel” both after the famous firework that it resembles, but also after the wife of the paper's second author.

Such systems are very rare and arise from “bulls-eye” collisions between two galaxies of similar mass. Shockwaves from the collision compress reservoirs of gas in each galaxy and trigger the formation of new stars. This creates a spectacular ring of intense emission, and lights up the system like a Catherine wheel firework on bonfire night.

Galaxies grow through collisions but it is rare to catch one in the process, and extremely rare to see a bull's-eye collision in progress. Fewer than 20 systems with complete rings are known.

Kathryn's Wheel was discovered during a special wide field survey of the Southern Milky Way undertaken with the UK Schmidt Telescope in Australia. It used a narrow wavelength optical region centred on the so-called red “H-alpha” emission line of gaseous hydrogen. This rare jewel was uncovered during a search of the survey images for the remnants of dying stars in our Milky Way. The authors were very surprised to also find this spectacular cosmic ring, sitting remotely behind the dust and gas of the Milky Way in the constellation of Ara (the Altar).

Residual image of the collision, made by subtracting the red image from the CTIO H-alpha image, which mostly cancels the contributions from normal stars and is effective in highlighting just the areas of active star formation. 

The newly discovered ring galaxy is seven times closer than anything similar found before, and forty times closer than the famous ‘Cartwheel’ galaxy. The ring is located behind a dense star field and close to a very bright foreground star, which is why it had not been noted before. There are very few other galaxies in its neighbourhood; the odds of a collision in such an empty region of space are very low.

Professor Parker said “Not only is this system visually stunning, but it’s close enough to be an ideal target for detailed study. The ring is also quite low in mass – a few thousand million Suns, or less than 1% of the Milky Way – so our discovery shows that collision rings can form around much smaller galaxies than we thought.”

Smaller galaxies are more common than large ones, implying that collisional rings could be ten times as common as previously thought.

Colour image of the collision, made by combining the CTIO H-alpha image with red and blue images in alternative colours. In this rendition the bright star to the south has been left in place while the active ring of star formation centred on the elongated galaxy at the centre of the image has been rendered a fiery orange.

[rad-press]

Hubble Finds That the Nearest Quasar Is Powered by a Double Black Hole

Astronomers using NASA's Hubble Space Telescope have found that Markarian 231 (Mrk 231), the nearest galaxy to Earth that hosts a quasar, is powered by two central black holes furiously whirling about each other.

The finding suggests that quasars — the brilliant cores of active galaxies — may commonly host two central supermassive black holes that fall into orbit about one another as a result of the merger between two galaxies. Like a pair of whirling skaters, the black-hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy's population of billions of stars, which scientists then identify as quasars.

Scientists looked at Hubble archival observations of ultraviolet radiation emitted from the center of Mrk 231 to discover what they describe as "extreme and surprising properties."

If only one black hole were present in the center of the quasar, the whole accretion disk made of surrounding hot gas would glow in ultraviolet rays. Instead, the ultraviolet glow of the dusty disk abruptly drops off towards the center. This provides observational evidence that the disk has a big donut hole encircling the central black hole. The best explanation for the observational data, based on dynamical models, is that the center of the disk is carved out by the action of two black holes orbiting each other. The second, smaller black hole orbits in the inner edge of the accretion disk, and has its own mini-disk with an ultraviolet glow.

"We are extremely excited about this finding because it not only shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black holes via the nature of their ultraviolet light emission," said Youjun Lu of the National Astronomical Observatories of China, Chinese Academy of Sciences.

"The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies," added co-investigator Xinyu Dai of the University of Oklahoma.

The central black hole is estimated to be 150 million times the mass of our sun, and the companion weighs in at 4 million solar masses. The dynamic duo completes an orbit around each other every 1.2 years.

The lower-mass black hole is the remnant of a smaller galaxy that merged with Mrk 231. Evidence of a recent merger comes from the host galaxy's asymmetry, and the long tidal tails of young blue stars.

The result of the merger has been to make Mrk 231 an energetic starburst galaxy with a star-formation rate 100 times greater than that of our Milky Way galaxy. The infalling gas fuels the black hole "engine," triggering outflows and gas turbulence that incites a firestorm of star birth.

