# Definitions of Astronomical Objects ## Overview of the Universe - Everything that exists is contained within the **universe**, including: - Galaxies - Stars - Solar systems - Planets - Planetary satellites - Comets - Many more celestial structures

The universe is a vast collection of billions of galaxies and represents the largest known structure

- The **observable universe** is the portion of the universe from which electromagnetic radiation has had sufficient time to reach Earth since the universe's formation - The universe is continuously expanding ## Hierarchy of Cosmic Structures ### Galaxies - A **galaxy** is a cluster of billions of stars held together by gravity - Earth and our Solar System are located in a spiral galaxy called the **Milky Way** - The Sun takes approximately 230 million years to complete one full revolution around the galaxy - Galaxies are moving away from one another - The greater the distance between galaxies, the faster they recede from each other ### Stars - **Stars** fuse hydrogen into helium through nuclear fusion, releasing vast amounts of energy as electromagnetic radiation - Stars form from gas and dust pulled together under the force of gravity - The **Sun** is the star closest to Earth - The Sun is classified as an average-sized, small-mass star - Total mass of the Sun: $M_{\text{Sun}} = 2 \times 10^{30}$ kg

Nuclear fusion in stars converts mass into energy according to Einstein’s equation E = mc²

### Solar Systems - A **solar system** consists of a star and the gravitationally bound objects that orbit it - Earth is the third of eight planets orbiting the Sun - Earth takes 365.25 days to complete one orbit around the Sun - Objects orbiting the Sun include: - Planets - Planetary satellites - Comets - Asteroids ### Planetary Satellites - **Planetary satellites** are bodies that orbit a planet - The **Moon** is a planetary satellite of Earth - The Moon takes 27.3 days to orbit Earth - This orbital period equals the time it takes to rotate once on its own axis (tidal locking) - **Artificial satellites**, such as GPS and communication satellites, orbiting Earth are also classified as planetary satellites ### Comets - **Comets** are celestial objects made from ice and rock - Comets travel in elliptical orbits around the Sun - Comets originate from the **Oort Cloud** - A distant spherical shell of icy objects surrounding the solar system - Example: Halley's Comet completes one revolution around the Sun every 76 years ## Scale of the Universe ### Age and Size - Evidence suggests the universe was created **13.8 billion years ago** - The observable universe spans a diameter of approximately **93 billion light-years** ### The Light-Year

One light-year is the distance light travels in one Earth year

- $1 \text{ light-year} = 9.5 \times 10^{15} \text{ m}$ - Speed of light in a vacuum: $c = 3.00 \times 10^{8} \text{ ms}^{-1}$ ### Comparative Distances | Object | Distance (km) | Distance (m) | Distance (light-years) | |--------|---------------|--------------|------------------------| | Diameter of Earth | 6,450 | $6.45 \times 10^{6}$ | 0.00000000068 | | Distance from Earth to the Moon | 250,000 | $2.5 \times 10^{8}$ | 0.000000026 | | Distance from Earth to the Sun | 150,000,000 | $1.5 \times 10^{11}$ | 0.000016 | | Distance from the Sun to Alpha Centauri (nearest star) | 40,000,000,000,000 | $4 \times 10^{16}$ | 4.24 | | Diameter of the Milky Way galaxy | 950,000,000,000,000,000 | $9.5 \times 10^{20}$ | 100,000 | | Thickness of the Milky Way galaxy | 9,500,000,000,000,000 | $9.5 \times 10^{18}$ | 1,000 | | Distance to Canis Major Dwarf Galaxy (nearest galaxy) | 2,400,000,000,000,000,000 | $2.4 \times 10^{21}$ | 250,000 | | Diameter of the observable universe | 880,000,000,000,000,000,000,000 | $8.8 \times 10^{26}$ | 93,000,000,000 | ### Putting Distances in Context - Astronomical distances are so vast that standard units (metres or kilometres) become impractical - Light-years provide a more manageable unit for expressing cosmic distances - Example: If you could travel at the speed of light, it would take: - 1.3 seconds to reach the Moon - 8.3 minutes to reach the Sun - 4.24 years to reach the nearest star beyond our Sun

The values of a light-year and the speed of light are provided in the data booklet during examinations

## Summary of Key Terms | Term | Definition | |------|------------| | **Universe** | The totality of all matter, energy, space, and time | | **Observable Universe** | The region of the universe from which light has had time to reach us | | **Galaxy** | A gravitationally bound system of billions of stars | | **Star** | A massive, luminous sphere of plasma undergoing nuclear fusion | | **Solar System** | A star and all the objects gravitationally bound to it | | **Planet** | A large body orbiting a star, massive enough to be rounded by its own gravity | | **Planetary Satellite** | A natural or artificial body orbiting a planet | | **Comet** | An icy body that releases gas and dust when near a star | | **Light-Year** | The distance light travels in one year: $9.5 \times 10^{15}$ m |