Tag Archives: light

Color is Weird

Yesterday on twitter, I ran across a perplexing image: yellow and magenta in a pair of windows that appeared to reflect blue and green, respectively, onto the sidewalk when illuminated by white light from the Sun.

What is going on?! I decided to put on my “I’m-an-astronomer-who-understands-light” cap and see if I could work it out. The rather circuitous tweet stream begins here, in which I made several errors: I tried to explain a complicated thing before I was fully awake, my brain kept switching key concepts like “absorb” vs. “reflect” and adding vs. subtracting colors of light, and I made a couple assumptions without explicitly stating them (spoiler: they turned out to be incorrect!).

Illustration of how colors of light add together. Source: Wikimedia commons

After much discussion on twitter, I concluded the paper posters in the window were reflecting their true colors (yellow and magenta) in all directions and leaving only their complementary light colors (blue and green) to reflect directly on the sidewalk. If you’re an astronomer, you might recognize this as analogous to Kirchhoff’s laws of spectroscopy: a nebula viewed from most lines of sight shows emission lines, but when it’s viewed in front of a bright object instead, you see absorption lines.

Yeah, that was wrong.

I took a closer look at the photo later that day, and realized the rectangles in the window were not posters at all, but looked more like transparent cellophane! That made things easier, I postulated:

Wrong again, though perhaps a bit closer.

The real explanation appeared today, when the original photographer returned to collect more data.




Trust me, you definitely want to play all those short videos (no sound required). The viewing angle changes everything! THAT’S SO COOL! Have you ever wondered why the cry of the scientist is “more data!”? It turns out that viewing something from more than one perspective can be very instructive, or should I say… illuminating? It’s enough to make this astronomer wish we had a way to fly halfway across the galaxy with a fleet of telescopes. Alas, space is way too big for that.

So there you have it: a learning experience, a more nuanced understanding of color, and a scientific quest all rolled into a handful of tweets.


Washed Out

Earlier this summer, I visited the Hayden Planetarium in New York City. It was a joy to see fellow astronomers present a live tour of the night sky in a state-of-the-art facility. The best moment of any planetarium show always comes midway through, once everyone has adjusted to the low light and is starting to get acquainted with the constellations twinkling above. The narrator pauses. The lights fade further still. As the dome transforms to inky black and countless new stars obscure once-familiar constellations, the audience audibly gasps.

A show at the Hayden Planetarium. Image: amnh.org.

When I ask students in my introductory astronomy class if they have seen the Milky Way, very few hands pop up. The night sky as our ancestors saw it is a novelty, a luxury; something foreign to gasp at.

That night in Manhattan, the planetarium show was followed by telescope viewing. As I headed outside and looked up to get my bearings, I felt very disoriented: in spite of mostly-clear skies, I could count the number of visible stars on my hands. And two of those were actually planets. I was struck by Neil deGrasse Tyson’s embarrassing admission that, as a child, seeing a dark night sky while camping reminded him of the Hayden Planetarium.

It is one thing to read about pervasive light pollution. Yes, our cities are growing and getting brighter, and it is increasingly impossible to see very many stars at night. Intellectually, I know this, and I know that it is a huge waste of money and energy; a large-scale, slow-growing problem with no easy fix. But experiencing it is something different altogether. It is deeply distressing. I fear we are literally blinding ourselves from the Universe’s past and our species’ future when we block our only window to the starry heavens above.

Light pollution in North America as viewed from space. Image: SPL / Barcroft Media

Also this summer, I traveled to northern Vancouver Island in Canada. My room faced northeast, toward the water, with expansive windows and no blinds or curtains. Night was dark. Not “it’s difficult to see, can you shine a light over this way?” dark, but truly, utterly, all-encompassingly DARK. There was no telltale glow on the horizon of some distant city or rogue gas station. There was no streetlight casting orange light in an unwanted direction.

