Hubble has taken some of the most breath-taking images of space that we know of, giving us
clear views of galaxies millions of light years away.
And yet, it struggles to take clear images of celestial bodies much closer to us like
Pluto and other trans-neptunian objects.
Surely that doesn’t make sense, how can it see something so clearly so far away, and
the yet the most detailed view of Pluto is this?
Well the answer is a lot simpler than you may think, it’s all about the angular diameter
of the object in question.
This fantastic image is the Andromeda galaxy, a galaxy found roughly 2 million light years
away from us.
But even though it is so far away, use a high exposure camera and you’ll find that it
is 6 times bigger than the Sun or a full moon in our sky.
The reason for this?
It is big, 220 thousand light years across, and at that size, 2 million light years away
is relatively close to us.
Its size and distance from us means that it covers 3° across the night sky.
It is so big in fact, that when Hubble looked at it, it couldn’t see the whole thing in
one go, it couldn’t zoom out enough.
So, this image is actually a mosaic of hundreds of Hubble exposures, and even then, they can’t
fit the whole galaxy in to this image.
Now, most galaxies aren’t as close or as big as Andromeda, but they are still big enough
for Hubble to image them and see interesting details.
Here’s the Pinwheel galaxy, 21 million light years away.
Now notice how it covers the gap between some foreground stars found in our galaxy.
That’s to give you some idea of how big it is in the sky.
If you could see it clearly with your naked eye, it would appear just smaller than the
moon in the sky.
I hope this is giving you some perspective.
Even though it is so far away, it is huge, and so, still has a large apparent size in
our sky.
Just as a side note, I can only image how beautiful it would be if all galaxies local
to us were bright enough to be visible to the naked eye.
Now on to Pluto.
To describe the size of something in the sky, astronomers use degrees, arcminutes and arcseconds.
60 arcminutes in a degree, and 60 arcseconds in an arcminute.
So Andromeda was three degrees across, and the Moon is about 30 arcminutes across.
The Pinwheel galaxy is about 28 arcminutes across.
Pluto is 0.11 arcseconds across at its closest approach to us.
That makes it ridiculously small in our night sky, a wonder that Hubble can resolve any
details on it at all.
But again, closer and larger planets, which have bigger apparent sizes, can be resolved
more easily by Hubble.
That’s why sending New Horizons to Pluto back in 2015 was so exciting, we really didn’t
know what to expect before we got there as we don’t have a telescope powerful enough
to resolve any details on Pluto, and so everything New Horizons sent back was a revelation.
I’ll finish off this point by showing you something you can perhaps relate to a little
easier than these mind-boggling distances and sizes.
In this picture, you can see objects at various distances and sizes.
I want you to think of this flower as Pluto.
It’s small and blurry at this distance.
And yet, there’s some buildings in the background, much further away than the flower, which are
clear in this image.
Imagine these are the galaxies Hubble images.
So, there you have it!
How Hubble can image distant galaxies so easily, but not Pluto.
I hope that clears that up!
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