Milky Way as seen by the human eye: where exactly do those photons come from?

Milky Way as seen by the human eye: where exactly do those photons come from?

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I'm asking here about the diffuse glow seen by the human eye, when looking at the Milky Way.

Where do those visible photons come from exactly?

  • the surface of a star?
  • starlight bouncing off something (dust or a molecule)?
  • emitted by gas in the interstellar medium?
  • all of the above?

I believe that the answer is mainly the first (directly from stars), although there will certainly be some of each of the others. The diffuseness comes from the fact that there are many more relatively dim stars than rods in the retina of the human eye. My argument for this is that magnified photographs of most of the Milky Way at least, break down into individiual stars, as seen in this image from

How to See the Farthest Thing You Can See

By: Bob King September 9, 2015 11

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Step by step, we explore the farthest things visible with the naked eye, from the most distant star to the hinterlands of the Andromeda Galaxy.

Saturn, the most distant, bright naked-eye planet, lies 950 million miles from Earth at mid-month.
Efrain Morales

Whenever I point out the planet Saturn to the public and tell them it's nearly a billion miles away, they're aghast. It doesn't seem possible to see that far. And yet at that distance, we're barely out the door, cosmically speaking. The unaided eye can do so much more.

Back on Earth, we're lucky to see beyond 12 miles (20 km) at sea level due to haze, dust, and the curvature of the Earth getting in the way. To peer further, we must go higher. From the top of Mt. Everest it's theoretically possible to see 210 miles (339 km) to the horizon.

But once we lift our gaze, we easily see the space station at night 250 miles up, the edge of the auroral oval 500 miles away, the Moon at 240,000 miles, and of course the planets — all the way to Saturn — with the naked eye. On dark nights, some even manage to snare a look at dim Uranus 1.7 billion miles away.

When we depart the Solar System to plumb the starry depths, the tremendous distances to the stars make using miles and kilometers cumbersome. Astronomers prefer the light-year, the distance light travels in one year's time moving at 186,000 miles per second (300,000 km/sec).

The light-year is not only a more practical way to describe distances, it also gives a clue as to how long light has traveled to reach our eyes. This scale starts close to home but takes us all the way out to the Andromeda Galaxy, the most distant object most people can see with the unaided eye.
Bob King

Since there are about 31,536,000 seconds in a year, that tallies up to just shy of six trillion miles in one year — a distance well beyond the most remote planets and asteroids but only a quarter the way to the nearest star system beyond the Sun, Alpha Centauri.

Using light-years also allows us to more fully appreciate the vastness of the universe. When we gaze at Alpha Centauri, we see the light that departed the star 4.4 years ago and traveled across a chasm of space 26 trillion miles wide before finally touching our retinas.

Deneb, which heads up the Northern Cross or marks the tail of Cygnus, is one of the intrinsically brightest stars in the sky. Located approximately 1,550 light-years away, the light you see tonight left on its journey to Earth around 465 A.D. during the sacking of Rome.

Three of the most distant naked-eye stars appear near one another in the northern sky on fall nights: Mu and Nu Cephei and Rho Cassiopeiae, perhaps the most distant star visible without optical aid.

But Deneb's distance pales to that of the most remote stars visible with the unaided eye. Several of the most distant are easily visible from the outer suburbs and countryside on fall nights in the northern sky. If you turn your gaze toward the familiar W of Cassiopeia, you'll find the dim point of light Rho Cassiopeiae 2.5° west of Beta. Currently at magnitude +4.7, nothing about its visual appearance offers a hint of the star's true nature.

The Sun shrinks to a minute disk next to the enormity that is Rho Cassiopeiae. The star, 450 times bigger than our Sun, is a likely candidate to go supernova within the next 50,000 years. Click image to learn more about this amazing ball of nuclear fire.
Gauloiq / Wikipedia Français

Yet from a careful study of its light, we know that Rho is a hypergiant star radiating 550,000 times more light than the Sun with a girth 40% larger than the orbit of Mars! Indeed, this is the reason we can see it with the naked eye in first place despite its spectacular distance of 8,200 light years.

Nearby Mu Cephei, better known as the "Garnet Star," and Nu Cephei (magnitudes +3.9 and +4.3 respectively), are both grand supergiants visible from remote realms of the Milky Way galaxy. Can we go further?

Of course! Just squeeze a bunch of stars into ball called a globular star cluster and your eyes can telescope across 25,000 light-years. The Milky Way's largest globular cluster, Omega Centauri, visible from the far southern U.S. and points south, packs 10 million into a spherical swarm 150 light-years wide some 15,000 light years away. At 4th magnitude, the cluster's plainly visible to the naked eye as a patch of glowing mist nearly as big as the Full Moon.

Globulars Omega Centauri (left) and the Hercules Globular M13 (right). Each is visible with the naked eye across distances of 15,000 and 25,000 light-years, respectively.
ESO (left) / N. A. Sharp, REU program, NOAO, AURA, NSF

Similarly, the Great Globular in Hercules (M13) lies 25,000 light-years from Earth and contains up to 300,000 stars. At magnitude +5.8, it shines just a hair above the standard naked-eye limit of +6.0, but I've seen it many times from dark, country skies as a small, milky patch with averted vision. As you whirl your scope in its direction and revel in its blizzard of stars, know that your ancestors were chipping away at projectile points 50,000 years ago during the Stone Age when the photons from those stars began their incredible journey to your eye.

25,000 light-years takes us a quarter way across the galaxy. To make the next leap, we must journey to the southern hemisphere, where the Milky Way's two brightest satellite galaxies, the Magellanic Clouds, reside. This duo of irregular dwarf galaxies orbit our own galaxy the Large Magellanic Cloud lies more than six times farther away than the Hercules cluster at about 160,000 light-years, while the Small Magellanic Cloud is around 200,000 light-year away.

If you use the top half of the W of Cassiopeia — the Alpha, Beta, and Gamma stars — as an "arrow", it points directly to the Andromeda Galaxy, a small, 4th-magnitude fuzzy patch of light close northwest of Nu Andromedae. Map the sky facing east-northeast around 9:30 p.m. local time. See photo below.

