MyCn18: An Hourglass Nebula

Credit: R. Sahai and J. Trauger (JPL), WFPC2, HST NASA

Explanation: The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star’s life occurs as its outer layers are ejected – its core becoming a cooling, fading White Dwarf. Astronomers have recently used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the “hourglass”. The unprecedented sharpness of the HST images has revealed surprising details of thenebula ejection process and may help resolve the outstanding mystery of the variety of complex shapes and symmetries of planetary nebulae.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Light Bridges on the Sun

Credit: G. Scharmer, L. Rouppe van der Voort (KVA) et al., SVST

Explanation: Bridges the length of a planet can form on the Sun in a matter of hours. Known as light bridges, these structures may form as large sunspot groups decay. Above, one of the sharpest photographs of the Sunever taken shows two such light bridges that appeared late last month. The 5000-kilometer long bridges connect moderately dark penumbral regions across the cool abyss of two dark sunspot umbras. A movie shows that material tends more to rise from below and fall rather than to cross the light bridges. Bright bubbling granules surround the sunspot group. The impressive details on this recently released picture from the Swedish Solar Vacuum Telescope were made possible by new adaptive optics that correct for the blurring of the Earth’s atmosphere. What eventually became of the light bridges? As days progressed, the bridge region expanded to fill the void as the sunspots moved apart and decayed.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Antares and Rho Ophiuchi

Credit & Copyright: D. Malin (AAO), AATB, ROE, UKS Telescope

Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the upper left. Rho Ophiuchi lies at the center of the blue nebula on the right. The distant globular cluster M4 is visible just below Antares, and to the left of the red cloud engulfingSigma Scorpii. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Sungrazer

Credit: LASCO, SOHO Consortium, NRL, ESA, NASA

Explanation: Arcing toward a fiery fate, this Sungrazer comet was recorded by the SOHO spacecraft’s Large Angle Spectrometric COronagraph (LASCO) on Dec. 23rd, 1996. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 5 million miles of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Spots and blemishes on the image are background stars and camera streaks caused by charged particles. Positioned in space to continuously observe the Sun, SOHO has detected 7 sungrazing comets. Based on their orbits, they are believed to belong to a family of comets created by successive break ups from a single large parent comet which passed very near the sun in the twelfth century. The bright comet of 1965, Ikeya-Seki, was also a member of the Sungrazer family, coming within about 400,000 miles of the Sun’s surface. Passing so close to the Sun, Sungrazers are subjected to destructive tidal forces along with intense solar heat. This comet, known as SOHO 6, did not survive.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

A Halo Around the Moon

Credit & Copyright: Juan Carlos Casado

Explanation: Have you ever seen a halo around the Moon? This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similarSun Halo may be visible during the day. The town in the foreground of the above picture is San Sebastian, Spain. The distant planet Jupiter appears by chance on the halo’s upper right. Exactly how ice-crystals form in clouds remains under investigation.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

NGC 3314: When Galaxies Overlap

Credit: W. Keel and R. White, (U. Alabama, Tuscaloosa), Hubble Heritage Team (STScI/ AURA), NASA

Explanation: Can this be a spiral galaxy? In fact, NGC 3314 consists of two large spiral galaxies which just happen to almost exactly line-up. The foreground spiral is viewed nearly face-on, its pinwheel shape defined by young bright star clusters. But against the glow of the background galaxy, dark swirling lanes of interstellar dust are also seen to echo the face-on spiral’s structure. The dust lanes are surprisingly pervasive, and this remarkable pair of overlapping galaxies is one of a small number of systems in which absorption of visible light can be used to directly explore the distribution of dust in distant spirals. NGC 3314 is about 140 million light-years away in the southern constellation of Hydra. Just released, this color composite was constructed from Hubble Space Telescope images made in 1999 and 2000.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

A Green Flash from the Sun

Credit & Copyright: Juan Carlos Casado

Explanation: Many think it is just a myth. Others think it is true but its cause isn’t known. Adventurers pride themselves on having seen it. It’s a green flash from the Sun. The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green. The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. Agreen flash is also visible for a rising Sun, but takes better timing to spot. A slight variant of this was caught in the above photograph, where much of the Sun was still visible, but the very top appeared momentarily green. TheSun itself does not turn partly green, the effect is caused by layers of the Earth’s atmosphere acting like a prism.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

The North America Nebula

Credit & Copyright: Jason Ware

Explanation: Here’s a familiar shape in an unfamiliar location! This emission nebula is famous partly because it resembles Earth’s continent of North America. To the right of the North America Nebula, cataloged as NGC 7000, is a less luminous Pelican Nebula. The two emission nebula measure about 50 light-years across, are located about 1500 light-years away, and are separated by a dark absorption cloud. The nebulae can be seen with binoculars from a dark location. Look for a small nebular patch north-east of bright star Deneb in the constellation of Cygnus. It is still unknown which star or stars ionize the red-glowing hydrogen gas.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Filaments In The Cygnus Loop

Credit: William P. Blair and Ravi Sankrit (Johns Hopkins University), NASA

Explanation: Subtle and delicate in appearance, these are filaments of shocked interstellar gas — part of the expanding blast wave from a violent stellar explosion. Recorded in November 1997 with the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope, the picture is a closeup of a supernova remnant known as the Cygnus Loop. The nearly edge-on view shows a small portion of the immense shock front moving toward the top of the frame at about 170 kilometers per second while glowing in light emitted by atoms of excited Hydrogen gas. Not just another pretty picture, this particular image has provided some dramatic scientific results. In 1999, researchers used it to substantially revise downward widely accepted estimates of distance and age for this classic supernova remnant. Now determined to lie only 1,440 light-years away, the Cygnus Loop is thought to have been expanding for 5 – 10 thousand years.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Blue Marble 2000

Credit: R. Stockli, A. Nelson, F. Hasler,
NASA/ GSFC/ NOAA/ USGS

Explanation: This newly released digital portrait of our planet is reminiscent of the Apollo-era pictures of the “big blue marble” Earth from space. To create it, researchers at Goddard Space Flight Center’s Laboratory for Atmospheres combined data from a Geostationary Operational Environmental Satellite (GOES), the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and the Polar Orbiting Environmental Satellites (POES) with aUSGS elevation model of Earth’s topography. Stunningly detailed, the planet’s western hemisphere is cast so that heavy vegetation is green and sparse vegetation is yellow, while the heights of mountains and depths of valleys have been exaggerated by 50 times to make vertical relief visible. Hurricane Linda is the dramatic storm off North America’s west coast. And what about the Moon? The lunar image was reconstructed from GOES data and artistically rescaled for this visualization.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)