What is the Sun?

So, now that we know that heliophysics plays such an important role in our everyday lives, we are brought to a simple, but fundamental question that we need to understand if we’re going to truly understand the Sun-Earth connection: What is the Sun?

From Encyclopædia Britannica, sun” is defined as “star around which Earth and the other components of the solar system revolve“. Okay, sounds right, but then what exactly is a star? Again from Encyclopædia Britannica, star is “any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources“. So, putting those two definitions together we could say that the Sun is the massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources around which Earth and the other components of the solar system revolve.

Simple, right? Let’s break that down a bit, starting with the simplest part, the end. The Sun is the center of our solar system and all of the planets, including the Earth, revolve or orbit around it. Of course, we didn’t always know or believe that, but that’s a story for a different post. Heading back to the beginning of the definition, we see the word “massive“. Massive here is used in the most literal meaning of the word. As mentioned in the previous post, the Sun makes up more than 98% of the total mass in the solar system. It’s 333,000 times more massive than the Earth, meaning it is made up of that much more matter. In literal dimensions, you could line up more than 100 Earths across the equator of the Sun and fit over 1 million Earths inside the Sun. That’s pretty huge when we think on a human or even a global scale, but keep in mind that the Sun is just a mid-size star!

The Earth shown next to the limb of the Sun and an erupting solar flare. The sunspots frequently visible on the surface of the Sun are usually about the size of a few Earths. Credit: NASA

So, next we’ll focus on the “body of gas” part of the definition. So, in elementary school you probably learned that there are three forms of matter: solid, liquid, and gas. Well, that’s not entirely true (“gasp“), there’s actually a fourth state of matter known as plasma. Really, it can be considered a special type of gas, an ionized gas, but since that ionization causes it to act in ways that are very different than a regular gas, it should be (and usually is) considered its own state of matter. Unlike regular gases, plasmas are electrically conductive and respond to electromagnetic forces. You’re probably extremely familiar with plasmas and their applications, without even realizing it: lightning, electric sparks, fluorescent and neon lights, and some TVs. So the Sun is actually a body of ionized gas (or plasma). Finally, the most important parts of the definition that truly define a star are “self-luminous” and “shines by radiation derived from its internal energy sources“. These statements refer to the fact that the Sun and other stars actually produce their own light (or radiation), unlike planets and the Moon which simply reflect light from the Sun. So what are these internal energy sources that are mentioned? Well, the inner cores of stars are the only natural environments (that we know of) where it’s hot enough (27,0000,000° F) and high enough density (150 g/cm³, roughly 10 times denser than lead) for nuclear fusion to take place. Nuclear fusion is the combination of smaller atoms of elements to make larger ones. In the cores of stars like our Sun, the lightest element, hydrogen (H), is converted into the second lightest element, helium (He), in a three-step process known as the proton-proton or “pp” chain” (cue giggling).

This image shows the steps of the proton-proton (pp) chain, the nuclear fusion reaction that converts hydrogen (H) into helium (He) in the core of the Sun. In the first step two protons collide to produce deuterium, a positron, and a neutrino. In the second step a proton collides with the deuterium to produce a helium-3 nucleus and a gamma ray. In the third step two helium-3s collide to produce a normal helium-4 nucleus with the release of two protons.

This image shows the steps of the proton-proton (pp) chain, the nuclear fusion reaction that converts hydrogen (H) into helium (He) in the core of the Sun. In the first step two protons collide to produce deuterium, a positron, and a neutrino. In the second step a proton collides with the deuterium to produce a helium-3 nucleus and a gamma ray. In the third step two helium-3s collide to produce a normal helium-4 nucleus with the release of two protons. Credit: NASA

In this reaction, four hydrogen atoms are converted into a single helium atom. That sounds all well and good, but it doesn’t quite explain why that conversion results in the massive energy production that we all know occurs in the Sun. The secret to that comes from what is probably the most famous equation in all of science: the mass-energy equivalence equation (E=mc²).

The mass-energy equivalence equation, derived by Albert Einstein in 1905, explains the energy release of the nuclear fusion reactions that power stars. Credit: Pax on both houses

You’ve probably seen that equation once or twice, and you probably know that it was introduced by arguably the most famous scientist ever Albert Einstein, and you probably incorrectly think that that’s why Einstein won the 1921 Nobel Prize in Physics (he actually won it for a different discovery he made in 1905, something known as the photoelectric effect). Anyway, this simple equation is the reason why the Sun is the most powerful energy source known to man. Remember those four little hydrogen atoms? The combined mass of those four hydrogen atoms (4 x 1.6605402 x1027 kg = 6.64187×1027 kg) is actually less than the mass of the one new helium atom ( 6.64648×1027 kg). Note that the difference is miniscule, a mere 4.61×10-30 kg, but looking at Einstein’s equation, that tiny mass is multiplied by a really large number, the speed of light squared (8.98755179×1016 m2/s2). That means that the total energy released by each pp chain is 4.14326137×10-13 kg·m2/s2 (or Joules). That’s 1×10-19 kW·h, the basic unit by which electric companies charge for usage. So each reaction is actually liberating only a tiny bit of energy, but when you multiply that by the enormous amount of mass in the Sun and mind-boggling number of reactions that are occurring in the core, you can see the energy add up.

The “random walk” that a photon of light takes to get from the core to the surface of a star can take tens or even hundreds of thousands of years! Credit: askamathematician.com

But the really crazy fact is that even once the light is emitted from the core in the form of a photon, it takes tens or even hundreds of thousands of years (estimates range from 17,000 to 1 million) for the photon to actually reach the surface of the Sun and escape out into space. That’s because the photon can be randomly emitted in any given direction, not necessarily towards the surface. Then, because the Sun is so dense, it doesn’t travel very far before the photon bumps into another atom and is absorbed. Then that photon is emitted again in a random direction. This all adds up to a really, really long “random walk” through the Sun to the surface.

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