What is the Sun Made Of? What is the Sun Made Of? By Tim Sharp, Reference Editor | August 17, 2012 05:33pm ET MORE The sun is a big ball of hot gases. The gases are converted into energy in the sun's core. The energy moves outward through the interior layers, into the sun's atmosphere, and is released into the solar system as heat and light. NASA's Solar Dynamics Observatory saw sunspot AR 1520 before the solar flare erupted from it on July 12, 2012. Credit: NASA/SDO (via Twitter @NASA_SDO) Most of the gas — about 72 percent — is hydrogen. Nuclear fusion converts hydrogen into other elements. The sun is also composed of about 26 percent helium and trace amounts of other elements — oxygen, carbon, neon, nitrogen, magnesium, iron and silicon. These elements are created in the sun's core, which makes up 25 percent of the sun. Gravitational forces create tremendous pressure and temperatures in the core. The temperature of the sun in this layer is about 27 million degrees F (15 million degrees C).

Hydrogen atoms are compressed and fuse together, creating helium and a lot of energy. This process is called nuclear fusion. The energy, mostly in the form of gamma-ray photons and neutrinos, is carried into the radiative zone. Photons can bounce around in this zone for about a million years before passing through the interface layer, or tachocline. Scientists think the sun's magnetic field is generated by a magnetic dynamo in this layer. The convection zone is the outermost layer of the sun's interior. It extends from about 125,000 miles (200,000 km) deep up to the visible surface or the sun's atmosphere . Temperatures cool in this zone, enough for heavier ions — such as carbon, nitrogen, oxygen, calcium and iron — to hold onto their electrons. This makes the material more opaque and traps heat, causing the plasma to boil or "convect." The convective motions carry heat quite rapidly to the surface, which is the bottom layer of the sun's atmosphere, or photosphere. This is the layer where the energy is released as sunlight.

The light passes through the outer layers of the sun's atmosphere — the chromosphere and the corona — before reaching Earth eight minutes later. Abundance of elements Astronomers who have studied the composition of the sun have catalogued 67 chemical elements in the sun. There may be more, but in amounts too small for instruments to detect. Here is a table of the 10 most common elements in the sun: Element What is the Sun made of? | Cool Cosmos What is the Sun made of? The Sun is a huge, glowing sphere of hot gas. Most of this gas is hydrogen (about 70%) and helium (about 28%). Carbon, nitrogen and oxygen make up 1.5% and the other 0.5% is made up of small amounts of many other elements such as neon, iron, silicon, magnesium and sulfur. The Sun shines because it is turning hydrogen into helium via the process of nuclear fusion in its extremely hot core. This means that as time goes on, the Sun has less hydrogen and more helium. For more about nuclear fusion and how the Sun shines, see Spitzer's IRrelevant Astronomy video Fission vs. Fusion .

Continue the conversation on All about the Sun All About the Sun There's an old joke that goes like this: An International gathering of world leaders in science met in Geneva to discuss their latest findings. Each nation took a turn sharing its latest developments. The leader of one nation stood before the group and declared that they had devised a spacecraft which would allow their astronauts to fly directly to the sun. He was met with boisterous laughter from the audience, to which he boldly replied, "I know what you're thinking, but we have a plan. We're going to fly the ship at night!" Just about everybody knows that you can't fly directly to the sun, for several reasons, not the least of which is that you can't even land there because there is no there to land on. The sun is a star, which is an enormous ball of burning gases. If you remember from your science lessons in school about the three states of matter (solid, liquid, gas) you remember that gases are not solids. In order to land and stand on any celestial body, it would need to be solid because we are solid.

