June 30, 2012
NGC 2818, a planetary nebula in Pyxis.
NGC 2818 is a planetary nebula about 6.5 light-years across, located some 10,400 light-years away in the southern constellation Pyxis.
The structure of NGC 2818 contains the glowing gaseous outer layers of the central sun-like star that were expelled into interstellar space during the final stages of its life, after it ran out of fuel to sustain the nuclear reactions in its core.
Planetary nebulae fade gradually over tens of thousands of years. The hot, remnant stellar core of NGC 2818 will eventually cool off for billions of years as a white dwarf. Our own Sun will undergo a similar process, so, this planetary nebula could well offer a glimpse of the future that awaits our own Sun in about five billion years.
NGC 2818 is often heralded as one of the Milky Way’s few planetary nebulae to be discovered as a member of an open star cluster (NGC 2818A). Recent investigations, however, suggest that it is merely a chance alignment, as the objects are actually located at varying distances along the line-of-sight. The case is just an example of a superposed pair.
Moreover, there has yet to be a single established case of a Galactic planetary nebula discovered in an open cluster. Planetary nebulae have been detected in several globular star clusters in our Milky Way. These densely-packed, gravitationally-bound groups of 100,000s to millions of stars are far older than their open cluster counterparts.
Theoretical models predict that planetary nebulae can form from main-sequence stars of between 8 and 1 solar masses, which puts their age at 40 million years and older.
The colors in the image of NGC 2818 represent a range of emissions coming from the clouds of the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen.
June 29, 2012
NGC 891, a beautiful edge-on spiral galaxy
NGC 891 (also known as UGC 1831 and Caldwell 23) is a spiral galaxy about 100,000 light-years in diameter with an H II nucleus and a mass of about 320 billion Sun masses. It is located 27 million light-years away in the constellation Andromeda and is a member of the NGC 1023 group of galaxies in the Local Supercluster (or Virgo Supercluster). In August 1986 Supernova SN 1986J was discovered.
The edge-on galaxy reveals its plane of dust and interstellar gas, in the middle intersected by areas with dark, obscuring dust. The bulge and the disk are surrounded by a flat and thick cocoon-like stellar structure. One can also also distinguish young blue star clusters and characteristic pink red starforming areas.
NGC 891 appeared previously to be very similar to our own Milky Way Galaxy: a spiral galaxy seen edge-on with a flat, thin, galactic disk and a central bulge. However, recent high-resolution images of NGC 891 show unusual filamentary patterns of gas and dust, that extend hundreds of light-years above and below the center line.
Astronomers believe these filaments are the result of the ejection of material due to supernovae or intense stellar formation activity. Stars cause powerful winds by lighting up when they are born, or exploding when they die, that can blow dust and gas over hundreds of light-years in space.
Investigations in infrared give rise to suspects that NGC 891 has a bar. This bar, if it exists, cannot be seen, as it is edge-on from our perspective. The bar hypothesis is also supported by the fact that the billions of stars orbiting the center appear to be moving too fast to just be travelling in circles.
June 28, 2012
Messier 55, a large globular cluster in Sagittarius
Messier 55 (also known as NGC 6809) is a large globular cluster with a diameter of 96 light-years, held together in a tight spherical shape by gravity. It is located only 17,600 light-years away in the constellation Sagittarius.
The cluster contains around 100,000 stars, mostly old red stars, but also an unusual number of blue stragglers (stars that appear younger than the population from which they formed), and has a mass of about 269,000 Sun masses. Only about 55 variable stars have been discovered in the central part of Messier 55.
Messier 55 is one of the 160 globular clusters encircling our Milky Way, mostly toward its bulging centre. Observations of globular clusters’ stars reveal that they originated around the same time (over 10 billion years ago) and from the same cloud of gas.
As this formative period was just a few billion years after the Big Bang, Messier 55 has a low metallicity, what means that nearly all of the gas in the original cloud was hydrogen, along with some helium and even much smaller amounts of heavier chemical elements such as oxygen and nitrogen. (Our Sun, and the Solar System are just 4.6 billion years old and are infused with heavier elements created in earlier generations of stars, which exploded in supernovae.)
The stars in M55 are among the oldest in the Universe. Astronomers study Messier 55 and other globular clusters, to learn how galaxies evolve and stars age.
As well as the stars of Messier 55, this VISTA image also records many galaxies lying far beyond the cluster.
June 27, 2012
NGC 3576, The Statue of Liberty Nebula
NGC 3576 (also known as The Statue of Liberty Nebula and ESO 129-EN5) is a bright emission nebula about 100 light-years across, located some 9000 light-years away in the Sagittarius arm of our Milky Way in the constellation Carina.
This giant nebula of glowing gas is an H II region where stars are currently forming at a high rate. H II (H-two) regions are where stars are born from condensing clouds of hydrogen gas (they are named for the large amounts of ionized atomic hydrogen they contain.) These regions are characterized by hot, young, massive stars which emit large amounts of ultraviolet light and ionize the nebula.
Episodes of star formation during the last several million years are thought to contribute to the complex and suggestive shapes. Powerful winds from the nebula’s embedded, young, massive stars shape the looping filaments, while hydrogen, sulfur, and oxygen, energized by intense ultraviolet radiation, making the nebula glow. NGC 3576 also contains scattered small dark nebulae known as Bok Globules, which are very dense, opaque clouds of gas and dust, and potential sites for the formation of new stars.
Because NGC 3576 is very dense, many of the stars are hidden from view. A cluster of stars is visible in infrared observations, but not enough young, massive stars have been identified to explain the brightness of NGC 3576. Astronomers have found a large flow of ionized gas in radio observations and huge bubbles in optical images that extend out from the edge of the HII region. Taken with the X-ray data, this information hints that powerful winds are emerging from this hidden cluster.
