Cool, but how does define the "whole galaxy"? I assume there's not a hard defined edge. How do you decide what should be made brighter for the effect in the photo?
Almost everything competing with Andromeda in that region of the sky is in the Milky May, and there's no problem differentiating between the two galaxies. In other words, galaxies are blurry but the Milky Way and Andromeda are sufficiently far apart that you can unambiguously partition 99.99999% of stars as being in one or the other.
It’s just a photoshop composite of some picture of the andromeda scaled to match the size of the moon, not a well principled simulation of a wide field survey telescope with otherworldly dynamic range. Don’t worry about it.
A galaxy doesn't have a hard edge, nor does for example the solar system. But - as far as we know - the density of stars drops really steeply at a certain radius. For practical purpose, we can take this radius as the "size" of the galaxy. The galaxy doesn't grow further than that, even if you keep exposing the photos longer.
Of course, there can be plenty of stars outside of that radius, but in a comparatively small number.
Hey bud, my four year old asked a similar question to chkaloon. I couldn’t answer it because I didn’t know and your answer really helped me explain it to her. In the four year old’s wonderful little parlance, you are ‘a really cool dude’.
You’ve joined people like her grandma and grandpa, a ‘pleese ossifer girl’ who wrote me a speeding ticket and gave her a tattoo, and her paediatrician in the realm of ‘really cool dudes’. In my wildest dreams, one day I’ll become a really cool dude too. :)
Seriously, thank you. You made today’s home daycare really really cool. And you’re infinitely less expensive than the ‘pleese ossifer girl’ so you even get old dad bonus marks.
Edit - If you happen to work in a role where an absolutely glowing thank you letter from a very happy parent would help your career, my email address is in my profile.
Thank you for your kind answer. I am a physicist with a background in astronomy, but I am working in the chip industry - I don't think a letter would be career-helping.
I do enjoy though answering these kind of questions about physics and astronomy, so feel to ask more.
Honestly friend, I will never be able to thank you enough.
My four year old has lost a lot during the pandemic. I like to show her things that I know she will like and space is one of her passions. Unfortunately, she’s four so her questions can go from low level, to profound, to areas where I just don’t have a bloody clue.
You have already really helped me help her. Can I ask for a favour? If you have any links you like, videos you enjoy or films/book recommendations, would you mind sharing them? My kid’s interest in astronomy far exceeds my knowledge and at this point, I don’t even know what questions to ask! :)
Edit - Sorry not to add this originally, but don’t worry about things like age appropriateness or what have you. She’s into lots of things that I’m baffled by, from My Little Pony to astronomy.
I don't know how a four year old would take it, but take a look at the YouTube channel, "Science And Futurism With Isaac Arthur". Many times when I watch his videos, I'm amazed by things I'd never considered. As a plus, the channel is family friendly.
There's not, and we've been redefining where we really think the border is, even within the past year or so. There's a lot more globular clusters - relatively small clusters of stars, etc. - that are gravitationally bound to galaxies than we previously knew. These can extend quite a distance out.
Their relatively low density and distance from the main portion of the galaxy means that even if you brightened them, they'd largely just look like other stars in the sky and would be hard to distinguish from being part of the general background of stars our galaxy provides, so there wouldn't necessarily be much point in doing so.
The thing is so wide that light needs to travel 140,000 years (!) to get from one side to the other. That’s just so hard to wrap my head around. I never even really considered that our own galaxy is 100,000 light years across.
Think of all the human history that occurred over 100,000 years — that’s how long it takes for light to go from one side of the Milky Way to another.
Its cool to visualize a little bubble around the Solar System that represents the 100 or so years we've been emitting radio waves into space. The signal would be very faint (power falls as 1/r^2) but visually if you zoom out on the milky way its a tiny little bubble around our system that barely reaches into our home galaxy. It's a humbling visualization.
> I never even really considered that our own galaxy is 100,000 light years across.