Oh yeah, for us to fly anywhere near the sun we would have to build a spacecraft that could withstand over 1,000,000 degrees Kelvin AND keep the people inside protected from not only the extreme heat, but the intense light and solar radiation. Human beings were built to withstand the intensity of the sun from the distance of earth, not any closer. Even at this distance, too much exposure to the sun can be dangerous. Besides, we don't really need to fly to the sun to study it, we can make observations from afar (it's much safer). The Source of the Sun's Tremendous Energy You would probably think of the sun as very old, as it is estimated that the "birth" of the sun happened 4.5 billion years ago. Things that grow old usually die, but 4.5 billion years is young for our sun. It is estimated that there is enough fuel in the interior of the sun to keep that lamp burning for about another five billion years, growing twice as bright as it is now. The source of the sun's fuel is hydrogen and helium gases.

Through a special chemical reaction, called nuclear fusion , the hydrogen gas is "burned" releasing an enormous amount of energy in the form of light and heat. Two isotopes of the element Hydrogen (tritium and deuterium) collide with each other under extreme heat in the interior of the sun. The two atoms smash into each other so hard that several things happen: 1. Like cars smashing into each other in a high speed crash, the atoms lose pieces of themselves, atomic particles 2. Unlike anything else we know of in the universe, however, an ENORMOUS amount of energy is released into the surrounding area, on the order of 450 times the amount of energy required for a fusion reaction to initiate (talk about a big return on your investment!) 3. The atomic particles from the Hydrogen atoms that were released during the collision are fused together, forming an entirely new molecule called Helium Of course, there's a LOT more than just two atoms smashing into each other in the center of the sun. There's so many nuclear fusion reactions happening inside the sun at any given time that we don't have a number big enough to count them!

The light and heat energy travels from the core of the sun to its exterior (the photosphere) where we see it from earth (during the day). It takes a million years for the energy from a single nuclear fusion reaction in the center of the sun to reach the surface. Lecture 2: Why Does the Sun Shine? WHY DOES THE SUN SHINE? ``The Sun is a mass Of incandescent gas, - They Might Be Giants, `Why Does the Sun Shine?' Key Concepts The Sun shines because it is hot. The Sun remains hot because it is powered by nuclear fusion in its core. When hydrogen is fused into helium, mass is converted into energy. (1) The Sun shines because it is hot. The Sun is, by far, the nearest star to Earth. The distance from Earth to Sun is 150 million kilometers (km), which equals 1 astronomical unit (AU), which equals 8.3 light minutes (that is, the distance traveled by light in 8.3 minutes).

By contrast, the distance from Earth to Proxima Centauri (the next nearest star) is 40 trillion kilometers, which equals 270,000 astronomical units, which equals 4.2 light years (ly). It's much easier to study the nearby Sun than the much-more-distant other stars. The Sun is made of dense gas. The average density of the Sun is about 1400 kilograms per cubic meter, 40 percent greater than the density of water. The Sun remains gaseous (rather than liquid or solid) at these high densities because of its extraordinarily high temperature, which prevents the ions within the Sun from bonding together. The gas of which the Sun is made is mostly ionized hydrogen and ionized helium, the two simplest chemical elements. The Sun thus contains positively charged hydrogen nuclei, positively charged helium nuclei, and negatively charged electrons. Because photons scatter readily from free electrons, the Sun is opaque (you can't see through the Sun to see the stars behind it). As you know from Astronomy 161, a hot, dense, opaque object (such as the Sun, or the filament of a light bulb) produces a continuous blackbody spectrum of light.

The temperature at the Sun's surface is 5800 Kelvin; the Sun's blackbody spectrum thus peaks at a wavelength of 500 nanometers, in the visible range of the spectrum. [For a review of blackbody radiation, see chapter 5 of the textbook, or the relevant Astronomy 161 web page.] The luminosity of the Sun (that is, the amount of energy it radiates away per second in the form of light) is very large, thanks to the Sun's high temperature and large surface area. The total luminosity of the Sun is 3.9 x 1026 watts. (That is, 390 trillion trillion watts.) (2) The Sun remains hot because it is powered by nuclear fusion in its core. The Sun has been shining away for 4.6 billion years. For well over 3 billion years, according to the fossil record, life has existed on Earth. The fact that life was not scorched or frozen out of existence during that time span indicates that the luminosity of the Sun has been fairly steady. What energy source has kept the Sun hot for billions of years, instead of cooling off like an unplugged iron?