June 26, 2012
The Tadpole Galaxy, a violently disrupted galaxy
The Tadpole Galaxy (also known as UGC 10214 or Arp 188) is a disrupted barred spiral galaxy located about 420 million light-years away toward the northern constellation Draco. Its most dramatic features are an incredibly long trail of stars and massive, bright blue star clusters, reflecting the essence of our dynamic, restless and violent Universe.
Its distorted shape was caused by a small intrudor, a very blue, compact, galaxy which crossed in front of the Tadpole Galaxy and was slung around behind the Tadpole by their mutual gravitational attraction. During this close encounter, tidal forces drew out the galaxy’s stars, gas, and dust, forming the conspicuous tail which is more than 280,000 light-years long.
Seen shining through the Tadpole’s spiral arms, the intruder (estimated to lie about 300 thousand light-years behind the much more massive Tadpole) is likely a hit and run galaxy that is now leaving the scene of the accident.
Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long tidal tail of stars. Each of these clusters contains up to a million stars and will redden with age to become in time globular clusters similar to those found in essentially all halos of large galaxies, including our own Milky Way.
Two prominent clumps of young bright blue stars are visible in the tidal tale and are separated by a gap. These will likely become dwarf galaxies that orbit the Tadpole when it has lost its tail as it grew older, like its terrestrial namesake.
Behind the Tadpole is another compelling picture: thousands of faint galaxies stretching back to nearly the beginning of time. They are a myriad of shapes and represent fossil samples of the Universe’s evolution.
June 25, 2012
Venus, the Morning or Evening Star
Venus (also known as the Morning or Evening Star) is the second planet from the Sun. After the Moon, it is the brightest natural object in the night sky, and with temperatures that reach 465 degrees C (870 degrees F), it is the hottest world in our solar system.
Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar to the Earth, and is often described as Earth’s “sister” or “twin”, but it has no natural satellite. The diameter of Venus is 12,092 km (only 650 km less than the Earth’s) and its mass is 81.5% of the Earth’s. The similarity in size and density between Venus and Earth suggests they share a similar internal structure: a core, mantle, and crust.
About 80% of the Venusian surface is covered by flat, smooth plains that are marred by thousands of volcanoes, ranging from about 0.8 to 240 kilometers wide, with lava flows carving long, winding canals up to more than 5,000 kilometers in length, longer than on any other planet. The sulfur in the atmosphere may indicate there is ongoing volcanic activity and even some recent eruptions.
Venus has two highland “continents”, one (Ishtar Terra) lying in the planet’s northern hemisphere and the other (Aphrodite Terra) just south of the equator. Its six mountainous regions include Maxwell Montes, which lies on Ishtar Terra. This 870 kilometers long mountain is the highest one on Venus, its peak is 11 km above its surface.
The planet has just about a thousand impact craters which are evenly distributed across its surface, demonstrating the surface is relatively young, approximately 300–600 million years old. These craters range from 3 km to 280 km in diameter.
Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid, preventing visual observation of its surface. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days. High in its atmosphere is an ozone layer, but it has a very weak magnetosphere.
The atmospheric pressure at the planet’s surface is 92 times that of the Earth and this extremely dense atmosphere consists mostly of carbon dioxide, with a small amount of nitrogen. Studies have suggested that billions of years ago, the atmosphere of Venus was much more like Earth’s than it is now, and that it previously possessed oceans, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.
Although the surface conditions on the planet are no longer hospitable to any Earthlike life that may have formed prior to this event, the possibility that a habitable niche still exists in the lower and middle cloud layers of Venus can not yet be excluded.
The Venusian year — the time it takes to orbit the sun — is about 224.7 Earth days long. Its rotation is by far the slowest of any of the major planets, namely 243 Earth days, which normally would mean that days on Venus would be longer than years. However, because of Venus’ curious retrograde rotation, the time from one sunrise to the next is only about 117 Earth days long.
June 24, 2012
ESO 498-G5, a spiral within a spiral
ESO 498-G5 (AM 0922-245) is a spiral galaxy located 100 million light-years away in the constellation of Pyxis.
Its luminous spiral arms with dark filaments wind all the way into the centre, so that ESO 498-G5’s core looks a bit like a miniature spiral galaxy. Astronomers refer to the distinctive spiral-like bulges as disc-type bulges, or pseudobulges. The bright elliptical centres of many spiral galaxies, which appear as glowing masses, are called classical bulges.
Star formation is still going on in disc-type bulges and has ceased in classical bulges. Classical bulges look much like a miniature version of an elliptical galaxy, embedded in the centre of a spiral, while disc-type bulges look like a second, smaller spiral galaxy located at the heart of the first — a spiral within a spiral.
The similarities between types of galaxy bulge and types of galaxy go beyond their appearance. Just like giant elliptical galaxies, the classical bulges consist of great swarms of stars moving about in random orbits. Conversely, the structure and movement of stars within disc-type bulges mirror the spiral arms arrayed in a galaxy’s disc. These differences suggest different origins for the two types of bulges: while classical bulges are thought to develop through major events, such as mergers with other galaxies, disc-type bulges evolve gradually, developing their spiral pattern as stars and gas migrate to the galaxy’s centre.
June 23, 2012
The Tarantula Nebula, an H II region in the Large Magellanic Cloud
The Tarantula Nebula (also known as 30 Doradus and NGC 2070) in the Large Magellanic Cloud (LMC), is an expanding emission nebula, H II region and stellar nursery, some 650 to 1,000 light-years across that lies 160,000 light-years away in the southern constellation Dorado.
The nebula is extremely luminous, and is the largest and most active starburst region known in the Local Group of galaxies. It is thought to contain more than half a million solar masses in gas and hosts some of the most massive stars known.