The Gaia telescope has helped us revise a lot of numbers around the Milky Way - we're actually likely the biggest kid on the block. The estimate the size of the Milky Way to be 170,000 to 200,000 light years across.
I think a more apt comparison would be how many solar systems away we are. I calculate we are only 143-4550 solar systems away, depending on how you define the solar system (using the orbit of Sedna or Neptune as the radius).
Yeah, I don't know where I got whatever number I was using for sun-alpha centauri distance (I was converting from Milky Ways, I think).
Today I calculate 1600 - 7300 solar systems, using wikipedia AUs. Maybe my calculator is broken, though...
if the sun, which is huge compared to the earth, were the size of a pea, the nearest star would be ~120 miles away to scale. this is a typical distance between stars and it’s why when andromeda collides with the milky way there is little chance of stars colliding.
I’ve heard, I think many years ago, this fact there is not much risk or star collisions when galaxies collide. But I wonder, given more recent knowledge of how common exoplanets are, if there is a more significant risk of disruptions to the planetary systems that surround the stars. These planetary systems are much larger than the stars themselves.
Stars are still sparse enough in both galaxies that most stars still wont have much effect on most other stars' planetary systems. On the scales of the galaxies a star system isn't much bigger than that system's star. A star affecting another's planetary system would pass so close it and the host star would likely form a gravitational relationship and would be considered a "collision". Space is stupid big and stupid empty.
I was going to say much the same thing until I saw your comment, which already expressed what I was going to. This has also struck me for a while as being interesting.
When making this comparison, I think it is interesting to note that both the distances between Earth to Alpha Centauri and the Milky Way to Andromeda aren't "typcial".
To quote Douglas Adams, Earth is "Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy". Intersteller distances are much smaller in galactic cores. For example, at the center of Andromeda, the average intersteller distance is 860 AU (compared to 266,900 for Earth-Centauri)[1].
Alternatively, Andomeda huge and on a colision course with the milky way, but not our closest neighbor. Both the Milky Way and Andromoeda have many smaller and closer neighbors, Some of which are of appreciable size. For example, the Large Magellanic Cloud is a "smaler" galaxy with about 30 billion stars and is 170kly or about one milky way diameter away from us.[2]
This is an entirely uneducated guess, but look closer at the spin over time, and consider the objects low density.
Think timeline of a highly elliptical orbit, but with energy and work concentrated on angular momentum.
A semi decent amateur telescope (I recommend starting with a dobsonian) and a drive to an area with low light pollution (perhaps the harder requirement) will fill your eyes with the many wonders of the night sky, M31 (Andromeda) included :)
The scale is so big fo me it's not possible to actually comprehend it. The distance to the nearest star is so vast I can't get a grip on it mentally. just one lightyear is unimaginably huge.
So I can only think about interstellar and intergalactic distances relative to this measure, which itself I can't comprehend.
There should be devices to help make artificial occultation of bright stars... maybe even glasses? Maybe could it be done with a LCD panel with really tiny pixels?
This is one of the thing which made my really sad about been a human - our lifespan is poorly short.
See, if my lifespan is 4.5 billion years, then I can just sit here and wait until Andromeda become a daily light show when it finally got close enough.
Look through a live telescope yourself will give you a very VERY different feeling when compare to look at a picture on NASA website.
I have an amateur telescope, 130EQ. I still remember the feeling it give to me when the first time it brought Saturn into my view, and then again for Jupiter, stars and clusters ...
For me, the best way to get over all the unhappiness, anger and depressions, is to sit down in the pitch dark of the night, setup the telescope and hunting for the stars.
Sadly I managed to broke it's tripod few years back :(
I also remember the first time I managed to see Saturn through my own telescope.
It took forever to get it in frame, and I wasn't even sure what I was looking for until I finally centered it. I knew it was brighter than the other stars and that it was probably a planet but not sure which one it was.
