Astronomy Picture of the Day

The Bubble Nebula

Explanation: Blown by the wind from a massive star, this interstellarapparition has a surprisinglyfamiliar shape.Cataloged as NGC 7635, it is also known simplyas TheBubble Nebula.Although it looks delicate, the 10 light-year diameterbubble offers evidence ofviolent processes at work.Above and right of the Bubble's center is a hot,O-type star, several hundred thousandtimes more luminous and approximately 45 times more massivethan the Sun.A fierce stellar wind and intense radiation from thatstar has blasted out thestructure of glowing gasagainst denser materialin a surrounding molecularcloud.The intriguing Bubble Nebula lies a mere11,000 light-years away toward the boastful constellationCassiopeia.A false-colorHubble palette was used to createthis sharp image andshows emission from sulfur, hydrogen, and oxygen atoms in red,green, and blue hues. The image datawas recorded using a small telescope underclear, steady skies, from Mount Wilson Observatory.

Earth and Moon from MESSENGER

Explanation: What does Earth look like from the planet Mercury?The robotic spacecraft MESSENGER found out as it looked toward the Earth during its closest approach to the Sun about three months ago. The Earth and Moon are visible as the double spot on the lower left of the above image. Now MESSENGER was not at Mercury when it took the above image, but at a location from which the view would be similar. From Mercury, both the Earth and its comparatively large moon will always appear as small circles of reflected sunlight and will never show a crescent phase. MESSENGER has zipped right by Mercury three times since being launched in 2004, and is scheduled to enter orbit around the innermost planet in March of 2011.

The Local Fluff

Explanation: The stars are not alone. In the disk of ourMilky Way Galaxyabout 10 percent of visible matter is in the form of gas, called theinterstellar medium (ISM). The ISM isnot uniform,and shows patchiness even near ourSun. It can be quite difficult to detect thelocal ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however, absorbs some veryspecific colors that can be detected in the light of thenearest stars. A working map of the localISM within 10 light-yearsbased on recent observations is shown above. These observations show that ourSun is moving through aLocal Interstellar Cloud as this cloud flows outwards from theScorpius-Centaurus Association star forming region. Our Sun may exit the Local Interstellar Cloud, also called the Local Fluff,during the next 10,000 years. Much remains unknown about the localISM, including details of its distribution,its origin, and how it affects theSun and theEarth.

Hole in the Sun

Explanation: This ominous, dark shape sprawling across the face of the Sunis a coronal hole --a low density region extending abovethe surface where the solar magnetic field opens freely intointerplanetary space.Studied extensivelyfromspace since the 1960s inultraviolet andx-ray light, coronal holes are known to be the source ofthe high-speedsolar wind, atoms and electronswhich flow outward along the openmagnetic field lines.During periods of low activity,coronal holes typically coverregions just above the Sun's poles.But this extensive coronal holedominated the Sun's northern hemisphere earlier this week, capturedhere in extreme ultraviolet light by cameras onboardthe Solar Dynamics Observatory.The solar wind streaming from this coronal holetriggered auroral displayson planet Earth.

Brighter Than Mars

Explanation: Even though you may have just read an emailclaiming Mars will be incredibly bright tonight,the brightest star on the horizon is not Mars.From central Iran on August 24th,the brightest star in thistwilightdesert skyviewis Venus, aka the Evening Star.But a bright Mars is in the picture, just above and rightof more brilliant Venus.Despite claims in the internet'sannually returning Mars Hoaxthat Mars will be as big and bright as the Full Moon,this celestial scenario is very similar to the western sky you cansee tonight.Along with Mars, the still beautiful vista includes Spica,alpha star of theconstellationVirgo, above and leftof Venus.Farther right of Venus,Saturn peeks through the sunset'sfading glow just above the clouds.Near the opposite horizon, the Full Moon illuminating thedesert is about 400,000 times brighterthan Mars.

M27: Not a Comet

Explanation: While hunting for comets in the skies above 18th century France,astronomer Charles Messierdiligently kept a list of the thingshe encountered that were definitely not comets.This is number 27 on hisnowfamous not-a-comet list.In fact, 21st century astronomers would identify it as aplanetarynebula, but it's not a planet either, even though it mayappear round andplanet-like in a small telescope.Messier 27 (M27) is an excellent example of agaseous emission nebula createdas a sun-like star runs outof nuclear fuel in its core.The nebula forms as the star's outer layers are expelled intospace, with a visible glow generated by atoms excited by the dyingstar's intense but invisibleultraviolet light.Known by the popular name of theDumbbellNebula, the beautifullysymmetric interstellar gas cloud is over 2.5 light-years across andabout 1,200 light-years away in theconstellationVulpecula.This impressive colorcomposite highlights details withinthe well-studied central region and fainter, seldom imagedfeatures in the nebula's outer halo.It includes narrowband images recorded usingfilters sensitive to emission from oxygen atoms, shown inblue-green hues, and hydrogen atoms in red.