The Tarantula Nebula owes its name to its glowing filaments that resemble spider legs. They extend from a central ‘body’ where a huge compact cluster of hot stars, known as R136, illuminates and shapes the nebula. This name, of the biggest spiders on the Earth, is also very fitting in view of the gigantic proportions of the celestial nebula.
R136, only between one and two million years old, has an approximate diameter 35 light-years and an estimated mass of 450,000 solar masses, suggesting it will likely become a globular cluster in the future. These thousands massive stars are blowing off material and producing intense radiation along with powerful winds. The gas has been heated to millions of degrees by these stellar winds and also by supernova explosions. X-rays are emitted from shock fronts formed by this high-energy stellar activity.
The Tarantula Nebula also contains an older star cluster, known as Hodge 301, with an age of 20–25 million years. The most massive stars of this cluster have already exploded in supernovae.
In the outskirts of the Tarantula are the remains of a star that exploded and was seen with the unaided eye in February 1987. Supernova SN 1987A, as it is known, is the brightest supernova since the one observed by Kepler in 1604 and is known to be surrounded by a ring.
June 22, 2012
Cepheus B, a molecular cloud and stellar nursery
Cepheus B is a molecular cloud (mainly consisting of cool molecular hydrogen) and a stellar nursery in our Milky Way, located about 2,400 light-years away from Earth. It is part of a large and active region of star formation in the constellation Cepheus.
There are hundreds of very young stars inside and around the cloud, ranging from a few million years old outside the cloud to less than a million years old in the interior.
While astronomers have long understood that stars and planets form from the collapse of a cloud of gas, the question of the main causes of this process has remained open for a long time.
One option was that the cloud cools, gravity gets the upper hand, and the cloud falls in on itself. The other possibility is that a “trigger” from some external source (like radiation from a massive star or a shock from a supernova) initiates the collapse. Some studies have noted a combination of triggering mechanisms in effect.
Researchers found that star formation in Cepheus B is mainly triggered by radiation from one bright, massive star (HD 217086) outside the molecular cloud. Radiation from this star drives a compression wave into the cloud-triggering star formation in the interior, while evaporating the cloud’s outer layers. They saw indeed a wave of star and planet formation that is rippling through the cloud.
Analysis revealed slightly older stars outside the cloud, and the youngest stars with the most protoplanetary disks in the cloud interior, exactly what is predicted from the triggered star formation scenario.
By combining the data from the Chandra X-ray Observatory and the Spitzer Space Telescope, researchers have shown that radiation from massive stars may trigger the formation of many more stars than previously thought.
June 21, 2012
Messier 10, a bright globular star cluster in Ophiuchus
Messier 10 (M10, NGC 6254) is a bright, very large globular cluster of stars, some 14,300 light-away in the central region of the constellation Ophiuchus.
The cluster spans about 83 light-years, while the brighter core (which can be seen visually) is only less than half as large, about 35 light-years. Its estimated mass is 200,000 solar masses and it contains 4 variable stars.
Messier 10 is receding from us at 69 km/sec. and is currently located about 16,000 light-years from the Galactic Center. It completes an orbit around the Milky Way galaxy about every 140 million years, during which it crosses the plane of the galactic disk every 53 million years.
In terms of the abundance of elements other than hydrogen and helium – the so-called metallicity – Messier 10 is “moderately metal–poor”. The abundance of iron is only 3.5% of the abundance found at the surface of the Sun.
Because binary stars are, on average, more massive than normal stars, the binaries tend to migrate toward the center of the cluster. The core region also contains a concentration of blue straggler stars, most of which formed 2–5 billion years ago.
For its size, Messier 10 should appear about two thirds the size of the Moon in the night sky. However, its outer regions are extremely diffuse, and even the comparatively bright core is too dim to see with the naked eye.
Hubble, which has no problems seeing faint objects, has observed the brightest part of the centre of Messier 10 in this image (released on June 18, 2012), a region which is about 13 light-years across.
M10 was discovered by Charles Messier in 1764, who described it as a “Nebula without stars”, and William Herschel was the first to resolve it into stars.
June 20, 2012
NGC 55, a barred irregular galaxy in Sculptor
NGC 55 (Caldwell 72) is a bright barred irregular galaxy about 70 thousand light years across, located 5.4 light-years away in the southern constellation Sculptor. It is likely that NGC 55 and its neighbor NGC 300 orbit each other and form a gravitationally bound pair.
Along with NGC 300, NGC 55 is part of the Local Group of Galaxies that also includes the Andromeda galaxy (M31), the Magellanic Clouds, and 40 other galaxies. (Both galaxies have traditionally been identified as members of the Sculptor Group, however, recent distance measurements indicate that the two galaxies actually lie in the foreground.)
NGC 55 is nearly edge-on and appears asymmetrical, like a cigar. Its bulge is diffuse, broad and somewhat elongated. The bright core is crossed with clouds of gas and dust, and it has a lot of pinkish active star forming regions, and young blue star clusters. Among all these stars are 143 Cepheids.
NGC 55 is thought to be similar to our galactic neighbour, the Large Magellanic Cloud (LMC), although the LMC is seen face-on, whilst NGC 55 is edge-on.
June 19, 2012
Titan, Saturn’s largest moon
Titan is, with a diameter of 3,200 miles (5,150 kilometers), Saturn’s largest moon and the second largest moon in our solar system (only Jupiter’s moon Ganymede is larger), roughly 50% larger than our own Moon and 80% more massive. Titan orbits Saturn at a distance of about 745,000 miles (1.2 million km), taking almost 16 days to complete a full orbit..
This cold world — the temperature at Titan’s surface is about -289 degrees Fahrenheit (-178 degrees Celsius) — is the only moon known to have clouds and a dense hazy, planet-like atmosphere, impenetrable by telescopes and cameras. Its atmospheric pressure is about 60 percent greater than the Earth’s.
