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Star A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. When the hydrogen fuel at the core is exhausted, a star of mass 0.4 times greater than the Sun's[2] will expand to become a red giant. In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[3] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Elements and Component . Did you ever wonder what stars are made of? You might not be surprised to know that stars are made of the same stuff as the rest of the Universe: 73% hydrogen, 25% helium, and the last 2% is all the other elements. That’s it. Except for a few differences here and there, stars are made of pretty much the same stuff.

After the Big Bang, 13.7 billion years ago, the entire Universe was a hot dense sphere. The conditions inside this young Universe were so hot that it was equivalent to being inside the core of a star. In other words, the entire Universe was like a star. And for the brief time that the Universe was in this state, nuclear fusion reactions converted hydrogen into helium to the ratios we see today.

The Universe kept expanding and cooling down, and eventually the hydrogen and helium cooled down to the point that it could actually start collecting together with its mutual gravity. This is how the first stars were born. And just like the stars we have today, they were made up of roughly 73% hydrogen and 25% helium. These first stars were enormous and probably detonated as supernovae within a million years of forming. In their life, and in their death, these first stars created some of the heavier elements that we have here on Earth, like oxygen, carbon, gold and uranium.

Stars have been forming since the Universe began. In fact, astronomers calculate that 5 new stars form in the Milky Way every year. Some have more of the heavier elements left over from previous stars; these are metal-rich stars. Others have less of these elements; the metal-poor stars. But even so, the ratio of elements is still roughly the same. Our own Sun is an example of a metal rich star, with a higher than average amount of heavier elements inside it. And yet, the Sun’s ratios are very similar: 71% hydrogen, 27.1% helium, and then the rest as heavier elements, like oxygen, carbon, nitrogen, etc. Of course, the Sun has been converting hydrogen into helium in its core for 4.5 billion years.

Stars everywhere are made of the same stuff: 3/4 hydrogen and 1/4 helium. It’s the stuff left over from the formation of the Universe, and one of the most elegant pieces of evidence to help explain how we’re here today.

We have written many articles about stars here on Universe Today. Here’s an article about how metallic stars can yield planets, and some identical stars that might not be so identical.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References: NASA Bluffton University

A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion. All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin. And once the fusion reactions begin, they exert an outward pressure. As long as the inward force of gravity and the outward force generated by the fusion reactions are equal, the star remains stable. Clouds of gas are common in our galaxy and in other galaxies like ours. These clouds are called nebulae. A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. The majority of the gas in nebulae consists of molecules of hydrogen and helium--but most nebulae also contain atoms of other elements, as well as some surprisingly complex organic molecules. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova. The source of the organic molecules is still a mystery. STAR BIRTHS are started when the interstellar matter in gas clouds, such as the Eagle Nebula shown here, compresses and fuses Image: Hubble Space Telescope STAR BIRTHS are started when the interstellar matter in gas clouds, such as the Eagle Nebula shown here, compresses and fuses.

Irregularities in the density of the gas causes a net gravitational force that pulls the gas molecules closer together. Some astronomers think that a gravitational or magnetic disturbance causes the nebula to collapse. As the gases collect, they lose potential energy, which results in an increase in temperature. As the collapse continues, the temperature increases. The collapsing cloud separates into many smaller clouds, each of which may eventually become a star. The core of the cloud collapses faster than the outer parts, and the cloud begins to rotate faster and faster to conserve angular momentum. When the core reaches a temperature of about 2,000 degrees Kelvin, the molecules of hydrogen gas break apart into hydrogen atoms. Eventually the core reaches a temperature of 10,000 degrees Kelvin, and it begins to look like a star when fusion reactions begin. When it has collapsed to about 30 times the size of our sun, it becomes a protostar. When the pressure and temperature in the core become great enough to sustain nuclear fusion, the outward pressure acts against the gravitational force. At this stage the core is about the size of our sun. The remaining dust envelope surrounding the star heats up and glows brightly in the infrared part of the spectrum. At this point the visible light from the new star cannot penetrate the envelope. Eventually, radiation pressure from the star blows away the envelope and the new star begins its evolution. The properties and lifetime of the new star depend on the amount of gas that remains trapped. A star like our sun has a lifetime of about 10 billion years and is just middle-aged, with another five billion years or so left. Margaret M. Hanson, an assistant physics professor at the University of Cincinnati, gives this response: Stars form from the gravitational collapse of large clouds of interstellar material. In fact, the space between stars is not empty; it is nearly empty, but not entirely. Interstellar matter, that found lying between the stars, is made from gas and dust. Granted, only about 10 percent of the mass in our Milky Way galaxy is made up of interstellar matter. But this material, as tenuous as it is, exerts a gravitational force, and as a result, it will begin to pull itself together. orion nebula Image: Hubble Space Telescope GALACTIC NURSERY. Many stars are born in the beautiful Orion Nebula. As this accretion continues, the gravity becomes increasingly strong because its strength rises as the mass increases and the distance of the individual atoms decreases. Eventually this interstellar matter entirely collapses in on itself. The material at the very center is compressed by the infalling material on the outside, pushing down to get to the center. And this compression heats up the center of the collapsing cloud. At some point, the temperature gets so extremely high at the center, it triggers a fusion reaction. All the material that has fallen in then evolves into a hot, bright star.

The star will continue to shine as long as there is hydrogen gas to fuse through nuclear reactions, and the gravitational pressure pushing inward keeps the atoms very hot and tightly packed at the center. For a more advanced, elaborate description, with wonderful pictures, see the Web site A Star Is Born, put together by Lee Carkner of the University of Colorado.