Impressive as these distances are, we can still do better. Much better. A fist and a half below the Cassiopeia's W, we finally arrive at the stopping point of human vision. Like so many objects in the astronomical world, its naked-eye appearance is deceiving. Just a bit of fuzzy fluff like a shard of Milky Way gone adrift. But that little spot has a name that says it all: Andromeda Galaxy.

This photo shows how Cassiopeia makes easy work of finding Andromeda. You can use the bottom half of the W to wander over to another wonderful sight — the Double Cluster in Perseus. Shining around magnitude +3.5, the clusters appear like a brighter fuzzy spot.
Bob King

Its enormous disk, some 220,000 light-years across, or more than twice the size of our galaxy, lies 2.5 millionlight-years from Earth. Amazingly, we can see it without optical aid from a moderately dark sky. Why? Andromeda's close as galaxies go, and a trillion stars cram its fuzzy disk. That's a lot of candlepower. But all those suns are so distant, they blend into a smooth, unresolved haze, even in large amateur telescopes.

When you've found the galaxy, spend a few metaphysical minutes pondering your place in the cosmic vastitude. Consider any number of potential Andromedid life forms looking back at your spiral galaxy, the Milky Way.

If we could find a suitable location equidistant from both the Milky Way Galaxy (left) and Andromeda Galaxy (right), we'd be able to appreciate their true sizes. Andromeda is more than twice the size of our home. Both are spiral galaxies with majestic, star-forming arms wound around dense cores of older stars.
Bob King

As you soak in the view, you may notice a bit of structure to the galaxy even even without optical aid. The center, where more of Andromeda's stars are concentrated, appears distinctly brighter than the more lightly-populated outer disk. Use averted vision — looking to this side and that rather than directly at the object — to appreciate its large apparent size. Under dark skies, the galaxy spans nearly 3° or six side-by-side full Moons.

The Andromeda Galaxy (M31) in all its stellar glory! The two smaller, fuzzy glows are two of its satellite galaxies.
Frank Barrett /

Some keen-eyed amateurs under the darkest skies have netted even more distant prey like the Triangulum Galaxy (2.7 million light-years) and even the M81–M82 pair in Ursa Major (11 million light-years), but most of us ordinary folk hit our limit at Andromeda. To go beyond requires optical aid, and that would spoil the fun.

Or would it? Next week, we'll return to Andromeda and discover how much we can discover there using only a pair of binoculars.

Get your observing groove on with Sky & Telescope's 2016 Observing Calendar!

Most Of Us Can't See The Milky Way Anymore. That Comes With A Price.

The silver ribbon of stars that wraps the night sky has long been an awe-inspiring sight for anyone who cares to look up. But that’s not the case anymore for people who live under a fog of light pollution.

A new analysis using satellite data and sky brightness measurements has found that the Milky Way is hidden from more than one-third of humanity, including 60 percent of Europeans and nearly 80 percent of North Americans. The research was reported Friday in the journal Science Advances.

The researchers calculated several degrees of light pollution, starting from the level at which artificial light obscures astronomical observations up to the level at which the midnight sky is as bright as it is at twilight. Their calculations show that more than 80 percent of the world and more than 99 percent of U.S. and European populations live under light-polluted skies.

This level of pollution may have negative consequences, ranging from harming animals’ life cycles to affecting human health and even psychology by taking away one of the most positive experiences that’s naturally available, experts said.

There Are Now People Who’ve Never Seen The Milky Way

The proliferation of light pollution started in the 1950s and 60s and has continued to expand every year, said Chris Elvidge, a scientist with the U.S. National Oceanic and Atmospheric Administration and a co-author of the study.

"For several generations, people in large urban centers have had their view of the Milky Way blocked," Elvidge told The Huffington Post. “This is an aesthetic loss, and perhaps a spiritual loss in terms of feeling a connection to the cosmos.”

Losing that connection could have major consequences when it comes to psychological health. The night sky presents one of the few universal situations in which all humans can experience a profound sense of awe. And awe, psychologists are increasingly finding, is a special emotion that can impact our cognition and behavior in unique and unexpected ways.

“Fleeting and rare, experiences of awe can change the course of a life in profound and permanent ways,” wrote Dacher Keltner and Jonathan Haidt in 2003 in one of the first psychological looks on this long-neglected emotion. Reviewing historical examples of people whose lives were transformed thanks to awe, Keltner and Haidt suggested that “awe-inducing may be one of the fastest and most powerful methods of personal change and growth.”

That’s why losing the chance to gaze at a vast sky may not be a small matter.

“The bright night sky and its stars has long been a profound source of awe and inspiration, which we know to stir creativity, generosity, good will and innovation,” Keltner told HuffPost. “Losing a clear night sky will harm our capacity for wonder and put a dent in our spirit of common cause.”

In more recent investigations of the effects of awe, researchers have elicited the emotion in the lab and observed that people’s perception of time appears to expand. Compared with people experiencing other emotions, those who experienced awe felt that they had more time, said Melanie Rudd of the University of Houston. “As a result, they started doing things that are good for your subjective well-being, like helping others and choosing experiences over material goods.” Having a greater perception of time and being present is particularly important in today’s culture, because people often feel rushed, Rudd said.

Rudd and her colleagues have also found that the best way to elicit awe in people is by putting them in nature -- at the foot of the Swiss Alps or on top of the Grand Canyon, for example. But for people who live in large, populous cities and don’t have a canyon in their backyard, looking at the night sky is one of the few ways to evoke the feeling of awe.

“The sky is right there. It's very accessible,” Rudd said. “But if the light pollution is getting in the way, then you are taking away a very nice source of awe for people."

Where Can We Still See The Milky Way?

Even at a distance, pollution from large cities casts a wide curtain of brightness on surrounding areas. “Light pollution is one of the most pervasive forms of environmental alteration,” the researchers wrote in their analysis of global light pollution. “It affects even otherwise pristine sites because it is easily observed during the night hundreds of kilometers from its source in landscapes that seem untouched by humans during the day.”

