Monthly Archives: October 2019

Space, planets and galaxies have interested Simon Lewis for as long as he can remember. The Greendale resident talks to Devon Bolger about his passion for astronomy and work as an astrophotographer at the West Melton observatory

What was it that first attracted you to astronomy?

Even as a kid, Ive always been interested in science and technology. As a boy, I was always pulling things apart and trying to fix them, nothing was safe in my house. I started to grow this passion for space and our universe so this is kind of a natural progression for me because Ive taken that interest in science and technology and applied it to astrophotography which is what I specialise in.

What do you like about astrophotography?

Astrophotography really includes a large part of science and astronomy because youre trying to look at different type of objects using cameras and you have to try to understand the dynamics of what youre looking at, the size and scale of what youre looking at, and also the type of object youre looking at and even when it might be visible. You have to put some effort into it and thats the bit I like, theres a science behind it. You have to understand the night sky and the object your aiming for and then kind of frame it up from there to create something thats pleasing to the eye. Everybody that does astrophotography say its very much an art form in its own right. Theres result thats the correct one to aim for, you just give it the finish that is your interpretation of the subject and one which makes you happy.

Is there anything that you find particularly exciting when you photograph it?

I like to look at things that arent quite mainstream. There are definitely major celestial objects that are commonly photographed and if you picked up an astronomy magazine youd see pictures of them in there but I like to photograph more low key objects that are just as interesting. Theyre not always as photogenic perhaps but theyre very interesting in their own right. For example, theres an object near the Southern Cross called the Corona Australis that is whats called a dark nebula, which is actually dust that is blocking the light from stars so if you look at it you cant actually see any light through it because theyre being blocked by the dust. I quite like taking photographs of them and think theyre nice to look at because theyre not photographed very often.

What is your favourite constellation?

Not a constellation in particular, as our southern night sky is really fantastic there is so much to look at, so choosing one as a favourite is really hard. If you look directly to the south of us youve got Carina which is a fabulous constellation which has the Carina Nebula and whats called the Homunculus which is a bubble of gas round a huge star thats a 100 times bigger than ours. Its a really beautiful area to photograph. Orion is also a really amazing constellation to look at too. Its in our summer evening sky and it has a number of beautiful objects within it to image. If you look up and to the right from the stars on the belt youll see the Orion Nebula. Its so big and so bright you can see it without a telescope even by naked eye as a fuzzy patch, you can really see it very clearly in a pair of binoculars.

Do you think are aliens out there?

Well, Im always seeing strange and weird things out there at night. There are always satellites, space junk and meteors passing by us, but I think it would be a very lonely place and a very, very strange occurrence if human life was the only life in the universe. The problem I think is actually the distances involved and as an astronomer youre acutely aware of how far away some of these objects are. Even in our own galaxy, which is around 200,000 light-years from edge to edge, our radio signals have only managed to get around a hundred light-years distant from earth so far, so Im not surprised we havent run into anybody yet.

You are from England, how long have you lived in the district for?

I visited New Zealand in 2008 while I was living in Europe and I settled in West Melton after doing some travelling and decided I loved New Zealand and wanted to live and work here, so Ive been in Selwyn since I first arrived here. My met my wife here andwe moved out to a lifestyle block in Greendale in 2016, basically because its a beautiful place to live, its amazingly dark at night and we have some land for my wifes horses and room for my observatory.

Could you tell me about the work you do at the West Melton observatory and for the Canterbury Astronomical Society?

Im on the societys committee and I hold a couple of roles. Im the webmaster and handle the website as well ticketing for events and Facebook updates. Im also the membership officer. I also manage the content on the website and write the newsletter that we publish online monthly, we also publish a written magazine too. I play an active role in our public night programmes. Every Friday night we get about 80 to 90 people visiting the observatory. I might drive one of the telescopes or just wander around with binoculars talking to the kids and showing them whats in the sky and explain what theyre seeing. The kids love it, theyre always asking things like: Oh can we see a black hole? So you can really have a lot of fun just wandering around talking to them and explaining the night skies. We do that from the end of March until the end of September and during July we also do KidsFest where we are open every night if its clear, for 15 nights in a row.

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Passion for astronomy is out of this world - Otago Daily Times

Lake Effect's Bonnie North speaking with astronomy contributor, Jean Creighton.