What about chemical sources, such as the burning of hydrogen? (We know there's plenty of hydrogen in the Sun.) Burning one kilogram of hydrogen to form water, by the reaction 2H + O -> H2O + energy, releases 140 million joules of energy. (The `joule' is the standard unit of energy in the metric system. It is the energy required to keep a 1 watt light bulb lit up for 1 second. 140 million joules is equivalent to 33,000 Calories or 40 kilowatt-hours.) If the Sun consisted entirely of hydrogen (which it doesn't), and if you could find a source of oxygen with which to burn it (never mind where all that oxygen would come from), it could keep the Sun at its present luminosity for a mere 20,000 years. A more potent energy source is needed to keep the Sun shining for billions of years. What about nuclear fusion? Four hydrogen nuclei can be fused together into a helium nucleus, accompanied by the release of energy: 4H -> He + energy. Fusing one kilogram of hydrogen into helium releases 630 trillion joules of energy.

That's nearly 5 million times what you'd get by burning the same hydrogen to get water. Thus, fusion of hydrogen will keep the Sun shining at a constant luminosity not for 20 thousand years, but for as much as 100 billion years. (3) When hydrogen is fused into helium, matter is converted into energy. The Sun, like most stars, is powered by the fusion of hydrogen into helium (what the textbook misleadingly calls `hydrogen burning'). You may well ask, ``Where does all that energy come from when hydrogen is fused into helium? Isn't energy conserved?'' There is another question you can ask. The total mass of four hydrogen nuclei is 6.693 x 10-27 kilograms. The mass of a helium nucleus is 6.645 x 10-27 kilograms. Thus, there is a loss of 0.048 x 10-27 kilograms every time a helium nucleus is formed by fusion. The loss is small, but real. You may then ask, ``Where does that mass go when hydrogen is fused into helium? Isn't mass conserved?'' Albert Einstein provided us with an answer to our questions.

He pointed out that mass can be converted into energy, and vice versa. The `exchange rate' between mass and energy is given by the famous equation E = m c2 where E = energy, m = mass, and c = the speed of light in a vacuum (300,000,000 ometers/second). Thus, the total amount of MASS+ENERGY is conserved. Every second, as we have seen, the Sun creates E = 3.9 x 1026 joules of energy. To balance the books, every second the Sun destroys m = E/c2 = 4.3 x 109 kilograms of mass. This mass loss, equivalent to more than 4 million tons per second, is accomplished by fusing 600 million tons of hydrogen into 596 million tons of helium. It should be noted that fusion of atomic nuclei is difficult to initiate. Atomic nuclei are positively charged; thus each nucleus repels each other nucleus. In order to fuse two nuclei together, they must be brought very close together, so that the strong (but short-range) nuclear force that holds nuclei together can do its work.

In order for two positively charged nuclei to overcome their electric repulsion and come very close, they have to collide at high speeds. High speeds for individual nuclei imply that the gas in which they exist is very hot (more than 10 million degrees Kelvin). These high temperatures are not found at the Sun's surface, but only in its hot, dense core. How can the Sun "burn"? StarChild Question of the Month for August 2001 Question: If there is no oxygen in space, how does the Sun "burn"? Answer: The Sun does not "burn", like we think of logs in a fire or paper burning. The Sun glows because it is a very big ball of gas, and a process called nuclear fusion is taking place in its core. Nuclear fusion occurs when one proton smashes into another proton so hard that they stick together...and release some energy as well. This energy then heats up the other materials (other protons and electrons and such) nearby. This heating eventually grows out from the center (or core) of the star to the outside, finally leaving the surface and radiating out into space to be the heat and light we know stars emit.