The binary black holes are predicted to spiral together and collide within a few hundred thousand years.

Mrk 231 is located 581 million light-years away.

The results were published in the August 14, 2015, edition of The Astrophysical Journal.

[rad-press]

Gaue Crater on Ceres

NASA's Dawn Spacecraft took this image of Gaue crater, the large crater on the bottom, on Ceres. Gaue is a Germanic goddess to whom offerings are made in harvesting rye.

The center of this crater is sunken in. Its diameter is 84 kilometers (52 miles). The resolution of the image is 450 feet (140 meters) per pixel.

The image was taken from a distance of 915 miles (1,470 kilometers) on August 18, 2015.

[rad-press]

Urvara Peaks

NASA's Dawn spacecraft took this image that shows a mountain ridge, near top left, that lies in the center of Urvara crater on Ceres. Urvara is an Indian and Iranian deity of plants and fields. The crater's diameter is 101 miles (163 kilometers).

This view was acquired on August 19, 2015, from a distance of 915 miles (1,470 kilometers). The resolution of the image is 450 feet (140 meters) per pixel.

[rad-press]

The Lonely Mountain

NASA's Dawn spacecraft spotted this tall, conical mountain on Ceres from a distance of 915 miles (1,470 kilometers). The mountain, located in the southern hemisphere, stands 4 miles (6 kilometers) high. Its perimeter is sharply defined, with almost no accumulated debris at the base of the brightly streaked slope.

The image was taken on August 19, 2015. The resolution of the image is 450 feet (140 meters) per pixel.

[rad-press]

Twin Jet Nebula, the wings of the butterfly

The shimmering colours visible in this NASA/ESA Hubble Space Telescope image show off the remarkable complexity of the Twin Jet Nebula. The new image highlights the nebula’s shells and its knots of expanding gas in striking detail. Two iridescent lobes of material stretch outwards from a central star system. Within these lobes two huge jets of gas are streaming from the star system at speeds in excess of one million kilometres per hour.

The cosmic butterfly pictured in this Hubble Space Telescope image goes by many names. It is called the Twin Jet Nebula as well as answering to the slightly less poetic name of PN M2-9.

The M in this name refers to Rudolph Minkowski, a German-American astronomer who discovered the nebula in 1947. The PN, meanwhile, refers to the fact that M2-9 is a planetary nebula. The glowing and expanding shells of gas clearly visible in this image represent the final stages of life for an old star of low to intermediate mass. The star has not only ejected its outer layers, but the exposed remnant core is now illuminating these layers — resulting in a spectacular light show like the one seen here. However, the Twin Jet Nebula is not just any planetary nebula, it is a bipolar nebula.

Ordinary planetary nebulae have one star at their centre, bipolar nebulae have two, in a binary star system. Astronomers have found that the two stars in this pair each have around the same mass as the Sun, ranging from 0.6 to 1.0 solar masses for the smaller star, and from 1.0 to 1.4 solar masses for its larger companion. The larger star is approaching the end of its days and has already ejected its outer layers of gas into space, whereas its partner is further evolved, and is a small white dwarf.

The characteristic shape of the wings of the Twin Jet Nebula is most likely caused by the motion of the two central stars around each other. It is believed that a white dwarf orbits its partner star and thus the ejected gas from the dying star is pulled into two lobes rather than expanding as a uniform sphere. However, astronomers are still debating whether all bipolar nebulae are created by binary stars. Meanwhile the nebula’s wings are still growing and, by measuring their expansion, astronomers have calculated that the nebula was created only 1200 years ago.

Within the wings, starting from the star system and extending horizontally outwards like veins are two faint blue patches. Although these may seem subtle in comparison to the nebula’s rainbow colours, these are actually violent twin jets streaming out into space, at speeds in excess of one million kilometres per hour. This is a phenomenon that is another consequence of the binary system at the heart of the nebula. These jets slowly change their orientation, precessing across the lobes as they are pulled by the wayward gravity of the binary system.

