<p>People have always looked for patterns in the skies, sometimes even seeing structures where none exist (such as a <a href="https://earthsky.org/tonight/orions-belt-points-to-sirius/" rel="nofollow noreferrer">hunter and his dog</a>). One specific kind of structure that astronomers care about is one which is <em>gravitationally bound</em>.</p>
<p>One effect of being gravitationally bound is that the gravitational attraction is strong enough to overcome the expansion of the universe, as explained in <a href="https://astronomy.stackexchange.com/a/109/28710">this answer</a> and <a href="https://astronomy.stackexchange.com/questions/11726/does-the-universe-expand-at-the-same-rate-everywhere-in-the-universe">this answer</a> and perhaps others. In our <a href="https://en.wikipedia.org/wiki/Local_Group" rel="nofollow noreferrer">local group</a>, the two biggest galaxies, namely the Milky Way and the Andromeda Galaxy, are so much gravitationally bound to each other that they are on a collision course and are expected to merge in another 4-5 billion years. Also, the <a href="https://en.wikipedia.org/wiki/Sagittarius_Dwarf_Spheroidal_Galaxy" rel="nofollow noreferrer">Sagittarius dwarf galaxy</a> has orbited several times around the center of the Milky Way, and in the course of that has passed through the Milky Way several times, leaving behind milky trails that loop up and back; it is also expected to eventually just merge with the Milky Way.</p>
<p>As <a href="https://astronomy.stackexchange.com/a/32513/28710">this answer</a> says, the Virgo supercluster is not gravitationally bound. Generally, within &quot;structures&quot; that are not gravitationally bound one can find substructures (e.g. galaxies) whose distances are increasing at a rate proportional to their distance, that proportionality constant being the Hubble constant. In other words, the relative motion of those substructures is governed by the expansion of the universe.</p>

CC BY-SA 4.0
2024-11-21T09:41:24.843Z
Consequently, are you of the opinion that everything in my list above the Local Group is ultimately erroneous, because whether we are located inside it is very much a matter of opinion? Apologies for requesting a summary of such a well-written answer – I promise I have read it in full.
RokeJulianLockhart
CC BY-SA 4.0
2024-11-21T13:03:46.140Z
Ah, well, I believe so, it is certainly consistent. But I would say that requires a deeper knowledge of astronomical structures than I possess.
Lee Mosher
CC BY-SA 4.0
2024-11-20T19:37:18.010Z
2024-11-21T14:21:34.620Z
2024-11-21T14:21:34.620Z
Lee Mosher
<p>Thus far, I've ascertained that we're inside (in ascending order of size):</p>
<ol>
<li>Earth <s>(expectedly <code>Sol c</code>, although <a href="http://www.exoplanetkyoto.org/exohtml/Sol_d_(Earth).html" rel="nofollow noreferrer">this</a> seems to think it's <code>d</code>?)</s> <a href="https://astronomy.stackexchange.com/questions/58914/what-is-the-largest-structure-that-contains-earth#comment136512_58914"><sup>[1]</sup></a></li>
<li>The <em>Solar System</em></li>
<li>The <em>Milky Way</em></li>
<li>The <em>Local Group</em></li>
<li>The <em>Virgo Supercluster</em></li>
<li>The <em>Laniakea Supercluster</em></li>
<li>The <em>Pisces–Cetus Supercluster Complex</em></li>
</ol>
<p>However, I'm interested in whether, as an example, the PCSC comprises the Hercules–Corona Borealis Great Wall, or whether it's distinctly separate. I've attempted to read <a href="https://en.wikipedia.org/w/index.php?title=Pisces%E2%80%93Cetus_Supercluster_Complex&amp;oldid=1253830973#/media/File:Superclusters_atlasoftheuniverse.gif" rel="nofollow noreferrer">the relevant maps provided by Wikipedia</a>, but I simply don't understand them.</p>
<p>Considering how many larger structures (like the aforementioned) exist beside the PCSC, I'll be surprised if it doesn't reside within, or upon the periphery of, a larger structure.</p>