People, including scientists, sometimes say that the Sun "burns hydrogen" to make it glow. But that is just a figure of speech. Hydrogen really doesn't burn, it fuses, into helium. So no oxygen is required! The Sun | Scholastic The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute. How large is the sun? Pretty big! Its diameter is about 860,000 miles. You could line up 109 Earths along the diameter of the sun! How heavy is the sun? Now that's a heavy question! The sun weighs 4.4 followed by 30 zeros pounds! By comparison, the earth weighs 1.3 followed by 25 zeros pounds. So the sun's weight equals 340,000 Earths! How big is the core of the sun? The sun is about 860,000 miles in diameter. (That would take our car 1.5 years to drive across!) What we call the core of the sun is the inner part where the nuclear energy is released. This is the source of the energy that we see as light from the sun.

The core is about one-fourth the size of the sun, or about 200,000 miles across. That is still pretty big! Earth is only about 8,000 miles across. How did the sun form? We believe that the sun formed from a big cloud of gas and dust. The cloud was dark and cold, and its gravity caused it to fall together. The gas that collected at the center got hot from all the other gas falling on it. When nearly all the gas and dust had fallen together, the center became very hot and very dense — our sun. Some of the leftover gas and dust going in orbit around this baby sun became the planets. How does the sun get its gases? Most of what the universe is made of is gas — mostly hydrogen gas. When the sun formed, it formed from a cloud of gas. However its gravity holds it together very strongly, making it hot and dense. How far away are we from the sun? Earth (and we) are about 93,000,000 miles from the sun. To give you an idea about how far that is, suppose we could build a highway and drive a car to the sun.

Let's drive 65 miles per hour. It would take over 160 YEARS to get there! How hot is the sun? The surface of the sun is about 10,000 degrees Fahrenheit (5,770 degrees Kelvin). That may seem mighty hot, but it is even hotter deeper inside the sun. What is the temperature of the sun's core? The sun's core is extremely hot — about 27 million degrees Fahrenheit (15 million degrees Kelvin). That is because of the tremendous pressure of the weight of the sun pressing down on the core — equal to 333,000 Earths. In pounds, that is 9 followed by 29 zeros! How do you measure how hot the sun is if you can't get that close to measure? One way is to study the sun's spectrum — that is, break up its light with a prism. A rainbow is a spectrum created when sunlight goes through raindrops, which act as little prisms. If we do this with a special device called a spectrograph, we can see that there are thin dark lines throughout the sun's spectrum.

Scientists have examined the different elements on earth, so we are able to figure out what kind of gas, say hydrogen, gives rise to which dark lines. It turns out that every element has a unique set of dark lines, like fingerprints. We can use them to figure out what the sun and stars are made of. They also show us how hot the gases are. Another way is to look at sunlight and measure how much light is in the blue part of the spectrum, in the yellow, in the red. The hotter something is, the more light it gives off in the blue compared to the red. All these methods show us that the sun's surface is about 10,000 degrees Fahrenheit! How close can you get to the sun without burning? Not very close! Have you ever gotten a sunburn? That's right here on earth with the atmosphere to protect you. In space, you would have no protection from the sun's ultraviolet light (on earth the ozone layer protects us). If you could go into space without a spacesuit, you would get burned badly by the sun. How close have people gotten to the sun? So far people have circled Earth in spacecraft and the shuttle, but that is only a few hundred miles up.

Years ago NASA astronauts went to the moon, which is about 250,000 miles away. But that is it. We have not yet visited any other planets or gotten close to the sun. For comparison, the sun is 93,000,000 miles away — much, much further than the moon. How do scientists determine the layers of the sun? The outer layers, which we can see, are the corona, the chromosphere, and the photosphere. We can use the techniques I described above to measure their temperatures. Inside the sun there are also layers, but we can't see them. We know about them only from detailed mathematical models of the interior of the sun. We test these models to see if they accurately represent how the sun looks and behaves. They give us our best "guess" of what is going on inside the sun. What is the next closest star to Earth besides our sun? That would be the double star Alpha Centauri. It is the brightest star in the constellation Centaurus, which unfortunately we can't see from the U.S. since it is in the southern sky. Alpha Centauri consists of two stars in orbit around each other.

