How Does a Star Work?

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Explore the Inner Workings of Stars

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The Sun is a star, like the trillion or so other stars in the Milky Way Galaxy. Here, it is seen through an ultraviolet-sensitive telescope onboard the SOHO spacecraft, which studies the Sun exclusively. The curved "handle" on the upper right is a prominence (an outburst of hot gas) rising up from the solar "surface". NASA/European Space Agency

 The idea that the Sun is a star (and indeed, is our closest one!) is not a new one. It goes way back to a time when the Greeks were seeking to understand and explain things in nature. The first person credited with the idea that the Sun is a star (and that other stars might also be like the Sun), was the philosopher Anaxagoras in the 5th century BCE. The idea kicked around for centuries, but it wasn't until astronomers of the Renaissance and thereafter began to see the Sun for what it is. The scientific "proof" of the Sun's "starhood" really didn't come until the 1700s, when astronomers could look at sunlight through instruments called prisms (and later, spectroscopes) and use the characteristics of the light to determine the distance between Earth and the Sun. 

Figuring out how the Sun worked took quite a bit more work, but eventually, the modern view of the sun (and all stars) was developed. It's based on Albert Einstein's ideas about the equivalency of mass and energy (expressed in the famous E = mcequation), plus the work of other famous astronomers. Sir Arthur Eddington (who suggested that the core of the Sun was incredibly hot and under very high pressures). 

Later on, Dr. Cecelia Payne-Gaposchkin, discovered that the Sun was made mostly of hydrogen), and ideas about how atoms could fuse together in the centers of stars suggested by Hans Bethe, Subramanyan Chandrasekhar (and others), eventually coalesced into a pretty solid theory (backed by continuous observations) about the central "engine" of the Sun. We can use what these astronomers developed to understand not just the Sun, but all stars. So, in a sense, the Sun stands in as a "template" for the inner workings of every star you see in the sky.

The process the Sun now undergoes to create heat and light is called nuclear fusion, and this makes each star its own nuclear reactor. It occurs in the solar core, where temperatures are close to 16 million Kelvin and the cores are filled with a dense "soup" of hydrogen atoms. The pressure in this region results in a core density 150 times denser than water. Under the influence of these conditions, atoms of hydrogen are in very close quarters with each other, they move very fast, and constantly smack into each other at 9.7 x 1037 times per second!

Each "smack" takes two hydrogen atoms and fuses them together to make one helium atom. That reaction emits light and heat. Essentially, the process of fusion exchanges some of the mass of the atoms to create energy (in the form of heat and light). Multiply it by all the atoms fused together each second, and you get an incredible amount of heat and light. 

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Lighting up the Cosmos: One Star at a Time

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A cutaway of the interior of the Sun. Most stars have similar types of zones, including the cores where nuclear fusion takes place. NASA/MSFC

The heat and light generated by the nuclear fusion at a star's core then travel out through the rest of the solar structure: the radiative zone above the core, to the convective zone, and then out through the photosphere (the visible surface of the Sun). From there, heat and light travel out through space to warm and light up the planets.

The process of fusion is one that all stars undergo. It's how they ALL give off light and heat, and as far as astronomers have been able to determine, this process has occurred in all stars, beginning with the first stars created some 13.7 billion years ago.

Imagine you're a newborn star in your stellar creche. At first, you don't have enough mass to "turn on" fusion, but you do glow from the heat produced as all the hydrogen contracts under the force of gravity. Eventually, if you get enough hydrogen, conditions at the core will be compressed and hot enough to ignite fusion. The more mass you have, the faster your fusion process will occur and the amount of fuel you have also determines the length of time you exist.

Stars fuse hydrogen in their cores through much of their lives. When they've fused all the hydrogen into helium, they begin fusing helium atoms to form carbon. Then, they fuse carbon, and so on, until — in the largest, most massive stars — they try to fuse iron. At that point, the process stops because it takes MORE energy and pressure to fuse iron than the star has.

Massive stars reach this state very quickly. The most massive ones fuse all their fuel in perhaps just a few million years. Others may last longer — a few tens of millions of years. But, in the end, the most massive ones eventually reach the end of their fuel. Their cores collapse and their atmospheres bounce off the cores and rebound out to space. What's left in the cores is a collection of highly compressed core material called "degenerate matter".  

Astronomers call these massive stardeaths "supernovae" (the event that formed the Crab Nebula) and the really massive ones (such as the star about to explode in Eta Carinae) as hypernovae.

The Sun won't end in quite the same way. It's a middling-mass dwarf, which means that its fusion process doesn't occur at the fast rate we see in much more massive stars. The Sun's fusion process "turned on" some 4.5 billion years ago, and will continue until it slowly dies, turns into a planetary nebula, and then ends as a white dwarf, tens of billions of years from now. 

So, to summarize what we know about how stars work: the process of nuclear fusion is what causes stars to shine and emit radiation. When you experience the Sun's light and heat, or look at dark sky filled with stars, you're watching the process of fusion occurring. The light you see is generated within each star, and it's a process that has gone on almost as long as the universe has existed.