The Sun




Never we know but in sleet and in snow,

The place where the great fires are,

That the midst of the earth is a raging mirth

And the heart of the earth a star.

— G. K. Chesterton, “A Child of the Snows”

Well, it is pretty hot in the Earth’s core, but there is a real star in our skies every day.

Mankind’s names for it vary across the inhabited lands of Earth.

In Mandarin, it’s called Tàiyáng, and in Japanese Taiyō. In Arabic, they call this star Al-Shams, while in Swahili it is Jua.

In the West, ancient Romans called it Sol. The Germanic tribes who fought them called it a name that has come down to us in English as the Sun.

We take the Sun – a G-type main-sequence star – for granted.

What if it disappeared?

Yeah, bad news for you and me.

That video explores a little bit of the Sun-Earth connection, but what exactly is the Sun? What’s going to happen to it, and us, in the future?

The Sun is a star

The Sun’s life-giving energy comes to us from some 80,000 km below the visible surface of this big ball of fiery gas and plasma. There, in the solar core, nuclear fusion transforms over 600 million tonnes of hydrogen into helium every second.

The thermal energy released by this, the equivalent of an explosion of 9.192×1010 megatons of TNT per second, then radiates up through the Sun’s layered interior until it is released into space as sunlight. Some of it also turns into kinetic energy (PDF) that blasts tiny particles – the solar wind – out into space.

Besides size and energy production, the most noticeable difference between the Sun and Earth is texture. Earth is stony; the Sun is a gas. This means there’s no definite outer boundary on the Sun. Instead, it has a photosphere.

We might not recognize that word, but we’ve all seen what it describes.

Hi, I'm the photosphere.  Please excuse the spots - I can't help it.  Source
Hi, I’m the photosphere. Please excuse the spots – I can’t help it. Source

The photosphere is the Sun’s visible surface…the first point, since energy first left the core, where there aren’t enough negatively charged hydrogen ions to absorb visible light. That’s right: Everything in this star below its photosphere is invisible to human eyes!

This photosphere is about 250 miles thick and ranges in temperature from 7600 to 13,240 degrees Fahrenheit. It forms the lowest layer of the solar atmosphere and is what scientists study with terrestrial and space-based telescopes that can pick up everything from radio waves to gamma rays.

The next layers of the atmosphere – the chromosphere and corona – are what you see when the photosphere is in shadow during a total eclipse of the Sun.

You can also install a coronagraph on your telescope.
Or you can install a coronagraph on your telescope.

The top of the Sun’s corona blows off into space as the solar wind. This interacts with everything that orbits the Sun and expands outward in a “space bubble” called the heliosphere.

By the way, contrary to what you might think, the Sun is not the biggest cohesive thing in the Solar System – the heliosphere is.

The termination shock is where solar wind particles drop below the speed of sound.  Both Voyager crafts have passed that point.  The heliosphere is beyond that, and apparently isn't precisely what scientists thought it was, but Voyager 1 has passed it...probably.  Image by NASA
That blue-gray fuzzy oval is the heliosphere. The perfectly round blue layer called the termination shock is where the solar wind drops below the speed of sound. Both Voyager crafts have passed that point. The heliopause is further out. Apparently it isn’t precisely what scientists thought it was, but Voyager 1 probably passed it in 2013. (By the way, the solar apex is, oddly enough, what they call the direction the Sun is traveling in through the galaxy.) Image by NASA

Earth and the Sun

As we saw above, life on Earth’s surface depends on the Sun either for photosynthesis or life-supporting heat. However, there is deadly radiation in the solar wind.

Why isn’t Earth therefore as barren as sun-blasted Mercury or Mars? For that matter, why isn’t there photosynthesizing, heat-loving life on Mars?

A major part of the answer involves magnetism.

Movement of plasma inside the Sun gives it a strong and dynamic magnetic field that’s responsible for such things as sunspots and solar flares. Part of this field extends outwards through the Solar System as the interplanetary magnetic field.

Movements of molten rock in our planet’s core turn Earth into a magnet, too. Of course, this is very small in scale compared to the Sun. However, Earth’s magnetic field is strong enough to protect us from the worst effects of those solar particles that are whizzing around our neighborhood in response to the interplanetary magnetic field.

Dr. Nikolai Tsyganenko, USRA/NASA/GSFC
Yay, Earth! Dr. Nikolai Tsyganenko, USRA/NASA/GSFC

Mercury has a magnetic field, too, but it’s leaky. The solar wind is actually funneled right down onto the planet’s surface by magnetic “tornadoes” (technically, they aren’t tornadoes at all but magnetic vortices called flux transfer events). Earth has such events, too, but the thickness of our atmosphere protects us.

Mars missions and examination of Martian meteorites have shown that the Red Planet, early in its history, used to have both a thick atmosphere and a magnetic field that was about 1/10 as strong as Earth’s. Probably not coincidentally it also had liquid water flowing on its surface.

Today, what little magnetism remains on Mars is less than 1/3,000 that on Earth. It’s pretty spotty, too, but may still be enough to hold the planet’s modern atmosphere in place. That atmosphere, however, is too thin to keep the planet warm or to prevent the intense solar wind from blasting the bleak Martian surface.

The Sun keeps us Earthlings alive and healthy, but it could also kill us if not for our world’s very effective shielding.

So remember to say “Thanks, ma!” the next time you’re out there watching pretty lights in the sky.

The future of the Sun

In some 4-5 billion years, that incredible nuclear furnace in the Sun’s core is going to run out of hydrogen. Then what?

The Sun won’t immediately go dark, of course, and it doesn’t have enough mass to explode as a nova or supernova. Here’s what Wikipedia says will happen:

  • The core’s nuclear fusion stops
  • The core starts to contract
  • This contraction brings in enough hydrogen to get fusion going in a shell around the core. The helium made this way falls into the core
  • The Sun’s outer layers expand and it becomes a red giant (which is really orange). About 6.5 billion years from now, it will be over two hundred times as big as it is now
  • Because the Sun loses mass during this process, Earth may or may not survive this phase
  • By 7.5 billion years from now, the giant Sun has lost about a third of its mass
  • Helium that has piled up in the core suddenly ignites
  • The Sun shrinks from hundreds of times its current size to just 10 times that size as it continues to burn helium
  • After about a million years, the helium is all gone. The Sun repeats the whole process of expansion, but faster
  • After some 20 million more years, the Sun gets unstable, with increasingly powerful thermal pulses
  • Old Sol finally sheds its outer layers and then turns into a long-lived white dwarf

Of course people have made YouTube videos of this! For example, here’s one that endorses the idea that Earth will be engulfed during the red giant phase.

Who knows if or what kind of descendants of ours will be around to see at least the start of this process…oh wait, it’s already begun.

Life on Earth only has about a billion more years to go, according to the SciShow guy:

Fortunately, on the human time scale, there’s still plenty of time to bask in the Sun’s warmth on summer vacation, or to study it and try to figure out ways to harness its energy.

Our life-giving Sun is about midway through its life as a “normal,” i.e., main-sequence star. Researchers are studying it closely, but there are still plenty of things still to learn.

Update, June 7, 2020: Much has been learned about the sun in the six years since I did this post — its appearance up close, for instance.


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