They are nearly twins, and are also very similar to our sun. It takes 80 years for the two stars to orbit each other. There is a third star, called Proxima Centauri, which may be in orbit around the other two stars. It is a dim, cool star, but happens to lie a little closer to us than the other two. Thus the name Proxima. If there is no oxygen in outer space, how does the sun burn? Don't things need oxygen in order to burn? There is no oxygen in space. Then how can the sun be burning? The answer to that is that the sun isn't burning the way we usually think of things. The energy and light from the sun come from nuclear reactions deep inside the sun that occur at incredibly high temperatures millions of degrees! That energy keeps the entire sun hot, although at the surface it is ONLY about 10,000 degrees Fahrenheit. Anything that hot will glow and give off light, and that is why the sun is bright and looks like it is on fire. Why is the sun orange in the morning and bright white in the afternoon?

If you look at the sun when it is first rising in the morning or setting in the evening, it is passing through more of the earth's atmosphere. The air scatters light, but it scatters blue and green light the most. Why is the sky blue? Because that is all the blue sunlight that the air has scattered! In the middle of the day, the sun seems white because not that much blue light is scattered. But in the morning and evening, most of the blue and green light are scattered. If you take blue and green away from white, you get orange and red. You may have noticed that the sun seems dimmer in the morning and evening than in the middle of the day. That is correct, because some of the light has been scattered away. Why doesn't the sun look like the other stars? Actually the sun is very much like the other stars. But it is much closer. The nearest star other than the sun, Alpha Centauri, is over 200,000 times farther away! If we put the sun at that distance, it would look just like Alpha Centauri! Is Earth moving away from the sun? No, I don't think Earth is moving away from the sun.

It has been in orbit at about the same distance for a long, long time or life would not have survived here. What is going to happen to the sun? We think that in about 4.5 billion years, the sun will run out of hydrogen in its center. The source of the sun's energy (and most other stars) is nuclear reactions that change hydrogen into helium. Then the center of the star will collapse, but only part way. The outer part will be blown out like a balloon, and our sun will become a "red giant." The surface will look red and will be a little cooler than it is now, but it will grow large — it will probably expand all the way out to the orbit of Mercury. At this point, the sun can use nuclear reactions of the helium that it made earlier, and convert it to carbon. When it runs out of nuclear reactions that can occur in the center, the sun will not be able to make much energy. So it will collapse, but we think that it will become a "white dwarf" — a small but hot star that would be only about the size of the earth!

Then the white dwarf will slowly give off its heat, becoming dimmer and cooler, until eventually all that would be left is a star cinder. All this would take many millions of years. When the sun becomes a red giant, it will be a whole lot hotter in the solar system! It will probably vaporize Mercury, and Venus and Earth would be charred. The outer planets would probably have their clouds burned away. We think that only really massive, big stars can become black holes. When they get old, they will go through similar (although more complex) changes. But when a very massive star collapses, its gravity can be so great that it will compress its mass to a point — a black hole. A black hole still has the same gravity as the star that made it. So it will affect only what is nearby, just like the star, except that if you get too close, zoom down the hole! But if a star had planets, they could probably continue in their orbits if nothing else changed. And no, the whole galaxy wouldn't be sucked up. We think that there may be big black holes in the center of many galaxies, but they are still not big enough to do that.

If the sun is so hot, why doesn't it just burn up? A star can exist because the heat it generates inside, due to nuclear reactions, pushes outward, but this is balanced by its gravity, which pulls inward. For a star like our sun, these forces are in balance. We think that a really big star, once it uses up all its nuclear fuel, will collapse and nothing can stop it. Gravity will take over, and all the matter in the star will collapse to a point. Near that point, the gravity is so strong that even light can't escape. That is why it is a BLACK hole — no light. Although this may sound pretty crazy, we think we have found black holes at the centers of some galaxies! The sun doesn't burn up because its source of power is nuclear energy. In the center of the star, there are tremendous temperatures (12 million degrees!) and pressures that help produce nuclear fusion. Hydrogen is being converted into helium, and energy is given off. The sun has a LOT of hydrogen. We know that the sun is about 4.5 billion years old, and we compute that it has about another five billion years to go before it runs out of hydrogen.