When people think of astronomers, several names come to mind: Isaac Newton, Galileo Galilei, or Carl Sagan all white men. But throughout history, women and other people of color have made huge contributions to our understanding of the cosmos.

Astronomy contributor Jean Creighton says that highlighting that diversity in the field is necessary for both kids and adults.

"It's so important to break those barriers, no matter who you are, no matter what you do, so that the stereotypes that people put in their heads are set aside," says Creighton.

She recalls that when she was a graduate student, most of her professors were male and "there were certain things that [she] felt they really couldn't answer."

Creighton notes that while diversity in astronomy has improved, there is still some ground to be gained.

Her advice? Look, ask and see. "If you can't see those people, they might be there anyway," notes Creighton. "And if they're not, yes, then it's your responsibility to try and become one."

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How To Increase Diversity & Representation In Astronomy - WUWM

VIPER will have four key instruments to detect water. The Neutron Spectrometer System will detect wet areas below the surface. Once an area looks promising, the rover will deploy its TRIDENT drill to dig into the soil. Two other instruments will analyze the soil samples.

On the mission, VIPER will collect different kinds of soil from different areas of the Moon, letting scientists map out exact locations where water is likely located.

With the Artemis missions beginning in the 2020s, water will be a key resource on the Moon. Deposits of ice water will be crucial for the longevity of a human settlement. Its not just drinking water thats in demand, either. Eventually, by mining the water ice, astronauts may be able to extract its hydrogen and oxygen and then use it to create rocket fuel. That could reduce the amount of fuel and supplies astronauts have to bring with them to the Moon, and even support future missions to Mars.

It can be used, and we need to use it, not just for life support but maybe even rocket fuel, said NASA administrator Jim Bridenstine.

Astronomers found water ice near the Moons south pole back in 2009, and scientists think its a promising spot to find more. And its not just NASA looking into the resources that might be hidden there. In September, India attempted to send their first lunar rover to the south pole to map the area and explore the region for water. However, the lander malfunctioned and crashed into the surface.

Engineers are already testing models of VIPER to see how it will move and function on the surface of the Moon. The final VIPER rover is being created by various NASA research centers, along with commercial partner, Honeybee Robotics, a Brooklyn-based company.

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NASA's sending a rover to the Moon to find water for astronauts - Astronomy Magazine

Astronomers have spotted Strontium in the aftermath of a collision between two neutron stars. This is the first time a heavy element has ever been identified in a kilonova, the explosive aftermath of these types of collisions. The discovery plugs a hole in our understanding of how heavy elements form.

In 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European VIRGO observatory detected gravitational waves coming from the merger of two neutron stars. The merger event was named GW170817, and it was about 130 million light years away in the galaxy NGC 4993.

The resulting kilonova is called AT2017gfo, and the European Southern Observatory (ESO) pointed several of their telescopes at it to observe it in different wavelengths. In particular, they pointed the Very Large Telescope (VLT) and its X-shooter instrument at the kilonova.

The X-shooter is a multi-wavelength spectrograph that observes in Ultraviolet B (UVB,) visible light, and Near Infrared (NIR.) Initially, X-shooter data suggested that there were heavier elements present in the kilonova. But until now, they couldnt identify individual elements.

This is the final stage of a decades-long chase to pin down the origin of the elements.

These new results are presented in a new study titled Identification of strontium in the merger of two neutron stars. The lead author is Darach Watson from the University of Copenhagen in Denmark. The paper was published in the journal Natureon 24 October 2019.

By reanalysing the 2017 data from the merger, we have now identified the signature of one heavy element in this fireball, strontium, proving that the collision of neutron stars creates this element in the Universe,said Watson in a press release.

The forging of the chemical elements is called nucleosynthesis. Scientists have known about it for decades. We know that elements form in supernovae, in the outer layers of aging stars, and in regular stars. But theres been a gap in our understanding when it comes to neutron capture, and how heavier elements are formed. According to Watson, this discovery fills that gap.

This is the final stage of a decades-long chase to pin down the origin of the elements,says Watson.We know now that the processes that created the elements happened mostly in ordinary stars, in supernova explosions, or in the outer layers of old stars. But, until now, we did not know the location of the final, undiscovered process, known as rapid neutron capture, that created the heavier elements in the periodic table.

