Archives mensuelles : mars 2023

Seeing Titan

Shrouded in a thick atmosphere, Saturn’s largest moon Titan really is hard to see. Small particles suspended in the upper atmosphere cause an almost impenetrable haze, strongly scattering light at visible wavelengths and hiding Titan’s surface features from prying eyes. But Titan’s surface is better imaged at infrared wavelengths where scattering is weaker and atmospheric absorption is reduced. Arrayed around this visible light image (center) of Titan are some of the clearest global infrared views of the tantalizing moon so far. In false color, the six panels present a consistent processing of 13 years of infrared image data from the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft orbiting Saturn from 2004 to 2017. They offer a stunning comparison with Cassini’s visible light view. NASA’s revolutionary rotorcraft mission to Titan is due to launch in 2027. via NASA

NGC 4372 and the Dark Doodad

The delightful Dark Doodad Nebula drifts through southern skies, a tantalizing target for binoculars toward the small constellation Musca, The Fly. The dusty cosmic cloud is seen against rich starfields just south of the Coalsack Nebula and the Southern Cross. Stretching for about 3 degrees across the center of this telephoto field of view, the Dark Doodad is punctuated near its southern tip (upper right) by yellowish globular star cluster NGC 4372. Of course NGC 4372 roams the halo of our Milky Way Galaxy, a background object some 20,000 light-years away and only by chance along our line-of-sight to the Dark Doodad. The Dark Doodad’s well defined silhouette belongs to the Musca molecular cloud, but its better known alliterative moniker was first coined by astro-imager and writer Dennis di Cicco in 1986 while observing Comet Halley from the Australian outback. The Dark Doodad is around 700 light-years distant and over 30 light-years long. via NASA

Which star created this bubble? It wasn’t the bright star on the bubble’s right. And it also wasn’t a giant space dolphin. It was the star in the blue nebula’s center, a famously energetic Wolf-Rayet star. Wolf-Rayet stars in general have over 20 times the mass of our Sun and expel fast particle winds that can create iconic looking nebulas. In this case, the resulting star bubble spans over 60 light years, is about 70,000 years old, and happens to look like the head of a dolphin. Named Sh2-308 and dubbed the Dolphin-Head Nebula, the gas ball lies about 5,000 light years away and covers as much sky as the full moon — although it is much dimmer. The nearby red-tinged clouds on the left of the featured image may owe their glow and shape to energetic light emitted from the same Wolf-Rayet star. via NASA

Yes, but can your green flash do this? A green flash at sunset is a rare event that many Sun watchers pride themselves on having seen.  Once thought to be a myth, a green flash is now understood to occur when the Earth’s atmosphere acts like both a prism and a lens. Different atmospheric layers create altitude-variable refraction that takes light from the top of the Sun and disperses its colors, creates two images, and magnifies it in just the right way to make a thin sliver appear green just before it disappears. Pictured, though, is an even more unusual sunset. From the high-altitude Cerro Tololo Inter-American Observatory in Chile one day last April, the Sun was captured setting beyond an atmosphere with multiple distinct thermal layers, creating several  mock images of the Sun.  This time and from this location, many of those layers produced a green flash simultaneously. Just seconds after this multiple-green-flash event was caught by two well-surprised astrophotographers, the Sun set below the clouds. via NASA

Reports of powerful solar flares started a seven-hour quest north to capture modern monuments against an aurora-filled sky. The peaks of iconic Arctic Henge in Raufarhöfn in northern Iceland were already aligned with the stars: some are lined up toward the exact north from one side and toward exact south from the other. The featured image, taken after sunset late last month, looks directly south, but since the composite image covers so much of the sky, the north star Polaris is actually visible at the very top of the frame. Also visible are familiar constellations including the Great Bear (Ursa Major) on the left, and the Hunter (Orion) on the lower right. The quest was successful. The sky lit up dramatically with bright and memorable auroras that shimmered with amazing colors including red, pink, yellow, and green — sometimes several at once. via NASA

