Would anything be different if there was a moon-sized galaxy in the sky?
April 9, 2023 6:46 AM Subscribe
I was idly wondering if there would be any implications of there being another galaxy that was close enough to see- say, a spiral galaxy the same size as the Milky Way that took the same amount of space in the sky as the moon. Clearly it’s still very far away, but that’s a lot of mass! Would anything be different for the denizens of the Milky Way, from a physical or cosmological standpoint?
Best answer: The galaxy we're in is pretty hard to make out in the sky unless it's very dark; it's nowhere near as bright as the moon. The physics would require the hypothetical neighbouring galaxy to be many orders of magnitude brighter than a normal galaxy. So it would be ridiculously dense by galactic standards, or filled with supernovae. We'd probably need to be in a universe with different fundamental physical rules.
posted by pipeski at 7:52 AM on April 9, 2023
posted by pipeski at 7:52 AM on April 9, 2023
Response by poster: Huh, I had no idea Andromeda was so big! I would say I can call my idle curiosity sated.
posted by rockindata at 12:29 PM on April 9, 2023
posted by rockindata at 12:29 PM on April 9, 2023
Best answer: Hmmm. Somewhere in the past six months or so I saw a decent treatment, in video form, of what the collision of our two galaxies (Milky Way and Andromeda) will look like, both from outside and inside (on Earth). It might have been this segment from a longer PBS Nova episode, but it could have also been some Youtuber, e.g. Scott Manley. Searching for this kind of thing on Youtube is a soul-sucking enterprise because of all of the terrible clickbait out there -- no, NASA is not "worried", no this didn't "just happen". Ugh. But this is a nice short visualization of what it will look like over the next 7 billion years. Of course, we won't be here after 5 billion years due to the evolution of our own star ...
posted by intermod at 12:45 PM on April 9, 2023 [2 favorites]
posted by intermod at 12:45 PM on April 9, 2023 [2 favorites]
Best answer: Two of the closest galaxies to the Milky Way are the Large Magellanic Cloud and Small Magellanic Cloud. They are both satellite galaxies of the Milky Way. There are about 59 potential satellite galaxies of the Milky Way (galaxies close enough to be gravitationally bound, though not all of them are actually gravitationally bound). A couple of others are closer to use, but the Magellanic Clouds are the only objects of this class that are visible to the naked eye.
The Large Magellanic Cloud is has magnitude 0.4 making it an easy naked-eye object even from mildly light-polluted areas. It's full size is about 11°x9°, making it many, many times larger than the full moon. But the the full size isn't visible without some type of optical aid. However, even the extent of the object that is visible to the naked eye is many times larger than the full moon: In dark skies and with dark-adapted eyes, "its brightest region is a bar roughly 5° long by 1° wide. That makes it 10 times as long and twice as wide as the Full Moon."
The Small Magellenic Cloud is roughly 2 degrees in diameter to the naked eye - also many time larger than the moon, whose diameter is about 0.5 degree
Both Clouds are visible only from the Southern Hemisphere, roughly 20 degrees south or more for a good view.
This article has some stories about the clouds from people who live in areas that could see them.
As Hatashran says, the Andromeda Galaxy is visible to the naked eye, and its full extent - visible in telescopes etc, but not necessarily to the naked eye - is far, far larger than the full moon.
However, even the portion of the Andromeda Galaxy visible to the naked eye is "bigger than a full moon in the sky". It requires a very dark sky to see this extent (or really, to glimpse the galaxy at all) but remember that most of the earth for most of history has been under very dark skies - at least, during the portions of the month/night when the moon is down. The galaxy appears to have been noted in ancient Sumerian and Babylonia star sources, and was cataloged by the Persian astronomer Abd-al-Rahman Al-Sufi in 964 AD.
Here is a nice first-hand account of seeing the galaxy in dark-sky conditions, and a helpful guide for finding it.
TL;DR: There are in fact three nearby galaxies with visible extent - to the naked eye, under good conditions - larger than the full moon: The Large Magellanic Cloud, the Small Magellanic Cloud, and the Andromeda Galaxy.
