Lesson Objectives:- Defining astronomy
- What makes up the universe
- Time and distance
- The size and age of the universe
Astronomy is the study of celestial objects and phenomena to understand origins and evolution. Studying astronomy applies mathematics, physics and chemistry to explain the origin of celestial objects (stars, planets and more) and how they evolved.
Human beings have always been curious about the endless array of stars they can see dotting the night sky. The scale of the Universe is very large. Our planet Earth orbits the Sun, a star, and is part of the Solar System, which in turn is part of the Milky Way galaxy. The Milky Way galaxy is one of billions of galaxies in the universe.
The Universe consists of all matter and energy. This includes everything from the smallest particles to large clusters and systems.
A Star is a glowing ball of gas that generates heat and light. Our Sun is an example of a star. A planet is a large object that orbits a star and reflects light from the star. Earth and Venus are both examples of planets. A Moon is an object that orbits a planet. The Earth has one Moon. Other planets may have more than one Moon.
A Star System consists of a star (or more than one star) and any planets that orbit it. In our case, we have the Solar System that is a star system consisting of the Sun, planets, moons or small solar system bodies (asteroids and comets) that orbit around it.
A Galaxy is a collection of stars. Our galaxy is the Milky Way Galaxy, just one of billions of galaxies in the Universe!
Galaxies are not static. They move quickly. A light year is the distance light can travel in one year, estimated to be 10 trillion kilometers.
The Andromeda Galaxy, which has been photographed using the Hubble telescope, is estimated to be traveling through space about 2.5 million light years from Earth. A supercluster is a cluster of galaxies.
The universe, as stated before, is the sum total of all matter and energy. The universe is also known as the cosmos. Thus, it includes all galaxies, superclusters and everything in between. The observable universe is the part of the universe that can be viewed from the Earth, and is only a small part of the entire universe.
One of the most fascinating topics when studying astronomy is the concept of time in the universe. To understand this better, let's first see how we measure distance and movement in the universe.
Distances between planets and objects within our Solar system are measured in astronomical units. An Astronomical Unit (AU) is the Earth's average distance from the Sun, which is 93 million miles or 150 million kilometers.
A Light year (ly) is the distance light travels in 1 year, which is 6 trillion miles or 10 trillion kilometers. We use light years to describe the distances of stars and galaxies. A light year is used to measure distance, not time. A light minute is the distance that light would travel in one minute, for example.
When we see a distant galaxy using a telescope, what we are actually seeing is a 'snapshot' of that galaxy moving through the Universe. For example, some galaxies seen through the Hubble telescope are 12 billion light years away, meaning that what we are seeing is actually from more than 12 billion years ago.
The Universe as measured by human beings is about 14 billion years old. Given that objects in the universe are moving at fast speeds trillions of miles away from us, what we see when we look deep into space using a sophisticated telescope is still only a snapshot from the distant past of those celestial objects and systems.
We cannot see light coming from anything more than 14 billion light years away, since that was before the known Universe existed. Hence 14 billion light years is considered the boundary for the observable universe. As mentioned before, the observable universe is the part of the universe that can be viewed from Earth. It does not include the entire universe since the light from anything further than 14 billion light years away has not had the time to reach us yet.
How big is the Universe? Now that we have looked at various parts that make up the Universe and have some understanding of sizes and distances in relationship to each other, it is easy to see that the Universe is mind-blowingly large. How big exactly still remains unknown. However, to have a better perspective of the scale of the universe, let us look at just one galaxy - the Milky Way.
The Milky Way galaxy consists of over 100 billion stars. Our Sun is one such star and our planet, Earth, is one planet of many, orbiting one star. The Universe consists of billions of galaxies. In some ways, you can compare the number of stars in the observable universe to the number of grains of dry sand on all the beaches on Earth!
In the observable universe, there are roughly 100 billion times 100 billion stars.