Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is.
This is another comic with a Fact, the second in a row of these fact comics to use an Astronomy fact.
Our best approximation of the number of galaxies in the observable universe is about 200 billion (2 × 1011). That's a lot of galaxies, and here Randall exemplifies this by showing a small circle and estimating that when the comic's picture is viewed at a typical arm's length, expanded to full screen on your typical smartphone, the circle contains roughly 50,000 galaxies (that means of course not the small circle itself, but the volume defined by the viewer's eye, that circle, and an onward conical extension into deep space — and simultaneously back in time — to the respective limits of the observable universe). Most of those far-away galaxies are undetectable by even our most powerful astronomical instruments today, and comparatively few could be seen (let alone positively identified as such) by the naked eye. For example, in the Hubble Deep Field, an image of a small region in the constellation Ursa Major, about 3,000 visible galaxies can be identified.
Measuring in the mid-point of the lines, the circle is about one fortieth of the width of the frame of the comic. The absolute circle size depends on the display resolution, size and mode, but it can reasonably be taken to be 1mm diameter, or 0.5mm radius, giving a total area π r2 or about π/4 square millimeters. You're probably holding the phone about a half a meter away from your eye. The surface area of a sphere is 4 π r2. With a radius of one-half meter, that comes out to be π square meters. Thus, the area of the circle is about 1/4000000 of the area of the sphere, 200 billion galaxies divided by 4 million is the 50,000 average mentioned in the cartoon. A similar mathematics was used for the comic 1276: Angular Size, in which the projective sphere was at the Earth's own radius and cross-sectional areas of objects were compared, rather than an approximate count of objects within a given angular spread.
While galaxies usually are between 3,000 to 300,000 light-years across and contain between 10^8 (100 million) and 10^14 (100 trillion) stars, most are so far away from the Earth (upwards of billions of light-years) that they are invisible to the naked eye, or even through most telescopes. When magnified across such vast distances, even something as small as a pinhole expands to huge sizes, easily able to fit tens of thousands of galaxies.
The premise of this comic is that although galaxies are giant, space is unimaginably big and contains a vast number of things. Randall is apparently overwhelmed by this, as shown in the caption: Astronomy Fact: There are too many galaxies.
The title text is Randall reassuring his readers why not to worry of this overwhelming fact. He states that most galaxies only have few stars and probably no planets. However, as mentioned above each galaxy contains a huge amount of stars, and as evident from all his own comics about exoplanets, it is now clear that many of the stars in a galaxy also have planets orbiting them. Thus the number of stars and planets in that small circle is much more mind-bogglingly large, than the number of galaxies, and thus the reassurance is sarcasm.
In 975: Occulting Telescope Cueball expresses a similar sentiment about the number of stars.
- [An almost white panel with a caption at the top. Then a small circle, much smaller than for instance the letter O in the text is in the center of the panel. A bending arrow points to the circle and beneath the arrow is a caption.]
- Open this picture fullscreen on your phone and hold it at arm's length.
- There are 50,000 galaxies in this circle.
- [Caption below the panel:]
- Astronomy Fact: There are too many galaxies.
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I bet this was inspired by the image from the James Webb Space Telescope after fine alignment was complete: https://www.nasa.gov/sites/default/files/thumbnails/image/telescope_alignment_evaluation_image_labeled.png. That image shows the one star that JWST was focused on, along with a bunch of galaxies in the far background demonstrating that there are a lot of galaxies. Orion205 (talk) 04:52, 22 March 2022 (UTC).
- My thoughts exactly - definitely a JWST thing.220.127.116.11 07:09, 22 March 2022 (UTC)
- I do not really think so. Not directly. The image is some weeks old now, and also the knowledge of the number of galaxies comes from the Hubble Deep Field. Which Randall has mentioned before, at least in his Thing explainer. --Kynde (talk) 13:43, 22 March 2022 (UTC)
Speaking of JWST, isn't the "only a few stars, and probably no planets." just a nod to the fact that one of the missions of its infrared astronomy is to "see back in time to the first galaxies forming just a few hundred million years after the Big Bang."? i.e.: given the conical shape of the sector of universe covered by the pin-hole, most of the galaxies in it will be the furthest back, thus those of which we see the "oldest" picture and thus which much earlier in their process of forming stars and planets? DrYak (talk) 09:17, 22 March 2022 (UTC)
- Maybe but we cannot see them before the stars have formed and begun shining light. And by that time most of the planets would have formed. And also many many stars. So I do not think there is anything in the title text other than to make you think that what he says there is just plain wrong, and thus become even more overwhelmed! --Kynde (talk) 13:43, 22 March 2022 (UTC)
- I sort of thought it might be an(other) H2G2 reference, to whit...
