Introduction to Astronomy

• Astronomy is the study of celestial objects, space, and the physical universe.
• A celestial object is any natural object outside Earth's atmosphere, such as planets, moons, stars, asteroids, and comets.
• The universe encompasses all of space, time, and their contents, including planets, stars, galaxies, matter, and energy.
• A solar system consists of a star and all the celestial bodies that orbit it, including planets, dwarf planets, asteroids, and comets.
• An orbit is the curved path of an object around a star, planet, or moon, involving periodic elliptical revolution.
• Orbital period is the time it takes an object to complete one orbit around another.
• Rotational period is the time it takes an object to complete one rotation on its axis.
• A galaxy is a massive, gravitationally bound system of stars, remnants, gas, dust, and dark matter.
• A black hole is a region of spacetime with gravity so strong that nothing, including light, can escape.
• A nebula is an interstellar cloud of dust, hydrogen, helium, and other ionized gases.
• A supernova is the explosion of a star, making it extremely bright.
• A satellite is an object orbiting another object in space, either natural (like a moon) or artificial (like a spacecraft).
• Retrograde motion is when a planet appears to move in the opposite direction compared to other objects in its system, as seen from a specific point.

Distinguishing Celestial Objects

• A star is a luminous sphere of plasma held together by gravity.
• A planet orbits a star, is rounded by its own gravity, and has cleared its orbital path.
• A moon is a natural satellite orbiting a planet.
• An asteroid is a small, rocky body orbiting the Sun.
• A comet is a small, icy body orbiting the Sun, which develops a tail when nearing the Sun.
• A meteor is a bright streak of light caused by a meteoroid burning up in Earth's atmosphere.
• A meteorite is a meteoroid that survives atmospheric passage and impacts the ground.
• A meteoroid is a small rock or debris particle in the solar system.

Characteristics and Structure of the Sun

• The Sun is a G-type main-sequence star, specifically a yellow dwarf.
• The layers of the Sun, from the core outward, are: core, radiative zone, convection zone, photosphere, chromosphere, and corona.
• The solar cycle is approximately 11 years long, during which the Sun's magnetic activity varies.
• The heliosphere is the region of space dominated by the Sun's magnetic field and solar wind.
• A magnetosphere is the region around a planet controlled by its magnetic field.
• The Sun is located in the Orion Arm (or Local Spur) of the Milky Way galaxy.

Planets in Order from the Sun

• The planets in order from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Inner vs. Outer Planets

• Inner planets have low mass and density, with a high density overall.
• Inner planets are primarily composed of rock and metal.
• Inner planets are smaller in size.
• Mercury, Venus, Earth, and Mars are examples of inner planets.
• Outer planets have large mass and radius, with a low density overall.
• Outer planets are primarily composed of gas (hydrogen and helium).
• Outer planets are larger in size.
• Jupiter, Saturn, Uranus, and Neptune are examples of outer planets.

Revolution of Planets

• Planets revolve around the Sun in elliptical orbits.
• Planets move faster when closer to the Sun and slower when farther away.

Dwarf Planets

• A dwarf planet orbits the Sun.
• A dwarf planet has sufficient mass to achieve hydrostatic equilibrium (nearly round shape).
• A dwarf planet has not cleared its orbital neighborhood.
• A dwarf planet is not a satellite.
• The five dwarf planets in our solar system are: Pluto, Ceres, Eris, Makemake, and Haumea.

Luminous vs. Non-Luminous

• Luminous objects produce their own light, such as stars.
• Non-luminous objects reflect light, such as planets and moons.

Lunar vs. Solar Eclipse

• A lunar eclipse occurs when Earth passes between the Sun and Moon, casting Earth's shadow on the Moon.
• A solar eclipse occurs when the Moon passes between the Sun and Earth, casting the Moon's shadow on Earth.

Scientific Notation

• Scientific notation expresses very large or small numbers conveniently.
• In scientific notation numbers written in the format of a × 10^b, where 1 ≤ |a| < 10 and b is an integer.

Astronomical Unit and Light Year

• An astronomical unit (AU) is the average distance between Earth and the Sun, about 150 million kilometers.
• A light year is the distance light travels in one year, about 9.461 × 10^12 kilometers.

Phases of the Moon

• The phases of the Moon are: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, and Waning Crescent.

Life Cycle of a Star

• Stars are born in nebulas, large clouds of gas and dust.
• Gravity causes gas and dust to collapse, forming a protostar.
• The protostar becomes a main sequence star, fusing hydrogen into helium.
• When a star exhausts its hydrogen fuel, it expands into a red giant.
• Smaller stars eventually collapse into white dwarfs, which cool to become black dwarfs.
• Larger stars explode as supernovas, leaving behind neutron stars or black holes.

Classifying Galaxies

• Spiral galaxies have a central bulge, a flattened disk, and spiral arms.
• Elliptical galaxies have an ellipsoidal shape and a smooth appearance.
• Irregular galaxies have no defined shape or structure.

Seasons on Earth

• Seasons are caused by the 23.5 degree tilt of Earth's axis relative to its orbital plane.
• The hemisphere tilted towards the Sun experiences summer, while the hemisphere tilted away experiences winter.
• An equinox occurs when the Sun crosses the celestial equator, resulting in approximately equal day and night lengths.
• A solstice occurs when the Sun reaches its highest or lowest point in the sky at noon, marking the longest and shortest days.

Space Exploration Technology

• Technologies like telescopes, advanced spacecraft, and instruments for data collection have enhanced space exploration.

Space Probes

• A space probe is an uncrewed spacecraft designed to explore the solar system and transmit data to Earth.
• Voyager 1 and 2 visited Jupiter, Saturn, Uranus, and Neptune.
• The Curiosity Rover is currently exploring Mars.