Unit 1: Introduction to Astronomy , Unit 2: The Solar System

🌌 Unit 1: Introduction to Astronomy

🔭 1.1 What is Astronomy?

Astronomy is the science of studying everything outside Earth’s atmosphere. That includes stars, planets, moons, galaxies, and even the entire universe itself. Astronomers try to understand how things in space work, where they came from, and how they might change in the future. Instead of just guessing, astronomers observe the sky, do careful experiments, and use computer models to figure things out. Astronomy isn’t just about looking through a telescope—it’s also about thinking deeply, doing math, and asking big questions like, “How did the universe begin?” or “Are we alone in space?”

🧠 Branches of Astronomy

Astronomy has different parts, or "branches," that each focus on a certain area:

• Planetary Astronomy – This is the study of planets (like Earth or Jupiter), moons, and other small objects in space like asteroids and comets. Scientists look at what they’re made of, how they move, and whether they could support life.

• Stellar Astronomy – This branch studies stars: how they are born, what happens during their lives, and how they eventually die. Some stars explode in huge events called supernovae. Others collapse into tiny, super-dense objects like black holes.

• Galactic Astronomy – This focuses on galaxies, which are massive groups of stars, dust, and gas. Our galaxy is called the Milky Way. Astronomers study how galaxies form, how they move, and how they interact with one another.

• Cosmology – Cosmology is the study of the universe as a whole. It looks at the biggest questions, like how the universe began (the Big Bang), how it has changed over billions of years, and what might happen in the far future.

🏛 1.2 History of Astronomy

People have looked at the stars for thousands of years. Long before modern science, ancient civilizations like the Babylonians, Egyptians, Greeks, Chinese, and Mayans studied the sky. They used the stars to track time, create calendars, and plan farming seasons. These early astronomers noticed patterns in the sky, such as the movement of the Sun, Moon, and planets. Even without telescopes, they made amazing discoveries using only their eyes, math, and careful observations.

One important ancient Greek thinker was Ptolemy, who believed that Earth was at the center of the universe. This idea lasted for over 1,000 years until it was later proven wrong.

🔬 The Telescope Revolution

A big change happened in the 1600s when a scientist named Galileo Galilei improved the telescope and pointed it toward the sky. With this new tool, he saw things no one had ever seen before: mountains on the Moon, moons orbiting Jupiter, and stars that were invisible to the eye. His work helped prove that Earth is not the center of the universe.

Later scientists like Isaac Newton built better telescopes and explained how gravity works in space. Over time, our technology improved. We now have powerful space telescopes like the Hubble Space Telescope, which lets us see galaxies billions of light-years away.

Thanks to the work of early astronomers and the invention of the telescope, astronomy became a real science based on evidence, not just belief.

👁 1.3 Observational Astronomy

Observational astronomy is all about looking at space. It’s the part of astronomy where scientists collect data by watching the sky. Instead of just guessing or using imagination, astronomers use tools—mostly telescopes—to see planets, stars, galaxies, and more. They also use computers to take pictures, track movements, and even detect things we can’t see with our eyes, like radio waves or infrared light. By gathering this information, they can learn what things in space are made of, how far away they are, and how they behave over time.

🔭 Types of Telescopes

There are many different kinds of telescopes, and each kind helps us see a different part of the universe:

• Optical Telescopes – These are the most common and look like what most people picture when they hear “telescope.” They collect light and help us see stars, planets, and galaxies. There are two main types:

  • Refracting Telescopes – Use lenses to bend light and focus it.

  • Reflecting Telescopes – Use mirrors to collect and focus light (invented by Newton).

• Radio Telescopes – These collect radio waves instead of visible light. They help us study distant galaxies, black holes, and cosmic events we can’t see with normal light.

• Infrared, X-ray, and Gamma-ray Telescopes – These detect energy we can’t see with our eyes. They are often sent into space because Earth’s atmosphere blocks these types of radiation.

⚙ Telescope Mounts and Tracking

A telescope isn’t just about the lens or mirror—it also needs a strong base, called a mount, to hold it steady. Mounts are important because the Earth is always spinning, which means the sky appears to move. Without a good mount, objects in the sky would quickly move out of view.

There are two main types of mounts:

• Altazimuth Mount – Moves up and down (altitude) and side to side (azimuth). It’s simple and good for beginners.

• Equatorial Mount – Designed to follow the movement of the stars as the Earth rotates. It’s great for tracking objects over long periods, which is useful in photography or deep space observation.

Some advanced telescopes use computer-controlled mounts that can automatically find and follow objects in the sky. These tracking systems make it easier to study stars, planets, and galaxies without needing to constantly adjust the telescope manually.

