Feb 114 min read

Space is Colorful!

This article was written by azastroguy - Mark Johnston.

Is our Sun really yellow? Are most of the stars at night really white? What about those pretty pictures I see on the internet of nebulas and galaxies – are those colors real?

The Sun, photographed by the author. Color added in post processing.

Our Sun is classified as a yellow dwarf star, yet if you glance at it from the International Space Station, it’s white. It appears yellow because the shorter wavelengths (blue and violet) get scattered by the atmosphere. In fact, next time you’re in a plane with a window seat at altitude, take a quick glance at the sun, it’s noticeably whiter than it appears from sea level.

What about the stars, are they mostly white? Glancing at the stars on a clear dark night reveals a few brighter red/orange or blue stars, but the vast majority are white. Is that the real picture?

No. Read on to find out why!

Our eyes have two types of light detecting cells, rods and cones. The cones detect color but require more energy to ‘turn on’ than the rods do (the rods detect black and white). That’s why when you go to bed at night, suddenly everything in the room appears to be shades of grey, when in fact you know you have a blue bedspread, red curtains, and so on. So at night, the brightest stars like Betelgeuse (red), Arcturus (orange) or Rigel (blue) are seen in their natural color.

But fainter stars don’t provide enough light to turn on our cone receptors. When a photographer shoots the stars, the camera gets enough light to see the true color of most of the stars. Many are white, but many others are red, orange, yellow, or blue.

Kembles Cascade in the constellation Camelopardalis. Photo by the author

What about those beautiful astronomy pictures you see on the internet? Are those colors real? The answer is…sometimes. Let’s explore why.

There are two different ways astrophotographers create color images. The first is to either use a color camera, or use RGB filters with a mono camera and combine them in post processing, which is essentially the same thing. Those images are generally true to life, however it’s common for astrophotographers to play with contrast, hue, and saturation to make the object more dramatic. So the real object might look like the photo, or it might look like a paler version of the photo.

Objects emit light in one or more wavelengths. Typical broadband emission sources are stars, planets (reflected starlight), galaxies and globular clusters. With these types of objects a standard color camera works well.

True color image of the Lagoon and Trifid nebulas. Photo by the author

When you get to nebulas, it’s another story entirely. It gets tricky because there are different nebula types. For example, some of them are reflection nebula, and these are emitting reflected starlight off dust. In this case it concentrates much of light at the blue end of the spectrum, but it remains a broadband emitter.

Examples of reflection nebula include the Iris nebula, the Pleiades and the Witches Head nebula.

The Iris Reflection Nebula, taken by the author in true color

Narrowband Imaging and the Hubble Palette

Other objects emit most of their light in one or more very specific narrow wavelengths, like Hydrogen Alpha or Oxygen III.

Astronomers can use a monochrome camera, and use a special filter that notches away all wavelengths except that specific one. In that way they can capture a black and white image of one type of gas in a nebula.

A popular way to do this is with the so-called “Hubble Palette”. In this case 3 narrowband filters are used consecutively on the same object. Hydrogen Alpha (656nm), Oxygen III (501nm), and Sulphur II (672nm). You end up with 3 black and white images. Then in software you assign a color to each wavelength and combine them into a full color image.

The Hubble Palette assigns Red to S2, Green to Ha, and Blue to O3. So by looking at the image done this way you can now get a sense of which wavelengths are most strongly emitted by the object. You sometimes see this designated as “SHO” for Sulphur2/Hydrogen alpha/Oxygen3.

One advantage of narrowband imaging is it allows you to do astrophotography in light polluted places, because you notch out all the undesired wavelengths.

SHO images are ‘false-color’ but can be beautiful. Consider the Rosette nebula. Below are two photos. One in broadband RGB, approximating its true appearance, and the other in SHO.

Rosette nebula, taken by the author with a color camera

Rosette nebula by the author in the Hubble Palette, using data from Telescope.live

Which is a nicer photo? The first one is truer to natural color, but the second one is beautiful and provide information about which types of gas make up the nebula.

So what have we learned? RGB photos are closest to reality, but even then the red you typically see on a monitor is 630nm (deep red) vs the true red of the HA nebula at 656nm. Narrowband images can be strikingly beautiful and convey information you simply don’t get in regular color images.