There are two types of neutron capture: rapid and slow. Each type of neutron capture is responsible for the creation of about half of the elements heavier than iron. Rapid neutron capture allows an atomic nucleus to capture neutrons quicker than it can decay, creating heavy elements. The process was worked out decades ago, and circumstantial evidence pointed to kilonovae as the likely place for the rapid neutron capture process to take place. But it was never observed at an astrophysical site, until now.

Stars are hot enough to produce many of the elements. But only the most extreme hot environments can create heavier elements like Strontium. Only those environments, like this kilonova, have enough free neutrons around. In a kilonova, atoms are constantly bombarded by massive numbers of neutrons, allowing the rapid neutron capture process to create the heavier elements.

This is the first time that we can directly associate newly created material formed via neutron capture with a neutron star merger, confirming that neutron stars are made of neutrons and tying the long-debated rapid neutron capture process to such mergers,says Camilla Juul Hansen from the Max Planck Institute for Astronomy in Heidelberg, who played a major role in the study.

Even though the X-shooter data has been around for a couple years, astronomers werent certain that they were seeing strontium in the kilonova. They thought they were seeing it, but couldnt be sure right away. Our understanding of kilonovae and neutron star mergers is far from complete. There are complexities in the X-shooter spectra of the kilonova that had to be worked through, specifically when it comes to identifying the spectra of heavier elements.

We actually came up with the idea that we might be seeing strontium quite quickly after the event. However, showing that this was demonstrably the case turned out to be very difficult. This difficulty was due to our highly incomplete knowledge of the spectral appearance of the heavier elements in the periodic table,says University of Copenhagen researcher Jonatan Selsing, who was a key author on the paper.

Up until now, rapid neutron capture was much debated, but never observed. This work fills in one of the holes in our understanding of nucleosynthesis. But it goes further than that. It confirms the nature of neutron stars.

After the neutron was discovered by James Chadwick in 1932, scientists proposed the existence of the neutron star. In a 1934 paper, astronomers Fritz Zwicky and Walter Baade advanced the view that a super-nova represents the transition of an ordinary star into aneutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density.

Three decades later, neutron stars were linked and identified with pulsars. But there was no way to prove that neutron stars were made of neutrons, because astronomers couldnt obtain spectroscopic confirmation.

But this discovery, by identifying strontium, which could only have been synthesized under extreme neutron flux, proves that neutron stars are indeed made of neutrons. As the authors say in their paper, The identification here of an element that could only have been synthesized so quickly under an extreme neutron flux, provides the first direct spectroscopic evidence that neutron stars comprise neutron-rich matter.

This is important work. The discovery has plugged two holes in our understanding of the origin of elements. It confirms observationally what scientists knew theoretically. And thats always good.

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Astronomers See Strontium in the Kilonova Wreckage, Proof that Neutron Star Collisions Manufacture Heavy Elements in the Universe - Universe Today

Researchers have discovered streams of gas rotating in opposite directions around a distant, supermassive black hole. This finding offers new clues about how black holes grew so rapidly in the early universe.

This supermassive black hole lies at the heart of the spiral galaxy NGC 1068, or Messier 77, which is located approximately 47 million light-years from Earth. This black hole is hidden within a thick doughnut-shaped cloud of dust and gas.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, astronomers found that the black hole is actually surrounded by two counter-rotating disks of gas. The inner disk spans 2 to 4 light-years and rotates in the same direction as the galaxy, while the outer disk spans 4 to 22 light-years and spins in the opposite direction, according to a statement from ALMA.

"Thanks to the spectacular resolution of ALMA, we measured the movement of gas in the inner orbits around the black hole," lead author Violette Impellizzeri, an astronomer from the National Radio Astronomy Observatory (NRAO) who works at ALMA, said in the statement. "Surprisingly, we found two disks of gas rotating in opposite directions."

Earlier studies of NGC 1068 revealed that the black hole has a monster appetite. Gas from the region surrounding the black hole, also known as the accretion disk, falls into the black hole, is superheated and then is thrown back out into space at incredibly high speeds. This process makes it difficult for optical telescopes to see through the region around the black hole. Gas in counter-rotating disks is believed to be more unstable than gas in a single rotating accretion disk, according to the statement. Therefore, gas in counter-rotating disks is believed to fall into a black hole faster, which could help to explain how some supermassive black holes grow so quickly, Impellizzeri said.

