Module_2_Notes

School

Brock University *

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Course

1P92

Subject

Astronomy

Date

Oct 30, 2023

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Pages

Uploaded by BailiffFox3804

Module 2.1- The Origin of the Solar System Nebular Hypothesis The Sun and the bodies that orbit around it are thought to have begun as a nebula (an immense cloud of gas and dust in space; also called a “molecular cloud”). The nebula that became our solar system began as a large irregularly shaped mass of gas and dust in space. Within the nebula the pressure of the gases act outwards to cause it to expand while gravitational forces (forces that pull bodies towards each other) act to cause the nebula to collapse onto itself. The force of gravity prevailed over gas pressure and the nebula collapsed and began to spin.

As the diameter of the nebula was reduced, the rate of spin increased. Due to the interaction of the pressure and gravitational forces, as the nebula was spinning it became flatter and formed a broad disk as the nebula continued to collapse. As the density of the centre of the disk increased along with its temperature the core of the nebula became the protosun. As the protosun continued to increase in density and its temperature reached about 10 million degrees, nuclear fusion began within the protosun. Nuclear fusion is a reaction that involves the formation of new chemical elements by combining atomic nuclei. For example, through nuclear fusion Hydrogen nuclei and subatomic particles can combine to form Helium. The onset of nuclear fusion further increased the temperature of the protosun and at this stage it effectively

became a star, in the case of our solar system one we call Sol, or the sun. With further increase in temperature other elements with larger and larger nuclei (up to Iron on the periodic table) were produced by nuclear fusion. Within the cloud swirling eddies developed drawing matter towards their centres to form the protoplanets. As the protosun became even hotter gases were driven off the inner region of the Solar System. The protoplanets became solid planets and continued their orbit, governed by the initial spin of the nebula.

The Nebular Hypothesis is attractive because it explains many features of the Solar System. For example, the orbits of the planets lie in a plane with the sun at its centre. This plane is called the "orbital" plane or "ecliptic" plane and it is also the plane of the early disk-shaped nebula. The Nebular Hypothesis also explains why the planets mostly rotate in the same direction and their axes of rotation are nearly perpendicular to the orbital plane. This direction of rotation was inherited from the direction of spin of the eddies in the spinning nebula that formed the protoplanets.

In our solar system Venus and Uranus do not rotate in the same direction as the other planets. Venus’s rotational axis is at right angles to the plane of the planets (the ecliptic plane) but it rotates in the opposite direction compared to the other planets. Uranus rotates about an axis that is almost parallel to the plane of the planets. Modern thinking is that the rotations of both planets were affected by major collisions with other bodies very early in their history. Module 2.2- The Planets of the Solar System The image below shows the 8 planets and their relative sizes. The four small planets that are closest to the Sun are the "inner planets" and the four large planets that lie further from the Sun are the "outer planets".

The Inner Planets Earth-like, "rocky" planets: metallic cores, dominated by silicon and oxygen compounds. Name Diameter (km) Diameter as a % of Earth diameter Distance from Sun (AU) Mercury 4,880 38% 0.37 Venus 12,103 95% 0.72 Earth 12,742 100% 1.00 Mars 6,779 53% 1.52 * Astronomical Units (AU) : 1 unit is the distance from Earth to the Sun, 149,597,900 km. The Outer Planets “Gas Giants” with thick atmospheres that become denser and hotter towards their rocky cores. Name Diameter (km) Diameter as a % of Earth diameter Distance from Sun (AU) Jupiter 139,822 1,097% 5.2

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