Titan’s surface is geologically young; although mountains and several possible ice volcanoes have been discovered, it is smooth and few impact craters have been found. It is primarily composed of water ice and rocky material. Titan is the only object other than Earth for which clear evidence of liquid hydrocarbon lakes in its polar regions has been found, and recently also long-standing methane lakes, or puddles, in its “tropics”.
Titan’s complex atmosphere might consist of compounds similar to those present in the primordial days of the Earth’s atmosphere. Titan’s thick cloudy atmosphere is mostly nitrogen (95 percent), like Earth’s, but may contain much higher percentages of “smog-like” chemicals such as methane and ethane. The smog may be so thick that it actually rains “gasoline-like” liquids. Titan also has a presence of organic molecules that contain carbon, hydrogen, oxygen and other elements similar to what is found in Earth’s atmosphere.
These chemicals might indicate that this fascinating moon could harbor a primitive, exotic form of life or precursor to life.
It has been suggested that life could exist in Titan’s lakes of liquid methane, just as organisms on Earth live in water. Such creatures would inhale H2 in place of O2, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide. But methane-based life forms are only hypothetical, scientists have not yet detected this form of life anywhere.
Titan’s climate, including wind and rain, also creates surface features similar to those of Earth, such as sand dunes, rivers, lakes, seas and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan’s methane cycle is viewed as an analog to Earth’s water cycle, although at a much lower temperature.
June 18, 2012
NGC 7635, an intriguing bubble in Cassiopeia
NGC 7635, also known as the Bubble Nebula and Sharpless 162 (Sh2-162) is an emission nebula and H II region, with a diameter of 10 light-years, surrounded by a giant molecular cloud. It is located 7100 to 11000 light-years away (sources differ) in the constellation Cassiopeia.
It is created by the fierce stellar wind (4 million miles per hour or 7 million kilometers per hour) and intense radiation from a massive hot, young, blue central star, BD+60 2522 (also known as SAO 20575), which is thought to have a mass of 10-40 Solar masses and causing the “bubble” to glow.
The molecular cloud is able to limit the expansion of gas in the bubble, but is torn to pieces by the scorching radiation from the central star of the Bubble. That radiation heats denser areas within the cloud and makes them glow as well.
The surface of the bubble is not uniform because as the shell expands outward it encounters regions of cold gas within the molecular cloud, which are of different density and therefore arrest the expansion by differing amounts, resulting in the rippled appearance.
It is this gradient of background material that the wind is encountering that places the central star off center in the bubble. There is more material to the northeast of the nebula than to the southwest, so that the wind progresses less in that direction, offsetting the central star from the geometric center of the bubble.
To the right of the central star is a ridge of much denser gas. The lower left portion of this ridge is closest to the star and so is brightest. The ridge forms a V-shape, with two segments that are aligned at the brightest edge.
The region between the star and ridge reveals several loops and arcs. The origin of this bubble-within-a-bubble” is unknown. It may be due to a collision of two distinct winds: the stellar wind colliding with material streaming off the ridge.
June 17, 2012
NGC 613, a large elongated spiral galaxy
NGC 613 is an elongated barred spiral galaxy over 100,000 light-years across, located about 64 million light-years away in the southern constellation Sculptor. The outer tips of the spiral arms curve in opposite directions as if to show the direction of spiral motion.
In contrary to most barred spirals, NGC 613 has many arms that give it a tentacle-like appearance. It has a huge central bar, and radio emission indicates the presence of a massive black hole at the center of the galaxy. NGC 613 is laced with prominent cosmic dust lanes and bright star forming regions.
Studies of this galaxy in the radio wavelengths indicate that the nucleus is extremely active. And there is some evidence for an accelerated and collimated jet of gas being expelled from the center of the galaxy.
Astronomers studying this galaxy are interested in the processes that take place along the bar of the galaxy. One recent paper suggests that this galaxy could have two bar-like structures in its center.
June 16, 2012
The Red Rectangle Nebula, a unique protoplanetary nebula
The Red Rectangle Nebula (HD 44179), so called because of its red color and unique rectangular shape, is a bipolar protoplanetary nebula, located about 2,300 light-years away toward the constellation Monoceros.
Protoplanetary nebula are formed by old stars, on their way to becoming planetary nebulae. In a few thousand years, once the expulsion of mass is complete, a very hot white dwarf star will remain and its brilliant ultraviolet radiation will cause the surrounding gas to glow.
The star at the centre of the Red Rectangle, known as MWC 922, was similar to our Sun but is now ejecting its outer layers to make the nebula, and giving it the distinctive shape. The shedding of the outer layers began about 14,000 years ago and is ejected from the star in two opposing directions.
It also appears that the star is a binary system, a close pair of stars that orbit each other with a period of about 10 1/2 months. Interactions between these stars have probably caused the ejection of the thick dust disk that obscures our view of the binary. It is an infrared source; stars surrounded by clouds of dust are often strong infrared sources because the dust is heated by the starlight and radiates long-wavelength light.
The disk has funneled subsequent outflows in the directions perpendicular to the disk, forming the bizarre bi- conical structure we see as the Red Rectangle. The reasons for the periodic ejections of more gas and dust remain unknown.
Seen from space, the Red Rectangle is not really rectangular, but has an overall X-shaped structure with additional complex structures of spaced lines of glowing gas, a little like the rungs of a ladder, which is interpreted as arising from outflows of gas and dust from the binary star in the center.
Astronomers are not yet certain which types of molecules are producing the red color that is so striking in the Red Rectangle, but suspect that they are hydrocarbons that form in the cool outflow from the central star.