Using ALMA's extremely capable zoom lens, astronomers were able to observe the molecular gas around the black hole in NGC 1068 in great detail. cPreviously, counter-rotation has only been observed in galaxies thousands of light-years away from the galactic center.

This ALMA image shows two disks of gas moving in opposite directions around a black hole in the galaxy NGC 1068. The colors represent the motion of the gas: blue is material moving toward us, red is moving away. The white triangles show the accelerated gas that is expelled from the inner disk forming a thick, obscuring cloud around the black hole.

(Image credit: ALMA (ESO/NAOJ/NRAO), V. Impellizzeri; NRAO/AUI/NSF, S. Dagnello.)

However, in this study, the counter-rotation around NGC 1068 was seen occurring on a much smaller scale, only tens of light-years from the central black hole, the researchers said.

"We did not expect to see this because gas falling into a black hole would normally spin around it in only one direction," Impellizzeri said. "Something must have disturbed the flow because it is impossible for a part of the disk to start rotating backward all on its own."

To explain the backward flow of gas observed in NGC 1068, the astronomers suggest that gas clouds may have fallen out of the host galaxy or, alternatively, a small, passing galaxy on a counter-rotating orbit may be captured in the accretion disk, according to the statement.

While the outer disk currently appears to be in a stable orbit around the inner disk, the researchers expect that it will eventually fall onto the inner disk.

"The rotating streams of gas will collide and become unstable, and the disks will likely collapse in a luminous event as the molecular gas falls into the black hole," co-author Jack Gallimore, a professor of physics and astronomy at Bucknell University in Lewisburg, Pennsylvania, said in the statement. "Unfortunately, we will not be there to witness the fireworks."

These findings were published Oct. 14 in the Astrophysical Journal Letters.

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How Ancient Black Holes May Have Grown So Quickly in the Early Universe - Space.com

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One of the major unanswered questions in astronomy is how our modern system of galaxies evolved into its present-day configuration in the first place. Now, researchers have found evidence of a massive galaxy that formed when the universe was far younger than it today, with a very different configuration than the galaxies we see in the modern era.

Astronomer Christina Williams, who authored the study, was working with the Atacama Large Millimeter Array (ALMA) when she observed an extremely faint galaxy in an area where no galaxy had previously been known to exist.

It was very mysterious because the light seemed not to be linked to any known galaxy at all, said Williams, a National Science Foundation postdoctoral fellow at the Steward Observatory. When I saw this galaxy was invisible at any other wavelength, I got really excited because it meant that it was probably really far away and hidden by clouds of dust.

So it was. And its discovery may help astronomers solve a longstanding problem with existing theories of galaxy formation. Because its obviously impossible for astronomers to create a bottle universe and then watch to see how galaxies form, we have to rely on computer models that generate results based on initial preconditions. If the model doesnt produce a universe that looks like the one we live in, you know the model is incorrect in some fashion.

Antenna galaxies NGC 4038 & 4039 mid-merger. Blue areas are areas of star formation. Image from Wikipedia

At present, theories suggest that star formation peaked about 3.5B years after the Big Bang, at a redshift value (expressed in terms of z) of 1.9. Redshift values do not scale linearly; they increase quickly as we approach the beginning of the universe. The cosmic microwave background radiation, which dates to ~389,000 years after the Big Bang, has a z value of 1089. The highest redshift galaxy yet detected is GN-z11, which is observed as it existed some 13.4B years ago, 400M years after the Big Bang, and has a redshift value of 11.09. Light from this newly detected galaxy (as yet unnamed) has traveled some 12.5B years to reach us and has an observed redshift value of z = 5.5 with a range of +/- 1.1.

One of the challenges for existing theories of early galaxy formation is that early galaxies appear to have gotten very big, very fast. There is a body of evidence suggesting that at redshift values of 3 or less, these rare-but-massive galaxies may account for half of the cosmic star formation rate density (CSFRD). Optical and near-infrared galaxies account for the other half of observed stars. Beyond z > 3, however, the situation is unclear. While a bare handful of these large, dust-obscured galaxies have been observed at greater redshift distances, the authors write that they trace only the very tip of the star formation rate (SFR) distribution at early times The total contribution of dust obscured star formation, and therefore the census of star formation in the early universe, is unknown.