Finally got it in frame and focused, saw the rings and jumped up and down exclaiming "it's Saturn, it's Saturn!". I was super excited and kept having to readjust the telescope because of Earth's rotation as it moved out of frame.
It was great night with my family and they still remember it to this day.
I know, right? All that aiming and panning is part of the experience which makes the fun.
To someone who don't have it before, we're basically looking at a blur dot slightly larger than a peanut. But when you do it yourself, the memory you'll receive is priceless.
I strongly recommend everybody try it out when they have chance.
> See, if my lifespan is 4.5 billion years, then I can just sit here and wait until Andromeda become a daily light show when it finally got close enough.
Well, not quite. It’d be quite static as the merger would take a long time on the scale of a human lifetime.
In theory, anything is possible, sure. Problem is, in order to live forever one must be indestructible. This is because for the modern human the mean time to the death in an accident is way below one thousand years. (Indestructibility would bring with it its own issues.)
Effective indestructibility can emerge through sheltering the human body and interacting with the outside world through avatars, e.g. robots etc and extensive body monitoring to provide rapid responses in case of a medical emergency.
With the exponential growth rate in technological sophistication, we could, in the forseeable future, have levels of personal safety and medical care that are many orders of magnitude greater than they are now.
If the humans alive today make it to the point where the average life expectancy is pushed to, say, 1000 years, then they stand a decent chance of living to see technology advance to levels that provide extreme (e.g. billions of years) life extension.
I think it's technically possible to explore our own galaxy in a human lifetime by accelerating to an appreciable fraction of c, just need to develop fusion rockets.
Fusion rockets are not nearly enough for that. You'd still need to carry an impossible amount of propellant, deal with friction and live through the months-long acceleration/deceleration cycles, most of which at non-relativistic speeds.
If you got a ramscoop engine, maybe you'd be able to grab enough propellant to push you at 1g for years at a time while cleaning your path of particles that'd impact your shielding.
Even at the speed of light, taking into account acceleration and deceleration you could cover ~50 light years in a lifetime. The Milky Way is ~150000 light years across. In day to day terms it's like walking 10km relative to exploring the whole Earth.
I'm sure someone better informed could weigh in with more details, but IIRC, taking time dilation into account, you could explore infinitely if you could somehow travel at the speed of light. Some large fraction of c would be sufficient to cross the galaxy in a lifetime, relative to you the traveller.
In 50 years experienced by the explorer inside the ship, at a more "realistic" speed of 0.9c the ship would travel ~115 light years, and at 0.99c it would do ~350 light years (excluding gravitational time dilation). Anything higher than that would probably be a bit incomprehensible for the human mind. At 0.99999c the ship would travel ~11000 light years, all of which would be fully experienced by the people at home. That's for the dash, the exploration itself comes on top of that.
And while at the speed of light time passes infinitely slow for you, it passes infinitely fast for the universe, which would very abruptly end your exploration.
We will not be exploring any significant portion of our galaxy without rewriting our current theory, inventing a game changing technology, or maybe discovering some phenomena.
"And given that it’s 2.5 million light years away, the surprising amount of sky space Andromeda takes up speaks to just how ridiculously large a 140,000 light year diameter galaxy is."
Well, not really. I mean, that doesn't tell me anything about its hugeness, because apparent size is based on a ratio. They're making a ratio with light years on both sides, so they effectively cancel out. 2.5 million / 140,000 is 17.8. So Andromeda is only 17.8 times as far away from us as its diameter. (which is, to me, a very interesting takeaway)
You could substitute "millimeters" for "light years" and it would appear the same size, i.e. a 459 foot diameter object that is 8200 feet away would have the same apparent size as Andromeda.
Not trying to criticize the article, but thought it was an interesting way they put it.
That's of course true, but I think it's a pretty big part of the context that "the object" is a galaxy, i.e. a collection of approximately 1 trillion (1,000,000,000,000) stars.
It's more or less obvious that such an object cannot be very close, and thus it's apparent size in our sky really does convey something about its size.
