While laying back in the hot tub the other night and sipping very dry martinis, my wife and I were looking up at the stars and commenting on how the light has travelled such a great distance, and how we were essentially looking back in time. She asked me, and I’m asking you, the masses:
What is the most distance object that can be seen from the Earth with the naked eye and when did the light that we’re just now seeing leave the object? In other words, how far back in time are we able to see?
The Andromeda galaxy is about 2 million light-years distant. That’s the farthest object I can think of off the top of my head. I’m sure there’s other, more distant galaxies that are naked eye objects.
M33 is naked-eye visible on nights with ideal seeing for people with perfect vision, according to what I read. M31 is the farthest the rest of us can see.
As I understand it, visible to the naked eye under ideal seeing conditions (* indicates at the edge of visibility) are:
Six major planets (Mercury, Venus, Mars, Jupiter, Saturn, Uranus*)
One minor planet/large asteroid: Vesta*
One satellite: the Moon
Note: The four Galilean satellites of Jupiter are, IIRC, all bright enough, at close opposition, to be naked-eye visible but are obscured by Jupiter’s relative brilliance.
~6,000 stars, all supergiants, giants, or relatively nearby main sequence stars of K and up spectra.
Several nebulae, number unknown to me.
One globular cluster: Omega Centauri*
Four galaxies: the Magellanic Clouds, M31 in Andromeda, and M33 in Triangulum*.
Although 2.3 million light-years is often quoted as the distance to the Andromeda Galaxy, I believe that more recent observations suggest a distance of more like 2.9 million light-years.
The Andromeda Galaxy has a “peculiar” velocity of 100km/s towards the Milky Way. If I used the Google calculator correctly that would mean that Andromeda was a bit more distant when the light we see left it (but just by about 1000 light-years).
A type 1a supernova with an absolute magnitude of -19.5 would be just visible to the naked eye, mag. 6, at a distance of 4,104,097 light years.
Using m = M + 5log(d) - 5 ( M = absolute magnitude, d is in parsecs (3.2 LY))
You’d have to know where to look, and catch the star in the first few hours after it exploded.
The reason why you can’t see them is obvious. All stars emit light with a certain power. If the energy flux from the star reaching your eye is above a certain limit ( 8,8*10^-11 w/m2) you can see the star. If not, you can’t see it.
If we make some assumptions we can easily calculate the amount of visible stars. If we assume that all stars emit light like our Sun, that is, with P=3,910^26W, and that there are 3,510^-3 stars per cubic meter in the space surrounding us, we can calculate that the amount of visible stars is 3700. Not all stars emit light like Sun and their density in space is not constant, but I hope this crude model helps somebody…
I put together a chart of the maximum distance at which various types of stars would be visible to the naked eye back in this thread: Naked eye viewing
Hasn’t s Doradus flared up in historical times bright enough to be a naked-eye object, and isn’t that in the Lesser Magellanic Cloud? Can’t find the reference at the moment. (That’s barely a tenth of the distance to the Andromeda galaxy, but a single star rather than a galaxy or cluster.)
Slightly off topic, but this site posts a new Astronomy picture of the day every day, with an explanation of each one.
Some of them are photos of the things talked about in this thread. And many of them are very interesting photos. I found one that I liked so much I used it as my PC wallpaper.