Please note: The Academy will be closed on Thanksgiving day.
By Dan Brady and Ryan Wyatt
This past Thursday at Nightlife, Dan Brady presented Colors of the Cosmos in the Morrison Planetarium. We thought we’d also share it with our Science Today readers. Click through the links to see the colorful images and enjoy!
The phrase “Colors of the Cosmos” makes for a catchy title, but real scientific value lies behind those alliterative words. Generally, we can divide our exploration of space pantones into two groups: the kind we see directly with our eyes, and the kind we use special instruments to reveal. This article is brimming with visual aids, so be sure to click on all the links below to appreciate the full beauty of this topic.
Just looking at the night sky reveals some subtle variations that are often obscured by San Francisco’s city lights and persistent fog. Orion occasionally shines through these obstacles, revealing some noticable color differences. Contrast the red of Betelgeuse with the blue of Rigel down below. Or on either side of Orion, the white Sirius with Taurus’s bloodshot eye, Aldebaran. These are subjective experiences: visible for all of human history, they’ve inspired myths and curiosity, but in the last few centuries, we’ve been able to focus some questions into answers.
In fact, most of what we know about the Universe comes from studying light from far away destinations. Not all of it is directly visible, however: either because our eyes are not sensitive to it (think radio waves, x-rays, and infrared), or because our atmosphere has shielded us from detecting the subtleties. Color, or the colors we see with our eyes, offer important clues to understanding the Universe, but from now on we may also use some representative colors to help us visualize the stuff beyond our normal perception.
The way we observe star clusters is a great example of how our digital telescopes can objectively quantify light in brand new ways. NGC2547 lies in the constellation Vela (not visible to us in the Northern Hemisphere) and while this photo is full of color, it is actually a composite of several black and white images. Just like the pixels in your phone or TV, the light we see is a combination of red, green, and blue wavelengths. Telescopes don’t work this way: they have to take photos of red, green, and blue light one at a time, with filters covering their digital sensors. These give us raw, black and white images—but the information is separated into the three primary colors of light. Look closely, and you’ll notice significant differences between each of these three filtered images. Then we add the appropriate colors, merge the images, and, voila: a complete, color image of a nearby group of stars.
Astronomers are interested in the individual wavelengths as well as the combined color image: the various stars in NGC2547 can emit very different wavelengths of light from even their closest neighbors. By measuring and comparing the difference in light output in different colors, astronomers can quantify the stars’ colors and even determine the age of a star cluster. (NGC2547 turns out to be between 20 and 35 million years old, BTW.)
At this point, it’s fair to ask yourself, “So what?” Colors sure are pretty, and astronomers can turn colors into numbers, but how important are they? Well, are you in your work place? Look up. What kind of lights do you see? Fluorescent? Incandescent? Halogen? We all know these lights shine with different colors. Neon signs are the clearest example—so, okay, maybe I should have had you imagine Las Vegas. Apologies. We can compare this everyday light sources to more distant examples…
Take a look at these different images from the Orion Nebula (pictured, above right). Light bulbs shine in different colors because there are different elemental gases inside them. And the same thing goes on with these formative stars: the colors (or wavelengths of light) that we don’t see tell us what gases and elements are inside, because their atoms absorb specific wavelengths as they pass through. The red stuff is hydrogen, seventy five percent of our visible universe by mass. The rest are elements that make up you and me: carbon, oxygen, nitrogen, and all the others in the periodic table.
In stars, colors reveal something even more meaningful: temperature. We know that some things burn red or white hot, but more specifically, we know that anything in the Universe of a certain temperature glows most brightly at a specific color (or wavelength of light). Knowing how color relates to temperature allowed us to calculate the ages of stars in NGC2547, but we use this relationship to pin down temperatures of much cooler objects as well. Information like this has been hidden to us until very recently—and one sky survey, called 2MASS, has revealed that there are many more light sources than we imagined hanging out in the infrared spectrum. And stars aren’t the only bright things out there: even the dust in our galaxy glows dimly in the spaces between the stars.
Asking “why is the sky blue?” even fits in here: when the white light of our sun hits our atmosphere, shorter-wavelength (blue) light scatters more than longer-wavelength (red or green) light. This basic childhood mystery turns out to have serious implications when we look at exoplanets orbiting other stars. Just in the past few months, astronomers have announced the colors of some of these distant planets—and they provide clues as to what’s on the surface. We’re not quite at the stage where we can see what’s fashionable in alien clothing trends or whether the leaves on their trees are green or purple, but we’re getting closer with each new generation of telescope.
Okay, the briefest of reviews: colors are everywhere in the cosmos; we quantify them using scientific instruments; different colors mean different temperatures or different energetic molecules emitting light; and a lot of the colors (or wavelengths of light) in the Universe are invisible to us humans.
Now it’s time to look at the rest of the electromagnetic spectrum… The wavelengths of light we cannot see!
Gamma rays are the most energetic waves in physics—carrying around a billion times more energy than visible light—so they usually come from the most powerful things in the Universe. In this map from the FERMI sky survey, the brightest sources lie along the galactic plane, where there’s the most stuff close to our telescopes. But other sources include pulsars and supernovae: collapsed or exploded stars that shoot x- and gamma-rays deep into the night sky. These sources show up as bright spots away from the streak of our Milky Way.
But the biggest questions in cosmology today come from radiation that is all but invisible to most of us, although it permeates every inch of the Universe. And to see it, we zoom way out: past our local group of galaxies, through the brightly colored galaxies of the Sloan Digital Sky Survey, the Two Degree Field Survey, and even past the oldest primordial stars known as quasars. It’s here that we can finally see the oldest image of the entire Universe, the Cosmic Microwave Background (CMB).
In the image of the CMB taken by the Planck space telescope, the colors represent variations in the temperature and density of the early Universe: blue-black corresponds to the coolest, densest parts of the image, and red marks the hottest, least dense regions. The midpoint of this color representation, the bright green, has a temperature associated with it: 2.7 Kelvins above absolute zero, the current average temperature of the cosmos. Amazingly, the difference between the hottest part of the image and the coolest works out to only one part in a hundred thousand! The darkest blue is only one-one-hundred-thousandth cooler than the reddest red.
This remarkable cosmic observation also has some fans on the Internet, most famously Randall Munroe of the webcomic xkcd—you can even buy t-shirts on his website to express how warmly you feel about this discovery. The overall shape of the curve in the xkcd comic tells us the temperature (the aforementioned 2.7 Kelvins), and the tiny variations indicate the temperature above and below that average.
This temperature map is the oldest image of the Universe because it’s the farthest back we can look back in time. Before this time, three hundred and eighty thousand years after the Big Bang, the Universe was so hot and dense that light couldn’t even travel through: this image dates from the precise moment when the Universe was cool enough to let electrons and protons come together to form the first hydrogen atoms.
The Planck CMB is as far back and as far out as we can go, so let’s head back home, diving in through the quasars, distant galaxies, towards our local group and the Milky Way. The colors we’ve explored have ranged from the extremely hot, where energetic gases form new stars or signal the death of the old; to the frigid, blackness of space, where temperatures hover near absolute zero and shelter secrets from the origins of the Universe. We’ve seen the visible colors of nebulae and distant planets, and the false colors of the Milky Way using x-rays, infrared, and microwaves. All these colors are visible only with the collaboration of astronomers all around the world. Their hard work has revealed a Universe that has been hidden until very recently—but here all along.
Dan Brady is a planetarium presenter at the California Academy of Sciences. He earned his BS in Physics from UCLA and has taught science since 2008.
Ryan Wyatt is the director of the Morrison Planetarium and Science Visualization at the California Academy of Sciences.