If you could see the galaxy as brightly as in the photos, there's nothing to indicate its size, at least not directly. You are saying it can't be close because it has a trillion stars, and we know stars are big, therefore it is big, therefore it must be far away, and then back to therefore it is big. Isn't that kind of circular?
I could imagine ancient people seeing a galaxy that size (if it was bright enough), and guessing it was a few hundred miles away and under a hundred miles across.
I'm the one that made the original composite that set this whole thing off[1]! By "whole thing" I mean it's been this amazing meme-like recurring wave that's ebbed and flowed continuously in the 6 years since I made it. It's so wonderful to see. I'm not even an astronomer, I just used to like looking at Nasa's Astronomy Picture of the Day[2], which gave me the very simple idea to paste Andromeda into a picture of the moon. I'd seen a composite of the moon pasted next to Andromeda on APOD, and just wondered what it would be like the other way round. Of course my original image has been corrected many times since then by people with actual astronomy credentials. This is my favourite, corrected for both size and to be in the true colour spectrum: https://i.imgur.com/2pbfvJa.jpg
Beautiful. It was a great idea: congratulations on it spreading the way it has.
My reaction on seeing this, probably like that of many other people, was, “wow, that would be so cool”. But of course if it looked like this normally we wouldn’t think much of it at all, just like all the other gorgeous sci-fi skies we see in movies or read about in books: multiple moons, two suns, close proximity to a ringed planet, etc. If these were our sky, they would be unremarkable.
We are surrounded by incredible beauty and so rarely stop to contemplate it. But the beauty is there nonetheless.
Your idea is a great reminder to pause and look up. There are deep lessons in the sky.
I'm surprised no one has corrected for relative brightness. This photo would only be realistic if Andromeda was an incredibly unusual super-bright galaxy, or if we could see a lot more light.
The former is physically unlikely although we'd see a very bright core if we lived next to a quasar. But the latter would turn the entire sky into a blaze of stellar glory. Far more stars would be visible, and we'd also see more nebulas between them. And other galaxies.
So the entire sky would be much busier. The Moon would be blindingly bright, the Sun would be unbearable - unless we had HDR eyes and maybe protective membranes for daylight - and Andromeda wouldn't stand out as much among all the other sky candy.
I also got thinking about this from APOD years ago. It's not just Andromeda that's visibly large (but too faint) to see, many famous nebulas are comparably big. The Orion nebula is over one degree across (The moon is about 1/2 degree). I'd love to a see a night sky full of all these at once and to imagine the mythologies that would have formed around them. A red-hued Mars became the god of war, what would Andromeda have been?
While the visuals would be stunning, what's interesting to me is how much our knowledge of the cosmos has advanced in the past 6 years between that article being written and now.
We now know the Milky Way is probably nearly 200,000 light years across[1]. And that it's not only much larger, but much more massive - probably larger than Andromeda[2].
When we talk about these collisions being billions of years away, we're also really only talking about the main portions of the galaxy - all of the globular clusters are going to start interacting way sooner. We're already seeing star formation in the Milky Way's globular clusters as a result of interaction with the Large Magellanic Cloud, even though we're 2.4ish billion years from the primary collision occurring [3].
73 comments
[ 0.27 ms ] story [ 187 ms ] threadThis comment is too persnickety to actually be asked in good faith imho.
Of course, there can be plenty of stars outside of that radius, but in a comparatively small number.
You’ve joined people like her grandma and grandpa, a ‘pleese ossifer girl’ who wrote me a speeding ticket and gave her a tattoo, and her paediatrician in the realm of ‘really cool dudes’. In my wildest dreams, one day I’ll become a really cool dude too. :)
Seriously, thank you. You made today’s home daycare really really cool. And you’re infinitely less expensive than the ‘pleese ossifer girl’ so you even get old dad bonus marks.
Edit - If you happen to work in a role where an absolutely glowing thank you letter from a very happy parent would help your career, my email address is in my profile.
