Structure of the Universe

【Basic Earth Science】Structure of the Universe: How Far Does Our World Extend?

Hello everyone! Welcome to the world of Earth Science. Starting today, we will be studying the "Structure of the Universe." When you hear the word "universe," you might think, "It sounds difficult..." or "It's too vast to imagine." But don't worry! We'll start with our familiar solar system and gradually widen our perspective to the broader world.
As you read, try to imagine we are on an expedition, discovering where our home, Earth, fits into the vast cosmos.

1. The "Rulers" Used to Measure the Universe

The universe is so immense that the "km" (kilometers) we use in daily life become inconveniently large. Instead, let's learn two special units used specifically for space.

① Astronomical Unit (AU)

An Astronomical Unit (AU) is defined as the average distance from the Sun to the Earth, represented as "1." 1 AU is approximately 150 million km. This is mainly used to describe distances within our solar system.

② Light-Year (ly)

A light-year (ly) is the distance light travels in one year. Light travels at about 300,000 km per second (that’s seven and a half times around the Earth!). Since it travels at this speed for an entire year, it represents an incredibly long distance. It is used to measure the distances between stars outside of our solar system.

Key Point: Remember that a light-year is a unit of "distance," not "time"!

2. Members of the Solar System

Our Earth belongs to the solar system. It is a collection of the Sun at the center and the celestial bodies that orbit around it.

Planets of the Solar System

There are eight planets in order of distance from the Sun. You might already know a mnemonic for this!

Planets are generally divided into two groups based on their characteristics.

(A) Terrestrial Planets (Mercury, Venus, Earth, Mars)

• Characteristics: Small in size, composed mainly of rock and metal, and have high density.
• Surface: They have a solid ground you can land on.
• Moons: Few or none.

(B) Jovian Planets (Jupiter, Saturn, Uranus, Neptune)

• Characteristics: Huge in size, composed mainly of gas and ice, and have low density.
• Surface: No distinct solid surface.
• Other: All of them have "rings." They have many moons.

Fun Fact: Pluto used to be considered a planet, but it is currently classified in a separate category called a "dwarf planet."

Smaller Bodies in the Solar System

Beyond the planets, the solar system is home to many unique members:

• Asteroids: Small, rocky bodies that are mostly clustered between Mars and Jupiter.
• Trans-Neptunian Objects: Bodies that orbit even further away than Neptune. Pluto is included in this group.
• Comets: Made of "ice and dust." As they approach the Sun, they evaporate and create a long tail. They are sometimes described as "dirty snowballs."

3. Appearance of the Sun

The Sun is a star that shines on its own. It is the only star in our solar system and is vital for providing energy to Earth.

The Sun's Surface and Atmosphere

Looking at the Sun in detail, we can observe several characteristic phenomena:

• Photosphere: The surface of the Sun that we usually see. The temperature is about 6000K (Kelvin).
• Sunspots: Dark spots seen on the photosphere. They appear dark because they are cooler than their surroundings (about 4000K).
• Chromosphere: A thin layer of atmosphere just outside the photosphere.
• Corona: The outermost atmosphere, which is extremely hot (over 1 million K) and very thin.
• Prominence: Gas that looks like a flame erupting from the edge of the Sun.

Common Misconception: People often think sunspots are "pitch black," but they are actually glowing brightly. They only look black because the surrounding area is so much brighter!

4. The Galaxy and the Expansion of the Universe

If we zoom out from the solar system, we see even larger structures.

The Galaxy (Milky Way Galaxy)

A massive collection of hundreds of billions of stars like our Sun is called a galaxy. The galaxy we live in is called the Galaxy (or the Milky Way Galaxy).
Viewed from above, the Galaxy has a spiral shape (a barred spiral galaxy), and from the side, it looks like a convex lens.

Hierarchical Structure of the Universe

The universe increases in scale through the following steps: Earth → Solar System → Galaxy → Galaxy Groups/Clusters → Superclusters → Large-Scale Structure of the Universe

As you can see, celestial bodies in the universe aren't just scattered randomly; they form groups and are arranged in an orderly fashion.

It might feel difficult at first, but you'll be fine! Start by imagining the nested structure: "Earth is part of the solar system, and the solar system is part of the Galaxy."

Summary: Things to Remember!

1. Units: Use "Astronomical Units" for the solar system and "light-years" for interstellar space.
2. Planets: Remember the difference between "Terrestrial" (rocky, small) and "Jovian" (gaseous, large).
3. The Sun: Sunspots are cooler regions. The surface (photosphere) is about 6000K.
4. Structure: Many stars collect to form a "Galaxy."

Great work! That’s all for today's lesson. The next time you look up at the night sky, I hope you remember to think, "How far away is that star?" and "That, too, is a part of our Galaxy."