CC BY-SA 4.0
2024-11-14T16:46:04.290Z
See [Exoplanet Naming Conventions](https://en.wikipedia.org/wiki/Exoplanet_naming_convention) for why I guess they're calling Earth  Sol d.   That said, as the first planet around Sol discovered, it _probably_ would be retroactively pedantically Sol b.
notovny
CC BY-SA 4.0
2024-11-14T17:02:41.717Z
The largest structure is undoubtedly the universe itself.
planetmaker
CC BY-SA 4.0
2024-11-14T17:06:46.153Z
@planetmaker, is that a structure?
RokeJulianLockhart
CC BY-SA 4.0
2024-11-14T19:50:58.287Z
Do you define a structure as a group of gravitationally bound objects?
Connor Garcia
CC BY-SA 4.0
2024-11-14T20:15:37.350Z
@ConnorGarcia, I asked because *I* cannot reasonably define anything in this field — I prefer to defer to authority. If I am to be asked, [`en.wikipedia.org/wiki/List_of_largest_cosmic_structures`](https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures#) states that “This list refers only to coupling of matter with defined limits, and not the coupling of matter in general (such as, for example, the cosmic microwave background, which fills the entire universe). All structures in this list are defined as to whether their presiding limits have been identified”. Does that answer?
RokeJulianLockhart
CC BY-SA 4.0
2024-11-14T20:16:33.573Z
@notovny, are they named in accordance to when they were discovered, rather than their orbital proximity to their nearest star?
RokeJulianLockhart
CC BY-SA 4.0
2024-11-14T20:37:13.437Z
@RokeJulianLockhart Yes, because it's very likely that we haven't discovered all the planets around most stars that we know have planets, and later discoveries may break the ordering.
notovny
CC BY-SA 4.0
2024-11-14T23:18:31.310Z
How exactly PCSC could comprise Hercules–Corona Borealis GW, if the latter is, like, 10 times longer (if exists at all)? (rhetoric q.) More to the point, all these "structures" are more or less arbitrary groupings. If some scientists fancied even larger one, then why not.
Mithoron
CC BY-SA 4.0
2024-11-15T16:17:54.023Z
@Mithoron, I wasn't aware that debate existed over whether such structures exist. Irrespective, I believe that I reasonably presume that at least at some point, the areas of these structures become quite well-defined. As an example, the periphery of the Milky Way is certainly so, even if, as an example, the Local Group's is less so. Consequently, if you answer this question by stating the largest structure with an agreed-upon boundary that ultimately contains Earth, I'll accept that. Such grievances with the premise are exactly why I thought to ask this question.
RokeJulianLockhart
CC BY-SA 4.0
2024-11-17T19:51:12.587Z
We're not inside Earth (unless you're in a hole). We're on it.
Starship
CC BY-SA 4.0
2024-11-17T23:59:46.207Z
@Starship, certainly, although what are you responding to?
RokeJulianLockhart
CC BY-SA 4.0
2024-11-18T00:00:25.793Z
@RokeJulianLockhart "Thus far, I've ascertained that we're inside (in descending order of size):1.Earth"
Starship
CC BY-SA 4.0
2024-11-18T02:44:16.000Z
In science fiction planets often have Roman numerals in the order of their semi-major orbital axis.  Thus Earth is Sol III in science fiction.  And no doubt when astronomers know exactly how many planets are in star systems, they were replace their current designations with Roman numerals to be consistent with science fiction.  Anyway the practice of giving planets lower case Latin letters in order of discovery can't apply to Earth, since Earth and several other objects were discovered to be planets in the modern sense at the same time, when the Heliocentric theory was proved.
M. A. Golding
CC BY-SA 4.0
2024-11-23T01:54:19.613Z
Isn't that ascending order of size?
Acccumulation
CC BY-SA 4.0
2024-11-23T14:36:46.963Z
@Acccumulation, it is — thanks. Remediated in [`/revisions/58914/6`](https://astronomy.stackexchange.com/revisions/58914/6#:~:text=we're%20inside%20(in-,ascending,-order%20of%20size):).
RokeJulianLockhart
CC BY-SA 4.0
2024-11-23T20:57:58.890Z
@planetmaker undoubtedly? Be more imaginative
Rabbi Kaii
<positional-astronomy>
What is the largest structure that contains Earth?
RokeJulianLockhart
# What is the largest structure that contains Earth?

(Asked by: username_0 on 2024-11-14)

Thus far, I've ascertained that we're inside (in ascending order of size):

1. Earth ~~(expectedly `Sol c`, although [this](http://www.exoplanetkyoto.org/exohtml/Sol_d_(Earth).html) seems to think it's `d`?)~~ [[1]](https://astronomy.stackexchange.com/questions/58914/what-is-the-largest-structure-that-contains-earth#comment136512_58914)
2. The *Solar System*
3. The *Milky Way*
4. The *Local Group*
5. The *Virgo Supercluster*
6. The *Laniakea Supercluster*
7. The *Pisces–Cetus Supercluster Complex*

However, I'm interested in whether, as an example, the PCSC comprises the Hercules–Corona Borealis Great Wall, or whether it's distinctly separate. I've attempted to read [the relevant maps provided by Wikipedia](https://en.wikipedia.org/w/index.php?title=Pisces%E2%80%93Cetus_Supercluster_Complex&oldid=1253830973#/media/File:Superclusters_atlasoftheuniverse.gif), but I simply don't understand them.

Considering how many larger structures (like the aforementioned) exist beside the PCSC, I'll be surprised if it doesn't reside within, or upon the periphery of, a larger structure.