The light the bit thats reaching us is probably caused by stars heating the gas clouds that sit between ourselves and the distant galaxy. The galaxy itself is completely obscured by this fog, though astronomers estimate its the approximate size of the Milky Way. Its far more active than our home, though. Rates of star formation may be up to 100x higher than the Milky Way is currently experiencing.

Star formation rates this high could explain how the early universe got so big, so fast, but we need to find a lot more galaxies like this to fully explain the implied rates of star formation in the early universe.

Our hidden monster galaxy has precisely the right ingredients to be that missing link, Williams explains, because they are probably a lot more common. The launch of the James Webb Space Telescope in 2021 should help shine more light on just how prevalent these large galaxies are.

Feature image by James Josephides, YouTube.

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Astronomers Find Massive Dust-Cloaked Galaxy From the Early Universe - ExtremeTech

BD +20 307 should not have been so bright. For such an old binary star system, it should have appeared cool in our scopes. Instead, it looked hot.

The discovery of this, made over a decade ago, signalled to astronomers that something incredible had happened: BD +20 307, a solar system over 300 light-years from Earth, was the scene of a violent cosmic accident.

Exactly what had taken place was up for debate, but it was definitely something powerful enough to spew out a hot, cloudy aftermath of swirling dust and debris encircling the distant star system in a giant ring.

Early analysis suggested asteroids or planetesimals running into one another may have been the cause. Subsequent spectroscopic observations refined the hypothesis, and then in 2010, scientists concluded the excess of dust we were seeing was, in all likelihood, the result of actual worlds colliding.

That giant dust bowl surrounding BD +20 307? Countless tiny fragments left behind by two dead exoplanets that just couldn't avoid each other in all the vast emptiness of space.

Artist's impression of worlds colliding in BD +20 307. (NASA/SOFIA/Lynette Cook)

"A catastrophic collision of two rocky, planetary-scale bodies in the terrestrial zone is the most likely source for this warm dust," astronomers explained in a paper, acknowledging that evidence of such "cataclysmic impacts" was rare.

Rare they may be, but simulations suggest these run-ins do occur. For what it's worth, you might only be reading this because our own planet once had such an encounter. The Moon, perhaps, was another beneficiary.

Regardless of local history, the hypothetical explanation for what transpired far away in BD +20 307 just got supported by a new study led by astrophysicist Maggie Thompson from the University of California, Santa Cruz.

Most spectacularly of all, the new research indicates that in the past decade or so since the last observations, the hot, dusty legacy of this violent collision seems to have gotten either hotter or dustier.

New infrared readings taken with SOFIA (the Stratospheric Observatory for Infrared Astronomy) and its Faint Object Infrared Camera indicate the luminosity of the dust ring 300+ light-years away has actually brightened about 10 percent in recent years.

That's a significant burst in brightness, and it could mean there's more dust grains absorbing more heat from the starlight of BD +20 307's two stars or it could mean something else has happened to heat the same amount of dust up to a hotter temperature (such as hotter or closer stars), although such developments are extremely unlikely in a short timeframe of only years.

If the former eventuality is true and the surface area of the dusk disk visible to us here on Earth is somehow greater it might mean the chaotic, billowing fallout from the two worlds colliding is still unfolding.

It's an intriguing possibility, although the team notes we don't have enough evidence to know for sure just why this luminosity flux exists.

On all the data available, the researchers maintain the collision between planetary-scale bodies is "still the most likely origin for the system's extreme dust", but BD +20 307's strange brightening warrants plenty more investigation yet.

"The warm dust around BD +20 307 gives us a glimpse into what catastrophic impacts between rocky exoplanets might be like," Thompson says.

"We want to know how this system subsequently evolves after the extreme impact."

The findings are reported in The Astrophysical Journal.

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Astronomers Have Measured The Devastating Aftermath of Two Exoplanets Colliding - ScienceAlert

There's a lot happening in the night sky in the upcoming days and weeks.

First off, the Orionids Meteor Shower will peak this week. The shower actually stretches from late September through November, but is scheduled to peak between midnight and dawn Tuesday, October 22. It's a minor shower producing around 20 per hour at its peak. The meteors will appear to radiate from the constellation Orion, and originate from the comet Halley.