🌞 Unit 2: The Solar System

🌍 2.1 What is the Solar System?

The solar system is made up of the Sun and everything that moves around it, including planets, moons, asteroids, comets, and dust. The word "solar" comes from "Sol," which is the Latin name for the Sun. The Sun is a giant, glowing ball of hot gas—mostly hydrogen and helium—that gives off light and heat. It is the center of our solar system, and its gravity pulls all the other objects toward it, keeping them in orbit. The solar system is like a family of space objects, all moving around the Sun in huge oval-shaped paths called orbits.

Even though the Sun takes up over 99% of the mass in the solar system, planets and other space objects still have important roles. Everything in the solar system moves in a predictable way, and scientists use these patterns to study space and understand where we came from.

🪐 2.2 The Planets

There are eight planets in our solar system, and they are split into two groups: the inner planets and the outer planets. The inner planets are closest to the Sun, and the outer ones are farther away.

🔸 Inner Planets (Terrestrial Planets)

These are small, rocky planets that are similar to Earth:

• Mercury – The smallest planet and closest to the Sun. It has no atmosphere and has very hot and cold temperatures.

• Venus – Almost the same size as Earth, but covered in thick clouds of carbon dioxide. It’s the hottest planet because of its strong greenhouse effect.

• Earth – The only known planet to support life. It has liquid water, an atmosphere, and a magnetic field that protects it.

• Mars – Known as the Red Planet because of its iron-rich soil. Mars has the largest volcano and canyon in the solar system and may have once had water.

🔹 Outer Planets (Gas and Ice Giants)

These are much larger than the inner planets and are made mostly of gas or ice:

• Jupiter – The biggest planet. It has many moons, a strong magnetic field, and a huge storm called the Great Red Spot.

• Saturn – Known for its beautiful rings made of ice and rock. It also has dozens of moons.

• Uranus – Spins on its side compared to other planets. It has a blue-green color because of methane gas in its atmosphere.

• Neptune – The farthest planet from the Sun. It’s known for its deep blue color and fast, stormy winds.

🪨 2.3 Dwarf Planets and Other Small Bodies

Not everything in the solar system is a planet. Some objects are too small or don’t meet the rules to be called planets. These are called dwarf planets. The most famous dwarf planet is Pluto, which used to be considered the ninth planet. But in 2006, astronomers changed the definition of a planet, and Pluto didn’t qualify because it shares its orbit with other icy objects.

Other dwarf planets include Ceres (in the asteroid belt), Eris, Haumea, and Makemake. They are important because they help us understand how the solar system formed.

There are also:

• Asteroids – Rocky objects that mostly live in the asteroid belt between Mars and Jupiter. They are leftovers from the early solar system.

• Comets – Made of ice, dust, and rock. They have glowing tails when they get close to the Sun.

• Meteoroids – Tiny bits of rock or metal in space. When they fall toward Earth and burn in the atmosphere, they become meteors or “shooting stars.” If they land on Earth, they’re called meteorites.

☄ 2.4 The Sun

The Sun is the most important object in our solar system. It’s a massive ball of hot, glowing gas that produces light and energy through nuclear fusion—a process that turns hydrogen into helium in its core. This releases a huge amount of energy that powers everything in our solar system, from plant growth on Earth to solar winds in space.

The Sun has several layers:

• Core – The center where energy is made.

• Radiative zone – Energy moves outward slowly.

• Convective zone – Hot gas moves in loops, like boiling water.

• Photosphere – The part we see as the Sun’s surface.

• Chromosphere and Corona – The outer layers of the Sun’s atmosphere, visible during solar eclipses.

The Sun also has features like sunspots (cooler dark spots), solar flares (explosions of energy), and solar wind, which can affect Earth’s magnetic field and cause auroras (northern and southern lights).

🌌 2.5 Formation of the Solar System

Scientists believe the solar system formed about 4.6 billion years ago from a big cloud of gas and dust called a solar nebula. This cloud began to collapse under its own gravity, and as it did, it started to spin. Most of the material gathered in the center and became the Sun. The rest of the dust and gas formed a flat, spinning disk around the Sun. In this disk, particles stuck together to form larger objects called planetesimals, which later became planets, moons, asteroids, and comets.

The inner part of the disk was hotter, so rocky planets formed there. The outer part was cooler, so gas and ice giants formed farther out. This is why we have rocky planets like Earth near the Sun and gas giants like Jupiter farther away.

Learning how the solar system formed helps scientists understand not just our own cosmic neighborhood, but also how other planetary systems form around stars in the universe.