# Constellations are Younger than Continents

At the Bay Area Solstice, I heard the song [Bold Orion](https://humanistculture.bandcamp.com/track/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.[^s6cc67g5ijp] 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.

![https://www.halcyonmaps.com/constellations-throughout-the-ages/](https://39669.cdn.cke-cs.com/rQvD3VnunXZu34m86e5f/images/e8f60ddba9e10dc5337cd94aa7e42353559f0d8ca724e23c.jpg)

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](https://simple.wikipedia.org/wiki/Orion_OB1_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.

[^s6cc67g5ijp]: 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.