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Explanation: What glows there? The answer depends: sea or sky? In the sea, the unusual blue glow is bioluminescence. Specifically, the glimmer arises from Noctiluca scintillans, single-celled plankton stimulated by the lapping waves. The plankton use their glow to startle and illuminate predators. This mid-February display on an island in the Maldives was so intense that the astrophotographer described it as a turquoise wonderland. In the sky, by contrast, are the more familiar glows of stars and nebulas. The white band rising from the artificially-illuminated green plants is created by billions of stars in the central disk of our Milky...

The enormous black hole at the center of our galaxy is having an unusually large meal of interstellar gas and dust, and researchers don't yet understand why. "We have never seen anything like this in the 24 years we have studied the supermassive black hole," said Andrea Ghez, UCLA professor of physics and astronomy and a co-senior author of the research. "It's usually a pretty quiet, wimpy black hole on a diet. We don't know what is driving this big feast." A paper about the study, led by the UCLA Galactic Center Group, which Ghez heads, is published today in...

Explanation: What's older than these ancient trees? Nobody you know -- but almost everything in the background of this picture. The trees are impressively old -- each part of the Ancient Bristlecone Pine Forest located in eastern California, USA. There, many of the oldest trees known are located, some dating as far back as about 5,000 years. Seemingly attached to tree branches, but actually much farther in the distance, are the bright orbs of Saturn (left) and Mars. These planets formed along with the Earth and the early Solar System much earlier -- about 4.5 billion years ago. Swooping down...

Explanation: Is there anything interesting to see in the direction opposite the Sun? One night last month, there were quite a few things. First, the red-glowing orb on the lower right of the featured image is the full moon, darkened and reddened because it has entered Earth's shadow. Beyond Earth's cone of darkness are backscattering dust particles orbiting the Sun that standout with a diffuse glow called the gegenschein, visible as a faint band rising from the central horizon and passing behind the Moon. A nearly horizontal stripe of green airglow is also discernable just above the horizon, partly blocked...

Explanation: You don't have to be at Monument Valley to see the Milky Way arch across the sky like this -- but it helps. Only at Monument Valley USA would you see a picturesque foreground that includes these iconic rock peaks called buttes. Buttes are composed of hard rock left behind after water has eroded away the surrounding soft rock. In the featured image taken in 2012, the closest butte on the left and the butte to its right are known as the Mittens, while Merrick Butte can be seen just further to the right. High overhead stretches a band...

Explanation: What are those streaks of light in the sky? First and foremost, the arching structure is the central band of our Milky Way galaxy. Visible in this galactic band are millions of distant stars mixed with numerous lanes of dark dust. Harder to discern is a nearly vertical beam of light rising from the horizon, just to the right of the image center. This beam is zodiacal light, sunlight scattered by dust in our Solar System that may be surprisingly prominent just after sunset or just before sunrise. In the foreground are several telescopes of the Bosque Alegre Astrophysical...

One of the most powerful things in the universe wears a sparkly purple coat. Well, at least it looks that way in this image combining data from NASA's Chandra X-ray Observatory and the Hubble Space Telescope. This is an image of the galaxy cluster RX J1532.9+3021... At the cluster's center is one of the most powerful black holes astronomers have ever found. The black hole is surrounded by hot gas, which appears here in purple. Normally, astronomers would expect dense, hot gas like this to form stars, but they haven't found any evidence of stars here. Instead, they think, the...

Explanation: There is a road that connects the Northern to the Southern Cross but you have to be at the right place and time to see it. The road, as pictured above, is actually the central band of our Milky Way Galaxy; the right place, in this case, is dark Laguna Cejar in Salar de Atacama of Northern Chile; and the right time was in early October, just after sunset. Many sky wonders were captured then, including the bright Moon, inside the Milky Way arch; Venus, just above the Moon; Saturn and Mercury, just below the Moon; the Large and...

Explanation: What will become of these galaxies? Spiral galaxies NGC 5426 and NGC 5427 are passing dangerously close to each other, but each is likely to survive this collision. Typically when galaxies collide, a large galaxy eats a much smaller galaxy. In this case, however, the two galaxies are quite similar, each being a sprawling spiral with expansive arms and a compact core. As the galaxies advance over the next tens of millions of years, their component stars are unlikely to collide, although new stars will form in the bunching of gas caused by gravitational tides. Close inspection of the...

Explanation: One of the last entries in Charles Messier's famous catalog, big, beautiful spiral galaxy M101 is definitely not one of the least. About 170,000 light-years across, this galaxy is enormous, almost twice the size of our own Milky Way Galaxy. M101 was also one of the original spiral nebulae observed with Lord Rosse's large 19th century telescope, the Leviathan of Parsontown. In contrast, this mulitwavelength view of the large island universe is a composite of images recorded by space-based telescopes in the 21st century. Color coded From X-rays to infrared wavelengths (high to low energies), the image data was...

Life as we know it exists on a cozy planet in a stable orbit around a sun shining brightly in its sky. But a new study hints that the most common life in the universe might exist deep inside eternal-night worlds far from any star, adrift in the icy dark of interstellar space. Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University estimate that "nomad" planets, ejected from their home stellar system and now free-floating through the Milky Way, could outnumber stars by as many as 100,000 to 1. Earlier estimates were more like a...

Most stars are born in clusters rather than singly, and there’s plenty of evidence that the Sun was too. For one thing, the material of the infant solar system (as preserved in the earliest meteorites) was enriched by fresh supernova debris from at least one very young, massive star (having 15 to 25 solar masses) that exploded less than 5 light-years away, no more than 2 million years after the Sun's formation. Today no such massive star exists within 300 light-years of the Sun. Clearly, the early solar system had stars close around it. But that was 4.57 billion years...

If there is life on other planets, an Australian team of astronomers has just found the region in our Milky Way galaxy where it is most likely to exist.. Unfortunately, technology does not yet exist to get man within reach of his nearest possible neighbours. According to Charles Lineweaver, writing in Science, there are four ingredients needed to create complex life: the presence of a host star (such as the Sun), enough heavy elements (carbon, oxygen and nitrogen) to form a planet, sufficient time for biological evolution (at least four billion years in the case of Earth) and the absence...

Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2002 August 14 Giant Emission Nebula NGC 3603 in Infrared Credit: 2MASS Project, UMass, IPAC/Caltech, NSF, NASA Explanation: NGC 3603 is the largest region of glowing gas in our Milky Way galaxy. Spanning over 20 light years across, the giant emission nebula (HII region) is home to a massive star cluster, thick dust pillars, and a star about to explode. NGC 3603 was captured above in infrared light...