The Sun. It’s right there in the middle of the Solar System. We’ve got spacecraft monitoring it 24 hours a day. You’d think we know just about everything there is to know about it, but the Sun harbors a great scientific mystery.

Before we go into that, let’s define some key terms.

• The photosphere: the surface layer of the Sun.
• The convective zone: the layer directly beneath the photosphere.
• The corona: the Sun’s “atmosphere,” located above the photosphere.

As heat radiates out from the Sun’s core, the temperature gradually decreases. In the convective zone, it’s several million degrees Celsius. In the photosphere, the temperature drops to merely a few thousand degrees Celsius.

That makes sense. As we get farther away from the source of all that heat, we’d expect the temperature to cool off a bit. So the corona should be cooler still, right?

Wrong. The temperature rapidly increases from a few thousand degrees to tens of thousands of degrees, and then peaks back in the range of several million degrees Celsius.

Why? I don’t know why. Nobody knows why, though it may have something to do with the Sun’s magnetic field. More recent research suggests “nanoflares” might have something to do with it.

But the truth is we do not know the reason for this temperature discrepancy. It is (at least for now) a scientific mystery.

Sources

The Sun’s Corona from NASA’s Imagine the Universe.

Explain the Coronal Heating Mystery from the Stanford Solar Center.

Layers of the Sun from The Sun Today.

Sun’s Magnetic Secret Revealed from Space.com.

Tiny “Nanoflares” May Solve Sun Mystery from Space.com.

As much as I’d like to give you a straightforward answer to this question, I can’t. I can’t just say the Sun is yellow or white or purple. To really understand the color of the Sun, we need to talk about absorption and emission spectra.

Light from the Photosphere

Atoms absorb light. To be more accurate, different kinds of atoms absorb specific wavelengths of light, letting all the other wavelengths continue on their merry way. The result is what we call an absorption spectrum.

The light we see coming from the Sun’s surface (a.k.a. the photosphere) has an absorption spectrum. The overall appearance is white or, according to some experts, a slightly greenish white.

But there’s more to the Sun’s color than just the photosphere.

Light from the Chromosphere

Sometimes, when an atom receives energy, it emits light. To be more accurate, different kinds of atoms emit light in specific wavelengths. We call this an emission spectrum.

The glowing plasma that surrounds the Sun is called the chromosphere. The chromosphere appears to be a red or pink color, due mainly to light emitted by hydrogen atoms.

The light of the chromosphere is not nearly as intense as the light from the photosphere, but that doesn’t mean the chromosphere doesn’t contribute to the Sun’s overall color—whatever that color is.

There’s one more factor that influences the Sun’s color, at least as far as our Earthly perception is concerned.

Earth’s Atmosphere

Earth’s atmosphere tends to scatter blue light (which is why the sky is blue). So we see the Sun as a yellow-orange color because most of the blue light has scattered away, while reds, oranges, and yellows follow a more or less direct path straight from the Sun to your eye.

The Color of the Sun

So what color is the Sun? It’s every color, or rather it’s every color except a few that were absorbed in the photosphere, plus a touch of reddish pink from the chromosphere, minus a whole lot of blue if you’re observing the Sun from Earth.

Sources

Lecture 9: The Sun’s Photosphere and Chromosphere from Dmitri Pogosian at the University of Alberta.

Spectra and What Scientists Can Learn from Them from NASA’s “Imagine the Universe.”

Photosphere from Wikipedia

Chromosphere from Wikipedia

What Color is the Sun? from Fraser Cain, publisher of Universe Today (YouTube video).

What Color is the Sun? from the Stanford Solar Center.

On Friday, we talked about the Carrington Event. A massive solar storm triggered a geomagnetic storm that wrecked the technological infrastructure of the time. Fortunately, this happened in 1859, and the infrastructure that was wrecked were telegraph wires and related equipment.

But what if something like the Carrington Event happened today? Several mini-Carringtons have caused blackouts, the most notable happening in Quebec in 1989, and in 2012 a frighteningly large solar storm just barely missed us. Even minor solar flare activity can wreak havoc on our modern technology.

The good news is that the National Oceanic and Atomospheric Administration (N.O.A.A.) has a Space Weather Prediction Center, located in Boulder, Colorado. Their job: to stare directly at the Sun (using satellites, space probes, and special telescopes). With sufficient warning from the N.O.A.A., we can protect our technologically advanced civilization from the Sun’s temper tantrums.

Thanks to the N.O.A.A., the power stays on, airplanes can navigate safely, and our satellites don’t get fried. In fact, the N.O.A.A.’s Space Weather Prediction Center is so good at what it does that most of us don’t even notice when a solar storm hits, unless it’s to check out the auroras.

P.S.: I said we can protect our technology, and we can; but we haven’t seen anything like the Carrington Event hit Earth since 1859. No one’s entirely sure how well prepared we are for something like that.

Sources

Education and Outreach Videos from the Space Weather Prediction Center.

S.W.P.C. Frequently Asked Questions from the Space Weather Prediction Center.

The Sun’s Wrath: Worst Solar Storms in History from Space.com.

Near Miss: The Solar Superstorm of July 2012 from NASA Science.