KING

Below is a list of planets, along with their position in the night sky you'll be able to see:

KING

Also, mark your calendars for November 11. That's when a rare transit of Mercury will occur. The planet closest to the sun is going to pass directly between the sun and earth on this day, almost like a miniature solar eclipse.

It's a fairly rare event that only happens once every couple years, and about a dozen times in a century. Unfortunately, you won't be able to see it unless you have a special solar filtering telescope.

KING

Check the forecast for cloud cover near you

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Exciting astronomy events coming up over the Pacific Northwest - KING5.com

Aviation and aerospace company Lockheed Martin will lead crewed flight operations and training, as well as develop the reusable Ascent Element for the lander. Draper, which is a not-for-profit research and development organization, will work on descent guidance and flight avionics. Northrop Grumman, an aviation company responsible the Pegasus rocket, will work on the technology that will bring the landing system down to the Moon.

We guided Apollo to the moon and back nearly 50 years ago, said Kaigham J. Gabriel, the President and CEO of Draper, in a press release. Were ready to do it again with the Blue Origin team for Artemis.

In May, Bezos announced the concept for Blue Origins soft lander, Blue Moon. That project utilizes a larger version of the lander, which can now be used as a model for the HLS.

This partnership is the latest addition to the Artemis program, in which many private aerospace companies have already partnered with NASA to develop new technologies that will help humans establish a presence on the Moon. Creating a reusable and reliable HLS will be key to getting humans to the lunar surface and back to Gateway, a proposed space station that orbits the Moon.

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Blue Origin teams up with other major aerospace companies for human l - Astronomy Magazine

Enlarge / A couple of different types of dark-sky-friendly LED streetlights.

Scott K. Johnson

I feel like were protecting the last tree, in a way. Thats what Flagstaff, Arizona, city council member Austin Aslan said at a recent meeting. The subject of that earnest statement might surprise you: it was streetlights. To be more specific, he was talking about a careful effort to prevent streetlights from washing out the stars in the night sky.

Flagstaff became the first city to earn a designation from the International Dark Sky Association in 2001. That came as a result of its long history of hosting astronomy research at local Lowell Observatory, as well as facilities operated by the US Navy. The city has an official ordinance governing the use of outdoor lightingpublic andprivate.

A few years ago, though, a problem arose. The type of dark-sky-friendly streetlight that the city had been using was going extinct, largely as a casualty of low demand. In fact, as of this summer, there are none left to buy. Meanwhile, the age of the LED streetlight has arrived with a catch: limited night-sky-friendly LED options.

If the city went out and just swapped lumens for the cheapest LED products out there, the astronomers would have marched on city hall with pitchforks and (night-vision-preserving) torches. And that might have been the least of their concerns, as the Navy informed the city last year that brightening of skies 10 percent over current conditions is not compatible with the [Naval Observatorys] mission.

The problem with LEDs boils down to blue light. Older streetlights are high-pressure sodium bulbs, which produce a warm yellow glow around a color temperature of 2,000 K. The bulbs Flagstaff relied on for most of its streetlights were low-pressure sodiuma variant that only emits light at a single wavelength (589 nanometers) near that yellow color, producing something resembling candlelight. Many of the LED streetlights on the market have much cooler color temperatures of 3,000 or even 4,000 K.

As Lowell Observatory Director Jeff Hall told Ars, As day turns to nightand your photopic cone-based vision turns into scotopic, rod-based vision, your sensitivity shifts a little bit blue. And so very blue-rich light at night comes off as really harsh and glaring and creates a lot of visible skyglow. So the less of that spectrum you touch, the better off you are for both visual observations, the night sky, as well as astronomy.

Hall continued: I see this wherever I go in my travels. By default, cities just put up, you know, 3,000 degrees CCT white, sometimes 4,000, which is this blue light. Just lumen for lumen [that] will create two-and-a-half to three times the skyglow of a high-pressure sodium system and, like, six times the skyglow of a low-pressure sodium system.

And the greater the skyglow around you, the harder it is to see the stars.

There are ways to build LED lights that change their natural color and mitigate this blue light problem. One way to do it is to simply throw a filter on the LED that blocks blue wavelengths from passing through. Of course, this significantly reduces the amount of light you produce for each watt of electricity. There are some aesthetic trade-offs, as wellof which not everyone is a fan.