Even protected areas such as national parks are not entirely safe from glimmering cities far away. For example, light from Las Vegas and Los Angeles can be seen from Death Valley National Park, the researchers wrote.

The researchers created an atlas of global light pollution that can be seen above, using dark gray to mark light-polluted sites that should be protected from future light increases. If sites are marked in blue, that means the sky is too bright for astronomical observations. Areas marked in yellow are places where people can’t see the Milky Way in the winter, and orange means even the brighter summer Milky Way is obscured by artificial light.

In areas marked in red, the night sky is as luminous as it is at twilight. “This means that, in places with this level of pollution, people never experience conditions resembling a true night because it is masked by an artificial twilight,” the researchers wrote.

The most light-polluted country is Singapore, where people live under skies so bright that the eye cannot fully adapt to night vision, the researchers said. Other countries with high levels of light pollution include Kuwait, Qatar, United Arab Emirates, Saudi Arabia, South Korea, Israel, Argentina, Libya, and Trinidad and Tobago. Countries with populations least affected by light pollution are Chad, the Central African Republic and Madagascar.

In Western Europe, only some areas -- most of them in Scotland, Sweden and Norway -- still enjoy a dark night. Among G-20 countries, Saudi Arabia and South Korea have the highest degree of light pollution, while India and Germany are exposed to the least light pollution.

In the United States, "the western U.S. and Alaska have the largest blocks of undeveloped, unpopulated lands where the night sky has largely been preserved,” Evlidge said.

For those who’d like to take a short break from the city for a stargazing trip, Elvidge suggests getting about 100 miles out.

Our eyes can see single specks of light

In dim-light conditions we can see plenty because, it turns out, our eyes are sensitive enough to pick up a single photon of light.

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For decades, researchers have wondered just how little light the eye can see. They now appear to have the answer. And it’s surprising. Our eyes can detect a single speck — what scientists call a photon or light particle, a new study suggests. If confirmed, this may allow scientists to use the human eye to test some basic features of physics on the super-small scale.

The new study also showed that the human eye detects single photons better when it has just seen another photon. This was “an unexpected phenomenon,” says Alipasha Vaziri. He is a physicist at Rockefeller University in New York City. Physicists study the nature and properties of matter and energy. Vaziri and colleagues described the results of their study July 19 in Nature Communications.

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Earlier experiments indicated that people can see blips of light made up of just a few photons. But there had been no surefire way to tell if the eye registers single photons. That’s because single photons are hard to produce reliably. But Vaziri and his co-workers were able to do it.

They used a technique with a long name: spontaneous parametric down-conversion, or SPDC. Scientists send a high-energy photon into a crystal. Once inside, the single photon turns into two low-energy photons. One of those new photons is diverted to someone’s eye. The SPDC system deflects the photon to a detector. That detector confirms each photon that is produced.

During the experiment, people watch for the very dim flash of a photon. The participants also listen for warning beeps. And there will be two. One of them accompanies the photon. The other does not. The participant does not know which beep corresponds to a light speck. Each viewer reports which beep they thought announced a photon, and how confident they were that they were right.

The scientists ran the experiment 2,420 times. Participants guessed the correct beep slightly more times than should occur just due to chance. That seemingly unimpressive success rate had been expected. The reason: Most photons will not make it all the way through to the retina at the back of the eye. That’s the part of eye sensitive to light. When the retina picks up photons, it will alert the brain, which can then form a visual image. That means that in most trials, a participant wouldn’t be able to see a photon associated with either beep.

But in trials where the participants indicated they were most certain of their choice, they were correct 60 percent of the time. Such a success rate would be unlikely if humans were unable to see a single photon. The chance of such a fluke would be one in 1,000.

“It’s not surprising that the correctness of the result might rely on the [viewer’s] confidence,” says Paul Kwiat. He is a physicist at the University of Illinois at Urbana-Champaign who was not involved with the research. Those trials where participants were more confident may represent the times when photons succeeded in making it through to their retinas, he suggests.

The data also indicate that single photons may be able to prepare the brain to detect more dim flashes that follow. Participants were more likely to correctly identify a photon if they had been sent one less than 10 seconds earlier.

The eye as a physics tool

Scientists hope to use the SPDC photon-scouting technique to test whether humans can directly observe quantum weirdness.

Quantum mechanics is a field of physics that deals with the way matter behaves on the scale of atoms or their even tinier building blocks. And that’s the arena our eyes may be able to help scientists understand.

All scientists agree that by normal standards, the quantum world is weird. For instance, photons can be in two places at once. Scientists describe this state as the photons being in a quantum superposition.

Some physicists wonder whether it might be possible to send such quantum states to someone’s eye. If humans could directly observe the strange quantum behavior, rather than having to use other fancy detectors, they might be able to better understand it.

But Leonid Krivitsky isn’t convinced. He is a physicist at the Agency for Science, Technology and Research in Singapore. He claims to be “pretty skeptical about this idea of observing quantumness in the brain.” By the time the brain realizes an eye has seen a flash of light, those photon signals will have lost any potential quantum properties, he suspects.

If true, why should anyone care if the eye can see a single photon? Vaziri says it may point to understanding our pre-electric-light society. Imagine, he says, that “you are somewhere outside of a city in nature, and on a moonless night.” You will have only stars by which to navigate. And on average, he says, the number of photons that get into your eye will be “approaching the single photon regime.”

Having eyes sensitive enough to see single photons, he says, may have had some evolutionary advantage. It may have helped us adapt to a night-time world that existed before the advent of light bulbs.

Power Words

(for more about Power Words, click here)

evolutionary An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more &ldquoadvanced,&rdquo just better adapted to the conditions in which it developed.

photon A particle representing the smallest possible amount of light or other electromagnetic radiation.

physicist A scientist who studies the nature and properties of matter and energy.

quantum mechanics A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.

quantum theory A way to describe the operation of matter and energy at the level of atoms. It is based on an interpretation that at this scale, energy and matter can be thought to behave as both particles and waves. The idea is that on this very tiny scale, matter and energy are made up of what scientists refer to as quanta &mdash miniscule amounts of electromagnetic energy.

quantum physics A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.

quantum superposition The condition in which a quantum system is in a few different states at the same time.

regime A system of government or an established organization that tends to establish rules or the normal or conventional way of looking at something or doing something.

retina A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that travel along the optic nerve to the brain, where a visual image is formed.


