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SYFY WIRE Bad Astronomy

A moment of the Universe

By Phil Plait
A foreground star (upper left) betrays its nature via diffraction spikes, an optical effect when a point source of light passes through a telescope.

Oh, humans.

We go about our lives, scurrying hither and yon, seeing only what’s directly in front of us, and far too rarely taking the time, even a moment, to consider what may lie exterior to our field of view, outside our experience, across the horizon.

Or above it.

So this I offer to you: A chance to see something greater than yourself, something on a scale far larger than you may have known.

The galaxy cluster Abell S0295, a vast structure containing dozens if not hundreds of galaxies. Credit: SA/Hubble & NASA, F. Pacaud, D. Coe

This patch of magnificence seen by the Hubble Space Telescope has the rather mundane name of ACO S +295, also known as the somewhat more lyrical Abell S0295. But the prosaic sound of its name belies its true nature: It is a cluster of galaxies, a massive, sprawling structure so immense it stretches our ability to comprehend.

Consider. When you go outside on a clear moonless, night, you can at best see a few thousand stars. These stars are almost to a one within a few hundred light years of you, with only a handful of powerhouses able to be seen from greater distance.

Yet our own galaxy, the Milky Way, is well over one hundred thousand light years across. We only see a pitiful portion of it. Although it contains several hundred billion stars in its expanse, we can only see a fraction of a fraction of them.

And even that doesn’t fully capture the essence of a galaxy, which also has planets, gas, dust, dark matter, and more. Galaxies are colossal objects, their true nature only becoming apparent to us a century ago.

A foreground star (upper left) betrays its nature via diffraction spikes, an optical effect when a point source of light passes through a telescope.

But look again at Abell S0295. Almost every single object you see in that image is a galaxy, an island universe unto itself. Stars betray themselves via their diffraction spikes, the colorful crosshairs, and I see perhaps two dozen of them, interlopers in Hubble’s view that are inside our own galaxy, coincidentally in our way.

Those stars are as much as a few thousand light years from Earth. But the galaxies in Abell S0295 are 3.5 billion light years distant, quite literally a million times farther away. If that’s difficult to grasp, imagine this: If you stood inside a room a meter away from a window, looking out on the night sky, and saw the International Space Station in orbit around the Earth gliding across your view, it would be about a million times farther away from you than the window.


Mind you, many of the galaxies in this image are even farther away, in the background behind the cluster. The Universe is a huge thing.

And it can be deceptive. Note that most of the galaxies in the cluster appear to be about the same size, as well as fuzzy and red. But then spy the glorious pair of blue spiral galaxies just off center. I suspect (though cannot easily confirm) that they are in the foreground, between us and the cluster. The sense of depth is lost in images of the Universe.

A galaxy in the cluster Abell S0295 gravitationally lenses a more distant galaxy, turning it from a spiral into a sinuous snake around it. Credit: SA/Hubble & NASA, F. Pacaud, D. Coe

Yet we can know some measure of the third dimension here. At the far upper right in the image is a galaxy that’s either a disk or flattened elliptical, and twisting around it is a blue snake, an oddly sinuous river of an object seemingly flowing around the galaxy.

That is a galaxy, likely a spiral similar to the Milky Way, far in the background of the cluster. As its light travels the Universe on its way to us, the gravity of the combined matter in the cluster galaxy bends it, warping space such that the light deflects around it. This is called gravitational lensing, and it can also create multiple images; I strongly suspect the two ends of the snake-like distorted background galaxy are actual duplicates of one part of it. Look closely at the blue knots of light at either end and you can see they are strikingly similar. Given the literal duplicity of lensing, this is unlikely to be coincidence.

Another deception: Abell S0295 is not one cluster. X-ray observations via the Chandra X-ray Observatory find it has two hearts, two distinct regions, one to the lower left and the other centered on our friend the gravitationally lensing galaxy. This implies it was once two separate clusters of galaxies, and they are colliding, merging into one entity. This has been seen many times with galaxy clusters, and indeed careful observations of such colliding clusters revealed they are riddled with unseen material that nonetheless affects them profoundly via gravity, confirming the existence of dark matter.

All this, all these wonders, from a patch of sky smaller than a grain of sand held at arm’s length.

The Universe has profound depths wrapped in layers both visible and hidden, close and unfathomably far, measurable yet still impossible to fit inside the boundaries of our minds.

The light from these various objects has taken hundreds, thousands, millions, and even billions of years to travel across the cosmos and end up here on Earth, an immense stretch of time compared to that in our daily lives. So I hope you take a moment to appreciate the beauty and knowledge it provides us, and allow it to give you an overwhelming sense of awe.