What's left is green, Hall said. And so you stand under this and it's like the zombie apocalypse, because everybody's green. They've gone to these in Hilo, Hawaii, and we were standing in a parking lot trying to talk to each other, and it's just like straight out of Night of the Living Dead.

Another way to do it is with phosphor coatings on the LED that absorb light of one wavelength and emit it at another wavelength. Lights known as phosphor-converted amber (PCA) shift all the light out of the blue and into the yellow part of the spectrum at the cost of some efficiency. The result is actually quite close to the ubiquitous high-pressure sodium streetlights were used to.

Narrow-band amber (NBA) LEDs provide a different option. These lights actually use a type of LED that only emits warmer colors from the start. In this way, they actually compare pretty well to the low-pressure sodium streetlights that recently went extinct. The range of wavelengths emitted is a little broader, but the practical effect is about the same.

The downsides of the NBAs are basically cost and efficiency. But both have improved considerably over the last few years. Flagstaff Traffic Engineer Jeff Baumannwho is in charge of the plan for replacing the citys 3,500 streetlightstold Ars that the available NBA options have recently climbed over 30 lumens per watt (on the ground), with efficiencies over 40 right around the corner. For comparison, the citys low-pressure sodium streetlights weigh in at about 50 lumens per watt.

Separately from all this wavelength wrangling, though, LEDs do have a strong natural advantagetheyre highly directional. That is, LED streetlights do a much better job of only lighting the street (rather than the adjacent homes). That means that fewer lumens coming out of the fixture can give the same result you had before.

Flagstaffs plan is generally to swap in NBA LEDs for all the low-pressure sodium lights, and PCA LEDs for the high-pressure sodium lights that are used along the busier streets (as theyre a little brighter). The better directionality of LEDscombined with resident requests for slightly dimmer lighting on residential streetsactually means that the total output of the citys streetlights is going to drop from about 29 million lumens to about 19 million lumens. Thats not unusual.

If you absolutely must use white LEDs, you could do what Tucson has done, Hall said. They... switched out their whole high-pressure sodium system to 3,000 degree white but reduced their lumen budget for street lighting from 480 million to, like, 170 million [lumens] or something. And you need to do that. For every white LED lumen, you're increasing your skyglow by a factor of about three, but they cut the lumen budget by about a factor of three. So overall, they managed to wash out the skyglowbecause theyve got a lot of observatories down there.

Of course, this isnt just an engineering optimization problem. Theres also public buy-in to contend with. In this case, the city of Flagstaff put up test sections of different fixtures around town so anyone interested could compare and provide feedback. And since public safety is the primary reason streetlights exist in the first place, perceptions (which can vary wildly) about how much or what kind of light qualifies as safe can force some compromises.

The astronomers in the community may point to studies suggesting that increasing lighting beyond a basic threshold level wont reduce crime or accidents, but this isnt always persuasive to those recalling tragic pedestrian collisions.

A single-track emphasis on energy efficiency might also push you toward the most efficientand therefore bluestLEDs you can find. But there are more knobs to turn than just color; without careful analysis, there can be a temptation to put in brighter lights than necessary so long as the overall wattage is lower than what you used to have. Its possible to choose to prioritize lighting color and still bring down energy usage by being careful about brightness levels.

It comes down to the fact that lighting choices dont just affect the things youre intentionally lightingthere are also the things you can avoid lighting. That means there are always ways to ensure that the pale stars of the night sky dont entirely disappear from your universe.

The trick is that its yet another thing you have to study up on in order to get it right. When Ars asked Jeff Hall whether he winces while walking through the outdoor lighting aisle at the local big box home improvement store, he had a solution in mind.

We certainly have had conversations internally about a dark-sky-compliant aisle, he said. There are so many choices to choose from. You know, people don't want to put a lot of duty cycles into it, even if they want to be helpful and be dark-sky compliant. They don't want to have to sit down and do four hours of research to figure out what light they should get. What they want is a box that's got an OK stamp on it. And there are fixtures that have an IDA (International Dark Sky Association) seal of approval.

Flagstaffs hope is basically to do that for cities by producing the first dark-sky ordinance updated to deal with LEDs. That could give other cities an example to follow, even if its not quite as easy as hitting up a dark-sky aisle at their local store.

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How Flagstaff Arizona switched to LEDs without giving astronomers a headache - Ars Technica