The two stars at the heart of the nebula circle one another roughly every 100 years. This rotation not only creates the wings of the butterfly and the two jets, it also allows the white dwarf to strip gas from its larger companion, which then forms a large disc of material around the stars, extending out as far as 15 times the orbit of Pluto! Even though this disc is of incredible size, it is much too small to be seen on the image taken by Hubble.

An earlier image of the Twin Jet Nebula using data gathered by Hubble’s Wide Field Planetary Camera 2 was released in 1997 (see image below). This newer version incorporates more recent observations from the telescope’s Space Telescope Imaging Spectrograph (STIS).

[rad-press]

Galaxy NGC 474: Cosmic Blender 

What's happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant. Regardless of the actual cause, the above image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with -- and accretions of -- smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).

[rad-press]

Shell Formation in NGC 474

A network of shells of various colors and thus stellar populations around  NGC 474. True color (combination of g and r bands) image obtained with the MegaCam camera at CFHT as part of the Atlas3D project, and processed by Coelum.

Abstract from a study published in 1991:

We present broad band optical (B, R, I) and H alpha images of the classic shell galaxy NGC 474 (Arp 227), and computer simulations of the shell forming process. We have selected NGC 474 as it has shells of unusually high surface brightness and it has a nearby interacting spiral companion, NGC 470. The merger model and the interaction model can explain various, but not all aspects of observed shell structure. According to the merger model, shells form as the result of a collision with another secondary elliptical or disk galaxy. The shells form from a phase wrapping of the disrupted secondary galaxy in the fixed potential well of the primary elliptical galaxy. According to the interaction model, the shells form as density waves in a dynamically cold component (thick disk) during a flyby interaction with another galaxy. Schombert & Wallin (1987) originally proposed an interaction model for the formation of the shells in NGC 474, based on the shell colours and close proximity of NGC 470. We compare our new, high S/N imaging data, with computer simulations of both the merger and interaction models to determine the most likely formation mechanism for the shells in NGC 474.

[rad-press]

The tumultuous heart of our Galaxy

X-ray view of the Galactic Centre.

This new image of powerful remnants of dead stars and their mighty action on the surrounding gas from ESA's XMM-Newton X-ray observatory reveals some of the most intense processes taking place at the centre of our galaxy, the Milky Way.

The bright, point-like sources that stand out across the image trace binary stellar systems in which one of the stars has reached the end of its life, evolving into a compact and dense object – a neutron star or black hole. Because of their high densities, these compact remnants devour mass from their companion star, heating the material up and causing it to shine brightly in X-rays.

The central region of our galaxy also contains young stars and stellar clusters, and some of these are visible as white or red sources sprinkled throughout the image, which spans about one thousand light-years.

Most of the action is occurring at the centre, where diffuse clouds of gas are being carved by powerful winds blown by young stars, as well as by supernovas, the explosive demise of massive stars.

The supermassive black hole sitting at the centre of the Galaxy is also responsible for some of this action. Known as Sagittarius A*, this black hole has a mass a few million times that of our Sun, and it is located within the bright, fuzzy source to the right of the image centre.

While black holes themselves do not emit light, their immense gravitational pull draws in the surrounding matter that, in the process, emits light at many wavelengths, most notably X-rays. In addition, two lobes of hot gas can be seen extending above and below the black hole.

Astronomers believe that these lobes are caused either directly by the black hole, which swallows part of the material that flows onto it but spews out most of it, or by the cumulative effect of the numerous stellar winds and supernova explosions that occur in such a dense environment.

This image, showing us an unprecedented view of the Milky Way's energetic core, was put together in a new study by compiling all observations of this region that were performed with XMM-Newton, adding up to about one and a half months of monitoring in total.

The large, elliptical structure to the lower right of Sagittarius A* is a super-bubble of hot gas, likely puffed up by the remnants of several supernovas at its centre. While this structure was already known to astronomers, this study confirms for the first time that it consists of a single, gigantic bubble rather than the superposition of several, individual supernova remnants along the line of sight.