J.N. Tinsley et al. Direct detection of a single photon by humans. Nature Communications. Vol. 7, July 19, 2016. doi: 10.1038/ncomms12172.

About Emily Conover

Physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

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Milky Way shining through an Arch on the California Coast [4000×7004] [OC]

The milky way is only vertical when facing south (215deg from Pfeiffer Beach to be precise). The keyhole faces west (255 if i'm generous and assuming you're standing as far north as possible shooting southerly).

This composition is impossible, just admit it's a composite instead of misleading people.

Yeah the descriptive gymnastics are always a good read to see the photog try to subtly admit these compositions are not real photos.

Not to mention the Milky Way doesn’t shine through the keyhole like that. No way it’s that bright. That’s most likely a photo taken at sunset with some generic Milky Way taken somewhere else layered over top

Yeah, this is definitely a composite, not a panorama or a time blend. That's perfectly fine, but he's being deceptive above it.
And there's no way it can "shine" through that hole.

Hmmm. yeah, I think you would have to be standing at an angle similar to this one at the very least to get the milky way in shot:

It's a very nice combination of shots and the editing is great. It's not real though.

Seriously this! Yes this is a great digital art, blending is meticulous, amazing effort in bringing the details out. But it’s stop there

u/mrcnzajac - should just admit that this is never going to be possible. All the explaination of stacking in just BS.

Entire thing is obviously photoshopped, should be in r/art. White noise here is manipulation. This is what an unshopped pic will tend to look like for comparison. Its a sunset pic of the arch with a milky way pasted on top. Still pretty though and makes a nice background, but I do wonder where the milky way pic is from and whether or not its original or borrowed without credit.

Exactly. I know this place and this is what i call FAKE NEWS !

The sad thing is, it even got a plat award

Thank you! Came here for this comment!

Is it me or that milky way is off as well? the composite seems way past zenith.

Plus sharp fog and waves are incompatible with long exposures.

Nice shot. But how much do u had to do after just taking the Pics?

Not the op but he did a lot. That is a shot of the arch is during sunset. He took that photo and took some shots of the Milky Way and superimposed it on the background. The Milky Way does not look like that facing the arch. The Milky Way is a more north south trajectory. That arch looks west. So it is a very cool picture to see, it is completely composed by the artist from pictures from different photo shoots.

Just think about where those photons came from

I’d like to start out by stating that there is no way the human eye can see the night sky exactly like this. You really see the Milky Way clearly with your naked eye if you are in an area with little light pollution like this one, it's just not as bright and colorful as the camera can capture it.

This is a vertical panorama of the Pfeiffer Beach archway in Big Sur on the Central California coast. The panorama consists of about 5 shots overlapping starting from looking straight at the sea stack all the way to almost completely overhead to capture a significant portion of the Milky Way.

How many stars can you see?

Sergio Garcia Rill wrote: “A west Texas sky from Mt. Locke in the Davis mountains near the McDonald Observatory … Even from this remote location, you can see the light coming from Fort Davis on the bottom of the image.”

What if you were far away from city lights, on a night with no moon and no clouds or haze. How many stars could you see with your unaided eye?

There’s really no definitive answer to this question. No one has counted all the stars in the night sky, and astronomers use different numbers as theoretical estimates.

Considering all the stars visible in all directions around Earth, the upper end on the estimates seems to be about 10,000 visible stars. Other estimates place the number of stars visible to the eye alone – surrounding the entire Earth – at more like 5,000. At any given time, half of Earth is in daylight. So only half the estimated number – say, between 5,000 and 2,500 stars – would be visible from Earth’s night side.

Plus, another fraction of those visible stars would be lost in the haze all around your horizon.

Chirag Upreti wrote on February 17, 2018: “Milky Way core, first light for 2018! A fortunate break in the weather coincided with a favorable moon phase today early morning. Impossible to resist, a buddy and I drove 3 hours to get to Montauk, the easternmost tip of New York State and the location of the Montauk Point Lighthouse. The night sky here is rated a Bortle Scale 4 (rural dark sky).”

Why can’t astronomers agree on the number of visible stars? It’s because we don’t all see the sky in the same way. Even under ideal conditions, there’s a fair amount of variation between how well people can see the stars – depending on things like the strength of your vision – and your age. As you get older, for example, your eyes become much less sensitive to faint light.

You also have to take into account the brightness of your night sky. Even on a moonless night, the glow of lights from Earth’s surface brightens the sky.

Still – far from city lights – under absolutely perfect conditions of darkness and sky clarity – a young to middle-aged person with normal vision should be able to see thousands of stars.

RodNell Barclay caught this image of the Milky Way in mid-February, 2018, while coming down from Ben Vrackie, a mountain in Scotland.

Bottom line: Estimates for the number of stars you can see with the eye alone on a dark moonless night vary, partly because eyesight and sky conditions vary.

And that Mishypothetized event meant what as to the Data sent back?

January 17, 2007
European Space Agency

"At the landing site we also saw rounded ice pebbles," says Jonathan Lunine, University of Arizona. The Surface Science Package (SSP) provided the final piece in this particular puzzle. The impact it detected when Huygens touched down indicated that the spacecraft had come to rest in compacted gravel. "Put it all together and it is clear that Huygens landed in an outflow wash," says Lunine.

The Gas Chromatograph and Mass Spectrometer (GCMS) instrument confirmed the nature of the liquid that shapes the surface of Titan. It detected methane evaporating from the Huygens landing site. "Methane on Titan plays the role that water plays on Earth," concludes Lunine. But there are still mysteries. It is not yet clear whether the methane falls mostly as a steady drizzle or as an occasional deluge.

