Image Credit & Copyright: NASA Spitzer Space Telescope of the core region of the Milky Way. This post is a little long for my usual writings but I think that you will enjoy it. If nothing else, the final few paragraphs should leave you with much to ponder.
If you’ve ever been to a dark sky location it’s not hard to be “wowed” by the night sky. As I’m sure some of you know, when you travel out to a dark sky location it’s even hard to make out the usual constellations because there are too many stars! It’s a pretty rare event for many as light pollution is an ever increasing problem that renders our night skies fainter and fainter, year after year. I always have a laugh when I look up because for some reason I think to myself how I feel like a fish in a fish bowl, looking out and wanting to wander around the living room……and preferably avoid the cat as well as the kitchen. But when you look up at night what are you actually seeing and how far away are they?
For this thought experiment we will use the baseline of extremely dark, clear, calm skies, with no Moon and all you have is your eyeballs. Well, glasses if you need them of course otherwise that’s just cruel.
Let’s start with busting a usual misconception, that when you look up at the night sky, many of the “stars” you’re seeing aren’t actually stars but in fact nebulae, galaxies etc. Unfortunately that’s completely untrue and I can almost guarantee that the points of light you’re seeing are, in fact, stars. Unless you know where to look and exactly what to look for it’s unlikely you’re seeing any galaxies at all. That is, of course, unless you live in a location where the Large and Small Magellanic Clouds (LMC & SMC) jump out at you from their positions in the southern hemisphere.
Let’s Begin: For starters, how many stars can you see in your night sky? Not stars as in the plane of the Milky Way but light that you can resolve into individual stars. The average person under perfect conditions and horizon can resolve individual stars down to an apparent magnitude of 6 or 6.5 without optical aid so the estimated answer to that would be about 2,500 stars at any one time. Let’s bump it up from there. Next we’re going to move out from your horizon to your entire hemisphere. Doing this roughly doubles that number, bringing the total number of stars that can be seen from your hemisphere to an estimate of about 5,000. Double that number again and the estimated grand total of stars that can be seen from the entire Earth with the naked (unaided) eye comes to roughly 10,000 though estimates range from a couple thousand less than that to about a thousand more. That’s just a tiny peek, a glance, at the incredibly rough estimate of 100 to 400 billion stars that are believed to inhabit our galaxy.
Now that we have a rough gauge of how many stars we can see, let’s find out how far away we can see without the use of optical aid. We can see things like the Sun, Moon and planets all the way out to Saturn which is about a billion miles away pretty easily. Uranus gets into the mag 5.5 range, thus if you know where to look and what it looks like you may spot it. Neptune on the other hand has never been seen with the naked eye as its apparent magnitude reaches a maximum of around 7.6. Remember that human eyes can see objects in space down to an apparent magnitude of about 6.5 (possibly 7 but it’s a stretch).
So what about stars? Most of those 10,000-ish stars visible from Earth are within roughly 4,000 light years. When you actually think about the fact that the Milky Way is about 100,000 light years in diameter it’s truly staggering just how far we CAN’T see in detail. The summer star Deneb in Cygnus the Swan is recognized as one of the most distant stars that are easily visible at a distance of roughly 2,600 light years. Herschel’s Garnet Star or Mu Cephei would be next at a magnitude of 4.2 and a distance of about 6,000 light years. Remember though, we’re talking about perfect conditions, and in those conditions its V762 Cassiopeia that takes the cake at an apparent magnitude of 5.8 and at a distance of roughly 16,000 light years. I’ve never even bothered looking for V762 Cass as of yet as there’s no chance from my red zone but it’s on my celestial bucket list for sure. And remember, I said most are within about 4000 light years, not all. The very large majority are within that range for sure.
So we’ve pushed the boundaries of naked eye visibility as far as stars but what about non-pinpoint objects such as star clusters and galaxies? It’s true; there are a few select galaxies; that with near perfect vision and sky conditions can be seen without optical aid. I’m going to skip over some of the close open and globular clusters and like Tom Brady, go deep straight away. The great globular cluster Omega Centauri at a distance of 15,800 light years is easily visible in southern skies at an apparent magnitude (mag) of 3.9. Beyond that is the summer cluster of M13 or the Hercules Cluster as its known. At a distance of 25,000 LY and with a mag of 5.8 can be seen in clear dark locations if you know where it is and what it looks like.