So it will run out of fuel some day — fortunately for us it won't be for a LOOOONG time. Why doesn't the sun drift off into another galaxy? Our galaxy is composed of billions of stars. They "hang together" because of their gravity. So our sun is kept in its path in our galaxy by the gravity of all those other stars. Have you seen a picture of a spiral galaxy? We believe that is what the Milky Way looks like. The sun is in an orbit in our galaxy. It is moving (the other stars are too) in a huge circle around the center of the galaxy. It takes about 200 million years for it to go around once! If the sun blows up, what effect will it have on the other planets? First, let me assure you that the sun is not going to blow up any time soon! In about five billion years, the sun will run out of energy. But that is nothing for us to worry about. If the sun became a supernova, the explosion would probably destroy the planets. Fortunately the sun is too small to become a supernova. If the sun were to run out of energy, it would quit putting out its light.

Then it would get very cold and dark here! The sun is, one way or another, the source of energy for all of us. Are sunspots like magnets? Sunspots are formed because of loops of magnetic fields in the sun. The loop, which has a north pole and a south pole like a magnet, somehow keeps that part of the sun's surface from being quite as hot as the rest, making it look dark compared to the rest of the sun. We are still not sure where the loops of magnetic field come from, except that they come up from inside the sun. Can you tell me how deep are sunspots? I think they are fairly shallow (not as deep as they are wide). Somehow the strong magnetic fields in the sunspots work to reduce the amount of light coming from inside the sun through the spot, so it is cooler than the surrounding surface of the sun and looks darker. But it's still pretty hot. The temperature is about 8,000 degrees Fahrenheit in the spot, and about 10,000 degrees at the surrounding surface of the sun! Does the amount of fusion in the core of the sun affect temperature?

The amount of nuclear fusion that happens inside the sun is the source of all the energy we get from the sun. It is also the source, one way or another, of nearly all the energy we use on the earth. So if the amount of fusion decreased, then the energy from the sun would go down. Probably the surface of the sun would be cooler. And it would DEFINITELY get cooler on the earth. Fortunately the sun has been undergoing nuclear fusion at a pretty constant rate for billions of years, and we believe that it will continue for several more billions of years. × Interior of the Sun (E = energy in watts m = mass in kilograms c = speed of light in meters/sec) Mass and energy are equivalent and can be converted into each other. In the 1930s, astronomers realized that Einstein's Theory held the key to how the sun and stars produce their energy: What actually happens to make the energy of the sun? Hydrogen fusion, often called the proton-proton chain, combines hydrogen nuclei into helium ones inside the sun. In addition to producing light in the form of gamma-rays, other particles such as neutrinos are produced (see below).

When 4 atoms of hydrogen are converted into one atom of helium, a small portion of the mass is converted to energy. Using this process, a star can produce large amounts of energy for a long time. This is an example of nuclear fusion. (From Nick Strobel Go to his site at for the updated and corrected version.) (Deuterium is an "isotope" of hydrogen ) A summary of how it works, showing all the reactions and how long it takes for them to occur, is to the left. (From U. Tenn Ast 162, In stars more massive than the sun, another reaction chain can be important in converting hydrogen to helium. In it, carbon, nitrogen, and oxygen isotopes are critical links although the final mix of isotopes is not modified. In analogy with terminology in chemistry, they are called catalysts. The reaction chain is the "C-N-O cycle". Why doesn't this happen on Earth? For two H nuclei, protons, to collide hard enough to overcome their natural electrical repulsion, they must be moving very fast.