They discovered the spectral signatures of anthracene and pyrene (hydrocarbons) in the ultraviolet light emitted by the nebula – potentially vital organic molecules for the formation of life. Until recently, it was thought that the ultraviolet light would quickly destroy these hydrocarbons.
June 15, 2012
Stephan’s Quintet (also known as Hickson Compact Group 92 and Arp 319) is a group of five galaxies (NGC 7317, 7318A, 7318B, 7319 and 7320) located in the constellation Pegasus. The name, however, is a bit of a misnomer because NGC 7320 (the blue one) is actually a foreground galaxy, located about 39 million light-years away, while the other members of the quintet reside 210-340 million light-years away.
The four yellowish galaxies are gravitationally interacting with each other and will eventually merge into a single big galaxy. Three of the galaxies have distorted shapes, elongated spiral arms, and long, gaseous tidal tails containing myriad star clusters, proof of their close encounters. These interactions have sparked a frenzy of star birth in the central pair of galaxies.
NGC 7320, the blue one at top right, is a dwarf galaxy, that is shown to have extensive H II regions, identified as red and blue dots, where bursts of star formation are occurring.
NGC 7319, at bottom right, is a barred spiral with a type 2 Seyfert nucleus and distinct spiral arms that follow nearly 180 degrees back to the bar. The blue specks in the spiral arm and the red dots are clusters of many thousands of stars.
Continuing clockwise, the next galaxy appears to have two cores, but it is actually two galaxies, NGC 7318A and NGC 7318B. Encircling the galaxies are young, bright blue star clusters and pinkish clouds of glowing hydrogen where infant stars are being born. These stars are less than 10 million years old and have not yet blown away their natal cloud.
NGC 7317, at top left, is a normal-looking elliptical galaxy that is less affected by the interactions.
These farther members are markedly redder than the foreground galaxy, suggesting that older stars reside in their cores. The stars’ light also may be further reddened by dust stirred up in the encounters.
Spied by Edouard M. Stephan in 1877, Stephan’s Quintet is the first compact group ever discovered.
June 14, 2012
NGC 5584, a big and beautiful galaxy in Virgo
NGC 5584 is a face-on spiral galaxy, more than 50,000 light-years across, that resides 72 million light-years away in the constellation Virgo. Its winding spiral arms are loaded with luminous young star clusters and thin dark dust lanes which appear to be flowing from the yellowish core, where older stars reside.
Among the galaxy’s myriad stars are 250 stars that vary in brightness and are classified as Cepheids. These are brilliant pulsating stars with a remarkable property — once the time it takes a Cepheid to brighten and fade is known, then it is possible to find how bright it actually is. When this information is combined with a measurement of how bright the star appears it is easy to work out how far away the star actually lies. This method is the most accurate and effective way to measure the distances to most nearby galaxies.
In addition to these Cepheids, NGC 5584 was also the site of SN 2007af, a type Ia supernova. These dramatic explosions of white dwarf stars are used as reference beacons for mapping the expansion, and acceleration, of the more remote Universe so this galaxy is a very valuable link between the two distance scales.
Once astronomers know accurate distances to galaxies near and far, they can determine the universe’s expansion rate. By studying many Cepheids in 8 galaxies (was one of them was NGC 5584) the team has been able to refine our knowledge of this expansion rate, expressed as a number known as Hubble’s constant, to an accuracy of 3.3 percent.
The value for the expansion rate is 73.8 kilometers per second per megaparsec. It means that for every additional million parsecs (3.26 million light-years) a galaxy is from Earth, the galaxy appears to be traveling 73.8 kilometers per second faster away from us.
Every decrease in uncertainty of the universe’s expansion rate helps solidify our understanding of its cosmic ingredients. Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and also helps astronomers tighten up a number of other cosmic properties, including the universe’s shape and its roster of neutrinos, ghostly particles that filled the early universe.
The reddish dots sprinkled throughout the image are largely background galaxies.
June 13, 2012
Sharpless 2-106, a bipolar emission nebula and HII region
Sharpless 2-106 (Sh2-106 or S106 for short) is a bipolar emission nebula and HII region. It measures about 2 light-years long by 0.5 light-year across and is located about 2,000 light-years away in the constellation Cygnus, in a relatively isolated region of the Milky Way.
Despite its appearance, Sh2-106 is not a planetary nebula, but a giant molecular cloud and star-forming region. Twin lobes of super-hot hydrogen gas (glowing blue) stretch outward from the central star, what creates the “wings”. This image also reveals ripples and ridges in the gas as it interacts with the cooler interstellar medium.
The massive, only 100,000 years old central star (S106 IR), which could be up to 15 times the mass of our Sun, is still shrouded in dust and gas at the centre of the nebula. It is responsible for the surrounding gas cloud’s hourglass-like shape and the turbulence visible within. Eventually its light will break free of the enveloping cloud as it begins the relatively short life of a massive star.
Dusky red veins surround the blue emission from the nebula. The faint light emanating from the central star reflects off of tiny dust particles. This illuminates the environment around the star, showing darker filaments of dust winding beneath the blue lobes.
Detailed studies of the nebula have also uncovered more than 600 hundred brown dwarfs which may someday create a small cluster. These “failed” stars weigh less than a tenth of our Sun. Because of their low mass, they cannot produce sustained energy through nuclear fusion like our Sun does.
June 12, 2012
ESO 593-8, an eagle in Sagittarius
ESO 593-8 (officially designated as ESO 593-IG008) is an impressive pair of interacting galaxies that looks like an eagle. The galaxies are located about 650 million light-years away in the constellation Sagittarius.
ESO 593-8 consists of a feather-like galaxy crossing, or piercing straight through, a companion galaxy, adorned with a number of bright blue star clusters.