One cloudless night as I lay in bed, I could see stars twinkling out the window, and woke my husband to drag him outside. We reveled in proper, glorious darkness. Like the Hayden Planetarium, there were so many stars that familiar constellations were hard to pick out. The sunrise, early though it was, reset my circadian clock. I reconnected with the night and felt whole.

We have a natural tendency to equate light with good, with safety and security, and with happiness. But we have slowly eased into modern life with an overabundance of this good thing. (Not to mention: studies have demonstrated that more light does not equal more safety.) I do what I can in my personal life to get lots of natural light during the day, use fewer lights at night, and take the irritating blue glow out of my backlit screens.

Of course, while it doesn’t make sense to shine excessively bright lights, it doesn’t make sense to turn them all off, either. Smart solutions exist for dark-sky friendly light fixtures. But I fear it will take a health or energy crisis to make us truly examine how we wash out the night. Until then, we must cherish every sliver of darkness we can find.

When Stars Explode

There’s a new supernova in the skies! Last week, students at the University of London Observatory discovered a strange bright spot in nearby galaxy Messier 82 (M82) during a routine observing training session. As undergraduate student Tom Wright put it, “One minute we’re eating pizza then five minutes later we’ve helped to discover a supernova. I couldn’t believe it.”

Words. Image credit UCL/University of London Observatory.
Two images of galaxy M82. The bottom one shows the location of the new supernova, dubbed SN 2014J. The overall galaxy appears dimmer in the bottom picture because the exposure time was shorter, so less light had time to reach the camera. Image from UCL/University of London Observatory.

What’s the big deal about a supernova? Well, to start with, all the elements in the Universe were formed deep inside stars, and spewed out into space through supernova explosions like this one. Take a moment to let that sink in.

This supernova is a special variety called “Type Ia” (type one-A). This means it is caused by a very dense white dwarf star collecting more mass than it can support and eventually going BOOM! We know this because we see signatures of telltale elements like Silicon in the spectrum of the explosion.

Type Ia supernovae are particularly useful because they are all physically very similar—white dwarf stars can only handle so much mass before they explode—so they are all roughly the same brightness. Astronomers love things that are all the same brightness, because they let us determine distances. How? Let’s pretend you’re staring into a huge, dark, empty room containing nothing but a handful of 100-Watt light bulbs. (Not a bad analogy for an astronomer’s life, really…) You’d like to know how far away the light bulbs are, but you don’t have a measuring tape, plus the room is really big. However, you know how much light each bulb is putting out (100 Watts), so you can figure out the ones that look dimmer are actually farther away. We call the 100-Watt light bulbs of the Universe, such as Type Ia supernovae, “standard candles” because they let us determine distance like this.

If you live in the Northern hemisphere and have access to good binoculars or a telescope, you can try seeing SN 2014J for yourself! It is close to peak brightness, and should be visible for another couple of weeks—the blink of an eye from an astronomical perspective.

Look for galaxy M82 with binoculars or a telescope near the dipper portion of the Big Dipper in Northern hemisphere skies. Image from Universe Today.

Even if you can’t spot the supernova in M82, the galaxy itself and neighboring galaxy M81 are a lovely sight. They’re also a great example of how light can be deceiving. The image below shows two images of these galaxies: one taken with visible light (inverted so the galaxies appear dark on a light background), and one taken with radio light. There is all kinds of gas and material connecting the galaxies together that you can’t see with your eye!

Two views of galaxies M81 (the larger one) and M82 (the smaller one above it). These pictures show the same region of space in two different flavors of light. The galaxies appear as two isolated collections of stars in visible light (left), while the multicolor radio image (right) shows gas connecting the galaxies. Observations like these are used to figure out how galaxies have interacted gravitationally in the past. Image from the SEDS Messier Catalog.

