We’ve been thinking about the common questions that astronomers get asked and one that was mentioned on the stargazerslounge astronomy forum was, how far can a telescope see? So we got to thinking about how much of the universe can we (us humans) see, with all of the fancy tools we’ve got at our disposal, starting with our eyes. The eyes are the best place to start because it wasn’t until about the late 1500s that we had any other option but our eyes, then we figured out how to make telescopes and these spawned the instruments that we now have available for visual astronomy. Of course there’s a whole lot more to astronomy than just the visible part of the electromagnetic spectrum and, as we’ve discovered in the last few decades, the more parts we “see” of the spectrum, the more we learn about the universe around us. For the purposes of this article we’ll look at what can be seen by the amateur astronomer.
With an eye or two
The eye appeared about 1/2 a billion years ago and the human version is one of the more later versions. Not really optimised for low light conditions and it can’t zoom much, but it is all we’ve had since modern humans appeared about 200,000 years ago. The good news is we have two of them so it’s like a pair of inbuilt binoculars. On a good day from Wellington, in New Zealand, I can see the Seaward Kaikoura mountain range in the South Island which is nearly a couple of hundred kilometres away. Up in an aeroplane you can see a few hundred kilometres in perfect conditions, though I don’t think I’ve ever been able to make out recognisable landmarks over about 250 kilometres. It’s when you look up that you can see some very awesome distances. The Sun, which happened to be out today rather than obscured by clouds, is about 150 million kilometres away. Saturn is easily visible to the naked eye and that’s about 1.2 billion kilometres when were at our closest and a 1.6 billion at the furtherest distance apart. Unfortunately that’s as far as we can see in our solar system.
We can see about 6000 stars in the night sky with the naked eye and most of them are within 1000 light years of us. Eta Carina and the mess that surrounds it is about 7500 light years away and it’s visible to the naked eye. In 1604 a star blew up (see the below picture from NASA) in Ophiuchus which Kepler observed, as well as a bunch of other people as it’s magnitude was about -2.5. This supernova is thought to have been from a star about 20,000 light years away. The bright globular clusters of 47 Tucanae and Omega Centauri are around 15,000 light years away and the Large Magellanic Cloud (LMC) is around 160,000 light years away. The two globular clusters are just visible to the naked eye as fuzzy looking stars and to see the LMC you need a reasonably dark sky. In the northern hemisphere, if it’s really dark and you’ve got a good eye, you can see the Andromeda Galaxy (M31), which is about 2.5 million light years away. And that’s it for the human eye, which is not too bad really.
We thought we would talk about binoculars as their own topic, since we like binoculars and we think people should get these before they invest in a telescope. Given we’re talking about the furtherest that can be seen, I won’t dwell on the stuff that’s less than 2.5 million light years, other than to say that with a pair of binoculars there’s a whole lot more that can be seen, for example with a good set you can see stars down to magnitude 9 which would mean about 200,000 stars would be visible, compared to the around 6000 to the unaided eye. Then there are all of the nebulae, globular clusters etc that are able to be seen as well. As far as pushing the limits and seeing how far we can see with binoculars then best options are the brighter nearby galaxies. Probably the best one is Sculptor Galaxy which is about 11 million light years away, through binoculars it looks like a grey smudge but it is certainly visible. M51 in the Northern sky can be seen through binoculars and that is just over 20 million light years away. Also in the Northern sky is NGC7331, a galaxy that looks a bit like Andromeda, which is about 40 million light years away. This is a tougher target with binoculars but with a good dark sky and a big pair of binos you should be able to see the faint smudge of it’s galaxies. Below is a picture of NGC7331 the author took in about 2009 (it doesn’t look like that with binoculars though).
With a telescope
The farthest thing spotted with the Hubble Space Telescope is a wee red splodge about 13.4 billion light years away near the dawn of the birth of the Universe. Given most of us don’t have a Hubble Space Telescope in the backyard then is probably worth thinking about what a more modest telescope would see. The object generally regarded as the most distant that the average backyard astronomer can see is the quasar 3C273, which is about 2.5 billion light years away. This is a huge distance, back when the light set out on its journey from that quasar to us we only had single celled organisms on Earth, complex life was still a long way off.
In summary, if anyone ever asks you, you can say the most distant thing we can see with the naked eye is Andromeda Galaxy at around 2.5 million light years, unfortunately if you are in the Southern Hemisphere, like us, then you’ll have to say the Magellanic Clouds are the most distant thing you can see.