The two components will probably merge to form a single galaxy in the future, and by then the evidence of the collision will have been disappeared. The newly formed galaxy might give future astronomers a hint, because this will be a brand new star-forming factory.
This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.
June 11, 2012
Messier 80, a globular star cluster in Scorpius
Messier 80 (NGC 6093) is a globular star cluster with a diameter of approximately 95 light-years, about 32,600 light-years away in the constellation Scorpius. It contains hundreds of thousands of stars, all held together by their mutual gravitational attraction, and is one of the densest known globular star clusters in the Milky Way Galaxy.
M80 contains a relatively large number of blue stragglers, stars that appear to be much younger than the cluster itself. It is thought these stars have lost part of their outer layers due to close encounters with other stars or perhaps the result of collisions between stars in the dense cluster.
Their large number in M80 indicates an exceptionally high stellar collision rate in the core of this globular cluster.
Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.
On May 21, 1860, a nova (T Scorpii) was discovered in Messier 80, briefly outshining the entire cluster.
June 10, 2012
Kronberger 61, a soccer ball in Cygnus
Kronberger 61 (the Soccer Ball) is an almost perfectly round planetary nebula located about 13,000 light-years away in the northern constellation Cygnus. The light of the nebula is primarily due to the emissions from twice-ionized oxygen.
A planetary nebula forms after nuclear fusion can no longer sustain the pressure of gravity in a dying sun-like star, and it becomes unstable, pulsates, and throws off a significant shell of gas from its outer layers. This expanding shell of thin ionized gas is what we see as a planetary nebula when its gas is ionized and glows due to the radiation still emitted by the central star.
Very few are this spherical. They’re usually elongated, have complex shapes and often look like butterflies.
This image of Kronberger 61 is showing the ionized shell of expelled gas resembling a soccer ball. The light of the nebula here is primarily due to emission from twice-ionized oxygen, and its central star can be seen as the slightly bluer star very close to the center of the nebula.
It is hoped that Kronberger 61 will help resolve a decades old debate, which is, are stellar companions key to the formation and structure of planetary nebulae? Explaining the puffs left behind when medium-sized stars like our Sun expel their last breaths is a source of heated debate among astronomers, especially the part that companions might play.
Planetary nebulae present a profound mystery. Some recent theories suggest that planetary nebulae form only in close binary or even planetary systems. On the other hand, the conventional explanation is that most stars, even solo stars like our Sun, will meet this fate. That might just be too simple.
Below the bright star at left is a barred spiral galaxy in the distant background, careful inspection will reveal several additional distant galaxies in the image.
June 9, 2012
ESO 510-G13, a heavily warped spiral galaxy
ESO 510-G13 is a heavily warped edge-on spiral galaxy about 150 million light-years away in the southern constellation Hydra. The galaxy illustrates that a galaxy’s disk is very fragile and can be easily distorted by gravity caused by interacting galaxies.
The most prominent features in this image are the galaxy’s heavily warped thin disk of stars, gas, and dust, its glowing central bulge, composed mostly of old stars, and its bright clusters of blue stars. The dark clouds of gas and dust in the disk stand out in this picture because they block the light of background stars, and because they are silhouetted from behind by light from the galaxy’s bright, smooth central bulge.
In the outer regions of ESO 510-G13, especially visible on the right-hand side of the image, the bent disk contains not only dark dust but also bright clusters of blue stars. These clusters are evidence that hot, young stars are being formed in the disk.
The strong warping of the disk indicates that ESO 510-G13 has recently undergone a collision with a nearby galaxy and is in the process of swallowing it. The formation of new stars is most likely triggered by this collision.
When galaxies collide, clouds of dust and gas smash together and are compressed in a process that takes millions of years, creating a favorable environment for star birth. Eventually the disturbances will die out, and ESO 510-G13 will become a normal-appearing single galaxy.
Much of ESO 510-G13 appears whitish because it contains stars of many different colors, which combine to create a white appearance in the image. However, near the dark band of dusty gas slicing through the middle of this galaxy, the starlight appears redder because the dusty gas blocks blue light more effectively than red light.
June 8, 2012
IC 1795, a bright emission nebula in Cassiopeia
IC 1795 (the Fishhead Nebula or Northern Bear Nebula) is a bright emission nebula with glowing gas and dark lanes of obscuring dust, about 70 light-years across and located just over 6,000 light-years away in the northern constellation Cassiopeia. The brighter region of the nebula is labelled NGC 896.
It is part of a complex of star forming regions that lie at the edge of a large molecular cloud along the Perseus spiral arm of our Milky Way, where star formation is most active. IC 1795 is attached to the right side of IC 1805 (the Heart Nebula).
Likely stars which have not yet emerged from their natal cocoon, and are presumed to be obstructed by interstellar dust and consequent absorption, are energizing the nebula and making it glow. This suggests that the stars inside are very young, and also probably quite massive, since the nebula is so prominent and bright.
An image of IC 1795 by WISE involving the infrared portion of the spectrum revealed several young stars which are a few million years old (see here). IC 1795 is also recognized as being a very strong source of radio emissions.
June 7, 2012
Mars, the “Red Planet”
Mars is the fourth planet from the Sun in the Solar System. It has approximately half the diameter of Earth.. Mars is often described as the “Red Planet” as the iron oxide (rust) prevalent on its surface gives it a reddish appearance.
Mars is a terrestrial planet with a thin carbon-dioxide-rich atmosphere, roughly 100 times less dense than Earth’s on average, but it is nevertheless thick enough to support weather, clouds and winds. It has surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons.