My four year old has lost a lot during the pandemic. I like to show her things that I know she will like and space is one of her passions. Unfortunately, she’s four so her questions can go from low level, to profound, to areas where I just don’t have a bloody clue.
You have already really helped me help her. Can I ask for a favour? If you have any links you like, videos you enjoy or films/book recommendations, would you mind sharing them? My kid’s interest in astronomy far exceeds my knowledge and at this point, I don’t even know what questions to ask! :)
Edit - Sorry not to add this originally, but don’t worry about things like age appropriateness or what have you. She’s into lots of things that I’m baffled by, from My Little Pony to astronomy.
https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g/fea...
Their relatively low density and distance from the main portion of the galaxy means that even if you brightened them, they'd largely just look like other stars in the sky and would be hard to distinguish from being part of the general background of stars our galaxy provides, so there wouldn't necessarily be much point in doing so.
Think of all the human history that occurred over 100,000 years — that’s how long it takes for light to go from one side of the Milky Way to another.
The Gaia telescope has helped us revise a lot of numbers around the Milky Way - we're actually likely the biggest kid on the block. The estimate the size of the Milky Way to be 170,000 to 200,000 light years across.
The distance from the Milky Way to Andromeda is only about 25 Milky Ways, but the distance from the Sun to Alpha Centauri is about 29 million Suns.
Also 266,900 earth orbits away
Other interpretations of the GP would imply the GP thought 267,000 / 5,000 is about 1.5. I'm pretty sure the GP isn't making that mistake.
That said, I think you might be interestested in the sister post I made to yours.
Or the RL version in Sweden: http://www.swedensolarsystem.se/en/
To quote Douglas Adams, Earth is "Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy". Intersteller distances are much smaller in galactic cores. For example, at the center of Andromeda, the average intersteller distance is 860 AU (compared to 266,900 for Earth-Centauri)[1].
Alternatively, Andomeda huge and on a colision course with the milky way, but not our closest neighbor. Both the Milky Way and Andromoeda have many smaller and closer neighbors, Some of which are of appreciable size. For example, the Large Magellanic Cloud is a "smaler" galaxy with about 30 billion stars and is 170kly or about one milky way diameter away from us.[2]
https://astronomy.com/magazine/ask-astro/2006/01/how-close-c...
https://en.wikipedia.org/wiki/Large_Magellanic_Cloud
https://youtube.com/watch?v=4disyKG7XtU
Most stars and matter will become part of the fused galaxy, "Milkomeda" is one proposed name. Some will be flung int intergallactic space.
The Sun will have entered its white dwarf phase. Earth will either be a cinder, or have been subsumed by or possibly be nova ejecta of the Sun.
So I can only think about interstellar and intergalactic distances relative to this measure, which itself I can't comprehend.
See, if my lifespan is 4.5 billion years, then I can just sit here and wait until Andromeda become a daily light show when it finally got close enough.
Look through a live telescope yourself will give you a very VERY different feeling when compare to look at a picture on NASA website.
I have an amateur telescope, 130EQ. I still remember the feeling it give to me when the first time it brought Saturn into my view, and then again for Jupiter, stars and clusters ...
For me, the best way to get over all the unhappiness, anger and depressions, is to sit down in the pitch dark of the night, setup the telescope and hunting for the stars.
Sadly I managed to broke it's tripod few years back :(
It took forever to get it in frame, and I wasn't even sure what I was looking for until I finally centered it. I knew it was brighter than the other stars and that it was probably a planet but not sure which one it was.
Finally got it in frame and focused, saw the rings and jumped up and down exclaiming "it's Saturn, it's Saturn!". I was super excited and kept having to readjust the telescope because of Earth's rotation as it moved out of frame.
It was great night with my family and they still remember it to this day.
To someone who don't have it before, we're basically looking at a blur dot slightly larger than a peanut. But when you do it yourself, the memory you'll receive is priceless.