**Tags:** positional-astronomy

> **Comments:**
> **username_1**: See [Exoplanet Naming Conventions](https://en.wikipedia.org/wiki/Exoplanet_naming_convention) for why I guess they're calling Earth  Sol d.   That said, as the first planet around Sol discovered, it _probably_ would be retroactively pedantically Sol b.  
> **username_2**: The largest structure is undoubtedly the universe itself.  
> **username_0**: @username_2, is that a structure?  
> **username_3**: Do you define a structure as a group of gravitationally bound objects?  
> **username_0**: @ConnorGarcia, I asked because *I* cannot reasonably define anything in this field — I prefer to defer to authority. If I am to be asked, [`en.wikipedia.org/wiki/List_of_largest_cosmic_structures`](https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures#) states that “This list refers only to coupling of matter with defined limits, and not the coupling of matter in general (such as, for example, the cosmic microwave background, which fills the entire universe). All structures in this list are defined as to whether their presiding limits have been identified”. Does that answer?  
> **username_0**: @username_1, are they named in accordance to when they were discovered, rather than their orbital proximity to their nearest star?  
> **username_1**: @username_0 Yes, because it's very likely that we haven't discovered all the planets around most stars that we know have planets, and later discoveries may break the ordering.  
> **username_4**: How exactly PCSC could comprise Hercules–Corona Borealis GW, if the latter is, like, 10 times longer (if exists at all)? (rhetoric q.) More to the point, all these "structures" are more or less arbitrary groupings. If some scientists fancied even larger one, then why not.  
> **username_0**: @username_4, I wasn't aware that debate existed over whether such structures exist. Irrespective, I believe that I reasonably presume that at least at some point, the areas of these structures become quite well-defined. As an example, the periphery of the Milky Way is certainly so, even if, as an example, the Local Group's is less so. Consequently, if you answer this question by stating the largest structure with an agreed-upon boundary that ultimately contains Earth, I'll accept that. Such grievances with the premise are exactly why I thought to ask this question.  
> **username_5**: We're not inside Earth (unless you're in a hole). We're on it.  
> **username_0**: @username_5, certainly, although what are you responding to?  
> **username_5**: @username_0 "Thus far, I've ascertained that we're inside (in descending order of size):1.Earth"  
> **username_6**: In science fiction planets often have Roman numerals in the order of their semi-major orbital axis.  Thus Earth is Sol III in science fiction.  And no doubt when astronomers know exactly how many planets are in star systems, they were replace their current designations with Roman numerals to be consistent with science fiction.  Anyway the practice of giving planets lower case Latin letters in order of discovery can't apply to Earth, since Earth and several other objects were discovered to be planets in the modern sense at the same time, when the Heliocentric theory was proved.  
> **username_7**: Isn't that ascending order of size?  
> **username_0**: @username_7, it is — thanks. Remediated in [`/revisions/58914/6`](https://astronomy.stackexchange.com/revisions/58914/6#:~:text=we're%20inside%20(in-,ascending,-order%20of%20size):).  
> **username_8**: @username_2 undoubtedly? Be more imaginative  

---

## 1 answers

### Answer by username_9 on 2024-11-20. Score: 3 | Accepted answer

People have always looked for patterns in the skies, sometimes even seeing structures where none exist (such as a [hunter and his dog](https://earthsky.org/tonight/orions-belt-points-to-sirius/)). One specific kind of structure that astronomers care about is one which is *gravitationally bound*.

One effect of being gravitationally bound is that the gravitational attraction is strong enough to overcome the expansion of the universe, as explained in [this answer](https://astronomy.stackexchange.com/a/109/28710) and [this answer](https://astronomy.stackexchange.com/questions/11726/does-the-universe-expand-at-the-same-rate-everywhere-in-the-universe) and perhaps others. In our [local group](https://en.wikipedia.org/wiki/Local_Group), the two biggest galaxies, namely the Milky Way and the Andromeda Galaxy, are so much gravitationally bound to each other that they are on a collision course and are expected to merge in another 4-5 billion years. Also, the [Sagittarius dwarf galaxy](https://en.wikipedia.org/wiki/Sagittarius_Dwarf_Spheroidal_Galaxy) has orbited several times around the center of the Milky Way, and in the course of that has passed through the Milky Way several times, leaving behind milky trails that loop up and back; it is also expected to eventually just merge with the Milky Way.

As [this answer](https://astronomy.stackexchange.com/a/32513/28710) says, the Virgo supercluster is not gravitationally bound. Generally, within "structures" that are not gravitationally bound one can find substructures (e.g. galaxies) whose distances are increasing at a rate proportional to their distance, that proportionality constant being the Hubble constant. In other words, the relative motion of those substructures is governed by the expansion of the universe.

> **Comments:**
> **username_0**: Consequently, are you of the opinion that everything in my list above the Local Group is ultimately erroneous, because whether we are located inside it is very much a matter of opinion? Apologies for requesting a summary of such a well-written answer – I promise I have read it in full.  
> **username_9**: Ah, well, I believe so, it is certainly consistent. But I would say that requires a deeper knowledge of astronomical structures than I possess.  

---

