NASA Astronomy Picture of the Day:

The stars are not alone. In the disk of our Milky Way Galaxy, about 10 percent of visible matter is in the form of gas called the interstellar medium (ISM). The ISM is not uniform and shows patchiness even near our Sun. It can be quite difficult to detect the local ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however, absorbs some very specific colors that can be detected in the light of the nearest stars. A working map of the local ISM within 20 light-years, based on ongoing observations and particle detections from the Earth-orbiting Interstellar Boundary Exporer satellite (IBEX), is shown here. These observations indicate that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region. Our Sun may exit the Local Cloud, also called the Local Fluff, during the next 10,000 years. Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth. Unexpectedly, IBEX spacecraft measurements indicate that the direction from which neutral interstellar particles flow through our Solar System is changing. APOD Year in Review: Night Sky Network Presentation for 2024

NASA Astronomy Picture of the Day:

A year in sunsets, from April 2023 to March 2024, track along the western horizon in these stacked panoramic views. The well-planned sequence is constructed of images recorded near the 21st day of the indicated month from the same location overlooking Cairo, Egypt. But for any location on planet Earth the yearly extreme northern (picture right) and southern limits of the setting Sun mark the solstice days. The word solstice is from Latin for "Sun" and "stand still". On the solstice date the seasonal drift of the Sun's daily path through the sky appears to pause and reverse direction in its annual celestial journey. Of course the Sun reaches a stand still on today's date. The 21 December 2024 solstice at 09:21 UTC is the moment of the Sun's southernmost declination, the start of astronomical winter in the north and summer in the south.

Photo by Wael Omar

NASA Astronomy Picture of the Day:

On the night of December 15, the Full Moon was bright. Known to some as the Cold Moon or the Long Night Moon, it was the closest Full Moon to the northern winter solstice and the last Full Moon of 2024. This Full Moon was also at a major lunar standstill. A major lunar standstill is an extreme in the monthly north-south range of moonrise and moonset caused by the precession of the Moon's orbit over an 18.6 year cycle. As a result, the full lunar phase was near the Moon's northernmost moonrise (and moonset) along the horizon. December's Full Moon is rising in this stacked image, a composite of exposures recording the range of brightness visible to the eye on the northern winter night. Along with a colorful lunar corona and aircraft contrail this Long Night Moon shines in a cold sky above the rugged, snowy peaks of the Italian Dolomites.

Photo by Giorgia Hofer

NASA Astronomy Picture of the Day:

After the Crab Nebula, this giant star cluster is the second entry in 18th century astronomer Charles Messier's famous list of things that are not comets. M2 is one of the largest globular star clusters now known to roam the halo of our Milky Way galaxy. Though Messier originally described it as a nebula without stars, this stunning Hubble image resolves stars across the cluster's central 40 light-years. Its population of stars numbers close to 150,000, concentrated within a total diameter of around 175 light-years. About 55,000 light-years distant toward the constellation Aquarius, this ancient denizen of the Milky Way, also known as NGC 7089, is 13 billion years old. An extended stellar debris stream, a signature of past gravitational tidal disruption, was recently found to be associated with Messier 2.

NASA Astronomy Picture of the Day:

What kind of strange galaxy is this? This rare structure is known as a polar ring galaxy, and it seems to have two different rings of stars. In this galaxy, NGC 660, one ring of bright stars, gas, and dark dust appears nearly vertical, while another similar but shorter ring runs diagonally from the upper left. How polar ring galaxies obtain their striking appearance remains a topic of research, but a leading theory holds that it is usually the result of two galaxies with different central ring planes colliding. NGC 660 spans about 50,000 light years and is located about 40 million light years away toward the constellation of the Fish (Pisces). The featured image was captured recently from Observatorio El Sauce in Chile.

Photo by Mike Selby

NASA Astronomy Picture of the Day:

What excites the Heart Nebula? First, the large emission nebula on the upper left, catalogued as IC 1805, looks somewhat like a human heart. The nebula glows brightly in red light emitted by its most prominent element, hydrogen, but this long-exposure image was also blended with light emitted by sulfur (yellow) and oxygen (blue). In the center of the Heart Nebula are young stars from the open star cluster Melotte 15 that are eroding away several picturesque dust pillars with their atom-exciting energetic light and winds. The Heart Nebula is located about 7,500 light years away toward the constellation of Cassiopeia. This wide field image shows much more, though, including the Fishhead Nebula just below the Heart, a supernova remnant on the lower left, and three planetary nebulas on the image right. Taken over 57 nights, this image is so deep, though, that it clearly shows fainter long and complex filaments.

