Summary: Stars type in cold, thick regions of room called molecular clouds. When the pressure of gravity pulling in ~ above the cloud is greater than the toughness of inner pressure pushing out, the cloud collapses right into a protostar.

You are watching: What happens when an interstellar cloud fragment shrinks

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

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

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

Molecular clouds space perfect star-forming regions due to the fact that the combination of this atoms right into molecules is much more likely in very dense regions.

This photo shows the Orion Nebula, one interstellar cloud in i m sorry star equipment - and possibly planets - room 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 system is recognized as the solar nebula.Click the picture to the left to see much more images that the Orion Nebula.

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A star forms when a molecule cloud collapses under its very own gravity creating a thick core continual by nuclear fusion. This happens only once the pressure of gravity pulling in exceeds the external push the pressure. High-density molecule clouds have stronger pressures of gravity pushing in, do it much easier to overcome the complete pressure within the cloud.SHOW METHE MATH

Once started, the fallen of the solar nebula continues since the force of gravity exerted top top the cloud grows stronger as the cloud shrinks in size. The universal law of gravitation is composed mathematically:


Fg is the force of gravitational attraction, M1 and also M2 are the masses of the two objects, and also d is the distance in between their centers. The prize G is known as the gravitational constant. Its worth is G = 6.67 · 10-11kg·m2/s2.

Before its please began, the gas that consisted of the solar nebula was more than likely spread the end over a about spherical region a few light-years in diameter. This gas was very low in density and extremely cold.

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

This is a snapshot of the Crab Nebula taken v the Hubble an are Telescope. The Crab Nebula was produced when a star exploded. Ours Solar system is assumed to have formed from corpuscle expelled native a supernova explosion end 4 billion year ago.


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

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

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

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

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As the please continues, thickness increases and the molecule inside start to interact much more and more. This extra interaction reasons the temperature come increase. Due to the fact that pressure depends on density and temperature (see display ME THE MATH), the pressure increases.SHOW METHE MATHOnce the press gets high enough, it begins to rival the strength of gravity and also the fallen of the cloud slow down. Eventually, the cloud the gas i do not care a protostar: an infant star that has actually not yet started to fuse hydrogen in that is core.

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Equilibrium of the Sun(click to enlarge)

A protostar is not taken into consideration a star until it gets its energy from nuclear combination instead that gravitational contraction. Gravitational convulsion is the process of a cloud"s gravity overcoming its inner pressure and also causing a collapse. The main point temperature of an object must with a minimum temperature prior to 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 right into carbon: 100 million °Kelvin

The price of combination increases until the amount of energy created in the core equals the lot of power radiated native the surface. The perfect balance in between energy produced and energy released stops the protostar indigenous collapsing further.

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Demographics the a 1-solar-mass Star(click to enlarge)
The variety of low-mass stars is much higher than the variety of high-mass stars. Minimum fixed of newborn star: 0.08 times the mass of the SunMaximum massive of newborn 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 time the massive of our Sun, there are 50 stars close come the massive of our Sun. Our sunlight is pretty mean in size and also mass.

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