Constellations are Younger than Continents
post by Jeffrey Heninger (jeffrey-heninger) · 2023-12-19T06:12:40.667Z · LW · GW · 22 commentsContents
22 comments
At the Bay Area Solstice, I heard the song Bold Orion for the first time. I like it a lot. It does, however, have one problem:
He has seen the rise and fall of kings and continents and all,
Rising silent, bold Orion on the rise.
Orion has not witnessed the rise and fall of continents. Constellations are younger than continents.
The time scale that continents change on is ten or hundreds of millions of years.
The time scale that stars the size of the sun live and die on is billions of years. So stars are older than continents.
But constellations are not stars or sets of stars. They are the patterns that stars make in our night sky.
The stars of some constellations are close together in space, and are gravitationally bound together, like the Pleiades. The Pleiades likely have been together, and will stay close together, for a few hundred million years. I think they are the oldest constellation.
The stars of most constellations are not close together in space. They are close in the 2D projection onto the night sky, but the distance to the stars is often dramatically different. They are on different orbits around the center of the Milky Way.
The sun and many of the nearby stars take about 230 million years to orbit the center of the Milky Way, but this is also not the relevant timescale for constellations to change.
The relevant timescale is determined by the differences between the velocities of stars in this part of the Milky Way.[1] This has been measured by astronomers: tracking small changes in the positions or brightness of large numbers of stars is a central thing that astronomers do.
Constellations change on a timescale of tens or hundreds of thousands of years. This is much faster than the movement of continents.
Orion is an unusual constellation. You can see above that the positions of its brightest 7 stars change more slowly than other constellations.
Many of the stars in Orion actually are related. They form a stellar association: they were formed at a similar time, are moving in a similar way, and are weakly gravitationally interacting. The stars of Orion will likely move around within the constellation, but many of them will remain close to each other for their entire life. Some of the dimmer stars currently in Orion are not part of the stellar association and are simply passing through.
The stars in the stellar association are young: at most about 12 million years old. Rigel (the brightest star in Orion) is 8 million years old. Alnilam is 6 million years old. Alnitak is 7 million years old. Saiph is 11 million years old.
These stars are also unusually large and bright. The larger the star, the shorter it lives. Most of the bright stars in Orion will not live to be 20 million years old.
Betelgeuse, usually the second brightest star in Orion, is special. It is noticeably red, and fluctuates dramatically in brightness. It formed in the stellar association about 8 million years ago, but is now leaving. It won't get very far. Within about 100,000 years, Betelgeuse will go supernova and shine as bright as the half moon for three months. Bright enough to be awesome but dim enough to not be dangerous.
Most constellations change as the stars move relative to each other with a time scale of tens or hundreds of thousands of years. Orion will last longer, for millions of years, before its bright stars burn out and go supernova. Neither of these is long enough to watch the continents rise and fall.
- ^
This is a kind of "temperature", if the stars themselves are treated as individual "atoms" in the "gas" of the Milky Way. The analogy is not perfect. Unlike atoms in a gas, stars almost never collide, and don't bounce off each other if they do, so there isn't "pressure" in the same sense.
The reason why stars in spiral galaxies mostly move along similar orbits is because the cloud of gas that was before the stars had pressure, and shock waves, and other fluid phenomenon that caused it to lose angular momentum and end up as a rotating disk. The stars merely inherited this pattern of motion. If anything happens that disrupts a lot of these orbits, like a collision with a similarly sized galaxy, the orbits will become randomized, and the galaxy will become elliptical instead of spiral. Roughly speaking, spiral galaxies are galaxies that have never had a major collision since most of their stars formed, and elliptical galaxies are galaxies that have.
22 comments
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comment by Shankar Sivarajan (shankar-sivarajan) · 2023-12-19T21:07:10.454Z · LW(p) · GW(p)
Related: I read that lots of cultures seem to independently consider there to have originally been seven Pleiades though one can only count six with the unaided eye, and have myths about what happened to the seventh. (link) Two of the bright ones are too close to resolve, and the idea is that the story dates back to a time when they were farther apart.
Replies from: dr_scomment by habryka (habryka4) · 2023-12-25T01:33:26.601Z · LW(p) · GW(p)
Promoted to curated!
It's Solstice/Christmas time! This is a fun and small post about Christmas/Solstice and how the world works. Also, what greater rationalist holiday tradition does there exist than endlessly adjusting all of our songs and speeches and writing until all fault is hammered out of them.
