There are <Carl Sagan voice>billions and billions</Carl Sagan voice> of stars in the universe. So why can’t we see the Milky Way shimmer the way it does in photographs?
Part of the reason why our eyes are so bad at seeing a full, glorious universe is the way light travels from a star to the Earth. Every star, including the Sun, constantly emits electromagnetic radiation. Some of that radiation is in the form of visible light, but that doesn’t mean it will still be visible when it gets to Earth. Light spreads out over distance, meaning that only a fraction of the light will actually arrive at our planet.
There’s also the matter of color. Stars emit light in subtly different colors depending on their temperature — cooler stars emit longer wavelength, redder light while hotter stars emit shorter wavelength, bluer or whiter light. Our eyes can generally see the hotter stars better than the cooler ones. And there’s also the simple fact that some stars are bigger than others and send out more light.
But the other part of the reason we can’t see gorgeous skies at night is because of the way our eyes work. Human eyes are made up of specialized cells called rods and cones that detect the intensity and color of visible light photons. When a photons enters your eye, the rods and cones convert that energy into a nerve signal that registers an image in your brain. By the time a star’s photons reach you, they are hardly representing an image at all.
Our eyes just aren’t sensitive enough to see the far off light that has lost a lot of intensity traveling millions of light-years to get to Earth. But cameras are.
Through long exposures and wide open f-stops, cameras can receive far more light in one shot than our eyes can see in one good look. A long exposure photograph can reveal stars we would never be able to see with the naked eye. The same applies when we look at a distant galaxy through a telescope — the telescope is much bigger, more sensitive, and can collect much more light before processing an image.
And things get more interesting when we bring together multiple types of light.
Most of the electromagnetic spectrum is invisible to us. On the long wavelength end, we can’t see infrared, radio waves, or microwaves. On the shorter end of the spectrum, we can’t see X-rays, gamma rays, or ultraviolet light. But once again, instruments can. This is what the galaxy below, designated NGC 1512, looks different in different wavelengths. The composite that pulls together ultraviolet, visible, and infrared image data in the middle.
Astronomers use specially designed instruments to see non-visible-light wavelengths to better understand the universe around us. And since most of these high and low frequency waves can’t get through the atmosphere to the Earth’s surface, many of these instruments are on high mountaintop observatories or on orbiting telescopes and spacecraft. Dim stars can emit infrared heat, something that only infrared cameras can see. High energy events like supernovas give off powerful gamma rays that only specially designed telescopes like NASA’s Fermi telescope can detect.
Astronomers can observe an object, like a distant galaxy, in all wavelengths from infrared to gamma to see different things. But they can also combine these pictures to create amazing composite images that show us the Universe in a way we could never see it with the naked eye.
I was disappointed when I learned that the astounding images that we get back from Hubble and other telescopes weren’t something I could ever see with just my own two eyes. But it’s easy to get excited again when I remember that science lets us see things that would otherwise be invisible to us, and they are beautiful. To me, finding hidden beauty in the sky is almost better than having a few more rods and cones.