The Comet, the Owl, and the Galaxy
2016 April 21
Explanation: Comet（彗星） C/2014 S2 (PanSTARRS) poses for a Messier（梅西耶天体） moment in this telescopic snapshot from April 18. In fact it shares the 1.5 degree wide field-of-view with two well-known entries in the 18th century comet-hunting astronomer's famous catalog. Outward bound and sweeping through northern skies just below the Big Dipper, the fading visitor to the inner Solar System was about 18 light-minutes from our fair planet. Dusty, edge-on spiral galaxy Messier 108 (upper right) is more like 45 million light-years away. A planetary nebula with an aging but intensely hot central star, the owlish Messier 97 is only about 12 thousand light-years distant though, still well within our own Milky Way galaxy. Astronomers expect the orbit of this comet PanSTARRS to return it to the inner Solar System around the year 4226.
NGC 7635: The Bubble Nebula
2016 April 22
Explanation: Blown by the wind from a massive star, this interstellar apparition has a surprisingly familiar shape. Cataloged as NGC 7635, it is also known simply as The Bubble Nebula（泡泡星云）. Although it looks delicate, the 7 light-year diameter bubble offers evidence of violent processes at work. Above and left of the Bubble's center is a hot, O-type star, several hundred thousand times more luminous and around 45 times more massive than the Sun. A fierce stellar wind and intense radiation from that star has blasted out the structure of glowing gasagainst denser material in a surrounding molecular cloud. The intriguing Bubble Nebula and associated cloud complex lie a mere 7,100 light-years away toward the boastful constellation Cassiopeia（仙后座）. This sharp, tantalizing view of the cosmic bubble is a composite of Hubble Space Telescope image data from 2016, released to celebrate the 26th anniversary of Hubble's launch.
Milky Way in Moonlight
2016 April 23
Explanation: A waning crescent moon, early morning twilight, and Al Hamra's city lights on the horizon can't hide the central Milky Way（银河） in this skyscape from planet Earth. Captured in a single exposure, the dreamlike scene looks southward across the region's grand canyon from Jabal Shams (Sun Mountain), near the highest peak in Oman, on the Arabian Peninsula. Mist, moonlight, and shadows still play along the steep canyon walls. Dark rifts along the luminous band of the Milky Way are the galaxy's cosmic dust clouds. Typically hundreds of light-years distant, they obscure starlight along the galactic plane, viewed edge-on from the Solar System's perspective.
M16: Pillars of Star Creation
2016 April 24
Explanation: Newborn stars are forming in the Eagle Nebula. This image, taken with the Hubble Space Telescope in 1995, showsevaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust. The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars. At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed. The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away. The pillars of creation（创生之柱） were imaged again in 2007 by the orbiting Spitzer（施皮策） Space Telescope in infrared light, leading to the conjecture that the pillars may already have been destroyed by a local supernova, but light from that event has yet to reach the Earth.
Credit from NASA
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