Replies from: zac-hatfield-dodds↑ comment by Zac Hatfield-Dodds (zac-hatfield-dodds) · 2023-12-26T01:21:55.653Z · LW(p) · GW(p)
In that spirit, my complaints about Time Wrote The Rocks:
We gaze upon creation where erosion makes it known,
And count the countless aeons in the banding of the stone.
Geological time isn't countless; the earth is only around 4.54 ± 0.05 billion years old and the universe 13.7 ± 0.2 billion years old. Seems worth getting the age of creation right! In the technical sense, there are exactly four eons: the Hadean, Archean, Proterozoic and Phanerozoic. It's awkward to find a singable replacement though, since "era" and "age" also have technical definitions.
The best replacements I've thought of are to either go with "ancient aeons" and trust everyone to understand that we're not using the technical definition, or "And see the length of history in" if a nod to chronostratigrapy seems worth the cost in lyricism.
Replies from: Raemon↑ comment by Raemon · 2023-12-26T19:47:14.395Z · LW(p) · GW(p)
My ornery Solstice opinions that arise in the face of this as "I dunno, I think there are two halves of rational ritual. One is being willing to let go of things that were pretty but not true. But I think another part is 'being willing to chill out about some things and let them be vaguely metaphorical and pretty." I realize aeon has something of a technical meaning, but it feels like one of the vaguer "technically technical" words, and I think it's just kinda fine for this phrase to obviously mean "a fucking long time, man."
Replies from: zac-hatfield-dodds↑ comment by Zac Hatfield-Dodds (zac-hatfield-dodds) · 2023-12-26T23:24:55.404Z · LW(p) · GW(p)
Agreed, "ancient aeons" would be my preferred edit.
Replies from: Raemoncomment by Aaron Bohannon (aaron-bohannon-1) · 2023-12-26T20:21:51.945Z · LW(p) · GW(p)
That is a fascinating observation that I never considered. I never took a course in astronomy. My interests are more aligned with with the philosophy of language. And I had a thought that some people might appreciate — one that is probably well-understood by most astronomers. I grew up around many people asking a seemingly sensible question: "What were those ancient humans smoking when they named these constellations? Ursa Major doesn't look like a bear. Draco doesn't look like a dragon. Scorpio doesn't look like a scorpion. And don't even get me started on Capricorn!"
Finally, a couple of months ago, I had a realization that has probably been long understood by many people: The reason that ancient humans named constellations was not because they looked up into the sky and thought to themselves. "Wow, that configuration of stars had a striking resemblance to Castor and Pollux from the story that I was told when I was a child!" They named the constellations simply because they needed a name for that point in the sky. They just needed to call them something... just like we need we names for our streets. How many people say, "Well, I've driven down Cygnus Lane a million times and have never once seen a swan. What a terrible name for that road!"
For a while, I worked for a civil engineering firm that designed subdivisions. They were responsible for laying out the shape of the plots of land and the streets for the developers... and also for naming the streets. Their proposed street names have to be approved by the county, but I can tell you this much: They aren't thinking all that hard about what to name those streets.
(Incidentally, my parents now happen to live in a town named Star, where all of the streets are named after stars and constellations. That was partly responsible for me getting a clue about all of this.)
Replies from: dr_s↑ comment by dr_s · 2023-12-28T08:42:23.282Z · LW(p) · GW(p)
To be fair, the star patterns do have vague shapes that inspire their name (in Orion the shape of a man with a club and a shield is quite recognizable). But of course pareidolia was applied out of necessity as you say; it was mnemonics probably as much as a good bit of storytelling fun. I would also add another possible mechanism: passing through multiple cultures. Maybe you inherited the constellation name from a previous culture, and mapped their God or hero of choice with one of yours because that makes it more familiar. But maybe some part of the analogy was lost in the mapping. Do it enough times and name and constellation may seem now associated in a completely arbitrary way.
comment by AndHisHorse · 2023-12-25T02:43:39.591Z · LW(p) · GW(p)
Perhaps, in a parallel to the kings earlier mentioned, this could be interpreted as Orion having seen the fortunes of continents rise and fall. Orion has seen the prominence of Africa as the source of humanity, and its subjugation by Europe; it has seen the isolation and the global power of the Americas; it has seen the mercantile empires of the West and its dark ages.
comment by YonatanK (jonathan-kallay) · 2023-12-31T22:48:02.606Z · LW(p) · GW(p)
Ever since first hearing the music of the Disney movie "Encanto" I've been sneering at the lyrics "stars don't shine they burn/ and constellations shift" because, no, of course constellations don't shift, without really stopping to think about it. Caught in my epistemic arrogance again!
comment by masasin (jean-nassar) · 2023-12-26T11:19:09.664Z · LW(p) · GW(p)
I love this post!