The GCMS also detected two isotopes of argon. Both have important stories to tell. The Ar40 indicates that the interior of Titan is still active. This is unusual in a moon and indicates that perhaps an insulating layer of water ice and methane is buried in the moon itself, close to the surface, trapping the heat inside it. Occasionally, this heat causes the so-called cryo-volcanoes to erupt. Icy 'lava' flows from these cryo-volcanoes have been seen from the orbiting Cassini spacecraft. Because Ar40 is so heavy, it is mostly concentrated towards the base of the atmosphere, so having Huygens on the surface was essential for its detection.

Daniel Gautier, Observatoire de Paris, France, thinks that the other isotope, Ar36, is telling scientists that Titan formed after Saturn, at a time when the primeval gas cloud that became the Solar System had cooled to about 40 ºK (-233 ºC).

The atmosphere of Titan held surprises too. "Huygens made a fantastic and unexpected discovery about the wind," says Gautier. At an altitude of around 60 kilometres, the wind speed dropped, essentially to zero. Explaining this behaviour presents a challenge for theoreticians who are developing computer models of the moon’s atmospheric circulation.

The Huygens Atmosphere Structure Instrument (HASI) provided the temperature of the atmosphere from 1600 kilometres altitude down to the surface. "This has helped put all the other data into context," says Coustenis. Huygens measured the composition profile of the atmosphere to be a mixture of nitrogen, methane and ethane. The methane and ethane provide humidity, as water does in Earth’s atmosphere. At the surface of Titan, Huygens measured the temperature to be 94 ºK (-179 ºC) with a humidity of 45 percent.

Even though the Huygens data set is now two years old, the discoveries have not yet stopped. "There are lots of surprises still to come from this data," says Francesca Ferri, Università degli Studi di Padova. In addition, Huygens gives planetary scientists a wealth of 'ground-truth' to complement and help interpret the observations still coming from Cassini. At the beginning of 2007, Cassini showed that liquid methane is present on Titan in lakes . "

Tips for Stargazing

Use Red Lights Only

Do not use bright white flashlights, headlamps, or cell phones. It takes 20-30 minutes for the human eye to fully adjust to very low light conditions. Bright lights delay this process. You can turn a regular flashlight into a red light by covering it with red cellophane, tape, fabric, paper, or similar materials.

Bring Food and Water

Plan ahead. There is no running water in most areas of the park.

Layer Up

Temperatures drop quickly in the evening. Bring extra layers of warm clothing.

Bring a Chair

You may be on your feet and looking up for long periods of time. A lightweight folding chair will help keep each person in your group comfortable and reduce strain. Do no trample vegetation and be aware of cacti in your area.

Watch Your Step

Cacti, nocturnal animals, and uneven surfaces may be difficult to see at night. Use a red light to check your viewing are for hazards.

Avoid the Moon

Bright moonlight reduces the number of stars you'll see. Check the moon's phase and rise and set times to find the best time to stargaze.

Interactive dark matter could explain Milky Way's missing satellite galaxies

Scientists believe they have found a way to explain why there are not as many galaxies orbiting the Milky Way as expected. Computer simulations of the formation of our galaxy suggest that there should be many more small galaxies around the Milky Way than are observed through telescopes.

This has thrown doubt on the generally accepted theory of cold dark matter, an invisible and mysterious substance that scientists predict should allow for more galaxy formation around the Milky Way than is seen.

Now cosmologists and particle physicists at the Institute for Computational Cosmology and the Institute for Particle Physics Phenomenology, at Durham University, working with colleagues at LAPTh College & University in France, think they have found a potential solution to the problem.

Writing in the journal Monthly Notices of the Royal Astronomical Society (MNRAS), the scientists suggest that dark matter particles, as well as feeling the force of gravity, could have interacted with photons and neutrinos in the young Universe, causing the dark matter to scatter.

Scientists think clumps of dark matter -- or halos -- that emerged from the early Universe, trapped the intergalactic gas needed to form stars and galaxies. Scattering the dark matter particles wipes out the structures that can trap gas, stopping more galaxies from forming around the Milky Way and reducing the number that should exist.

Lead author Dr Celine Boehm, in the Institute for Particle Physics Phenomenology at Durham University, said: "We don't know how strong these interactions should be, so this is where our simulations come in."

'By tuning the strength of the scattering of particles, we change the number of small galaxies, which lets us learn more about the physics of dark matter and how it might interact with other particles in the Universe."

'This is an example of how a cosmological measurement, in this case the number of galaxies orbiting the Milky Way, is affected by the microscopic scales of particle physics."

There are several theories about why there are not more galaxies orbiting the Milky Way, which include the idea that heat from the Universe's first stars sterilised the gas needed to form stars. The researchers say their current findings offer an alternative theory and could provide a novel technique to probe interactions between other particles and cold dark matter.

Co-author Professor Carlton Baugh said: "Astronomers have long since reached the conclusion that most of the matter in the Universe consists of elementary particles known as dark matter.

'This model can explain how most of the Universe looks, except in our own backyard where it fails miserably."

'The model predicts that there should be many more small satellite galaxies around our Milky Way than we can observe."

'However, by using computer simulations to allow the dark matter to become a little more interactive with the rest of the material in the Universe, such as photons, we can give our cosmic neighbourhood a makeover and we see a remarkable reduction in the number of galaxies around us compared with what we originally thought."

The calculations were carried out using the COSMA supercomputer at Durham University, which is part of the UK-wide DiRAC super-computing framework.

The work was funded by the Science and Technology Facilities Council and the European Union.

Light Pollution

Before we begin exploring where the Milky Way is located, let’s talk about light pollution. Light pollution is excessive or misdirected artificial light that sets a limit on the faintness of stars that can be seen or photographed. So if you live in or near a big city, you may only be able to see a few stars above you, similar to the chart shown. Unfortunately, light pollution can span for hundreds of miles and flush out a large majority of starlight where you are located. The Bortle Dark-Sky Scale shown here is a useful nine-level numeric scale that measures the night sky’s brightness of a particular location. As you have probably already figured out, darker is better. If you are trying to find a dark location far away from light pollution, begin by visiting the following helpful websites:

The sources above are fairly accurate however my advice is to use the maps as a basic starting point. For example, you may travel to a certain location, only to find that urban light pollution is reaching farther than the map displays. If you’re lucky, the night sky is much darker than you expected (hopefully this will be the case).

