The image is very nice, but the explanation of brightness is a bit confused:
> 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).
If we reverse the situation the analysis is clearly flawed. The Sun is not a particularly bright star in an absolute sense, but it's extremely bright to us. If you moved it a few hundred AU farther away, it would become difficult to distinguish from the background stars. Clearly distance makes a difference in apparent brightness.
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.
> If we reverse the situation the analysis is clearly flawed. The Sun is not a particularly bright star in an absolute sense, but it's extremely bright to us. If you moved it a few hundred AU farther away, it would become difficult to distinguish from the background stars.
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 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.
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!)
The explanation is confusing indeed because closer means not only "brighter" but also closer. But we can try to overcome this confusion by a thought experiment: lets compress Andromeda galaxy by a factor of k and divide distance by a factor of k. This way it would be k times close but it would have the same angular size.
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".
We do have a close-up view of a similar galaxy - our own. While the milky way, which is a part of our galaxy that does not resolve as individual stars, is not exactly bright, it appears quite a lot brighter than Andromeda. (It would appear brighter still if not partially obscured by dust in the galactic plane, which we are viewing edge-on. In particular, it blocks most of the galactic core.)
Double the distance, you get 1/4 the light. Double the distance, you get 1/4 the solid angle. The surface brightness of the object in your field of view therefore stays constant.
This took me a surprisingly long time to accept even though it seems intuitive based on the monitor example.
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.
Yup, all stars other than the Sun are below our ability to resolve as anything other than a point, so they do indeed look dimmer than the Sun to us.
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.
One thing to note here is that the ratio of the (distance to nearest galaxy)/(diameter of galaxies) is much smaller than (distance to nearest star)/(diameter of solar system). This is why galaxy collisions are significant but stellar collisions mostly are not.
Indeed. Galaxies are much "closer" to each other in this sense than are most smaller things in the universe.
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.
Should have posted this a couple weeks ago while Andromeda was in a good position for observation in the bay area-- its now setting pretty close to sundown.
Lots of exposures, which were slightly too long: I think those were on the order of 0.7 seconds (which is why the stars were elongated). I just went back out every ten minutes to keep the camera pointed at the target, and used the free software package siril to calibrate, register, and combine the images.
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. :) )
It's important to realize that the concept of a "galaxy" is arbitrary. Specifcally, it's not like stars suddenly stop and that's where the edge is. It's gradual.
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?
> It's important to realize that the concept of a "galaxy" is arbitrary. Specifcally, it's not like stars suddenly stop and that's where the edge is. It's gradual.
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.
I find it amazing it takes 7 bya for two close galaxies to merge, only twice as long as the entire life of the universe to date. It feels like we are just at the beginning of time.
Nitpick, "bya" means "billion years ago", probably not what you meant. Abbreviations for "billion years" include Gy (gigayear), Ga (giga-annum), and "byr" (less common in my experience.)
I believe we are. I think cosmically we just left the starting blocks compared to a slow heat death that will take place eventually. Maybe someone knows the stats for that eventuality.
We were completely kept in the dark, matter of fact - they said we were in the black! Whole deal seems to be spiraling out of control.
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!
It’s always humbling to look into the dark night sky and wonder about the significance of our existence on a pale blue dot dusty planet floating in a humongous space with giants such as our Sun and other magnificent beasts-black holes, extremely large stars, and the vast emptiness of space filled with dark matter. Truly humbling. Maybe if everyone just looked up in the sky for 5 minutes every night, there would be no war or conflict anymore.
> It’s always humbling to look into the dark night sky and wonder about the significance of our existence
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.
What’s humbling is even for all that, our effect on the universe will still be nonexistent and eventually every bit of evidence that could ever possibly point to our existence will turn to dust and will be indistinguishable from every other bit of dust.
Even if we persist for a billion years, we’re still temporary.
We're currently regretting that we've managed to increase the CO2 levels in our atmosphere to levels that may have substantial effects on climate. Some yokel-head environmentalist got linked to yesterday that we can get geothermal energy everywhere if we just dig far enough; just imagine the headlines years from now when we discover that we've been letting out all the heat from the part of the earth that keeps the magnetic field running that keeps our atmosphere from going the way of Mars. Putting the heat back in there will make getting rid of CO2 look like an elementary school project! I'd say leaving no trace might be a useful challenge for us...
I think you’re underestimating the amount of energy contained within the Earth. We’re not going to “let the heat out”.
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.
Say what you will about our insignificance compared to the cosmos, but currently we're waging several wars in the other direction against proteins and molecules, and I hope we win. We already rule several atomic and chemical reactions.
It's always amusing to me when articles like this come up, as if it's a "new development." I know it's more of a discovery but it's still always amusing to me we are discovering phenomena that likely has happened from before humans can write and will continue for long after everything this civilization disappears, the article titles always phrase it as if it like just happened recently.
You wouldn’t actually notice anything. When a large object is far away, all the light coming from it is concentrated into a smaller area than when it is up close. This means that the actual surface brightness of the object doesn’t change as it gets closer: the light appears brighter due to the closer distance, but spread out over a larger area. The Andromeda galaxy is already spread over an area of the sky wider than the full moon, and you’ve never noticed it; only the core is visible as a very faint star which turns fuzzy when you look at it in a telescope. You will remain essentially invisible for most of the rest of that 5 billion years. The shape of the visible portions of the Milky Way may slowly change over the last few hundred million years (and for long after), but if you’re nearsighted or live in a city then you’ve probably never seen that either. Obviously even if you could see them, the changes would all be spread out over such a long period that you would have trouble noticing anything. You can find some sped–up simulations on Youtube though.
Yeah, I'd love to have a night sky that looked like the 4th image in that sequence. I love long exposure photography of our current sky, but to be able to go out and capture something like that would be amazing!
TLDR: Billions of years from now, our Milky Way galaxy and the Andromeda galaxy are expected to merge. New data show that the outer galaxy boundaries have likely already started colliding. This post contains photos and video illustrating the impending merger and showing how the Andromeda galaxy will appear in Earth’s night sky over the next 7 billion years.
Then work on extending human lifespan. This is something the average HN contributor could be making progress towards. E.g. make a YC company working on capability advancement in biotech, or enabling technologies like atomically precise manufacturing.
M type dwarfs can easily have a lifespan in the main sequence spanning many trillions of years with even their red giant phases estimated to outlasting even the main sequence of stars of our size.
A steller mass black hole's estimated lifespan would of course make even these seem like an eyeblink by comparison.
Indeed, current theories of stellar evolution place the extinction of life on Earth in the range on hundreds of millions of years, due to the increase of its luminosity. For example here's a description of a better climate model [1] extending the time until the surface temperature reaches 70 degrees Celsius to one billion years, whereupon feedback of the water content of the atmosphere results in boiling away all liquid water.
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.
