How deep the Universe

How deep the Universe

How deep the Universe

Bad Astronomy
The entire universe in blog form
Jan. 17 2011 11:07 AM

How deep the Universe

The Universe is a big place.

I mean, really big. Big enough for anything. Literally, big enough for everything. Everything you see, everywhere you go, it's all inside. And there's room for all of it, with space to spare. I get used to it sometimes, and then, suddenly, I'm thrown into a state where I'm forced to remember just how much of the Universe there is.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Let me show you something:

[Click to galactinate, and while it may take a little while to download the entire 3500 x 2000 pixel image, it will definitely be worth your time.]

This is the nearby spiral galaxy NGC 1345 as seen by Hubble. Lovely, isn't it? You wouldn't even think it's a spiral at first; the arms are so faint compared to the sprawling core and inner regions. But it so happens the galaxy is close to our own, making fainter parts easier to observe.


Now there you go. Did you see that? What I said? "The nearby spiral...". "The galaxy is close to our own...". But it isn't.

Look. Let your eyes move to the top of the galaxy, just to the right of center. See that bright star? You can tell it's a star because it has those spikes going through it, an artifact of how point sources are seen by some of the Hubble cameras. Given how bright it is, that star is almost certainly in our own galaxy, and not some luminous giant in NGC 1345; it's just coincidentally superposed on the more distant galaxy. That means it's no more than a few thousand light years away, and given its deep red color, that means it's most likely a very cool and faint red dwarf, and therefore in all likelihood much closer even than that.

But even if it's only a thousand light years away, that's 10 quadrillion kilometers! That distance is impossible to imagine: it's more than 60 million times farther away than the Sun... and the Sun is hardly close. If you could fly an airplane to the Sun, it would take 20 years. Twenty years! And that star is millions of times farther away.

... and that star is the closest thing in that picture. I said NGC 1345 is nearby, and on a cosmic scale it is; it's part of a small cluster of galaxies a mere 85 million light years away: 850 quintillion kilometers. That's 850,000,000,000,000,000,000 kilometers.



But now let your eyes roam over the image. You can see dozens of smaller galaxies crowding the frame. Those are background galaxies much, much farther away than NGC 1345. I've extracted three of them here. Each looks to be a spiral galaxy -- the one on the upper right is edge-on, but the tell-tale dark dust lane across its middle is a dead giveaway that it's a disk galaxy -- and although the distances aren't known, it's safe to bet they are hundreds of millions of light years away. Maybe more. In my time on Hubble we'd routinely see background galaxies that were well over a billion light years away. Routinely. Mind you, each of these background objects is itself an entire galaxy, containing tens or hundreds of billion of stars, perhaps as big, rich, and diverse as our own Milky Way.

And in the course of things, this was a short exposure for Hubble, just a little over half an hour. I once worked on a Hubble image that had an exposure that lasted for days, and we saw objects so faint that the faintest star you can see with your naked eye would be ten billion times brighter. These objects were essentially as far away as anything we possibly can see. And yet the Universe is deeper even than that. It stretches on and on... and while it's finite -- it has an actual size -- in practical terms it's infinite. Why? Because it's expanding. If you could somehow hitch a ride on a photon, the fastest thing in the cosmos, you'd still never reach the edge of the Universe even if it had one. That's because the edge would be receding away from you faster than you could reach it. You'd forever be playing catch-up. Literally, forever.

I sometimes think it's fantastic that we can see anything at all when we gaze upwards. And yet, there it is. Splayed out for us to study, for us to explore.


Some people feel small, insignificant, when they look out into all that space, all that blackness. It's easy to feel that way, but it's not a fair assessment. It can be a struggle, and a mighty one, but it's worth the effort to seek out the awe and the grandeur in it as well. In all that vastness, all that depth, it's entirely possible there are trillions of planets like Earth, and maybe more. But none is this Earth. Nowhere else is there another you, another me.

In the end, when you make that effort, this is one of most important lessons you learn: we're a part of all this. A unique part. And that's a fine thing to know.

Image credit: ESA/Hubble & NASA