Astronomy Picture of the Day
APOD: 2018 January 12 - Blue Comet PanSTARRS
Explanation: Discovered with the PanSTARRS telescope on September 7, 2016, this Comet PanSTARRS, C/2016 R2, is presently about 24 light minutes (3 AU) from the Sun, sweeping through planet Earth's skies across the background of stars in the constellation Taurus. An inbound visitor from our Solar System's distant Oort Cloud, its beautiful and complex ion tail is a remarkable shade of blue. Still relatively far from the Sun, the comet's already well-developed ion tail is very impressive. Emission from unusually abundant ionized carbon monoxide (CO+) molecules fluorescing in the increasing sunlight is largely responsible for the pretty blue tint. This color image of the blue comet is a combination of data taken from two different telescopes during the night of January 7. Located at the apex of the V-shaped Hyades star cluster in Taurus, bright star Gamma Tauri is responsible for the glow at the bottom left corner of the frame.
APOD: 2017 September 23 - A Conjunction of Comets
Explanation: A conjunction of comets is captured in this pretty star field from the morning of September 17. Discovered in July by a robotic sky survey searching for supernovae, comet C/2017 O1 ASASSN is at the lower left. The visible greenish glow of its coma is produced by the fluorescence of diatomic carbon molecules in sunlight. Nearing its closest approach to the Sun, the binocular comet was only about 7.2 light-minutes from Earth. In the same telescopic field of view is the long-tailed, outbound comet C/2015 ER61 PanSTARRS at the upper right, almost 14 light-minutes away. Many light-years distant, the starry background includes faint, dusty nebulae of the Milky Way. The well-known Pleiades star cluster lies just off the top right of the frame.
APOD: 2016 December 8 - Whirlpool with Comets
Explanation: Not a comet, bright spiral galaxy Messier 51 is popularly known as the Whirlpool Galaxy. Just off the handle of the Big Dipper in northern skies, you can spot it at the upper left in this image from December 1st. The pretty 4 by 2.5 degree wide field of view does contain two comets though. Different in appearance, both comets are new visitors to the inner Solar System and are currently faint telescopic objects, highest above northern horizons in morning twilight. At lower left newly discovered comet NEOWISE (C/2016 U1) shows off a round fuzzy coma in the greenish light of diatomic carbon gas fluorescing in sunlight. Sunlight reflects from dust in the coma and stubby tail of comet Johnson (C/2015 V2) at upper right.
APOD: 2016 June 17 - Comet PanSTARRS in the Southern Fish
Explanation: Now approaching our fair planet this Comet PanSTARRS (C/2013 X1) will come closest on June 21-22, a mere 5.3 light-minutes away. By then its appearance low in northern hemisphere predawn skies (high in the south), will be affected by the light of a nearly Full Moon, though. Still the comet's pretty green coma is about the apparent size of the Full Moon in this telescopic portrait, captured on June 12 from the southern hemisphere's Siding Spring Observatory. The deep image also follows a broad, whitish dust tail up and toward the left in the frame, sweeping away from the Sun and trailing behind the comet's orbit. Buffeted by the solar wind, a fainter, narrow ion tail extends horizontally toward the right. On the left edge, the brightest star is bluish Iota Piscis Austrini. Shining at about fourth magnitude, that star is visible to the unaided eye in the constellation of the Southern Fish.
APOD: 2016 April 11 - The Comet and the Star Cluster
Explanation: Comet Linear has become unexpectedly bright. The comet, discovered in 2000, underwent a 100-fold outburst just a week before it passed a mere 14 lunar distances from Earth late last month. The comet was captured here last week at about magnitude 6 -- just bright enough to be seen by the unaided eye -- passing in front of the distant globular star cluster M14. Comet 252/P LINEAR is one of a rare group of comets that vacillate between the Earth and Jupiter every 5 years. How the comet will evolve from here is unknown, but hopes run high that it will remain a good object for binoculars in northern skies for the next week or two.
APOD: 2016 January 1 - Comet Catalina Tails
Explanation: A new year's treat for binoculars, as 2016 begins Comet Catalina (C/2013 US10) now sweeps through planet Earth's predawn skies near bright Arcturus, alpha star of Bootes. But this telescopic mosaic from December 21 follows the pretty tails of the comet across a field of view as wide as 10 full moons. The smattering of distant galaxies and faint stars in the background are in the constellation Virgo. Trailing behind the comet's orbit, Catalina's dust tail fans out below and left in the frame. Its ion tail is angled toward the top right, away from the Sun and buffeted by the solar wind. On January 17, the outward bound visitor from the Oort Cloud will make its closest approach to Earth, a mere 110 million kilometers away, seen near bright stars along the handle of the Big Dipper.