The Ultimate Guide to Editing a Milky Way Photo

Post-processing is an extremely subjective part of any photographer’s workflow. By putting in days of practice, each photographer eventually develops a characteristic look that can be seen throughout many of their photos, whether that be high contrast, low contrast, highly saturated, monochrome, bright exposures, dark and moody exposures, or anywhere in between.

While there is no correct way to process a photo, most landscapes scenes can be readily viewed with the human eye. Because of this, viewers of an image can, in theory, compare the image to the real-life scene to know how far post-processing techniques moved an photo away from “reality.” However, since the Milky Way cannot be seen with the human eye like it can be with modern day digital cameras, the range of different nuanced looks that can be applied to a photo of the Milky Way without being restrained by what it *should* look like is endless. So, with this tutorial, in order to finish off my comprehensive guide to planning, photographing, and post-processing Milky Way photos, I want to take a close look at the potential effects of some of the editing tools that can used to process an image of the night sky, instead of showing only one specific way to process an image.

The 3 Main Pillars of Milky Way Post-Processing

There are essentially three main pillars of post-processing that will affect the appearance of the Milky Way: White Balance, Contrast & Exposure, and Noise Reduction. So, I want to take a step-by-step look at each of them to show how different settings can affect the look of the Milky Way with regard to these three pillars. While any RAW processing software can edit a Milky Way photo, for this tutorial I’ll be using Adobe Lightroom and Adobe Photoshop. For the first part of our tutorial, we’ll be taking a look at the RAW file below, which was shot on a Nikon D750 at 24mm, 16,000 ISO, and f/2.8 for 10 seconds. I originally took this shot with the intention of stacking it with similar exposures, which is the reason for the high ISO and short shutter speed. This base file already has the lens profile correction for my lens added to it, which reduced some vignetting (darkening) that was visible in the corners of the image.

Based on how I processed the photo to my personal taste, the image below was my final result using only Lightroom. As we go through the editing process, I’ll try to show step-by-step how each adjustment affects both the base image and the final results. Keep in mind as we go along, however, that there is no incorrect way to process an image, and no correct order of operations when going through the editing process.

White balance is the single biggest thing that will affect the overall look of the Milky Way in your photo, so it is always my starting point when editing a night sky photo. Assuming you shot your night sky image as a RAW file—which is highly recommended due to how much flexibility it gives in the editing process—White Balance does not need to be set in-camera when you are out in the field.

White Balance, which is a color temperature measured in degrees Kelvin (K), ranges from blue at the cool end of the spectrum and orange on the warm end of the spectrum. In Lightroom’s Develop Module, the main White Balance slider is labeled as “Temp.” Another slider, which balances between green and magenta coloring, is labeled “Tint” and is located just below the Temp slider in the Develop Module.

Just based on personal taste, I find that I tend to make the White Balance relatively neutral in most of my Milky Way photos, but ultimately push the overall coloring slightly towards the blue and magenta ends of the White Balance spectrum. Depending on the ambient lighting conditions at my shooting location, the look I prefer normally results in a Temp of somewhere in the 3,700 K to 3,900 K range, with the Tint slider being set somewhere between +5 and +15. The photo below has a Temp of 3,786 K and a Tint of +10. These settings gave a good balance between a slightly blue sky, a yellow core of the Milky Way, a magenta colored Lagoon Nebula in the central core of the Milky Way, and the green airglow that was present on the night I was shooting.

Since all of this coloring is extremely subtle in an unedited RAW file, when choosing my White Balance I will first crank up the Saturation and Vibrance of the image as high as possible. Doing so exaggerates the colors in the image, allowing me to easily see the effects of a subtle change of the Temp or Tint sliders. Upping the Saturation and Vibrance to +100 gave me the image below.

For the sake of comparison, if I were to have chosen a Tungsten White Balance, which sets the Temp slider at 2,850 K, or Daylight White Balance, which sets the Temp slider at 5,500 K, I would get the images below. Note that not only does the overall color of the sky change, but the subtle colors in the core of the Milky Way, as well as the green airglow, are overpowered.

I will openly admit that designating one of the 3 Pillars as “Contrast and Exposure” is unbelievably broad. However, when focusing on strictly the Milky Way in a night sky image, Contrast and Exposure are very closely linked and are the major factors that affect the appearance of the Milky Way in your photo once you have chosen your White Balance. Given that it is meant to be processed, an unedited RAW photo of the Milky Way looks washed out and offers little detail of our main subject. So, each tweak we make going forward has the end goal of using contrast to bring out the desired amount of detail in the Milky Way, while also maintaining a correct exposure. When adding contrast to a Milky Way photo, I tend to do it gradually, evaluating how each adjustment and small tweak effects the overall image.

After adjusting White Balance, the Blacks slider in Lightroom is typically my next stop when processing a Milky Way image. Not only does it deepen the colors in the image, but it also serves as a way to bring out the subtle colors in the Milky Way and provide a bit of noise reduction to the sky. While adding contrast via the Contrast slider would have a similar effect with regard to bringing out colors, it would also brighten the highlights in the image at the same time. By using the Blacks slider, we can affect only the darker portions of the image for now, giving us more nuanced control. For the final processed image, I dropped the Blacks slider to -60.

If I were to take away that Blacks adjustment from the final image, it gives the Milky Way a look with less contrast, which may look washed out to some, but to others may give a more dreamy and ethereal feel.

In a similar way as the Blacks slider, the Whites slider in Lightroom allows us to selectively brighten parts of the image, effectively creating contrast when used in tandem with the Blacks slider. The Whites slider is typically what I tweak right after using the Blacks slider so that I can see the cumulative effect they have on the Milky Way. The image below shows our RAW file with the Whites slider set to +30.

Also, here is what the final Lightroom image would look like if the Whites had not been raised. Leaving the Whites slider at 0 does not have a huge overall effect in the image, but some of the glow of the Milky Way is removed, making it look slightly flatter.

After selectively adjusting contrast using the Blacks and Whites sliders, I used the general Contrast slider to further boost contrast in the image. By bring up the Contrast slider to +50, the Milky Way’s details become more isolated from the overall exposure, seemingly deepening the sky and the dust trails in the core of the Milky Way.