Just wanted to add that different cultures have different sets of constellations, but AFAICT the Pleiades was not one of them. For example, in the Greek-derived constellations that the astronomical community uses, it's part of Taurus.
The Pleiades are an open star cluster, and an asterism, but they aren't a constellation in the formal definition of the word.
comment by DylanD · 2023-12-25T15:36:06.416Z · LW(p) · GW(p)
I'd like to talk about Betelgeuse. As far as I've heard from astronomers and science communicators, Betelgeuse has at most 10,000 years left, not 100. However, we can't be sure of the exact timeframe. In other words, it could happen at any moment within these 10,000 years. It might even happen tomorrow. Its dimensions are astounding because if you were to replace the Sun with Betelgeuse, it would engulf most of the planets in our solar system. But precisely this is the reason for its relatively short lifespan. Our Sun has about 4 billion years "left".
I've also heard about constellations and "zodiac signs", meaning the Sun's position relative to a specific constellation in a given month. Over the 2,000 years since they were invented, the constellations have shifted, so when people say someone is, for example, a "Capricorn", they should actually be a "Sagittarius" now.
comment by Ben Sleeper (ben-sleeper) · 2023-12-25T18:20:01.065Z · LW(p) · GW(p)
In the context of the human timeline, this may be one of the first times this difference has provided meaning to us. Therefore, rationing the importance of the metaphor, in the previously static context, I am urged to bring forth a question: why did our ancestors see it the opposite way? I predict that they may be pointing to an undercurrent that has not mattered, until this very post.
Great post. Merry Christmas!
Replies from: aaron-bohannon-1, dr_s↑ comment by Aaron Bohannon (aaron-bohannon-1) · 2023-12-26T19:42:43.545Z · LW(p) · GW(p)
That's an interesting thought. At least since the time of Plato, Western philosophy has been based around the idea that some things never change. Plato called them "forms" or "ideals". Why would anyone think such a thing, though? How would we even come up with the idea that anything is truly immutable when everything that we see around us is always changing? It most likely originated from us observing the unfailing regularity in the position and motion of the sun and the moon and the stars. That was, I suspect, the source of the notion of (universal) objective truth.
Replies from: ben-sleeper↑ comment by Ben Sleeper (ben-sleeper) · 2023-12-27T04:41:08.571Z · LW(p) · GW(p)
Yeah, it does force one to wonder how that all squares. A thing is a thing is a thing… well you get the picture. Even things that aren’t things are things. Nothing is ever the same, but no thing ever changes? How do you rationalize a paradox like that if nothing truly ever demonstrates itself? And more deepities…
Your question/answer is a really good one. That notion seems to have been around before we could really describe/articulate it, and “the heavens” may well have been just that thing to help describe it.
↑ comment by dr_s · 2023-12-28T08:48:14.806Z · LW(p) · GW(p)
For a long time western philosophy literally believed the stars to be encased in a single rigid sphere surrounding the Earth. In fact it was a really big deal in Aristotelic cosmology that the heavy, corruptible, changing things are low (and thus fall to the Earth) while the substance of the skies is higher and incorruptible, thus unchangeable. It's why Copernican and Newtonian celestial mechanics were such a big deal. The mind-blowing part was that they suggested that everything everywhere followed the same laws, and the sky wasn't special in any way.
That said, this is just the view that had become mainstream in medieval Europe. If you had asked Democritus back in ancient Greece, he'd likely have told you that the stars were just other suns like ours, with other planets like ours, moving through the void, because that was the atomist view.
comment by Nathan Helm-Burger (nathan-helm-burger) · 2024-01-21T21:53:22.269Z · LW(p) · GW(p)
Related, sorta: https://i.imgur.com/nmTnRnd.jpeg
comment by tcheasdfjkl · 2024-02-22T19:40:09.836Z · LW(p) · GW(p)
Some lyric change ideas tossed around in a brainstorming session in the choir Discord:
- fjords and empires and all
- glaciers, sovereigns and all
- states and seas and stones and all
- seas and sovereigns and all
- seas and pyramids and all
↑ comment by Jeffrey Heninger (jeffrey-heninger) · 2024-02-22T20:52:39.399Z · LW(p) · GW(p)
I'm currently leaning towards
- kings and commonwealths and all
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