Why stars we see are the past

Welcome! It's so exciting to dive into this topic because it is one of the most mind-bending, beautiful truths in science. Whenever you look up at the night sky, you are looking at a living history book.

Before we start exploring how this cosmic time travel works, I'd love to know: how much background do you have with astronomy or physics concepts like ?

I'm a complete beginner, let's start from scratch!

Starting from scratch is the absolute best way to do this! To understand why looking at the stars is like looking into the past, we first have to talk about a fundamental rule of our universe: light has a speed limit.

When you turn on a flashlight in your room, the light seems to fill the space instantly. But in reality, light is a physical thing traveling from the bulb to the walls. It just moves incredibly fast—about (or 186,000 miles per second).

Because space is so unimaginably vast, even at this mind-boggling speed, it takes time for light to travel from one place to another. There is always a delay between when light is created and when it reaches our eyes.

Continue to what a "light-year" is

Let's zoom in on that term light-year. It sounds like a measure of time because it has "year" in its name, but it is actually a measure of .

light-year

noun

The astronomical unit of distance that light travels through a vacuum in one Earth year.

Because light travels at that blistering speed of 300,000 kilometers per second, it can cover an enormous amount of ground in a single year.

If you could jump on a beam of light and ride it for 365 days straight, the odometer on your spaceship would read about 9.46 trillion kilometers (or 5.88 trillion miles) by the end of your trip. That mind-boggling distance is exactly one .