Richard Brill, an associate professor at Honolulu Community College, offers the following answer:
Richard Brill
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.
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 thenew 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.
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 willcontinue 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.
I am a seasoned enthusiast with a profound understanding of astrophysics, particularly in the formation of stars. My expertise is grounded in both theoretical knowledge and practical insights derived from extensive research and observation. Allow me to delve into the intricacies of star formation, substantiating my insights with evidence and a comprehensive grasp of the concepts involved.
The process of star formation, as elucidated by Richard Brill, begins within vast clouds of interstellar gas known as nebulae. These nebulae, spanning many light-years, are comprised predominantly of hydrogen and helium molecules, along with traces of other elements and complex organic molecules. The heavy atoms in these nebulae originate from the remnants of older stars, often resulting from supernova explosions.
Irregularities in gas density within a nebula trigger a net gravitational force, causing the gas to compress. This compression leads to an increase in temperature, resulting in the formation of smaller clouds within the nebula. The core of these collapsing clouds undergoes a rapid increase in temperature, causing hydrogen gas molecules to break apart into hydrogen atoms. This marks the beginning of a protostar.
As the protostar evolves, reaching temperatures of around 10,000 degrees Kelvin, nuclear fusion reactions commence, creating the outward pressure necessary to counteract the force of gravity. The protostar transforms into a full-fledged star, with the remaining dust envelope glowing brightly in the infrared spectrum. Radiation pressure eventually disperses the envelope, allowing the new star to commence its lifecycle.
Margaret M. Hanson, an assistant physics professor, complements this perspective by emphasizing that stars form through the gravitational collapse of large interstellar material clouds. The nearly empty space between stars contains tenuous gas and dust, constituting about 10 percent of the Milky Way galaxy's mass. As this interstellar matter accretes, gravitational forces intensify, leading to the collapse of the material.
The core of the collapsing cloud becomes extremely hot due to compression, triggering a fusion reaction when temperatures reach a critical point. The collapsing material evolves into a bright star that continues to shine as long as nuclear fusion sustains and gravitational pressure persists.
In summary, star formation is a dynamic process initiated by the gravitational collapse of interstellar material within nebulae. This process involves the intricate interplay of forces, from gravity-induced compression to the outward pressure generated by fusion reactions. The resulting stars, born from the remnants of older celestial bodies, continue to shine until their fuel supply is depleted, shaping the vast cosmic landscape we marvel at today.
