فلك 10 - كواكب ومذنبات

The Solar System Components

• The Sun comprises 99.85% of the solar system's mass.
• Planets account for 0.135%.
• Comets represent 0.01% of the system's mass.
• Moons account for 0.00005%.
• Asteroids make up 0.0000002%.
• Meteors and meteorites, along with dust and gases, constitute a minuscule fraction of the overall makeup of the system.

Formation of the Solar System

• The prevailing theory, widely accepted by scientists, is the nebular hypothesis.
• This hypothesis proposes that the solar system originated from a nebula, a cloud of gas and dust, initially cool and rotating slowly.
• The nebula gradually contracted under its gravitational pull.
• Conservation of angular momentum increased the nebula's rotational speed as it shrank.
• The nebula flattened into a disk shape due to centrifugal forces.
• Eventually, the particles within the rings coalesced due to gravity, forming planets orbiting the central, contracting nebula, which evolved into the Sun.

Solar System's Age and Location

• The solar system is approximately 4.6 billion years old.
• Our solar system resides within the Milky Way galaxy.
• The sun is a typical star in this galaxy.

Planetary Day and Year

• A planetary day is the time it takes a planet to rotate once on its axis (from west to east).
• A sidereal day is the time for a planet to complete one full rotation relative to a distant star.
• A solar day accounts for the Earth's movement around the Sun; it's slightly longer than a sidereal day (approximately 24 hours).

Planetary Orbital Periods

• A planet's orbital period is the time taken to complete one orbit around the Sun.
• Orbital paths are elliptical, not circular.
• Inner planets have shorter orbital periods than outer planets.

Properties of the Sun

• The Sun is a typical, medium-sized star.
• It's a source of energy for our planet.
• It's in a state of hydrostatic equilibrium, balancing its inward gravitational pull with outward pressure from nuclear reactions.
• Primarily composed of hydrogen (73%) and helium (25%), with trace amounts of other elements.
• Density is slightly higher than water.
• Average distance from Earth is 149.6 million kilometers.
• Light from the Sun takes approximately 8.3 minutes to reach Earth.
• The Sun takes about 250 million years to revolve around the galaxy's center.

Internal Structure of the Sun

• The Sun's core is incredibly hot and dense (about 15 million degrees Celsius/Kelvin).
• Nuclear fusion reactions occur within the core, generating energy.
• Energy radiates outward through the radiative zone.
• Convection currents transfer energy to the surface in the convective zone.

Sun's Atmosphere

• The photosphere is the visible surface.
• The chromosphere is a layer above the photosphere.
• The corona is the outermost atmospheric layer.

Solar Phenomena

• Sunspots are dark, relatively cool areas on the Sun's surface, associated with strong magnetic fields.
• Prominences are large, looping structures of gas extending out from the Sun's surface.
• Solar winds are streams of charged particles emanating from the Sun.
• Auroras are colorful displays in Earth's upper atmosphere caused by interaction with solar wind.

Classification of Planets

• Inner planets (terrestrial): Mercury, Venus, Earth, Mars
• Outer planets (gas giants): Jupiter, Saturn, Uranus, Neptune

Planets' Characteristics (Specific examples)

• Mercury: Closest to the Sun, extremely hot during the day, extremely cold at night, slow rotation.
• Venus: Very hot due to thick atmosphere, thick atmosphere of CO2.
• Earth: Only known planet with life.
• Mars: Reddish color, thin atmosphere, evidence of past water.
• Jupiter: Largest planet, many moons, Great Red Spot (giant storm).
• Saturn: Known for its rings, many moons.
• Uranus: Tilted axis, rotates on its side.
• Neptune: Far from the Sun, cold, known for its bluish color.

Dwarf Planets and Other Objects

• Pluto is a dwarf planet, considered part of the Kuiper belt.
• Asteroids are rocky bodies orbiting the Sun between Mars and Jupiter.
• Comets are icy bodies with tails, originating from the Kuiper belt or Oort cloud.
• Meteors and meteorites: remnants from comets and asteroids.

Kuiper Belt and Oort Cloud

• The Kuiper belt is a region beyond Neptune containing icy bodies and dwarf planets.
• The Oort cloud is a far larger, hypothetical cloud of icy objects.

Earth's Atmosphere

• The Earth's atmosphere is a mixture of gases (nitrogen, oxygen, etc.).
• The atmosphere has distinct layers (troposphere, stratosphere, mesosphere, thermosphere, exosphere).
• The atmosphere protects life on Earth by filtering harmful radiation.

Origin of Meteors and Meteorites

• Meteors are small pieces of rock or metal burning up in Earth's atmosphere.
• Meteorites are pieces that survive atmospheric entry and land on Earth.

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