Another huge pocket of hot gas, designated the 'Arc Bubble' due to its crescent-like shape, can be seen close to the image centre, to the lower left of the supermassive black hole. It is being inflated by the forceful winds of stars in a nearby stellar cluster, as well as by supernovae; the remnant of one of these explosions, a candidate pulsar wind nebula, was detected at the core of the bubble.

The Galactic Centre through the emission of heavy elements.

The rich data set compiled in this study contains observations that span the full range of X-ray energies covered by XMM-Newton; these include some energies corresponding to the light emitted by heavy elements such as silicon, sulphur and argon, which are produced primarily in supernova explosions. By combining these additional information present in the data, the astronomers obtained another, complementary view of the Galactic Centre, which reveals beautifully the lobes and bubbles described earlier on.

In addition, this alternative view also displays the emission, albeit very faint, from warm plasma in the upper and lower parts of the image. This warm plasma might be the collective macroscopic effect of outflows generated by star formation throughout this entire central zone.

Another of the possible explanations for such emission links it to the turbulent past of the now not-so-active supermassive black hole. Astronomers believe that, earlier on in the history of our galaxy, Sagittarius A* was accreting and ejecting mass at a much higher rate, like the black holes found at the centre of many galaxies, and these diffuse clouds of warm plasma could be a legacy of its ancient activity.

[rad-press]

Departing Dione

NASA's Cassini spacecraft captured this parting view showing the rough and icy crescent of Saturn's moon Dione following the spacecraft's last close flyby of the moon on Aug. 17, 2015.

Cassini obtained a similar crescent view in 2005. The earlier view has an image scale about four times higher, but does not show the moon's full crescent as this view does.

Five visible light (clear spectral filter), narrow-angle camera images were combined to create this mosaic view. The scene is an orthographic projection centered on terrain at 0.4 degrees north latitude, 30.6 degrees west longitude on Dione. An orthographic view is most like the view seen by a distant observer looking through a telescope.

The view was acquired at distances ranging from approximately 37,000 miles (59,000 kilometers) to 47,000 miles (75,000 kilometers) from Dione and at a sun-Dione-spacecraft, or phase, angle of 145 degrees. Image scale is about 1,300 feet (400 meters) per pixel.

North on Dione is up and rotated 34 degrees to the right.

[rad-press]

Cassini's Closest Views of Dione II

As NASA's Cassini soared above high northern latitudes on Saturn's moon Dione, the spacecraft looked down at a region near the day-night boundary. This view shows the region as a contrast-enhanced image (top image) in which features in shadow are illuminated by reflected light from Saturn. Inset just above center is a higher resolution view -- one of the mission's highest-resolution views of the Saturnian moon's icy surface.

Territory seen here is just east of a crater named Butes, near an unnamed tectonic structure around 65 degrees north latitude, 25 degrees west longitude.

The broader view is from the spacecraft's wide-angle camera (WAC) and includes an inset view from Cassini's narrow-angle camera (NAC). The NAC view shows features about 10 times smaller than the WAC view. The unenhanced WAC view is the bottom image in this post.

The views were acquired in visible light at an altitude of 365 miles (588 kilometers) above Dione. The wide-angle camera image has an image scale of about 115 feet (35 meters) per pixel; the narrow-angle camera image has an image scale of about 12 feet (3.5 meters) per pixel.

[rad-press]

Cassini's Closest Views of Dione I

This two-in-one view of Dione from NASA's Cassini spacecraft includes the mission's highest-resolution view of the Saturnian moon's icy surface.

The view, from the spacecraft's wide-angle camera (WAC), includes an inset view, near center left, from the narrow-angle camera (NAC). The NAC view (also available here at its full resolution) shows features about 10 times smaller than the WAC view.

The wide-angle camera view has an image scale of about 105 feet (32 meters) per pixel; the narrow-angle camera view has an image scale of about 10 feet (3 meters) per pixel. Sunlight illuminates the scene from top. North on Dione is down. The views were acquired in visible light at an altitude of 334 miles (537 kilometers) above Dione.

The images were acquired simultaneously during a close flyby of the icy moon on Aug. 17, 2015.