It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is zero, and that any people you may meet from time to time are merely the products of a deranged imagination.
- ...but worlds-to-galaxies, by the same 'logic', rather than beings-to-worlds. But I'm not sure enough to mention it in the main article, because it disagrees on the basic 'number of planets' issue (indeed, whether the ((observable)) universe is infinite). But putting it out there because it's perfect Adamsonian logic. ;) 18.104.22.168 14:09, 22 March 2022 (UTC)
While space contains contains a vast number of things, it's still mostly empty. Like, on average, few atoms in whole square meter. Atoms of hydrogen, usually. Imagining so much of practically empty space is even harder than imagining all the stuff in it. -- Hkmaly (talk) 05:50, 22 March 2022 (UTC)
It took me half a minute to understand it: not galaxies within the circle, but within the region of the sky with the same apparent size from your position as the circle. 22.214.171.124 10:19, 22 March 2022 (UTC)
- And you can move that circle anywhere you like. Also looking down at Earth surface. Then there will still be 50,000 galaxies within that circle on the other side of the Earth. As long as you hold it at arms length. --Kynde (talk) 13:43, 22 March 2022 (UTC)
Another way to consider this is that the moon is a privacy screen for 1.5 million galaxies at any one time. 126.96.36.199 16:55, 22 March 2022 (UTC)
The size of the circle could be more accurately estimated by noting that the majority of phones have between a 4.7 to 5.8 inch diagonal (https://deviceatlas.com/blog/viewport-resolution-diagonal-screen-size-and-dpi-most-popular-smartphones). While aspect ratios vary, that seems a decent approximation for the diagonal of the image, and from that, the size of the circle could more accurately estimated from its pixel size relative to the image diagonal. Overkill, though, I know. 188.8.131.52 16:55, 22 March 2022 (UTC)
"Most are undetectable by even our most powerful astronomical instruments today," - surely the point is that they are all detectable by the most powerful instruments, otherwise we could not count how many (typically) there are. It's just that we need more than your 'average' telescope, i.e. Hubble/Webb. The HDF image shows 3,000 items in a solid-angle approximately 1/24millionth of the sky, by the way, which is (at 50cm radius) approximately an eighth of a mm². 50,000
starsgalaxies would be in ~2mm² of such an image, which is a circle almost 0.8mm in radius - if I haven't slipped up somewhere in the mental arithmatic (someone will tell me that I have, probably!). But that seems to relate well to the scenario suggested. 184.108.40.206 20:29, 22 March 2022 (UTC)
- "Surely the point is that they are all detectable by the most powerful instruments" - no, not necessarily. I figure that we can guess at the number of galaxies out there without being able to detect them all - by estimating the density of galaxies and considering the size of the universe, for example; and it may be that some galaxies are so far away and that the universe is expanding at such a rate that energy from those galaxies has not yet reached us and might never. - Brian Kendig (talk) 20:47, 22 March 2022 (UTC)
- Both those points are (differently) addressed by the fact that we're talking about the Observable Universe.
- We're already seeing to the edge of where galaxies are. In reality, as far back in time as we can, when galaxies and their composition of stars were about as young as they could be, meaning that there's not many things yet to be seen beyond (though whatever we do end up seeing is going to be interesting to science).
- Anything whose image will never reach us is beyond the limit of the O.U. and is, by definition, unknowable and uncountable.
- (Maybe the interface between the matter and antimatter remnants of the original actually balanced universe is beyond this effective limit? Or we may indeed have an infinite universe that, if we could see it all, means there's no patch of dark sky, because there's always something bright in whatever bit of the sky we look, at whatever scale or level of zoom.)