What I find particularly mind-boggling is how a galaxy some 12 million light years distant is “nearby” on a cosmic scale. Because light doesn’t travel instantaneously, we are seeing this supernova as it happened 12 million years ago. In contrast, every star in the night sky is located in our own Milky Way galaxy, which is about 100,000 light years across, so the light from these stars (and the planets orbiting them!) is “only” delayed by hundreds or thousands of years, not millions. If the planets in our Solar System are our next-door neighbors, and stars in our galaxy with their own planets are other cities, then M82 is an entirely different country.

I can’t help but wonder… is some alien civilization in our galaxy witnessing this distant explosion just as we are, at this very moment? Are intelligent creatures on a planet we have recently discovered also turning their telescopes to the heavens to study this supernova and learn more about the Universe we share?

Buckets of Starlight

Have you ever watched rain fall on the pavement? It makes a beautiful, seemingly random pattern. No drop is more or less likely to fall in a spot where another drop has fallen.

Let’s say you wanted to collect as much water as possible during a 30-minute rainfall. What kind of bucket would you make? Tall, short… wide, narrow… orange, green… hot, cold… raised above the ground, in a hole… what would you do?

This is the exact situation astronomers face when designing telescopes. Only instead of rain, we are trying to collect drops of light – called photons. And instead of setting a bucket in a rainstorm, we put a mirror in a dark location and hope for clear skies.

If you took a minute to think about it, you probably settled on using as big of a bucket as possible to collect the most rain. The width of the bucket, or its area on the ground, is more important than its depth, so long as it isn’t going to overflow. And the color, temperature, and height above the ground don’t really matter. This is exactly why astronomers are partial to huge telescopes! The most important part of a telescope is its primary mirror, which is like a bucket for light. Giant mirrors don’t really give us a more “zoomed in” view of the cosmos, but they do let us collect more light – just like a big bucket will collect more rain. Photons of light are always raining down from the cosmos. Bigger mirrors mean more photons, and more photons means fainter stars and galaxies are visible.

Four telescopes, each with an 8.2 m diameter mirror, that together compose the aptly-named Very Large Telescope (VLT). Image credit: Wikipedia via ESO.
The aptly-named Very Large Telescope (VLT) comprises four telescopes, each with an 8.2 m diameter mirror. Image credit: Wikipedia via ESO.

There is another way to get even more rain, or photons, too, of course: leave your bucket out for more than 30 minutes. Maybe even leave it out for hours on end, and carefully set aside all the water – or light – you collect until you can add it all up later. The faintest, most distant galaxies are only visible to those with a big light-bucket and a lot of patience.

The Hubble eXtreme Deep Field. Nearly every object here is a galaxy, containing billions of stars. Hubble spent over 20 days total staring at a tiny patch of sky with its relatively modest 2.4 m diameter mirror to create this image.
The Hubble eXtreme Deep Field. Nearly every object here is a galaxy, and each galaxy contains billions of stars. Hubble spent a total of over 20 days staring at a tiny patch of sky with its relatively modest 2.4 m diameter mirror to create this image.
Image credit: NASA, ESA, et al.

As it turns out, falling rain and incoming photons can both be described by a special set of mathematical rules called Poisson statistics. Each drop hits the ground in a random spot. But the sum total of all of these random events is predictable. So, If you make a graph of raindrops hitting a bucket, or photons hitting a telescope mirror, you get a special shape: a Poisson distribution.

Astronomers use this to know how long we need to stare at a distant star or galaxy, or how big a telescope we need to use. Poisson statistics tells us that the quality of telescope data for a star emitting N photons will be 1/√N. (We like to say the uncertainty, or “standard deviation,” is √N.) This is great news! If you spend some time looking at a star and collect 100 photons, your uncertainty will be 10 photons, which is 10% of 100. That’s not very precise. But if you stare at it longer – or trade up to a bigger telescope – and collect 1000 photons, the uncertainty will be about 32 photons, which is only 3%. Much better.

Image credit: wellhappypeaceful.com
Image credit: wellhappypeaceful.com

The next time you look up at the night sky, remember that your eyes are miniature telescopes. Or perhaps fun-sized light-buckets. How much starlight can you collect?