Without this contrast adjustment, the Milky Way in the final Lightroom image would have had slightly less punch and separation between the dark and light portions of the sky.

At this point in the editing process, we can see that the overall exposure of the Milky Way is a bit dark due to all of the adjustments made to add contrast. The dense path of stars in the core of the Milky Way does not glow at all, so I decided to raised the Exposure slider to +0.40 stops to get the overall brightness of the image closer to where I would want the final product to be.

Without this increase in exposure, the final file of Milky Way would look like it does below.

Although its designed purpose is to bring back detail in areas of an image affected by atmospheric haze or fog, the Dehaze slider in Lightroom effectively adds some serious contrast and punch to the Milky Way in night sky photos. Hidden away from the main panel in the Develop Module, Dehaze can be found in the Effects menu of the Develop Module. In addition to adding contrast, Dehaze also helps to bring out some of the colors in and around the Milky Way, including the green airglow in the bottom of the image. I bumped the Dehaze slider to +40, which was probably the most drastic adjustment thus far other than setting the White Balance. I try to carefully evaluate how much I increase the Dehaze slider, however, because pushing it too far makes the Milky Way look a bit grungier than the look I’m hoping for.

Without the increase in Dehaze, the final image has noticeably less contrast and color.

A Curves adjustment is a powerful tool that gives you a large amount of nuance control when editing a Milky Way photo. If you know how to use it properly, many of the adjustments above could be mimicked by making a Curves adjustment in Lightroom or Photoshop, as it allows you to selectively adjust shadows, midtones, and highlights of an image. When trying to add contrast to an image, you will want to make the diagonal line extending from the bottom left to the top right of the histogram overlay into an S-shape by dragging a point on the left half of the histogram down and dragging a point on the right side up. The more pronounced the S-shape, the more contrast that will be added to the image.

In playing around with Curves adjustments in both Lightroom and Photoshop in the past, I have actually found the Photoshop Curves adjustment to yield better results. Because of this, I typically don’t use the Lightroom Curves adjustment other than to check to see if either the “Medium Contrast” or “Strong Contrast” Curves presets yield an effect on the image that I like. If I have a more complicated image that requires only adding a Curves adjustment to a specific portion of the photo, I will bring the file into Photoshop and combine the Curves Adjustment with a Layer Mask. For our Milky Way-only image, I used the “Strong Contrast” preset to give a little more punch to the core of the galaxy.

The Highlights slider was one of the last ones that I touched while processing this Milky Way photo. After looking at the overall exposure, I wanted to get a little bit more glow out of the brighter parts of the Milky Way, so I boosted the Highlights slider to +30. This honestly didn’t make an enormous difference to this image. However, depending on the photo, the Highlights slider can help to be a fine adjustment tool to either add a little extra glow to the Milky Way, or to bring the exposure of the brightest parts of the image to avoid a dense area of stars to be blown out.

The Clarity slider is one of the more powerful tools in Lightroom, the effect of which can essentially be summed up as “edge contrast.” Boosting the Clarity slider provides punch and a more three-dimensional look to an image, and a little can go a long way. In Milky Way photos specifically, the dragging the Clarity slider to the right can make the stars seem to pop a bit more, while dragging it to the left is another way to give a dreamy, ethereal look to the sky, similar to an Orton Effect (which is discussed further below).

The image below shows the RAW file with the Clarity boosted to +40 to add a bit of pop to the stars.

And for a more ethereal look, this image below is the final Lightroom file with Clarity set to -40 instead of +40.

Since Milky Way photography is a constant balancing act between keeping your camera’s ISO low enough to get a relatively clean image and keeping your shutter speed short enough to prevent noticeable star trailing, noise reduction is often a necessary step in a night sky photography workflow. The Improve Photography article 5 Great Ways to Reduce Noise in Your Photos gives an in-depth comparison of the effects of different types of noise reduction.

The type of noise reduction you choose for your Milky Way photo may largely be dependent on where you ultimately want to display your photo. The photos that I plan to post on Facebook or Instagram will be compressed and, most likely, will be viewed on a small cell phone screen. If this is the case, I put very little effort into noise reduction and just boost the Luminance slider in the Noise Reduction Panel of Lightroom’s Develop Module to somewhere between +10 and +20.

If I know that the end goal of my photo is for it to be printed, I typically take noise reduction a bit more seriously because, depending on my print surface, the noise can end up being a lot more apparent. In these situations, I have two go-to methods.

Image Stacking

As detailed in 5 Great Ways to Reduce Noise in Your Photos, image stacking using a median filter in Photoshop is an extremely effective way to increase the signal-to-noise ratio in your photo. In Milky Way photography, stacking enhances faint details in the galaxy while smoothing out noise in the final photo by identifying the random luminance and color noise in each exposure and removing it. It can be time-intensive, but it is well worth the effort.

For a sky-only photo with no clouds, stacking is my preferred technique because the alignment process is quick and easy. With clouds or a foreground included, however, Photoshop can sometimes get confused about which pixels are supposed to be aligned, forcing you to align exposures manually. (If you do run into this problem, check out this tutorial on how to manually align sky exposures.) Lastly, image stacking essentially lets you to cheat in that aforementioned balancing act between your ISO and shutter speed, because it allows you to both boost your ISO and reduce your shutter speed. This results in more noise, but more pinpoint stars. After image stacking, your stars will remain pinpoint, but your noise will be greatly reduced.

Nik Dfine and Luminosity Masks

Another effective way to reduce noise in a Milky Way photo is to use a combination of Nik Dfine (Google’s free noise reduction Photoshop plug-in) to reduce noise, and a luminosity mask to selectively choose where to apply that noise reduction. If you are unfamiliar with luminosity masks, check out this in-depth article written by Brian Pex to get you started.

After loading your Milky Way image into Photoshop, open Nik Dfine and either let it analyze the noise profile in your image automatically, or do so on your own. Once Nik Dfine has finished analyzing the noise, close the plug-in and wait for Photoshop to create a new layer with the noise reduction applied. In all likelihood, Nik Dfine will do a fairly good job of smoothing out much of the noise in the image, but will also blur the boundaries between stars and the dark sky around them. In order to keep those edges crisp, we have to turn to luminosity masks.