Regarding your question as to how such nearby galaxies might affect each other, the Milky Way, or us personally, here is an example: "The ongoing Dark Energy Survey found a dark stream of interacting matter between the Large and Small Magellanic Clouds. Simulations performed by a team of scientists at the University of Arizona suggested that the two galaxies might be interacting with each other and might eventually merge."
We now have lots of examples of galaxies about to collide, mid-collision, and post-collision. The long & the short of it is, yes they do have a pretty huge impact on each other when the pass closely, collide, or merge. FWIW pretty much all large galaxies, including the Milky Way, are created by mergers of several or even numerous small galaxies. We can see the remnants of some galaxies the Milky Way has swallowed as strings of similar stars and such.
Finally, the Andromeda Galaxy and the Milky Way are in fact on a collision course. As they merge, they will undoubtedly look a lot like the other examples we have seen of large galaxies merging. But don't take my word for it - just wait around 4.5 billion years or so and you'll get to experience the merger of the Milky Way & Andromeda Galaxies first hand!
posted by flug at 2:55 PM on April 9, 2023 [10 favorites]
The Large Magellanic Cloud is has magnitude 0.4 making it an easy naked-eye object even from mildly light-polluted areas. It's full size is about 11°x9°, making it many, many times larger than the full moon. But the the full size isn't visible without some type of optical aid. However, even the extent of the object that is visible to the naked eye is many times larger than the full moon: In dark skies and with dark-adapted eyes, "its brightest region is a bar roughly 5° long by 1° wide. That makes it 10 times as long and twice as wide as the Full Moon."
The Small Magellenic Cloud is roughly 2 degrees in diameter to the naked eye - also many time larger than the moon, whose diameter is about 0.5 degree
Both Clouds are visible only from the Southern Hemisphere, roughly 20 degrees south or more for a good view.
This article has some stories about the clouds from people who live in areas that could see them.
As Hatashran says, the Andromeda Galaxy is visible to the naked eye, and its full extent - visible in telescopes etc, but not necessarily to the naked eye - is far, far larger than the full moon.
However, even the portion of the Andromeda Galaxy visible to the naked eye is "bigger than a full moon in the sky". It requires a very dark sky to see this extent (or really, to glimpse the galaxy at all) but remember that most of the earth for most of history has been under very dark skies - at least, during the portions of the month/night when the moon is down. The galaxy appears to have been noted in ancient Sumerian and Babylonia star sources, and was cataloged by the Persian astronomer Abd-al-Rahman Al-Sufi in 964 AD.
Here is a nice first-hand account of seeing the galaxy in dark-sky conditions, and a helpful guide for finding it.
TL;DR: There are in fact three nearby galaxies with visible extent - to the naked eye, under good conditions - larger than the full moon: The Large Magellanic Cloud, the Small Magellanic Cloud, and the Andromeda Galaxy.
Regarding your question as to how such nearby galaxies might affect each other, the Milky Way, or us personally, here is an example: "The ongoing Dark Energy Survey found a dark stream of interacting matter between the Large and Small Magellanic Clouds. Simulations performed by a team of scientists at the University of Arizona suggested that the two galaxies might be interacting with each other and might eventually merge."
We now have lots of examples of galaxies about to collide, mid-collision, and post-collision. The long & the short of it is, yes they do have a pretty huge impact on each other when the pass closely, collide, or merge. FWIW pretty much all large galaxies, including the Milky Way, are created by mergers of several or even numerous small galaxies. We can see the remnants of some galaxies the Milky Way has swallowed as strings of similar stars and such.
Finally, the Andromeda Galaxy and the Milky Way are in fact on a collision course. As they merge, they will undoubtedly look a lot like the other examples we have seen of large galaxies merging. But don't take my word for it - just wait around 4.5 billion years or so and you'll get to experience the merger of the Milky Way & Andromeda Galaxies first hand!
posted by flug at 2:55 PM on April 9, 2023 [10 favorites]
Response by poster: Even more great information to answer my idle curiosity, in multiple different ways! Thanks everyone!
posted by rockindata at 6:32 PM on April 9, 2023
posted by rockindata at 6:32 PM on April 9, 2023
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Having it visible might have led to different social development, but we're already at the point where it takes up far more sky space than the moon.
posted by Hatashran at 6:57 AM on April 9, 2023 [21 favorites]