Mars is known of Olympus Mons, the highest known mountain within the Solar System, and of Valles Marineris, the largest canyon. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Channels, valleys, and gullies are found all over Mars, and suggest that liquid water might have flowed across the planet’s surface in recent times. Water may even still lie in cracks and pores in underground rock,
Geological evidence suggest that Mars once had large-scale water coverage on its surface. Radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. Observations also show evidence that parts of the southern polar ice cap have been receding and have revealed possible flowing water during the warmest months on Mars.
The habitability of a world, favors planets that have liquid water on their surface and lie within the habitable zone. During perihelion Mars dips inside this region, but the planet’s thin atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet’s potential for habitability. Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.
Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. Tests have shown that the soil has a very alkaline pH and it contains magnesium, sodium, potassium and chloride. The soil nutrients may be able to support life but life would still have to be shielded from the intense ultraviolet light.
Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life, as these chemical compounds would quickly break down in the Martian atmosphere. It is remotely possible that these compounds may instead be replenished by volcanic or geological means.
Streaks are common across Mars and new ones appear frequently on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. The commonly accepted theories include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils. Several explanations have been put forward, some of which involve water or even the growth of organisms. Mars also has the largest dust storms in our Solar System. These can vary from a storm over a small area, to gigantic storms that cover the entire planet.
Mars has a dense, metallic core region about 1794 km in radius, consisting primarily of iron and nickel with about 16–17% sulfur. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the martian crust are iron, magnesium, aluminum, calcium, and potassium. The average thickness of the planet’s crust is about 50 km, with a maximum thickness of 125 km.
Although Mars has no evidence of a current structured global magnetic field, observations show that parts of the planet’s crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past. Also, Mars is nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Mars’ average distance from the Sun is roughly 230 million km and its orbital period is 687 (Earth) days. The solar day on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is 1 (Earth) year, 320 days, and 18.2 hours.
A Martian year is about two Earth years long. Martian surface temperatures vary from lows of about −87 °C (−125 °F) during the polar winters to highs of up to −5 °C (23 °F) in summers. The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low thermal inertia of Martian soil.
Mars has two moons, Phobos and Deimos, which are small and irregularly shaped, and may be captured asteroids.
June 6, 2012
Hickson Compact Group 59, a variety of galaxies
Hickson Compact Group (HCG) 59 consists of a variety of galaxies, about 180 million light-years away in the constellation Leo. There are three main galaxies, of which two are star-forming while the other one is quiescent.
The main galaxies are two large spirals, one face-on with smooth arms and delicate dust tendrils, and one highly inclined, as well as a strangely disorderly galaxy featuring clumps of blue young stars. In this image we can also see many apparently smaller, probably more distant, galaxies visible in the background.
Hickson Compact Groups are small, relatively isolated clusters of galaxies listed in a catalogue published in 1982 by the British-born Canadian astronomer Paul Hickson (1950–). A typical Hickson Compact Group contains four to five closely spaced members, up to half of which may be interacting or even merging. (The most famous groups on Hickson’s list of 100 objects are Stephan’s Quintet and Seyfert’s Sextet.)
HCGs most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Possibly compact galaxy groups and interacting galaxies play a significant role in galaxy evolution.
Hickson Compact Groups display many peculiarities, often emitting in the radio and infrared and featuring active star-forming regions. In addition their galaxies frequently contain Active Galactic Nuclei (AGN) powered by supermassive black holes. HCGs contain large quantities of diffuse gas and are dynamically dominated by dark matter.
June 5, 2012
Messier 100, a grand design spiral galaxy
Messier 100 (NGC 4321) is an example of a grand design spiral galaxy and is, with diameter of 160,000 light-years, one of the largest and brightest spiral galaxies in the sky. It is part of the Virgo Galaxy Cluster and is situated about 55 million light-years away in the southern part of the constellation Coma Berenices.
The galaxy has a brilliant core, two prominent arms of bright blue stars, several fainter arms and a few dust lanes. The blue stars in the arms are young hot and massive stars which formed recently from density perturbations caused by interactions with neighboring galaxies. Intense star formation activity was found to take place in a ring of starburst activity along the periphery of the galaxy’s innermost spiral arms.
Messier 100 has an active galactic nucleus — a bright region at the galaxy’s core caused by a supermassive black hole that is actively swallowing material, which radiates brightly as it falls inwards.
Despite its nearly perfect symmetric outline, this galaxy appears slightly asymmetric, as on the southern (lower) side of the nucleus more (or brighter) young stars have formed. A significant part of the galaxy’s mass may lie in the faint outer regions.
Astronomers discovered over 20 Cepheids and one nova in Messier 100, as well as (so far) five supernovae:
• 1901B, a type I, mag 15.6 in March 1901;
• 1914A of undetermined type, mag 15.7 in Feb/Mar 1914;
• 1959E of type I, mag 17.5 in Aug/Sep 1959, 58″E and 21″S of the nucleus, discovered February 21, 1960 and observed through June 17, 1960; and
• 1979C of type II, mag 11.6 on April 15, 1979, which however faded quickly.
• 2006X of type Ia was found on February 7, 2006, when it was at 15.3 mag and still rising. Supernova 2006X was located 12″W and 48″S of M100’s nucleus, and was discovered early before reaching its maximum.
M100 is accompanied by two satellite galaxies: NGC 4323 (bottom center) and NGC 4328 (bottom right).
June 4, 2012
The Iris Nebula, a bright reflection nebula
The Iris Nebula (LBN 487, VDB 139 and Caldwell 4) is a bright reflection nebula some 6 light-years across and about 1,300 light-years away in the constellation Cepheus. The nebula is often mistakenly labelled for its associated open star cluster NGC 7023, which is present in the triangular “top hat” just above center in the image.
A reflection nebula is an interstellar dust cloud that emits no light of its own but rather reflects the blue starlight of nearby bright stars. In this case, the dust cloud is illuminated by the hot, blue newborn star HD 200775 (also known as SAO 19158), which is 10 times the mass of our sun. Actually HD 200775 is a variable star and part of an eccentric binary system with a periodicity of 3.7 years.
