Milky Way and Andromeda galaxies are already merging (2020)
earthsky.org
earthsky.org
> But, of course, the galaxy isn’t nearly this bright. You need a dark sky to see it, and, even then, it’s a barely visible fuzzy patch of light. In order to appear as bright as in the image above, the Andromeda galaxy would need to be closer. If it were close enough to look so bright, it would appear even bigger on our sky’s dome.
Things closer by do not appear brighter, they merely appear bigger. The Andromeda galaxy will never appear as bright as in that image, not matter how close it is. As things get closer they do, of course, cast more light on us, but that light also comes from a larger solid angle, which means when we look at them, the brightness (per area) remains constant. It's why your computer monitor doesn't look any dimmer as you walk away. It just gets smaller in your field of view.
This breaks down for pointlike objects, like single stars - anything small or far away enough to look like a point to our eye cannot get any smaller, so changes in distance are only apparent as changes in brightness. But as the image demonstrates, the Andromeda galaxy is much, much larger than a pointlike object to the naked eye, and so this does not apply (it doesn't matter that the individual light sources are pointlike stars here - for a galaxy they average out due to density, but even if they didn't, as individual points get brighter the space between them gets larger so it all still cancels out when we look at overall apparent brightness for the whole group).
One thing that happens is that once the star fills a 2D area in the sky its brightness stops falling off at 1/r^2 and falls off at 1/1 (no falloff) in the near field where the area is a significant fraction of the overall field.
You missed my point about pointlike objects.
If you moved the sun 1 AU further away, it would of course cast 1/4 of the total light on the Earth, but as an object in the sky it would have the same surface brightness as it does now. It would just look smaller. I'm talking about the appearance of objects in the sky to a camera-like observer.
Once the distance increases to the point where the human eye cannot resolve it beyond a single point, then brightness starts diminishing with distance. This is the case for all single stars other than the sun. This is not the case for aggregate objects like galaxies that are large enough to see as non-points with the naked eye, like Andromeda, regardless of whether they are composed of individual pointlike stars or not.
The expressions "look so bright" and "appear even larger" refer to a change in our perception of Andromeda, i.e., the resolution of your eyes. In the distance, the bright objects blur with the dark background, so that we do not perceive a lot of structure. Closer in, the contrast increases so that we can distinguish brighter from darker structures. This is analogous to the fact that with binoculars we can see stars that we cannot see with the naked eye.
When a distant object gets closer, the area that the object occupies in our field of view becomes larger. So while its brightness under the same angle remains the same, its overall brightsness increases. Think of someone approaching with a torchlight from 100 meter to one centimeter from your eye.
This is what can be seen in the eight pictures in the article. The little Andromeda spot and its halo on the first picture is as bright as Andromeda in average in the subsequent pictures; only that it becomes larger. The 7th and 8th picture show the core of Andromeda and the Milkyway together and without the darker surroundings. This is the reason why the overall brightness of the pictures is higher. (Or in the binoculars analogy: Don't look with binoculars into the sun!)
And now lets figure what the k need to be to make Andromeda this bright. Does such k exists? I believe it does.
I'm not sure would it work, because my physics knowledge is limited, but if I'm right then it is a non-confusing way to interpret the confusing statement "needs to be closer to be brighter".
After thinking about it for a while I understand it now. The inverse-square law applies to a point source. As you move further away the intensity drops off for each point, but the points get closer together in your field of view to compensate.
One weird thing is that many of the stars in the sky would be as bright or brighter than the Sun, but their "disc" is just so tiny that we perceive it as being dim.
The inverse square law always applies, but it relates to the total amount of light received by a constant area observer (or density at a pointlike observer). Once you're projecting the image of the source onto a plane with a lens, that doesn't map to surface brightness.
Another place where the inverse square law breaks down is with beams of light, in the near field - consider something like a laser beam. While it will expand, and eventually follow the law once you get far enough, it very much doesn't at close range. And there's an interesting parallel here: to an observer, a collimated laser beam looks like a perfect pointlike object at infinity (modulo the diffraction limit, of course). Another way to see it is that focusing a beam of light in a certain way creates a virtual emitter much further away, and so you need to add that to the distance you plug into the inverse square law.
If we assume the diameter of the solar system is 100 AU (the size of Pluto's orbit) and Proxima Centauri is 4 LY away, the ratio is about 2500: The distance to Proxima Centauri is about 2500 times the diameter of the solar system.
If we assume the diameter of the Milky Way is 100k LY and the distance to Andromeda is 2.5 million LY, the ratio is 25. Not 2500 but 25.
Here is an image I took of it w/ a full frame camera on a tripod and a 200mm lens back on Feb 7th, https://files.catbox.moe/59swbu.jpg
I was pleased enough with that result and the images I took of some other targets with that technique, I got a tracking equatorial mount. Ironically, my first images from it were worse due to exposing the sensors banding that escaped calibration which had been covered up by all the frame to frame motion in my untracked images: https://files.catbox.moe/r0vlqt.jpg
(I've since figured out how to address the noise, https://files.catbox.moe/fu23xd.jpg , but now Andromeda sets too early to photograph until months from now. :) )
And while the intergalactic medium has very little matter (often quoted as ~1 atom per cubic meter) it's not evenly distributed and there are stars. In the Milky Way stars are on average ~5 LY apart. In the IGM it's likely this number is more like ~1000 but it's really ahrd to say for sure.
Do the math. 1 LY = ~9.5e15m. A cube 1000 LY on each side is ~8.6e56m^3. The Sun is ~1e30 kg. For simplicity let's say that's all hydrogen. Avogadro's Number tells you thats 6e23 atoms per gram or 6e57 atoms in a space of 8.6e56, which comes out pretty close to 1 atom per cubic meter while there still being stars.
So the gas halos of the Milky Way and Andromeda galaxies are arugably already "colliding". I'm not sure this is a useful description of a galactic collision. There are gas structures in the IGM. At what point do you consider something to have collided exactly?
Eh, the concept isn't that arbitrary, it's the borders that would be more arbitrary. It's like asking where does a river end and the ocean begin. No one would argue the Atlantic isn't an ocean and the Amazon isn't a river, but exactly where you draw that border requires some discrimination.
Either way, ya is indeed confusing.
I'm going to bring this up to Dr. Milkey. Way to go, nine_k, for bring this up! This is BIG! Bang the war drums, because we are going to bring these guys down to earth!
I don't find it humbling, quit the reverse. When I look into the night sky I note that human brains are the most intricately complex things we know of or have evidence for. It's shocking how lifeless and brainless and beneath humble the rest of the universe appears to be. We live and thrive in a hydrogen scrapheap, and as far as we know, we are all that is going on.
Even if we persist for a billion years, we’re still temporary.
Just like every gravity assist steals energy from the planet and gives it to the spacecraft. We’ve actually slowed down Jupiter… by a tiny, tiny amount. But still probably less than some random asteroid does all the time.
Be a sight to behold.
> The artist’s concepts below, released by NASA in 2012, show what will happen to Earth’s night sky as the Andromeda galaxy hurtles toward us.
[pictures]
Optimistic people in the comments suggesting that we may last until the end of the millenium.
A steller mass black hole's estimated lifespan would of course make even these seem like an eyeblink by comparison.
However, the sun will grow in energy output until going red giant, so the planet will be uninhabitable by then.
So while the sun may be nowhere near the end of its life, the time of Earth lying in its "Goldilocks zone" is much shorter.
[1] https://www.sciencedaily.com/releases/2013/12/131216142310.h...
Not that it will matter to any humans living billion of years from now, who will have the capability to move old blue Earth a safe distance out.
I plan on being around that long though.