Photo by Jeff Horne & Drew Evans

NASA Astronomy Picture of the Day:

This kilometer high cliff occurs on the surface of a comet. It was discovered on the dark nucleus of Comet Churyumov - Gerasimenko (CG) by Rosetta, a robotic spacecraft launched by ESA, which orbited the comet from 2014 to 2016. The ragged cliff, as featured here, was imaged by Rosetta early in its mission. Although towering about one kilometer high, the low surface gravity of Comet CG would likely make a jump from the cliffs by a human survivable. At the foot of the cliffs is relatively smooth terrain dotted with boulders as large as 20 meters across. Data from Rosetta indicates that the ice in Comet CG has a significantly different deuterium fraction -- and hence likely a different origin -- than the water in Earth's oceans. The probe was named after the Rosetta Stone, a rock slab featuring the same text written in three different languages that helped humanity decipher ancient Egyptian writing.

NASA Astronomy Picture of the Day:

Meteors have been flowing out from the constellation Gemini. This was expected, as mid-December is the time of the Geminid Meteor Shower. Pictured here, over two dozen meteors were caught in successively added exposures taken over several hours early Saturday morning from a snowy forest in Poland. The fleeting streaks were bright enough to be seen over the din of the nearly full Moon on the upper right. These streaks can all be traced back to a point on the sky called the radiant toward the bright stars Pollux and Castor in the image center. The Geminid meteors started as sand sized bits expelled from asteroid 3200 Phaethon during its elliptical orbit through the inner Solar System. Jigsaw Challenge: Astronomy Puzzle of the Day

Photo by Jakub Kuřák

NASA Astronomy Picture of the Day:

Awkward and angular looking, Apollo 17's lunar module Challenger was designed for flight in the near vacuum of space. Digitally enhanced and reprocessed, this picture taken from Apollo 17's command module America shows Challenger's ascent stage in lunar orbit. Small reaction control thrusters are at the sides of the moonship with the bell of the ascent rocket engine underneath. The hatch allowing access to the lunar surface is seen at the front, with a round radar antenna at the top. Mission commander Gene Cernan is clearly visible through the triangular window. This spaceship performed gracefully, landing on the Moon and returning the Apollo astronauts to the orbiting command module in December of 1972. So where is Challenger now? Its descent stage remains at the Apollo 17 landing site in the Taurus-Littrow valley. The ascent stage pictured was intentionally crashed nearby after being jettisoned from the command module prior to the astronauts' return to planet Earth.

NASA Astronomy Picture of the Day:

An intriguing pair of interacting galaxies, M51 is the 51st entry in Charles Messier's famous catalog. Perhaps the original spiral nebula, the large galaxy with whirlpool-like spiral structure seen nearly face-on is also cataloged as NGC 5194. Its spiral arms and dust lanes sweep in front of its smaller companion galaxy, NGC 5195. Some 31 million light-years distant, within the boundaries of the well-trained constellation Canes Venatici, M51 looks faint and fuzzy to the eye in direct telescopic views. But this remarkably deep image shows off stunning details of the galaxy pair's striking colors and fainter tidal streams. The image includes extensive narrowband data to highlight a vast reddish cloud of ionized hydrogen gas recently discovered in the M51 system and known to some as the H-alpha cliffs. Foreground dust clouds in the Milky Way and distant background galaxies are captured in the wide-field view. A continuing collaboration of astro-imagers using telescopes on planet Earth assembled over 3 weeks of exposure time to create this evolving portrait of M51. Watch: The 2024 Geminid Meteor Shower

Photo by The Deep Sky Collective

NASA Astronomy Picture of the Day:

Based on its well-measured orbit, 3200 Phaethon (sounds like FAY-eh-thon) is recognized as the source of the meteoroid stream responsible for the annual Geminid meteor shower. Even though most meteor shower parents are comets, 3200 Phaethon is a known and closely tracked near-Earth asteroid with a 1.4 year orbital period. Rocky and sun-baked, its perihelion or closest approach to the Sun is well within the orbit of innermost planet Mercury. In this telescopic field of view, the asteroid's rapid motion against faint background stars of the heroic constellation Perseus left a short trail during the two minute total exposure time. The (faint) parallel streaks of its meteoric children flashed much more quickly across the scene. The family portrait was recorded near the Geminid meteor shower's very active peak on 2017 December 13. That was just three days before 3200 Phaethon's historic close approach to planet Earth. This year, the night of December 13 should again see the peak of the Geminid meteor shower, but faint meteors will be washed out by the bright light of the nearly full moon. Watch: The 2024 Geminid Meteor Shower

Photo by Mikiya Sato

NASA Astronomy Picture of the Day:

What's the closest active galaxy to planet Earth? That would be Centaurus A, cataloged as NGC 5128, which is only 12 million light-years distant. Forged in a collision of two otherwise normal galaxies, Centaurus A shows several distinctive features including a dark dust lane across its center, outer shells of stars and gas, and jets of particles shooting out from a supermassive black hole at its center. The featured image captures all of these in a composite series of visible light images totaling over 310 hours captured over the past 10 years with a homebuilt telescope operating in Auckland, New Zealand. The brightness of Cen A's center from low-energy radio waves to high-energy gamma rays underlies its designation as an active galaxy. Astrophysicists: Browse 3,500+ codes in the Astrophysics Source Code Library