The star’s solar winds have cleared a bi-lobed zone surrounding the star that measures 5 x 2.5 light-years. In some central filaments the dust is converting invisible ultraviolet light into visible red light by photoluminesence. The blue nebula is surrounded by dark obscuring clouds of dust and cold molecular gas.
Near infrared observations indicate that The Iris Nebula may contain complex carbon molecules known as polycyclic aromatic hydrocarbons (PAHs).
June 3, 2012
A close-up of NGC 1569, a dwarf irregular starburst galaxy
NGC 1569 (also known as UGC 3056 and Arp 210) is a dwarf irregular starburst galaxy nearly 11 million light-years away in the constellation Camelopardalis.
NGC 1569 is a hotbed of vigorous star birth activity which blows huge bubbles and super-bubbles. The galaxy’s vigorous “star factories” are also manufacturing brilliant blue star clusters.
NGC 1569 shows large hot bubbles, or lobes extending above and below a disk of gas along the equator of the galaxy. Scientists found that bubbles contain hydrogen gas and oxygen equal to the oxygen contained in 3 million suns.
The bubble structure is sculpted by the galactic super-winds and outflows caused by a colossal input of energy from collective supernova explosions that are linked with a massive episode of star birth.
This galaxy had a sudden and relatively recent onset of star birth and supernova explosions, thought to have begun about 25 million years ago, perhaps triggered by a collision with a massive gas cloud. (The first supernovae blew up when the most massive stars reached the end of their lifetimes.)
The supernovae eject oxygen and other heavy elements at high velocity into the gas in the galaxy, heating it to millions of degrees. Hot gas boils off the gaseous disk of the galaxy to form the bubbles, which expand out of the galaxy at speeds of hundreds of thousands of miles per hour.
The environment in NGC 1569 is still turbulent and the supernovae may not only deliver the gaseous raw material needed for the formation of further stars and star clusters, but also actually trigger their birth.
NGC 1569 is exceptional in that its spectrum is blueshifted. This means that the galaxy is moving towards the Earth. In contrast, the spectra of most other galaxies are redshifted because of the expansion of the universe.
Because of their size, dwarf galaxies have relatively low gravity and matter can escape from them more easily. This property, combined with the fact that dwarf galaxies are the most common type of galaxy in the universe, makes them very important in understanding how the universe was seeded with various elements billions of years ago, when galaxies were forming.
This faint galaxy is well studied by professional astronomers, who are interested in the history of star formation within the galaxy. Because the galaxy is relatively nearby, the Hubble Space Telescope can easily resolve the stars within the galaxy. So NGC 1569 offers astronomers an excellent opportunity to study stellar populations in rapidly evolving galaxies.
June 2, 2012
Zw II 96, a luminous pair of merging galaxies
Zw II 96 (also II Zw 96) is a luminous infrared pair of merging galaxies containing powerful, young starburst regions. It is located about 500 million light-years away in the constellation Delphinus.
Zw II 96 is a good example of star formation that occurs during galactic mergers. As a starburst galaxy, Zw II 96 is producing new stars at a high rate. As the dust and gas from each galaxy collides, new stars are formed. The star formation is seen as bright blue collections of light.
The shape of the merging galaxies is unusual; a number of powerful young starburst regions hang as long, threadlike structures between the main galaxy cores. The system almost qualifies as an ultraluminous system, but has not yet reached the late stage of coalescence that is the norm for most ultraluminous systems.
Four primary infrared sources are found within Zw II 96. One (let’s call it source A) may be a galaxy nucleus, since it shows morphological hints of a stellar bar, and is in the throes of powerful starburst activity. Another (B) seems to be a relatively quiescent spiral galaxy. The remaining two sources (C and D) are very red with strong near-infrared emission lines from atomic and molecular hydrogen and helium. C and D are most likely highly obscured, luminous starburst regions.
A, C, and D are in a similar evolutionary stage, containing very young starbursts and seen before the final collapse and dynamical relaxation which simulations predict for major mergers.
These starburst regions are probably groups of super star clusters, rich in OB stars, which are ubiquitous in starburst galaxies. An important, yet open, question is whether these galaxies like Zw II 96 represent part of an evolutionary sequence, a stage through which most or even all infrared-luminous mergers will pass.
June 1, 2012
Munch crater on Mercury
Mercury’s Munch crater with a diameter of 58 kilometers (36 miles) is situated in the far northern part of the Caloris basin, the youngest large impact basin on Mercury.
The floor of the Caloris basin has been flooded with volcanic flows. The ejecta-blanket surrounding the Munch crater contains dark material that originated at depth and was excavated by the Munch-forming impact.
The presence of both bright and dark materials suggest that there is a diversity of rock types on and below Mercury’s surface, with different mineralogical compositions from those of the volcanic plains that comprise the majority of the floor of Caloris basin.
This image, created on February 10, 2012, is a portion of the MDIS global mosaic basemap that was acquired during MESSENGER’s first year in orbit. The scene shows a dramatic close-up of Munch crater, named for the Norwegian impressionist painter, printmaker, and draftsman Edvard Munch (1863-1944). (That is the one who painted “The Scream” which was sold at auction for $119.9 million, becoming one of the most expensive pieces of artwork in the world.)
The MESSENGER spacecraft (which entered orbit around the planet Mercury on March 18, 2011) is the first ever to orbit the planet Mercury, and the spacecraft’s seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System’s innermost planet. During the one-year primary mission, MDIS acquired 88,746 images and extensive other data sets. MESSENGER is now in a year-long extended mission, during which plans call for the acquisition of more than 80,000 additional images to support MESSENGER’s science goals.