- Actually, I tend to suspect that the real problem is that the universe has a wrap-around topology which means that looking far/long enough in any direction (barring expansion or even big-rip limits to what we could see) means we start seeing things from the opposite side of the entire field of view but from the other direction... Then everything closer to us (beyond that) then eventually, ourselves. And beyond us (*hand-wave away what happens if some strangeness of the situation means we're effectively the thing we see 28.5 gigaparsecs or more away in every direction at once and so can't be seen beyond... ;) *) a smaller copy of everything we's been peering between to see our current view....
- Not that we could (speed of light + age of universe + continuing expansion of universe + any acceleration of expansion + something else we probably don't know of yet), but I have an unhealthy affinity for the concept of a boundless but finite universe, like the 3D surface of a hypersphere where looking in any direction (including a time-component!) eventually wraps-around on a Great Circle analogue. ;-p
- ...anyway, the point being that if we have a problem with counting visible objects, it's more likely that we can be counting some things twice or more (cosmos-wide gravitational lensing?). Anything truly beyond visibility is irrelevent, and probably won't become any more relevent any time soon. But we can develop means to better see things that aren't that easy to see, yet can be seen with enough effort. 220.127.116.11 23:18, 22 March 2022 (UTC)
- Also note that the Hubble Deep Field depicts approximately 50,000 galaxies, not just stars. See . - Brian Kendig (talk) 20:50, 22 March 2022 (UTC)
- My error, now corrected. I was refering to the galaxies that both the comic and the HDF were enumerating, but I wrote stars - for reasons best known to my dithering mind at the time. But nice to see you're reading my waffle and actually paying attention. ;) 18.104.22.168 23:18, 22 March 2022 (UTC)
I measured the size of the circle compared to the frame width (it's 1/40 of the frame width). I left the other calculations alone, but someone should probably re-scale them, as they presume that the frame is 40mm wide, which seems small for "fullscreen on your phone". Or maybe the calculations are wrong; if I had to bet between Randall and some random Internet editor (myself being included in that category) I'd bet on Randall. -- Dtgriscom (talk) 10:29, 23 March 2022 (UTC)
I question the comment about the title text being 'sarcasm' on the basis that "we know that galaxies contain billions of stars and most of them will have planets" (I paraphrase). My view is that most of the 50,000 galaxies are likely to be very young, and we believe that the first generation of stars were large, few and short lived.
As to whether they would have had multiple planets, this is an open question. Certainly, there wouldn't have been rocky planets, as the stars will have formed from the elements formed in the original big bang, which was almost exclusively Hydrogen and Helium, with essentially no heavier elements to form anything like rocky planets.
Gas giant planets may have formed alongside the first massive stars, but we just don't know.OwenGwynne (talk) 12:00, 23 March 2022 (UTC)
Adding to the discussion about planets in galaxies: Most galaxies in the universe are Dwarf Galaxies (see also the external sources of the Wikipedia article). DG have very distinct physical properties than our Milky Way, they contain older and/or bigger (=hotter) stars, less heavy elements (>He) and are more compact, i.e., there are more stars in less space. They also seem to have more dark matter per visible mass than larger galaxies. All of these properties make it more improbable for the stars to form planets, and even more so small rocky ones. And it probably also makes it more difficult to create life, as the cosmic vicinity of each star is much more unstable.
Not relevant, but a point about the numbers involved. It's estimated that there are 200 billion galaxies in the observable universe - undoubtedly a large number. It is also estimated that there are 3 trillion trees on Earth, so maybe 15 trees for every galaxy.
The discussion ignores dwarf galaxies. Some have diameters as small as 100 light years and contain as few as 1000 stars. (Examples available at Wikipedia's "List of Satellites of the Milky Way".) Apparently the title text refers to dwarf galaxies, which could be 90% of all galaxies.
Of course, there is probably no obvious limit between a dwarf galaxy and a regular galaxy. At the other end of the scale, there is no obvious limit between a dwarf galaxy and a globular cluster (or just a bunch of stars) in intergalactic space.