Summary: Stars kind in cold, thick regions of room called molecule clouds. Once the force of gravity pulling in top top the cloud is greater than the strength of internal pressure pushing out, the cloud collapses right into a protostar.

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Sections:Where execute stars form?Why execute stars form?What slows the cloud collapse?Nuclear FusionMasses of newborn StarsRelated linksRelated Lessons:Astro-Chronology

Stars type in the densest regions of the interstellar medium, or ISM, referred to as molecular clouds. The ISM is the name provided to the gas and dust the exists in between the stars in ~ a galaxy. It is 99% gas and 1% dust, by mass.

Properties of ISM gas in our region of the Milky means Galaxy:70% hydrogen28% helium2% facets heavier 보다 helium
The Orion Nebula(click for an ext images)

Molecular clouds room perfect star-forming regions since the combination of these atoms right into molecules is much much more likely in really dense regions.

This photograph shows the Orion Nebula, an interstellar cloud in i beg your pardon star equipment - and also possibly planets - are forming. Our very own solar device presumably developed as gravity led to the please of a similar huge cloud that gas. The item of cloud that developed our Solar device is well-known as the solar nebula.Click the photo to the left come see more images of the Orion Nebula.

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A star develops when a molecule cloud collapses under its own gravity developing a dense core sustained by nuclear fusion. This happens only as soon as the pressure of heaviness pulling in above the outward push of pressure. High-density molecule clouds have actually stronger forces of heaviness pushing in, making it simpler to overcome the complete pressure in ~ the cloud.SHOW METHE MATH

Once started, the please of the solar nebula continues since the force that gravity exerted on the cloud grows stronger as the cloud shrinks in size. The universal regulation of gravitation is composed mathematically:


Fg is the pressure of gravitational attraction, M1 and also M2 space the masses that the 2 objects, and d is the distance in between their centers. The prize G is well-known as the gravitational constant. Its value is G = 6.67 · 10-11kg·m2/s2.

Before its fallen began, the gas that comprised the solar nebula was more than likely spread out over a roughly spherical region a couple of light-years in diameter. This gas was extremely low in density and also extremely cold.

Possible causes for the start of a collapse:Shock tide from a adjacent exploding starCollision the 2 molecular clouds
The Crab Nebula(click to enlarge)

This is a picture of the Crab Nebula taken v the Hubble space Telescope. The Crab Nebula was developed when a star exploded. Our Solar device is thought to have formed from particles expelled native a supernova explosion over 4 billion years ago.


As the solar nebula shrinks in size, its density, temperature and also shape all undergo dramatic changes.

HEATING: The temperature of the solar nebula increases as the collapses. As the cloud shrinks, that is gravitational potential energy is convert to the kinetic energy of separation, personal, instance gas corpuscle falling inward. These particles crash right into one another, converting your kinetic power into thermal energy.

SPINNING: favor an ice ska pulling in she arms as she spins, the solar nebula rotates faster and also faster as it shrinks in radius. This rise in rotation rate represents conservation of angular momentum. The rotation of the cloud may have been imperceptibly slow before its fallen began, but the cloud"s shrinkage makes quick rotation inevitable.SHOW METHE MATH

FLATTENING: The solar nebula has actually flattened into a disk. This flattening is a natural repercussion of collisions in between particles in a spinning cloud. A cloud might start with any type of size or shape, and different clumps of gas in ~ the cloud may be relocating in arbitrarily directions at random speeds. Once the cloud collapses, these different clumps collide and merge, resulting in a flattened rotating disk.

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As the please continues, thickness increases and also the molecules inside start to interact much more and more. This extra interaction reasons the temperature come increase. Because pressure depends on density and temperature (see show ME THE MATH), the pressure increases.SHOW METHE MATHOnce the press gets high enough, it starts to competitor the toughness of gravity and the fallen of the cloud slows down. Eventually, the cloud that gas becomes a protostar: an infant star that has actually not yet started to fuse hydrogen in that is core.

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Equilibrium that the Sun(click come enlarge)

A protostar is not considered a star till it it s okay its power from nuclear blend instead that gravitational contraction. Gravitational convulsion is the process of a cloud"s gravity overcoming its interior pressure and also causing a collapse. The core temperature of things must reach a minimum temperature before the atoms inside will fuse.

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Minimum temperature for:Hydrogen come fuse right into helium: 10 million °Kelvin3 helium nuclei come fuse right into carbon: 100 million °Kelvin

The price of fusion increases till the quantity of energy created in the core equals the quantity of energy radiated indigenous the surface. The perfect balance between energy produced and also energy released stops the protostar from collapsing further.

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Demographics the a 1-solar-mass Star(click to enlarge)
The number of low-mass stars is much greater than the variety of high-mass stars. Minimum mass of child star: 0.08 time the mass of the SunMaximum fixed of newborn star: 150 times the mass of the Sun

The diagram mirrors the relative number of stars because that every high-mass star. For every star 10-150 times the massive of our Sun, there are 50 stars close to the massive of our Sun. Our sun is pretty median in size and also mass.

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