Main Page

Welcome to the explain xkcd wiki!
We have an explanation for all 3231 xkcd comics, and only 72 (2.2%) are incomplete. Help us finish them!

 Go to this explanation

Latest comic

Star Formation

|< < Prev #3222 (March 20, 2026)

Title text: It's ok, I still have some nice, cool gas clouds that aren't collapsing. As long as nothing ionizes them, I can continue to enjoy their ... HEY! NO!!!

|< < Prev Random Explanation

Explanation

This comic depicts a simplified model of how structure forms in the universe, then pushes it to an absurd extreme. In cosmology, small density variations in the early universe grow over time: regions with slightly higher density attract more matter via gravity, eventually forming gas clouds, stars, and galaxies. Pressure, driven by temperature, resists collapse, so the evolution of a cloud depends on the balance between gravitational attraction and internal pressure; this is often described by the Jeans instability criterion.

In the first panel, the on-screen narrator (with a passing resemblance to the time-travelling sphere), and apparent creator of the universe, describes pressure waves moving through gas and causing it to clump, which is broadly accurate. In the second, the clouds begin to collapse under gravity as more gas falls in, again matching real astrophysical processes. However, the third panel exaggerates the outcome: as collapse proceeds, the gas heats up (via compression and radiation processes), increasing pressure and often slowing or fragmenting the collapse rather than causing a runaway “explosion.” The comic instead imagines a catastrophic, accelerating collapse, akin to forming a black hole or triggering an energetic outburst. In reality, only under specific conditions, such as in the formation of direct collapse black holes, can gas clouds avoid fragmenting and collapse rapidly into massive objects.

The final panel humorously frames this as the narrator lamenting the destruction of their “beautiful clouds,” as the process has gone out of control and produced an intense central object (likely a star or black hole) instead of stable clouds.

The title text continues this joke by referencing the importance of ionization. “Cool” gas clouds (low temperature, neutral gas) can remain stable or collapse slowly. If they are ionized (for example, by nearby stars emitting ultraviolet radiation), the gas heats up, increasing pressure and preventing or disrupting collapse. The narrator hopes to preserve some calm, neutral clouds but then reacts in horror as something ionizes them, ruining the delicate balance and ending their ability to “enjoy” stable gas clouds.

Transcript

[A glowing dot is in a field of clouds, speaking to itself.]

Dot: I think I did a good job with this universe.

Dot: Pressure waves dance through gas clouds.

Dot: They clump together and then pressure pushes them apart.

[The dot observes some more densely gathering of clouds.]

Dot: Oh weird, that big clump of clouds is staying together.

Dot: Their gravity is overcoming the pressure and more gas is falling in.

[More of the increasingly fragmentary clouds start to fall into a nascent star.]

Dot: It's not stopping!

Dot: The heat is rising but the collapse is only accelerating!

Dot: I messed up bad. I messed up bad.

[There is now a shining star in place of the dense field of clouds, with a few small clouds only at a distance.

Dot: NOOOOOOOO!!!

Dot: My beautiful clouds!

Dot: Ruined! It's all ruined!

---

        View comic discussion

New here?

Last 7 days (Top 10)

Rank Score Pages Changes Username 1 24 20 24 Victoria 2 23 19 23 TheusafBOT 3 14 7 19 DollarStoreBa'al 4 12 5 16 GSLikesCats307 5 10 6 11 Kynde 6 10 3 15 John Hawkinson 7 9 2 16 Fephisto 8 9 3 12 BunsenH 9 8 3 10 DKMell 10 7 4 7 Aprilfoolsupdate!

Lots of people contribute to make this wiki a success. Many of the recent contributors above have just joined. You can do it too! Create your account here.

You can read a brief introduction about this wiki at explain xkcd. Feel free to create an account and contribute to the wiki! We need explanations for xkcd comics, characters, What If? articles, and everything in between. If it is referenced in an xkcd comic, it should be here.

• If you're new to wiki editing, see the explain xkcd:Editor FAQ for a specific guidance to this wiki and the more general help on how to edit wiki pages. There's also a handy wikicode cheatsheet.

• Discussion about the wiki itself happens at the Community portal.

• You can browse the comics from the list of all comics or by navigating the category tree at Category:Comics.

• The incomplete explanations are listed here. Feel free to help out by expanding them!

Rules

Don't be a jerk!

There are a lot of comics that don't have set-in-stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to xkcd and, of course, only submit material that can legally be posted and freely edited. Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, post a message to the Admin requests board.