Summary: Stars form in cold, thick regions of room called molecular clouds. Once the force of gravity pulling in top top the cloud is higher than the toughness of inner pressure advertise out, the cloud collapses into a protostar.

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Sections:Where carry out stars form?Why carry out stars form?What slowly the cloud collapse?Nuclear FusionMasses of child StarsRelated linksRelated Lessons:Astro-Chronology

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

Properties of ISM gas in our region of the Milky method Galaxy:70% hydrogen28% helium2% aspects heavier than helium
The Orion Nebula(click for more images)

Molecular clouds are perfect star-forming regions since the mix of these atoms into molecules is much an ext likely in very dense regions.

This photo shows the Orion Nebula, an interstellar cloud in which star equipment - and also possibly planets - are forming. Our very own solar system presumably created as gravity caused the please of a similar large cloud that gas. The item of cloud that developed our Solar device is well-known as the solar nebula.Click the photograph to the left to see much more images the the Orion Nebula.

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A star develops when a molecule cloud collapses under its own gravity creating a dense core sustained by nuclear fusion. This wake up only as soon as the pressure of heaviness pulling in exceeds the external push that pressure. High-density molecule clouds have actually stronger pressures of heaviness pushing in, make it much easier to get rid of the full pressure in ~ the cloud.SHOW METHE MATH

Once started, the fallen of the solar nebula continues because the force of gravity exerted on the cloud grows stronger as the cloud shrinks in size. The universal regulation of gravitation is written mathematically:


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

Before its collapse began, the gas that consisted of the solar nebula was most likely spread the end over a about spherical region a couple of light-years in diameter. This gas was exceptionally low in density and also extremely cold.

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

This is a picture of the Crab Nebula taken v the Hubble an are Telescope. The Crab Nebula was created when a star exploded. Ours Solar mechanism is thought to have formed from corpuscle expelled indigenous a supernova explosion end 4 billion years ago.


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

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

SPINNING: like an ice skater pulling in her arms as she spins, the solar nebula rotates faster and faster together it shrinks in radius. This increase in rotation rate represents conservation of angular momentum. The rotation of the cloud may have actually been imperceptibly slow prior to its collapse began, but the cloud"s shrinkage makes quick rotation inevitable.SHOW METHE MATH

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

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As the fallen continues, thickness increases and the molecule inside start to interact more and more. This extra interaction reasons the temperature to increase. Because pressure relies on density and also temperature (see present ME THE MATH), the pressure increases.SHOW METHE MATHOnce the press gets high enough, it starts to competitor the stamin of gravity and also the please of the cloud slow down. Eventually, the cloud of gas i do not care a protostar: an infant star that has not yet begun to fuse hydrogen in its core.

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Equilibrium the the Sun(click come enlarge)

A protostar is not considered a star until it it s okay its energy from nuclear blend instead that gravitational contraction. Gravitational convulsion is the process of a cloud"s gravity overcoming its inner pressure and causing a collapse. The core temperature of an item must with a minimum temperature before the atom inside will certainly fuse.

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

The rate of fusion increases until the amount of energy developed in the core amounts to the amount of energy radiated from the surface. The perfect balance between energy produced and also energy released stops the protostar from collapsing further.

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

The diagram shows the relative number of stars for every high-mass star. For every star 10-150 times the massive of ours Sun, there space 50 stars close to the fixed of our Sun. Our sun is pretty mean in size and mass.

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