Cosmic Proportions

Everyone you’ve ever loved (or hated), everything you’ve ever held dear, every experience you’ve ever had – all of these have been confined to one tiny rock orbiting a giant ball of gas at 67,000 mph. From a distance, that tiny rock – Earth – is just a pale blue dot – or in the words of Carl Sagan – “a mote of dust suspended in a sunbeam.” On this speck, on this space pebble we call home, humankind resides in all of its glory. But what is our place in a universe so immense? Just how big is the cosmic darkness that encompasses us humans, as a speck upon a speck?

If we really want to grasp how big the universe is, which is largely impossible and genuinely mind-melting, we have to start small. And when I say small, I still mean pretty big.

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The Pale Blue Dot: in this image from NASA’s Voyager 1, the Earth sits in sun ray as a pale blue speck denoted by the blue circle. This photo was taken by Voyager 1 as it exited the solar system and turned back to gaze at its home planet, about 4 billion miles away.

If the Sun was a Basketball

Let’s begin with the solar system. It’s hard to imagine astronomically large distances, so let’s relate our solar system to something familiar – a basketball. If the Sun was a basketball (about 23 cm in diameter), then the Earth would be the size of a tiny bead, just over 2 mm wide. Since the average Earth-Sun distance is about 150 million km, this bead would sit about 25 m away from the basketball. That’s approximately half the width of a football field (and also apparently twice the height of a brachiosaurus – thank you, internet).

Hopefully, you are beginning to picture this strange scenario where there is a basketball on a football field. Keep in mind that Neptune, the most distant planet in the solar system (sorry Pluto), is 30 times farther from the Sun than the Earth but only 4 times the diameter of the Earth. Just think: Neptune would be the size of an ant over two football fields away from our basketball-sized Sun. Sure, planets are big, but the distances that separate them are enormous in comparison.

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A log distance scale of our solar system and surrounding objects. This scale is based on AU, where 1 AU is the Earth-Sun distance. AC +79 3888 is a star 17.6 ly away – in 40,000 years, Voyager 1 will come to its closest encounter with this star at 1.6 ly away.

In reality, the Sun is not a basketball. It’s actually about 1.4 million km in diameter. If you melted them down, 1.3 million Earths could fit inside the Sun, and our home star isn’t even the biggest star out there. The largest star ever discovered is UY Scuti, which boasts a diameter 1,708 times that of the Sun. This massive size means that UY Scuti could hold the volume of  6 quadrillion 489 trillion Earths. On our scale where the Sun is a basketball, UY Scuti would be 400 m wide (check out the 500m FAST telescope for comparison). We’re not even sure that UY Scuti is the largest star in existence – it just happens to be the largest star in the Milky Way. That being said, UY Scuti could eat the Sun for breakfast. But it’s also 9,500 light years away, so that’s not going to be a problem.

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A scale of commonly known stars for size comparison. You can see that the sun is represented in the center, but that it isn’t even big enough to merit a visible dot.

A Long Time Ago, in a Galaxy Not So Far Away…

What is a light year (abbreviated as ‘ly’), you ask? It must be a distance of time, you say? Not quite. Although the name may be misleading, a light year is the distance light travels in one year – that’s 63,241 Earth-Sun distances, or 9.5 trillion km to be exact. In keeping with our weird sports analogies, a distance of one light year equates to about 14,500 football fields away from our basketball/Sun. With that in mind, it sounds like UY Scuti is so far away that it must be at the edge of the universe. In fact, 9,500 ly is chump change in comparison to the size of our home galaxy. The Milky Way itself is fairly large, 100,000 ly across (at a minimum, some estimates report a larger diameter courtesy of a larger halo), but it’s puny when considering monstrous galaxies like  IC 1101, which is about 2 billion ly in diameter – that’s 20,000 times as large as the Milky Way.

The Universe for Scale

In the observable universe, there are an estimated 100 billion galaxies, each containing billions of stars. Even more astonishing is the fact that what lies between each star and galaxy is light years of empty space. The closest star to our solar system is Alpha Centauri, which lies 4.2 ly away, and the closest major galaxy to the Milky Way is Andromeda, located 2.5 million ly away. Multiply these numbers by 9.5 trillion km (the length of a light year) and you’ll see just how far interstellar objects are apart. Think of how vast the universe must be to allow for this. And you thought your commute to work was far.

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The Hubble Ultra Deep Field: the deepest image of the sky ever taken, a composite image of UV to infrared. This is a very small portion of the universe – only 2.4 arcminutes by 2.4 arcminutes (2.4 arcminutes is roughly 1/25th of a degree) – yet it contains an estimated 10,000 galaxies. That’s right, most points of light in this image are galaxies, each containing billions of stars.

The universe, as far as we can tell, is about 92 billion ly across. You may have heard that the universe is 13.8 billion years old, and this is true. But because space is expanding, the universe is larger than 13.8 billion ly across (if we didn’t know better, we may expect the age and the size to be the same, since the speed of light is the fastest that information can travel). Because of inflation, things that were 13.8 billion ly away may now be up to 46 billion ly away. That’s as far out as we can observe, since anything past that point is farther than light could travel to us given the age of the universe.

That’s right, the universe is big and it is only getting bigger. At the end of the day, no one really knows how big the universe is. We are pretty sure it’s flat, which means it would be infinitely wide and could expand forever. You may find yourself asking, “But what is it expanding into?” A cosmologist would say that space itself is expanding, but that is hardly satisfying to most folks. In a possibly infinite universe, there are infinite mysteries.

Small but Stellar

This is not to make you feel insignificantly small in a universe of boundless size. There is a deeper meaning behind all of this large-scale language!  In a breadth of space which we cannot fathom, in a universe of endless elegance, you exist. You are an integral part of something beyond human comprehension. Even the elements that make up our bodies were created in stellar explosions of unimaginable power. We are indeed made of star stuff; we contain the universe that so graciously contains us. And that, my friends, is wondrous.

About the author:

image04 Lauren Sgro is a PhD student in the Physics department at the University of Georgia. Her research focus is in astronomy, specifically debris disks around young stars that may tell us more about planetary formation. Despite the all-consuming nature of graduate school, she enjoys doing yoga and occasionally hiking up a mountain. You can’t reach her on Twitter, but you can email her at lauren.sgro25@uga.edu. More from Lauren Sgro.