Let’s leave the galaxy….sort of. The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are satellite galaxies to our own Milky Way and if you live in the southern hemisphere these two spiders on the wall are easily visible in dark locations. The LMC sits at a distance of 160,000 LY with a mag of about 1 and the SMC is about 200,000 LY away with a mag of roughly 3. These objects are the furthest things associated with our galaxy that we can observe without optical aid.
So we can see things outside of the Milky Way without binoculars or telescopes? Yea, the furthest object that you can observe readily in perfect conditions is the 2.5 million LY distant, 3.4 mag M31 Andromeda Galaxy. If you have near-perfect vision and are under perfect skies the 2.8 million LY distant, mag 5.7 M33 Triangulum Galaxy can be viewed. It’s been said that M81 Bode’s Galaxy at a staggering 12 million LY with a mag of 6.9 is also possible to view naked eye for some people but honestly I would have to see it to believe it! The problem with observing galaxies is first and foremost, you have to know what you’re looking for. They call em “faint fuzzies” for a reason. They do not resemble stars in any way and even through telescopes you truly have to get some experience behind you before you look at a galaxy and recognize it as such. You don’t see vast spiral arms and you don’t see color or nebulae running through them. All you can readily make out is the dim fuzzy core of these galaxies even in telescopes so naked eye viewing definitely takes some work. You might be keen to question as to why, for example M33 would be so difficult to observe, after all its apparent magnitude is 5.7? The problem is that M33 would be an apparent magnitude of 5.7 if all of its light was concentrated into a single, star-like point. The actual situation is that its brightness is spread out over the surface of the galaxy and that surface brightness comes in at a magnitude of 14.2 making it very difficult even under the best of conditions.
Now let’s have some real fun! So we’ve pushed well beyond the limits of star numbers and distances and realized what’s available to us as far as viewing the small handful of galaxies/satellite galaxies. There are objects in the universe however, that allow you to see massive distances out into space and back into time. They aren’t always there and they aren’t always in the same place twice. They are Supernovae and Gamma Ray Bursts.
Supernovae can, for a short time shine with the brightness of millions to billions times more brightly than a star and regularly outshine the entire galaxy they reside in. What’s that mean for us? Every so often (usually a few times a year) a supernova will explode in a galaxy millions of light years away. Of course you can’t see that galaxy with your eyes but where that galaxy lies, a new “star” will appear for a few days before fading away. During that time, you are viewing with your naked eye something that occurred before humans even existed on Earth.
Then there’s the gamma ray burst…….On Wednesday, March 19, 2008 at 06:12 UTC (02:12 EDT) gamma ray burst GRB 080319B was detected by the NASA Swift space telescope. Those lucky enough to be observing the constellation Bootes for the brief 30 seconds or so that it was mag 5.8 (naked eye visible) witnessed something so powerful and so bright that its light could be seen on Earth from a distance of 7.5 billion LY. That’s correct; this event took place when the universe was less than half its current age. When the Sun, the Moon and planets of our solar system were likely still a nebulae of long exploded stars, waiting to collapse and create…….us.
Another intriguing fact is that a whopping five GRB’s were detected that day…….It was also the day that the legendary Arthur C. Clarke died. The universe seemed to be delivering a fireworks show for their reclamation of one of the greatest science fiction writers in history.
THE NAKED EYE SHORT LIST
Herschel’s Garnet Star (Mu Cephei) 6000ly
Omega Centauri (NGC 5139) 15,800ly
V762 Cassiopeia 16,000ly
Hercules Cluster (M13) 25,000ly
M31 Andromeda 2.5mly
M33 Triangulum 2.8mly
GRB 080319B (7.5 billion LY away in Bootes)
Spitzer Space Telescope page for this image: http://www.spitzer.caltech.edu/images/1540-ssc2006-02a-A-Cauldron-of-Stars-at-the-Galaxy-s-Center