Venus and the Da Vinci Glow

On March 23 early evening skygazers could watch Venus and a young crescent moon, both near the western horizon. On that date Earth’s brilliant evening star, faint lunar night side and slender sunlit crescent were captured in this telephoto skyscape posing alongside a church tower from Danta di Cadore, Dolomiti, Italy. Of course the subtle lunar illumination is earthshine, earthlight reflected from the Moon’s night side. A description of earthshine, in terms of sunlight reflected by Earth’s oceans illuminating the Moon’s dark surface, was written over 500 years ago by Leonardo da Vinci. On March 24, from some locations the Moon could be seen to occult or pass in front of Venus. Around the planet tonight, a waxing lunar crescent will appear near the Pleiades star cluster. via NASA

Outbound Comet ZTF

Former darling of the northern sky Comet C/2022E3 (ZTF) has faded. During its closest approach to our fair planet in early February Comet ZTF was a mere 2.3 light-minutes distant. Then known as the green comet, this visitor from the remote Oort Cloud is now nearly 13.3 light-minutes away. In this deep image, composed of exposures captured on March 21, the comet still sports a broad, whitish dust tail and greenish tinted coma though. Not far on the sky from Orion’s bright star Rigel, Comet ZTF shares the field of view with faint, dusty nebulae and distant background galaxies. The telephoto frame is crowded with Milky Way stars toward the constellation Eridanus. The influence of Jupiter’s gravity on the comet’s orbit as ZTF headed for the inner solar system, may have set the comet on an outbound journey, never to return. via NASA

Spiral Galaxy NGC 2841

A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in planet Earth’s night sky toward the northern constellation of Ursa Major. This sharp image centered on the gorgeous island universe also captures spiky foreground Milky Way stars and more distant background galaxies within the same telescopic field of view. It shows off the bright nucleus of NGC 2841, along with its inclined galactic disk, and faint outer regions. Dust lanes, small star-forming regions, and young star clusters are embedded in the galaxy’s patchy, tightly wound spiral arms. In contrast, many other spirals exhibit broader, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, making it even larger than our own Milky Way. X-ray images suggest that extreme outflows from giant stars and stellar explosions create plumes of hot gas extending into a halo around NGC 2841. via NASA

How far can you see? The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy, over two million light-years away. Without a telescope, even this immense spiral galaxy appears as an unremarkable, faint, nebulous cloud in the constellation Andromeda. But a bright white nucleus, dark winding dust lanes, luminous blue spiral arms, and bright red emission nebulas are recorded in this stunning fifteen-hour telescopic digital mosaic of our closest major galactic neighbor. But how do we know this spiral nebula is really so far away? This question was central to the famous Shapley-Curtis debate of 1920. M31’s great distance was determined in the 1920s by observations that resolved individual stars that changed their brightness in a way that gave up their true distance. The result proved that Andromeda is just like our Milky Way Galaxy — a conclusion making the rest of the universe much more vast than had ever been previously imagined. via NASA

Can dust be beautiful? Yes, and it can also be useful. The Taurus molecular cloud has several bright stars, but it is the dark dust that really draws attention. The pervasive dust has waves and ripples and makes picturesque dust bunnies, but perhaps more importantly, it marks regions where interstellar gas is dense enough to gravitationally contract to form stars. In the image center is a light cloud lit by neighboring stars that is home not only to a famous nebula, but to a very young and massive famous star. Both the star, T Tauri, and the nebula, Hind’s Variable Nebula, are seen to vary dramatically in brightness — but not necessarily at the same time, adding to the mystery of this intriguing region. T Tauri and similar stars are now generally recognized to be Sun-like stars that are less than a few million years old and so still in the early stages of formation. The featured image spans about four degrees not far from the Pleiades star cluster, while the featured dust field lies about 400 light-years away. via NASA