One of the things that’s very difficult to appreciate is the relative sizes and distances of celestial objects. After all, their sizes and distances aren’t remotely small enough for us to appreciate. As a result, everything we see looks in some way compressed – one object in an image may just as easily be twice as distant, or ten times as near, as another with little way of knowing the difference.
But if we apply a little knowledge to these images, the relative distances seem to jump out at us, like this:
This is a Hubble Space Telescope image of ESO 318-13 taken with Hubble’s Advanced Camera for Surveys. ESO 318-13 is an irregular-shaped galaxy. That is, a galaxy that has no formal structure such as spiral arms or even a well-defined central nuclear region.
It’s a beautiful image, but what I really love about it is that in one shot we can discern a tremendous amount of perspective and depth. Let’s start with the nearby stuff and work our way outward.
The brightest objects are foreground stars in our own Milky Galaxy. Coincidentally the foreground star in the middle right of the image happens to line up with the middle of ESO 318-13, but keep in mind that the galaxy is itself a collection of stars millions of light years beyond.
If you look at the full-resolution version of the image, you’ll see individual stars within ESO 318-13. And if you look toward the right-edge of the galaxy you’ll spot a beautiful face-on spiral tens of millions of light-years in the distance behind the galaxy. For all of their grandeur, galaxies are mostly empty space, and can be transparent enough to allow background galaxies to easily be seen through them.
Pan around the image and you’ll see a lot more galaxies in the background, including a giant elliptical galaxy in the upper right-hand corner of the image. ESO 318-13 is a dwarf galaxy and may contain a few tens of millions of stars. But the elliptical galaxy in the background is a cosmic titan containing several hundred billion suns!
The distances to objects in this image are all relative – there isn’t anything that tells us exactly how far away they are just by looking at them. To get those measurements we need to use indirect methods such as spectroscopy. But in one image we can begin to get an idea of the sheer scale of the cosmos – keeping in mind that this is still a fairly local region of the universe!