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Formation of Galaxy – Solar Nebulas and Solar System

Grade 8
Jun 12, 2023


The matter dispersed throughout the universe after the Big Bang was drawn together by gravitational attraction to create galaxies. The mutual attraction between galaxies led to the clustering of galaxies. Despite the enormous distances between galaxy clusters, gravitation constantly connects them.

Being a long-range non-contact force, gravity acts between all celestial objects and controls the size and shape of the universe.


The Solar Nebulas:

The large empty regions of space between the stars contain everything needed to create planets, moons, and stars. Just as there are clouds in the sky, there are clouds in space. These clouds are called nebulas.


Nebulas (or nebulae) are mixtures of gases—mainly hydrogen and helium—and dust made of elements such as carbon and silicon.

Fig: The Horsehead Nebula is a cold, dark cloud of gas and dust. Observations suggest that it is also a site where stars form.


The force of gravity holds the material of a nebula together. There is a lot of room between the particles in most nebulas. Nebulas are less dense than air!

If gravity pulls on all the particles in a nebula, why don’t nebulas slowly collapse?

The specific relationship or balance between temperature and pressure in a nebula prevents it from drifting apart.


The average kinetic energy, or the energy of motion, of the particles in an entity, is measured by temperature. If a nebula’s particles have low kinetic energy, they move slowly, and the cloud’s temperature is extremely low. If the particles are moving quickly, the cloud’s temperature is high. As particles move around, they sometimes crash into each other. As shown in the figure, these collisions cause particles to push away from each other, which creates pressure.



Try to blow a balloon and gradually increase the pressure inside it.


While blowing up a balloon, we observe that the inside air pressure prevents a balloon from shrinking due to the pressure caused by the atmosphere.

A nebula’s outward pressure counteracts the gravitational force from within, preventing the cloud from collapsing. A nearby star exploding or two nebulas colliding can break the equilibrium between gravity and pressure in a nebula.

Formation of a Solar System:

The nonequilibrium of forces causes the solar nebula to contract. Smaller nebula regions are compressed or pushed together. These regions can become so dense that they contract under their gravity. As the matter collapses inward, the temperature increases, and the stage is set for stars to form.

The events that may have led to the formation of the solar system are shown in the Figure below. As the solar nebula collapsed (1), it began to rotate (2). The rotating cloud became hotter and denser in its center.

The gas and dust that rotated around the central part of the cloud formed a disk (3). This disk began to cool enough for dust particles to form. These particles began to collide and form larger particles from Planetesimals to Planets.

Body sizes like rocks and asteroids were created by particle collisions. These small bodies are called planetesimals. The size of planetesimals depended on their distance from the center of the solar nebula.


The size of planetesimals continued to increase until their gravity became strong enough to pull in the solid matter near their orbit. In this way, planetesimals grew into protoplanets (4). Eventually, the protoplanets became large enough to form planets and moons. Matter in the solar nebula was pulled together by gravity into spheres.

The sun, the planets, and most moons are spherical because a sphere is the only geometric form in which all points on the surface are an equal distance from the center.

As the planets were forming, gas and dust near the center of the solar nebula (5) grew denser and denser. As the center got hotter and denser, hydrogen atoms started to fuse to generate helium. Massive energy was generated during fusion, and this pressure was sufficient to counteract the gravitational pull of the Earth. At this point, when the gas stopped collapsing, our sun was born, and the new solar system (6) formed.

Formation Of Galaxy


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