How Vast Is Our Universe?

Does anyone have any idea—”How vast is our universe?” “How big is our sky?” Everyone would say it’s enormous or immense. Let’s try to view this enormity in a simpler way—

The units we regularly use to measure distance aren’t quite suitable for expressing the immensity of space. So, first, we need to choose a unit that’s adequately large. Scientists have chosen the distance between the Earth and the Sun as a unit of measurement. In conventional units, this distance is 1.5 x 10⁸ km = 150,000,000 km. In astronomy, this distance is called 1 AU (Astronomical Unit).

That is, 1 AU = 150,000,000 km.

Using this unit, the distance between the Sun and Pluto is about 80 AU, or 80 x 150,000,000 km = 6,000,000,000 km. What a massive distance that is! Isn’t it? Since Pluto is the outermost planet in the solar system, this distance can be called the radius of the solar system. (Although there is debate over considering Pluto a planet today, for now we are counting it as a planet only to illustrate the vastness of the solar system.)

The distance from the solar system to the nearest star is 300,000 AU. There’s no need to explain just how enormous that amount becomes if expressed in kilometers.

On a clear night, we can see the beautiful, cloud-like Milky Way across the sky. It’s nothing but a collection of millions upon millions of stars. Our familiar Sun is also a giant star, located at one edge of this very Milky Way. Now the question is—”How big is the Milky Way?”

To express the vastness of the Milky Way, we need to become familiar with a new unit, which is much larger than AU. This new unit is called a light year. The distance that light travels in a year is called one light year. It’s a huge distance because light travels 300,000 km every second. So, by calculation,

1 light year = 9.5 x 10¹² km = 9,500,000,000,000 km

Using this new unit, the diameter of the Milky Way is about 100,000 light years (or about 6,000,000,000 AU).

The Milky Way is a galaxy. No matter how immense this galaxy is, here’s the interesting part—there isn’t just one, but millions upon millions of galaxies in the universe. So, you can see, the vastness of the universe is beyond imagination.

To make sense of the scale of the universe, take a look at illustration 1. Although it is difficult to fully comprehend the size of the universe from the above information, the given illustration should provide a rough idea.

One more piece of information—the distance between the farthest known star in the universe and us is several billion light years.

Source: book-“Chandrasekhar And His Limit”