Since the stars are mostly bright points of light, they do not show much noise to begin with, so an easy way to reduce noise everywhere except the stars is to use a luminosity mask. Many photographers and online teaching resources sell or provide free downloads of Photoshop actions to create luminosity masks. However, for the purpose of reducing noise, they can be easily created in Photoshop with a few clicks without these actions:

  1. Click on the “Channels” tab in Photoshop and CMD/CTRL click on the RGB channel
  2. Next, at the bottom of the Channels tab click “Save Selection as Channel”. This will make a new channel appear showing the selection you just created. To select more bright areas, CMD/CTRL +SHIFT click on the RGB channel and save the selection. And to select fewer bright areas, CMD/CTRL+ALT+SHIFT click on the RGB channel
  3. CMD/CTRL click on the newly created luminosity channel
  4. Go back to the “Layers” tab and click “Add a Mask” and
  5. Invert the mask by using CMD/CTRL+I.

Other Techniques

Aside from these three main pillars of Milky Way post-processing, there are an endless amount of additional editing techniques that can be used to perfect your night sky image. While it would be impossible to dig in to every possibility here, there are a few common ones that are worth being aware of.

Foreground Processing

To create a truly compelling image with the Milky Way, it’s important to include some kind of foreground or midground in the scene to add some additional interest to the image. While the Milky Way itself is spectacular to stare at, it begins to look the same in each image when shown as its own composition instead of as a part of a larger shot.

For the image below, I used most of the ideas mentioned above to get the Milky Way looking the way I wanted before switching my focus to the foreground. While this process made the Milky Way look great, all of the added contrast resulted in a very dark foreground. To combat this problem, I raised the Shadows slider in the Develop Module in Lightroom to +100, which did help a bit, but not enough for my liking.

To bring the exposure of the foreground up a bit more, a valuable option is to utilize the adjustment brush function in Lightroom to selectively adjust the image. For this shot, I used an adjustment brush with Exposure set to +0.33 and Contrast set

to -100. Keep in mind that these were the sliders in the Adjustment Brush panel and not the Basic Panel of the Develop Module, so they affected only the area of the foreground that I painted over.

This shot was made all with one exposure on a Nikon D750, which not only handles high ISO settings very well, but also has fantastic dynamic range, meaning that the sensor can capture a wide range of shadows and highlights all in one exposure. Further, the D750 is able to capture detail in the shadows of an image without also introducing large amounts of noise. If using a camera that didn’t have these qualities, however, I would instead capture the sky with a proper exposure in one scene and the foreground with a proper exposure in another. While this strategy takes a bit more effort because care needs to be taken to blend the two exposures together in Photoshop with layer masks, it also yields a cleaner image because each exposure can be captured while taking noise into consideration.

Regardless of your chosen method to enhance the foreground in your image, keep in mind that your Milky Way photo was taken at night. To keep the image looking “realistic”, if that is your goal, the foreground shouldn’t be as bright as it would be in a daylight image.

Star Reduction

Another advanced technique to add another level of dimension to your Milky Way image is to use a technique referred to as “star reduction.” Star reduction is often used in deep sky astrophotography to make stars less prominent when the main focus of an image may, for example, be a nebula or galaxy behind those stars. In Milky Way photography, since the core of our galaxy features a very dense collection of stars, star reduction in post-processing provides some separation between stars to make them seem more distinct. In order to perform star reduction to your Milky Way, open up your image in Photoshop and follow the steps below:

  1. Use the Eyedropper Tool to select a star in the photo
  2. Click Select>Color Range
  3. Use the Eyedropper Plus or Eyedropper Minus tools to select stars with varying colors to either add or remove color ranges from the selection
  4. Adjust the Fuzziness slider in the Color Range menu to tweak the amount of the image the selection effects. The image below used a Fuzziness of 60. Once you choose your Fuzziness, click OK
  5. Once the selection is made, click Select>Modify>Feather and choose a radius of approximately 1 to 2 pixels
  6. Click Filter>Other>Minimum and choose a radius of approximately 0.5. In the dropdown menu, choose “Preserve Roundness”.

Astigmatism Removal

As mentioned in the lens selection section of the Ultimate Guide to Planning Your Milky Way Photography , some lenses suffer from saggital astigmatism, the effects of which appear as bright wingtips on either side of stars on the outer portions of the image. The degree to which this astigmatism can be seen varies by lens. However, if the astigmatism is noticeable enough to take away from the quality of the image, check out this video tutorial made by astrophotographer Tyler Sichelski over at Lonely Speck for a way to correct saggital astigmatism in Photoshop.

Orton Effect

The Orton Effect is something that has been running rampant through landscape photography lately. It can provide a subtle (or not so subtle), dreamy glow to an image, which can be especially enticing in Milky Way photography. While I have never personally used the Orton Effect in a photo, it has become popular enough that it is certainly worth mentioning when it comes to post-processing Milky Way photos. There are many different ways to produce the Orton Effect, ranging from complex luminosity masking to simple adjustments. A simple search for Orton Effect on YouTube yields a variety of results. The image with the Orton Effect applied below was done with a relatively simple process by doing the following:

  1. Duplicate the Milky Way image to a new layer in Photoshop
  2. Click Filter>Blur>Gaussian Blur and a value of 10 pixels
  3. Click Image>Adjustments>Brightness/Contrast and set Brightness to 20 and Contrast to 70
  4. Set the Opacity of the new Orton Effect layer to 30%

Final Thoughts

Post-processing is an extremely subjective process. There is no right or wrong way to edit a photo of the Milky Way, but hopefully this tutorial was able to help show some of the many different post-processing tools that are currently available to photographers that can help to make your Milky Way photos come alive. Milky Way photography can be an awe-inspiring and extremely tiring endeavor, but one I personally have found to be very rewarding over the past few years. I hope this has given some insight into how to put the finishing touches on all the effort that goes into making an image of the Milky Way. Good luck!

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