Ask most people what the Sun is, and the answer comes fast: a giant ball of fire. It is a tidy picture, and it is wrong in a way that turns out to matter.<\/p>\n
The Sun is not burning like a fire, and it is not made of ordinary neutral gas. Most of it is plasma: an ionized gas, the same broad state of matter found in lightning and neon signs.<\/p>\n
The familiar picture, and why it falls short<\/p>\n
Fire is combustion: a fuel reacts rapidly with an oxidizer, usually oxygen, and gives off heat and light. The Sun does not work that way. As NASA\u2019s StarChild<\/a> resource puts it, \u201cThe Sun does not \u201cburn\u201d, like we think of logs in a fire or paper burning\u201d. Its energy does not come from a chemical reaction, and it does not need an external supply of oxygen to keep going.<\/p>\n What the Sun actually does is fuse. The same NASA page continues: \u201cHydrogen really doesn\u2019t burn, it fuses, into helium. So no oxygen is required!\u201d In the Sun\u2019s core, immense temperature and pressure allow a chain of nuclear reactions that ultimately converts hydrogen nuclei into helium, releasing energy in the process.<\/p>\n Every second, the Sun converts around 4.26 million tonnes<\/a> of mass into energy. That is only a small fraction of the much larger amount of hydrogen involved in fusion; most of the original mass remains in the helium produced.<\/p>\n According to the US Department of Energy, the Sun has been fusing hydrogen for about 4.5 billion years<\/a>. Confusingly, physicists do still use the word \u201cburning\u201d in this context. A self-sustaining fusion reaction is called a burning plasma, and that same Department of Energy page describes the Sun that way. But it is burning in the technical sense of sustaining fusion, not combusting. There is no flame and no chemical fire.<\/p>\n What plasma actually is<\/p>\n So if the Sun is not a burning ball of ordinary gas, what is it? Plasma is often called the fourth state of matter, alongside solids, liquids and gases. Give a gas enough energy and collisions can knock electrons away from its atoms, creating a mixture of free electrons, positively charged ions and, in many cases, some neutral atoms as well.<\/p>\n Plasma earns its own category because those charged particles make it behave very differently from an ordinary neutral gas. It conducts electricity and responds strongly to electric and magnetic fields.<\/p>\n The Sun\u2019s plasma in practice<\/p>\n Inside the Sun, this plays out across an extraordinary range of temperatures. The core, where fusion happens, reaches about 15 million degrees Celsius<\/a>. The visible surface we see from Earth, called the photosphere, is far cooler at roughly 5,500 degrees Celsius<\/a>.<\/p>\n Stranger still, the thin outer atmosphere, the corona, climbs back up to around 2 million degrees Celsius<\/a>, making it far hotter than the surface below it.\u00a0<\/p>\n None of this behaves like a flame. The Sun behaves like a vast, magnetically active fluid made largely of charged particles, which is exactly what it is.<\/p>\n And plasma is not some rare arrangement. The DOE Office of Science says it makes up more than 99 percent of the visible universe<\/a>. It is found in stars, nebulae and the diffuse material between stars and galaxies. Ordinary neutral gas is familiar on Earth, but across the luminous universe, plasma is the dominant state of matter.<\/p>\n Lightning, neon, and the plasma hiding in plain sight<\/p>\n The same broad state of matter turns up much closer to home, and this is where the Sun stops feeling quite so exotic. The MIT center puts it well: \u201cThat branch of lightning that cracks the sky is plasma, so are the neon signs along our city streets. And so is our sun, the star that makes life on earth possible.\u201d<\/a><\/p>\n In a neon sign, an electrical discharge ionizes part of the gas and accelerates electrons through the tube. Collisions excite neon atoms, and those atoms emit their characteristic light as their electrons return to lower energy states.<\/p>\n