Why can we see distant galaxies with a telescope but not Pluto?

Until recently, the only views we had of Pluto were distant and blurred images taken from telescopes on Earth, or maybe a little better taken from the Hubble telescope. Because of the tiny size of Pluto, and its extreme distance from Earth, the only way to get a better image was to actually travel to Pluto.


On a special mission, NASA sent the New Horizons craft out to the edge of the solar system, and thankfully many of the secrets of the secretive Pluto have now been revealed, like this volcano on the surface of the frozen planet.

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Why Can’t we see that with a Telescope?

But that is small comfort to those astronomers among us who have two feet firmly stuck on the ground. I don’t plan on interplanetary travel any time soon!

So why can we see such beautiful images of spectacular deep space objects such as spiral galaxies and colorful nebulae, when we couldn’t even see Pluto properly within our own solar system?

How can an Earth bound telescope capture brilliant views of distant galaxies which are millions of light years from Earth, yet we were barely able to pick up Pluto, which although very remote, is many orders of magnitude closer to us?

The answer lies in the physical size of the deep space object, as well as the distance from Earth. We need to consider the issue in terms of mathematical parameters, but really it is a simple concept. The ratio between actual size and distance away is known as angular size, which is a measurement of the relative size of an object compared with the overall size of our picture of space.

In essence, despite the fact that galaxies and star clusters are many times more distant than Pluto, they are as we understand, made up of millions of individual stars spread over a much wider space in the universe. Hence, by many orders of magnitude in size, and despite the greater distance of separation, a galaxy actually covers a larger spatial portion of the sky.

How do we measure in Space?

Much in the same way as a compass is divided up by angular units called degrees, if you imagine the sky being divided up into angular units, then you can understand the maths. A circle consists of 360 degrees, and each degree can be divided into sub units called arc-minutes (there are 60 arc-minutes in a degree!) which can be further broken down into arc-seconds (yep – 60 arc-seconds in one arc-minute).

The Sun and the Moon

The best way to illustrate the spatial measurement of objects is to compare the Sun and the Moon. Now we know that the Sun is much larger than everything else, but have you ever wondered why the Sun and Moon appear roughly the same size? Although they are vastly different in terms of size and separation, both the Sun and Moon have roughly the same angular size in the sky. They share a similar ratio of size and distance which equates to a very similar angular measurement. The Sun is roughly 400 times more distant than the Moon, but is 400 times larger, hence equating to the same angular size of approximately half of one degree of the sky (0.5 degrees)

Pluto and the galaxies?

But by comparison, Pluto is a very small object – not much larger than the Moon in fact, and a lot further away from Earth. In terms of angular size, Pluto measures only 0.05 arc-seconds, which is many orders of magnitude less than the Sun and Moon.

A galaxy is also very distant, but if your telescope can pick it up, then that is because of the relatively massive size of the object, which typically measure up to 1 arc-second, or 20 times larger than Pluto in terms of spatial size.  Have a look at some of the magnificent visions of the more spectacular galaxies that are visible to any keen astronomer.

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But luckily for all of us, NASA has taken such great images of Pluto, because no matter how hard you try, you won’t get a good view from your backyard at home!



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