I strongly recommend everybody try it out when they have chance.
Well, not quite. It’d be quite static as the merger would take a long time on the scale of a human lifetime.
With the exponential growth rate in technological sophistication, we could, in the forseeable future, have levels of personal safety and medical care that are many orders of magnitude greater than they are now.
If the humans alive today make it to the point where the average life expectancy is pushed to, say, 1000 years, then they stand a decent chance of living to see technology advance to levels that provide extreme (e.g. billions of years) life extension.
If you got a ramscoop engine, maybe you'd be able to grab enough propellant to push you at 1g for years at a time while cleaning your path of particles that'd impact your shielding.
There's been a lot of speculation about scooping up hydrogen from the interstellar medium as fuel, but I think it's mainly wishful thinking.
https://en.wikipedia.org/wiki/Bussard_ramjet
https://en.wikipedia.org/wiki/Tau_Zero
And while at the speed of light time passes infinitely slow for you, it passes infinitely fast for the universe, which would very abruptly end your exploration.
We will not be exploring any significant portion of our galaxy without rewriting our current theory, inventing a game changing technology, or maybe discovering some phenomena.
Well, not really. I mean, that doesn't tell me anything about its hugeness, because apparent size is based on a ratio. They're making a ratio with light years on both sides, so they effectively cancel out. 2.5 million / 140,000 is 17.8. So Andromeda is only 17.8 times as far away from us as its diameter. (which is, to me, a very interesting takeaway)
You could substitute "millimeters" for "light years" and it would appear the same size, i.e. a 459 foot diameter object that is 8200 feet away would have the same apparent size as Andromeda.
Not trying to criticize the article, but thought it was an interesting way they put it.
It's more or less obvious that such an object cannot be very close, and thus it's apparent size in our sky really does convey something about its size.
That was my interpretation, at least.
I could imagine ancient people seeing a galaxy that size (if it was bright enough), and guessing it was a few hundred miles away and under a hundred miles across.
1. https://www.reddit.com/r/spaceporn/comments/1u0dxs/andromeda...
2. https://apod.nasa.gov/apod/astropix.html
My reaction on seeing this, probably like that of many other people, was, “wow, that would be so cool”. But of course if it looked like this normally we wouldn’t think much of it at all, just like all the other gorgeous sci-fi skies we see in movies or read about in books: multiple moons, two suns, close proximity to a ringed planet, etc. If these were our sky, they would be unremarkable.
We are surrounded by incredible beauty and so rarely stop to contemplate it. But the beauty is there nonetheless.
Your idea is a great reminder to pause and look up. There are deep lessons in the sky.
The former is physically unlikely although we'd see a very bright core if we lived next to a quasar. But the latter would turn the entire sky into a blaze of stellar glory. Far more stars would be visible, and we'd also see more nebulas between them. And other galaxies.
So the entire sky would be much busier. The Moon would be blindingly bright, the Sun would be unbearable - unless we had HDR eyes and maybe protective membranes for daylight - and Andromeda wouldn't stand out as much among all the other sky candy.
We now know the Milky Way is probably nearly 200,000 light years across[1]. And that it's not only much larger, but much more massive - probably larger than Andromeda[2].
When we talk about these collisions being billions of years away, we're also really only talking about the main portions of the galaxy - all of the globular clusters are going to start interacting way sooner. We're already seeing star formation in the Milky Way's globular clusters as a result of interaction with the Large Magellanic Cloud, even though we're 2.4ish billion years from the primary collision occurring [3].
Last year NASA also came out with a bunch of images on what the night sky will look like as the Andromeda collision occurs: https://kottke.org/19/10/behold-our-dazzling-night-sky-when-...
1. https://www.nbcnews.com/mach/science/milky-way-galaxy-may-be... 2. https://www.businessinsider.com/milky-way-is-much-more-massi... 3. https://www.universetoday.com/144556/an-upcoming-impact-with...