==> need very high temperatures (From Nick Strobel Go to his site at for the updated and corrected version.) To have enough collisions to generate significant energy requires high density ==> reactions can occur only in the centers of massive objects like stars or (at least so far) in special machines where they can be sustained for only a very short time because the energy released disrupts the continuing reaction Only about 10% of the mass of the sun has temperatures and pressures sufficiently high for nuclear reactions to occur. The core of the sun, where ) Nonetheless, we can study the reactions in the core by detecting neutrinos from the sun. Neutrinos react extremely weakly with other forms of matter. They can escape from the center of the sun virtually unimpeded and they carry away about 2% of the energy from the fusion reactions occurring there. (From Nick Strobel. Go to his site at for the updated and corrected version.) Neutrinos are NOT electromagnetic radiation or matter. They are another type of fundamental particle, similar to photons, but also different from them in some important ways such as their ability to travel through matter without interacting, so they can emerge from the center of the sun .

Neutrino Detectors Because neutrinos emerge from the very center of the sun, measuring how many of them escape should allow us to probe our understanding of the reactions taking place there! Although neutrino detectors have found fewer neutrinos than expected, we think it is the neutrino physics that was wrong and that our models for the interior of the sun are very accurate. animation by G. Rieke) If the sun produced energy more rapidly in its core, it would be hotter. Then the pressure would increase and the core would expand. The larger, lower density core would have fewer proton-proton collisions, reducing the rate of reactions, and causing the production of energy to decrease. Thus, the energy output is self-regulating. The structure of the sun adjusts until the gravitational "pull" towards its center is just balanced by the "push" of the gas pressure outward. Fortunately, this results in a very stable state, called hydrostatic equilibrium. Good link on the sun: Test your understanding before going on Aten, Egyptian god of the sun , How Was the Sun Formed? How Was the Sun Formed?

By Nola Taylor Redd, Space.com Contributor | January 17, 2013 05:22pm ET MORE The Sun: Our Perfectly Average Middle Aged Star Credit: Space.com In a wide expanse of space, gravity drew dust and gas together to create the young solar system. The sun formed first from the vast material, with the planets close behind. But how did a sea of swirling particles become the brightest star in our sky? Although it may look empty, space is filled with gas and dust. Most of the material was hydrogen and helium, but some of it was made up of leftover remnants from the violent deaths of stars. Waves of energy traveling through space pressed clouds of such particles closer together, and gravity causes them to collapse in on themselves. As the material drew together, gravity caused it to spin. The spin caused the cloud to flatten into a disk like a pancake. In the center, the material clumped together to form a protostar that would eventually become the sun. The young protostar was a ball of hydrogen and helium not yet powered by fusion.

Over the course of about 50 million years, the temperature and pressure of the material inside increased, jumpstarting the fusion of hydrogen that drives the sun today. The formation of the sun didn't take up all of the cloud it was born from. What was left continued to orbit the star, while planets formed from the leftover material. The sun is an average-size star , not too big and not too small. Its size makes it an excellent star to orbit, as it is neither large and fast-burning nor small and dim. Several billion years from now, the hydrogen inside of the sun will run out, and the star will swell up into a red giant with a radius extending to Earth's orbit. The helium at its core will also be consumed. The star will never be hot enough to burn the oxygen and carbon that are left behind, so the sun will fizzle out and become a white dwarf. Related: The Life-Giving Sun The Sun�s Structure and Nuclear Fusion The proton -proton chain. This is the principle fusion reaction in the Sun. Mass, in the form of hydrogen atoms , is converted to energy as described by Einstein's formula: E = mc2. What kind of fire is burning on the Sun?

And what keeps it burning steady? The power generator in the Sun is in its center, buried deeply within it. It is called "the core", with a radius close to one fourth of that of the star (see the figure above). In the core, pressures and temperatures are high enough to force fusion, that is, nuclear reactions whereby some nuclei merge to make others. It is the type of reaction that powers a hydrogen bomb. The most important reaction within the core of the Sun is the process called the "proton-proton cycle." photons In the proton-proton chain reaction , hydrogen nuclei are converted to helium nuclei through a number of intermediates. The reactions produce high-energy photons ( gamma rays ) that move through the "radiative layer" surrounding the core. This layer takes up 60 percent of the radius of the Sun. It takes a million years for energy to get through this layer into the "convective layer", because the photons are constantly intercepted, absorbed and re-emitted. In the core, the helium nuclei make up 62% of the mass (the rest is still hydrogen). The radiative and convective layers have about 72% hydrogen, 26% helium, and 2% heavier elements (by mass).

