All the Colors of the Sun


It is still not known why the Sun’s light is missing some colors. Here are all the visible colors of the Sun, produced by passing the Sun’s light through a prism-like device. The spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our white-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light. The dark patches in the above spectrum arise from gas at or above the Sun’s surface absorbing sunlight emitted below. Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun. Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth. Today, the majority of spectral absorption lines have been identified – but not all.

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Stars of a Summer Triangle


Rising at the start of a northern summer’s night, these three bright stars form the familiar asterism known as the Summer Triangle. Altair, Deneb, and Vega are the alpha stars of their respective constellations, Aquila, Cygnus, and Lyra, nestled near the Milky Way. Close in apparent brightness the three do look similar in these telescopic portraits, but all have their own stellar stories. Their similar appearance hides the fact that the Summer Triangle stars actually span a large range in intrinsic luminosity and distance. A main sequence dwarf star, Altair is some 10 times brighter than the Sun and 17 light-years away, while Vega, also a hydrogen-fusing dwarf, is around 30 times brighter than the Sun and lies 25 light-years away. Supergiant Deneb, at about 54,000 times the solar luminosity, lies some 1,400 light-years distant. Of course, with a whitish blue hue, the stars of the Summer Triangle are all hotter than the Sun.

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Planet Aurora


What bizarre alien planet is this ? It’s planet Earth of course, seen through the shimmering glow of aurorae from the International Space Station. About 400 kilometers (250 miles) above, the orbiting station is itself within the upper realm of the auroral displays, also watched from the planet’s surface on June 23rd. Aurorae have the signature colors of excited molecules and atoms at the low densities found at extreme altitudes. The eerie greenish glow of molecular oxygen dominates this view. But higher, just above the space station’s horizon, is a rarer red band of aurora from atomic oxygen. The ongoing geomagnetic storm began after a coronal mass ejection’s recent impact on Earth’s magnetosphere.

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Star Trails Above Table Mountain


Stars trail above and urban lights sprawl below in this moonlit nightscape from Cape Town, South Africa, planet Earth. The looming form of Table Mountain almost seems to hold terrestrial lights at bay while the stars circle the planet’s South Celestial Pole. This modern perspective on the natural night sky was captured in June 2014, the scene composed of over nine hundred, stacked 30 second exposures. The stunning result was chosen as the winner in the Against the Lights category, a selection from over 800 entries in The World at Night’s 2015 International Earth and Sky Photo Contest.

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Triple Conjunction Over Galician National Park


What are those bright objects hovering over the horizon? Planets — and the Moon. First out, the horizon featured is a shoreline of the Atlantic Ocean that occurs at the Galicia National Park in northern Spain. Next furthest out, on the left, is the Moon. Easily the brightest object on the night sky, the Moon here was in only a crescent phase. The next furthest out, on the right, is the planet Venus, while planet Jupiter is seen at the top of the triangle. The long exposure from our rapidly rotating Earth made all of celestial objects — including the far distant stars — appear as slight arcs. The featured image was taken last Sunday night. Although the Moon’s orbit has now taken it away from this part of the sky, the planets Venus and Jupiter can be seen superposed just after sunset until mid-August. The closest apparent separation of Venus and Jupiter will occur in one week, when the two planets will appear separated by less than the angular diameter of the Moon.

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Sharpless 308: Star Bubble


Blown by fast winds from a hot, massive star, this cosmic bubble is huge. Cataloged as Sharpless 2-308 it lies some 5,200 light-years away toward the constellation of the Big Dog (Canis Major) and covers slightly more of the sky than a Full Moon. That corresponds to a diameter of 60 light-years at its estimated distance. The massive star that created the bubble, a Wolf-Rayet star, is the bright one near the center of the nebula. Wolf-Rayet stars have over 20 times the mass of the Sun and are thought to be in a brief, pre-supernova phase of massive star evolution. Fast winds from this Wolf-Rayet star create the bubble-shaped nebula as they sweep up slower moving material from an earlier phase of evolution. The windblown nebula has an age of about 70,000 years. Relatively faint emission captured in the expansive image is dominated by the glow of ionized oxygen atoms mapped to a blue hue.

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Rings and Seasons of Saturn


On Saturn, the rings tell you the season. On Earth, today marks a solstice, the time when the Earth’s spin axis tilts directly toward the Sun. On Earth’s northern hemisphere, today is the Summer Solstice, the day of maximum daylight. Since Saturn’s grand rings orbit along the planet’s equator, these rings appear most prominent — from the direction of the Sun — when the Saturn’s spin axis points toward the Sun. Conversely, when Saturn’s spin axis points to the side, an equinox occurs and the edge-on rings are hard to see. In the featured montage, images of Saturn over the past 11 years have been superposed to show the giant planet passing from southern summer toward northern summer. Although Saturn will only reach its northern summer solstice in 2017 May, the image of Saturn most analogous to today’s Earth solstice is the bottommost one.

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M64: The Black Eye Galaxy


This big, bright, beautiful spiral galaxy is Messier 64, often called the Black Eye Galaxy or the Sleeping Beauty Galaxy for its heavy-lidded appearance in telescopic views. M64 is about 17 million light-years distant in the otherwise well-groomed northern constellation Coma Berenices. In fact, the Red Eye Galaxy might also be an appropriate moniker in this colorful composition. The enormous dust clouds obscuring the near-side of M64’s central region are laced with the telltale reddish glow of hydrogen associated with star forming regions. But they are not this galaxy’s only peculiar feature. Observations show that M64 is actually composed of two concentric, counter-rotating systems. While all the stars in M64 rotate in the same direction as the interstellar gas in the galaxy’s central region, gas in the outer regions, extending to about 40,000 light-years, rotates in the opposite direction. The dusty eye and bizarre rotation is likely the result of a billion year old merger of two different galaxies.

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M45: The Pleiades Star Cluster


Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure took over 12 hours and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer’s eyesight.

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APOD is 20 Years Old Today


Welcome to the vicennial year of the Astronomy Picture of the Day! Perhaps a source of web consistency for some, APOD is still here. As during each of the 20 years of selecting images, writing text, and editing the APOD web pages, the occasionally industrious Robert Nemiroff (left) and frequently persistent Jerry Bonnell (right) are pictured above plotting to highlight yet another unsuspecting image of our cosmos. Although the featured image may appear similar to the whimsical Vermeer composite that ran on APOD’s fifth anniversary, a perceptive eye might catch that it has been digitally re-pixelated using many of the over 5,000 APOD images that have appeared over APOD’s tenure. (Can you find any notable APOD images?) Once again, we at APOD would like to offer a sincere thank you to our readership for continued interest, support, and many gracious communications. If you consider yourself a fan of APOD, you might want to consider joining the Friends of APOD.

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