6. Space Physics

• The Earth is a planet.
• The Earth rotates on its axis, which is an imaginary line that passes through the North and South poles. The axis is tilted at an angle of 23.5 degrees.
• The Earth rotates from west to east, which means that the Sun and the Moon appear to rise in the east and set in the west.
• The Earth takes about 24 hours to complete one rotation, which is why we have day and night.
• The Earth orbits around the Sun, which is a star that provides light and heat to our planet.
• The Earth’s orbit around the Sun is not a perfect circle, but an ellipse, which means that sometimes the Earth is closer to the Sun and sometimes farther away. The tilt of the Earth’s axis and its orbit around the Sun cause the seasons, because different parts of the Earth receive different amounts of sunlight throughout the year.
• The Earth takes about 365 days to complete one orbit, which is why we have a year.
• The Moon is a natural satellite that orbits around the Earth. The Moon reflects the light from the Sun, which makes it visible in the night sky.
• The Moon takes about 28 days to complete one orbit around the Earth.
• The shape of the Moon changes as it orbits around the Earth, because we see different parts of its illuminated side. These changes are called phases of the Moon.
• The average orbital speed of an object is how fast it moves along its orbit. It depends on how far away it is from what it orbits and how long it takes to complete one orbit.
• We can calculate the average orbital speed using this formula:

  v = (2πr)/T

  Where v is the average orbital speed, r is the average radius of the orbit, and T is the orbital period (the time for one orbit).
• The Solar System is a collection of objects that orbit the Sun, which is the only star in our system and contains most of its mass.
• The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 
• The planets are divided into two groups: the inner rocky planets and the outer gas giants.
• There are dwarf planets (e.g. Pluto) and asteroids which also orbit the Sun, as well as moons that orbit the planets.
• The Solar System was formed about 4.6 billion years ago from a cloud of gas and dust that collapsed and rotated to form an accretion disc. The Sun formed at the center and the planets formed from the remaining material.
• The planets have elliptical orbits around the Sun, which means they are not always at the same distance from it. The time it takes for a planet to complete one orbit is called its orbital period.

   
• The Sun emits electromagnetic radiation. This radiation can be divided into different regions based on its wavelength and frequency, such as visible light, infrared, ultraviolet, X-rays, and gamma rays.
• The distance between objects in the Solar System is so large that it takes time for light to travel from one place to another. For example, it takes about 8 minutes for light to reach Earth from the Sun, and about 5 hours to reach Pluto.
• The gravitational force between two objects depends on their masses and their distance apart. This force keeps the planets in orbit around the Sun and it also affects their shapes and motions.
• Some planets have rings made of dust and ice particles that orbit them.
• Some planets have natural satellites or moons that orbit them.
• The Sun has a very strong gravitational field that pulls the planets towards it. The closer a planet is to the Sun, the stronger the pull and the faster the planet orbits around it. The farther a planet is from the Sun, the weaker the pull and the slower the planet orbits around it.
• When a planet is closer to the Sun, it has more kinetic energy (energy of motion) and less potential energy (energy of position). When a planet is farther from the Sun, it has less kinetic energy and more potential energy.
• The total energy of a planet in its orbit is always constant. This means that when a planet moves faster, it also moves closer to the Sun, and when it moves slower, it also moves farther from the Sun.
• The Sun is a star of medium size that consists mostly of hydrogen and helium.
• The Sun releases most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum. This energy comes from nuclear fusion of hydrogen nuclei into helium in the core of the Sun.
• Nuclear fusion is a reaction in which two or more atomic nuclei are fused together to form a larger and heavier nucleus.
• Nuclear fusion is the main source of energy for all stars, and it also produces most of the elements in the universe. The Sun fuses about 500 million metric tons of hydrogen each second.
• Nuclear fusion requires very high temperatures and pressures to overcome the repulsion between positively charged nuclei.
• Galaxies are huge collections of stars, gas and dust that are held together by gravity. There are billions of galaxies in the universe, each with billions of stars. 
• Our Sun is one of the stars in the Milky Way galaxy, which is shaped like a spiral.
• A light-year is the distance that light travels in one year, which is about 9.5 trillion kilometres.
• Stars are born from clouds of gas and dust in space, called nebulae. Gravity makes these clouds collapse and heat up, forming protostars.
• When protostars get hot enough, they start to fuse hydrogen atoms into helium atoms, releasing energy. This is called nuclear fusion and it makes stars shine.
• A protostar becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature in the centre of the star. This lasts for most of the star’s life. Our Sun is currently a stable star.
• Stars have different life cycles depending on their mass. Low-mass stars like our Sun live longer than high-mass stars because they use up their fuel more slowly.
• When a star runs out of hydrogen in its core, it starts to fuse helium and other heavier elements. This makes the star expand and cool down, becoming a red giant or a red supergiant depending on its mass.
• A red giant from a low-mass star eventually sheds its outer layers, forming a planetary nebula with a white dwarf star at its centre. A white dwarf is very hot and dense, but it gradually cools down and fades away, becoming a black dwarf.