The energy produced by fusion is then transported to the solar surface and emitted as light or ejected as high-energy particles. By the time the energy reaches the surface of the Sun, things have cooled down to 6000 degrees Kelvin , a temperature that corresponds to the sunlight we see. By now most of the hydrogen is in the atomic state and the density of the gas is low, similar to that of the gas in neon lights. The energy emitted from the hot surface, on average, is near 230 million watts per square meter. (On Earth's surface, we typically get about a millionth of that, to warm us.) The solar surface showing active regions and magnetic loops. (Courtesy of SOHO/EIT Consortium) Nuclear fusion, the source of all the energy so generously radiated by the Sun, does two things: it converts hydrogen into helium (or rather, makes helium nuclei from protons) and it converts mass to energy. The mass-to-energy conversion is described by Einstein's famous equation: E = mc2, or, in words, energy equals mass times the square of the velocity of light.

Because the velocity of light is a very large number, this equation says that lots of energy can be gained from using up a modest amount of mass. The energy created by the fusion processes within the core of the Sun (or any other star) exerts an outward pressure. Unless contained, such pressure would produce an explosion (as happens in the hydrogen bomb, on a much smaller scale). The inward pressure that keeps a star from exploding is the gravitational attraction of the gas mantle surrounding the core (which is most of the volume of the Sun, and is very hot but does not burn itself). The pressure of the energy generated in the solar core pushes outward and would cause the Sun to expand if it were not exactly balanced by the gravitational pressure of the Sun's outer layers. The outward pressure from the fusion reactions keeps the stars from collapsing. The inward pressure from gravitation keeps the stars from exploding. If the fusion reactions in the core become too weak, a star can and does collapse. Such collapse can provide new conditions in a core that result in new types of fusion reactions, so that expansion follows. If fusion reactions in the core become too strong, a star can and does explode. Such events can be observed.

When a star explodes it shines with extreme brightness for a while; it turns from an unnoticed to a "new" star, a "nova". Stars, like our Sun, where inward pressure and outward pressure is nicely balanced, fluctuate but little in brightness and give off a steady stream of energy. The balance is achieved by self-regulation: a slight decrease in fusion energy would result in contraction that would heat up the core and increase fusion rate, and vice versa. Other stars, where the balance is not so well tuned, pulsate noticeably. Living on a planet circling a pulsating star presumably would be difficult or impossible. The Crab Nebula. This is remnant of a supernova that occurred in the year 1054. When nuclear reactions in the core of a star no longer generate enough pressure to offset its weight , the star may explode, as happened here. (Courtesy: VLT) Thus, the reason that the Sun neither expands (from the ongoing explosion within) nor collapses (from its own weight) is that the two forces keep the balance. In the distant future, when this balance is disturbed because most of the hydrogen is used up, the Sun will expand.

This will be the end of the solar system as we know it. Photo of the Sun, demonstrating sunspots , dark areas of irregular shape on the surface. They are often large enough to see with the naked eye. (Courtesy: NASA ) Thus, on the whole, our star shines with a steady light. However, it has changed its output over geologic time. Also, it varies a bit on a number of cycles. The most obvious manifestation of this is the so-called " sunspot cycle " which describes periodic changes in the abundance of spots on the face of the Sun. The abundance of spots is related to the brightness of the Sun (more spots, more brightness). The variation is less than 1 percent, and the mechanisms are poorly understood. It has to do with changes in the magnetic field of the Sun and with convection within the outer layer of our star (not with processes in the core). Sunspot activity is closely in phase with ejection of solar plasma (protons mainly) and the ensuing atmospheric fireworks in the polar regions of Earth called " aurora ". The Aurora Borealis Charged particles from the solar wind collide with atoms in the Earth's atmosphere to produce the "Northern Lights". (Source: NASA )

Question: The sun is hot because of the nuclear transformation of hydrogen to which other gas?
Answer: Helium