The closing scene of this week’s episode of Cosmos was a spectacular view of the Milky Way galaxy rising, which NdG Tyson said might be seen “from a planet orbiting a star in a distant globular cluster.”
It shows a spiral extending almost as far across the horizon as you could see without moving your eyes, with the nucleus brighter than the full moon, and many times larger.
But we’re inside the Milky Way, looking at it “edge on” rather than top down, so it should be at least as bright as the nucleus would look from above, and yet it’s so dim that many people have never even seen it. If you’re out in the desert or something, it’s plainly visible, but nowhere near as bright as the moon.
The spiral he depicted was like one you see in telescopic photos, but my understanding is that they are the result of long exposures, and not what a human eye could see.
So was he taking artistic liberties, or could a galaxy really look like that to human eyes from the hypothetical planet he mentioned?
Would the Milky Way be dazzlingly bright from Earth if there were no dust clouds in the way?
I haven’t seen the new episode yet, but Carl Sagan did this in the original - his famous “Glorious dawn” quote. This may be the picture.
The nearest spiral galaxy to the Milky way is Andromeda - M31. It’s about 2.5 million light years distant. And at least in long-term exposures looks like this in comparison to the moon (of course M22 is much fainter). The nearest globular cluster M22 is only about 10,600 light years away, 2^8 times closer. That would make Andromeda truly gigantic and much brighter (were you as close to it), and though you probably still couldn’t read by it, it would certainly be bright enough to see details in the spiral arms.
I’ll try and find the NdG picture and post that after I watch GoT.
It would look a bit brighter than the Milky Way, but not much. Since we live inside the spiral, a lot of the light from the Milky Way is blocked by dust clouds, and by diffuse dust and gas in the interstellar medium. As seen from above this obscuration wouldn’t happen, so we would have an unobscured view of parts of the galaxy we can’t see right now.
But it wouldn’t be very bright, and certainly not as bright as in some of these artistic interpretations. We see the Andromeda Galaxy as a faint fuzzy blob; if it were closer that blob would look much larger, but the surface brightness per unit area of the sky (per steradian) would be about the same, so close-up the Andromeda Galaxy would look like a large, dim, fuzzy blob.
From the same distance and angle the Milky Way would look the same. On a dark moonless night it would be stunning - but from a city you’d be lucky to see it at all.
So that’s mostly long-exposure photo and a helping of artistic license. I don’t think we’d see as much color and gas clouds between the spiral arms. Also, that’s a top-down view of a spiral galaxy.
I reckon our view toward the center of the Milky Way wouldn’t be that much improved if there were no dust clouds. It’s just that we’re like 85% towards the “back” in the cheap seats.
The Aqulia Rift is a vast dust cloud between us and the brightest part of the galaxy, and it is one of hundreds of others.
Apart from the dust clouds, there is the effect of diffuse gas and dust; this is known as extinction, and is on the order of 1.8 magnitudes per kiloparsec in the plane of the galaxy. Extinction (astronomy) - Wikipedia)
The galactic centre is eight kpc away, so any stars that manage to peek out between the dust clouds will be about 14 magnitudes dimmer.
As seen from above starlight will only need to pass though a kiloparsec or less of galactic medium to reach the eye, so will be almost as bright as if extinction didn’t exist.
It would be a cool sight but as eburacum45 has pointed out not anything like the illustration shown. Think of it as the same brilliance of the Milky Way that we see in a clear sky but instead of a diffuse band of light in a fairly straight line we’d see a faint spiral shape with a slightly brighter center. Actually, I live in the Northern Hemisphere and have not seen the Galactic center except near the horizon. It might be that the central region, without its shroud of intervening dust and gas, would be nicely vivid but still a far cry from what’s pictured.
For illustrative purposes, here’s the Sombrero Galaxy, a galaxy that’s close to edge-on to us. See how much darker the edge is than the middle? That’s due to the same sort of dust clouds as block our view of our own Galaxy. Our view of our Galaxy is to the view from the outside as the dark portions of that image are to the light portions.
According to this thread, the Andromeda galaxy from 80,000 LY is no spectacular sight, being no more than a fuzzy patch of light, brighter than as seen from earth, but nothing like the long-exposure photos. Since most of the stars in any galaxy are dim red dwarfs, even a trillion of them in a galaxy as large as Andromeda will not look visually spectacular unless you are almost inside the galactic halo. Our galaxy has about 25% less brightness than Andromeda.
Now if there were a quasar nucleus, it should be very different. I imagine if we had 3C 273 lurking in the center of our galaxy, with 2 trillion solar luminosities, we might be able to read by it on Earth. :eek:
