<p>We are IN a Galaxy. It doesn't feel very warm to me when I go outside at night, compared with direct sunlight!</p>
<p>Your basic problem here is the enormous distance between stars compared with the distance between the Earth and the Sun, combined with the inverse square law of how much flux (power per unit area) a planet receives as a function of distance from a luminous source.</p>
<p>Another way of thinking about it is that the Sun occupied about 0.004% of the sky (when it is above the horizon), but the distant stars in a Galaxy, even though numerous, occupy a much smaller fraction because they are so far away and separated from each other by distances much larger than their individual diameters.</p>

CC BY-SA 4.0
2023-10-13T03:40:44.987Z
My understanding is there is a distance from the galactic core yielding a life zone; except for the fact you're fried by radiation without some truly exotic solution.
Joshua
CC BY-SA 4.0
2023-10-11T10:28:10.340Z
2023-10-11T10:28:10.340Z
ProfRob
<p>Even if the primary (galaxy around which the planet is orbiting, in this case) has an <a href="https://fermi.gsfc.nasa.gov/science/eteu/agn/" rel="noreferrer">active galactic nucleus</a>, so the galaxy is very bright, the interstellar or intergalactic planet would be very cold due to the inverse square law which says how light/energy from a point source decreases with distance: the light/energy is <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html" rel="noreferrer">inversely proportional</a> to the square of distance, so at distance 1 you get 1 unit of energy, at distance 2 you get 1/4 and at distance 3 you get 1/9;</p>
<p>The planet would have to orbit in the galactic disk for its orbit to be stable enough for a while, though it could easily be disturbed by a passing star; if the planet would have a high inclination, orbiting above and below the galactic disk, then the gravitational forces the planet would be subjected to would vary along its orbital path, which would change its orbit significantly during a single cycle.</p>
<p>An important thing to consider is that the <a href="https://sites.ualberta.ca/%7Epogosyan/teaching/ASTRO_122/lect24/lecture24.html" rel="noreferrer">orbital speed</a> for planets around a star decreases with distance from the star - <a href="https://en.wikipedia.org/wiki/Orbital_elements" rel="noreferrer">Keplerian orbits</a>;</p>
<p><img src="https://i.sstatic.net/rHKGG.jpg" alt="keplerian vs galactic orbit speed" /></p>
<p>In the case of galaxies, the orbital speed of stars (and therefore, of the interstellar planet in question) around the galactic center stays quite high (over 200km/sec) as distance increases from the center, and this also varies from galaxy to galaxy. Compare that to the <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/" rel="noreferrer">orbital speed of the Earth</a> around the Sun which is about 30km/sec, whereas Pluto orbits at about 4.7km/sec.</p>
<p>For this reason, besides being cold and dark, such an interstellar planet would have an unstable orbit, and it could be either captured or thrown out of the galaxy by <a href="https://en.wikipedia.org/wiki/Gravity_assist" rel="noreferrer">gravity assist</a> of a passing star.</p>
<p><img src="https://i.sstatic.net/tcKAN.jpg" alt="agn" /></p>
<p>The only places where the planet might get enough light/radiation would be in the jet stream of an active galactic nucleus (AGN), except that is in the galactic north and south pole directions, and an orbit above or below the galactic disk would be unstable, and passing through the jet stream from the poles would happen once in a few hundred million years(the night), depending on the orbit, and would last probably a few hundred years(the day), if such an orbit were stable, which it is not.</p>

CC BY-SA 4.0
2023-10-12T07:54:44.090Z
I became interested.  Is the blue curve an observation or a calculation?  Why does it have such an oscillatory character?
ayr
CC BY-SA 4.0
2023-10-12T08:03:58.010Z
@dtn the blue curve represents orbital speeds of stars around the galactic center at different radiuses, so it is observation data; at the [source](https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect24/lecture24.html) I put in the post, you can see the above diagram, and further below, another one, showing the orbital speeds of stars in 4 separate galaxies, and you will see they are all different, so this curve varies from galaxy to galaxy
jmarina
CC BY-SA 4.0
2023-10-12T09:56:11.687Z
Yes, I understand the meaning of this graph, but I still do not understand why the speed of rotation of stars around the center of the galaxy has such an oscillatory character.
ayr
CC BY-SA 4.0
2023-10-12T11:24:44.473Z
@dtn this is an unsolved problem in astronomy/astrophysics - [the galaxy rotation curve](https://en.wikipedia.org/wiki/Galaxy_rotation_curve) difference between expected and actual orbital speed as distance increases from the center is why dark matter was invented, to try to explain why galaxies rotate so fast
jmarina
CC BY-SA 4.0
2023-10-12T12:44:12.357Z
Why the planet should necessarily be interstellar? Why it can't orbit non-luminous source? like a brown dwarf. neutron star of even a black hole. at the same time it could be illuminated by the jet, as you propose. Moreover, the density of stars per unit of volume is different in different parts of galaxy/universe., so why jet, why central halo can't illuminate such a planet?
dEmigOd
CC BY-SA 4.0
2023-10-12T12:50:36.637Z
I know about this problem, it is mentioned in the article about dark matter on the wiki and there is also an example of such a curve: https://en.wikipedia.org/wiki/Dark_matter#/media/File:GalacticRotation2.svg But it was about another aspect (in the figure from your answer).  Let me clarify by attaching my drawing: https://i.sstatic.net/htRDi.png So in the picture I noticed a strange oscillation (like “oscillations of a spring”) and did not quite understand what it was? Error of measuring instruments or the uneven distribution of this dark matter in the galaxy?
ayr
CC BY-SA 4.0
2023-10-12T12:59:51.137Z
I can't see why you are arguing that various orbits are "unstable" - we see stars in such orbits. They aren't unstable.
ProfRob
CC BY-SA 4.0
2023-10-12T14:30:20.673Z
[orbital instability due to high inclination](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6677160/) for Keplerian orbits (inside a solar system) causes the orbits to change, and therefore they are unstable; in the case of interstellar orbits, this is likely different anyway, and not necessarily undesirable in this case;
jmarina
CC BY-SA 4.0
2023-10-12T14:35:47.660Z
@dEmigOd the question was about a planet orbiting a galactic center, so that would be an interstellar planet; you are correct that it could orbit a non-luminous source; this is in fact how we know the mass of the central black hole in our galaxy to be 4M solar masses although we cannot see it - because the star S2 and then several other stars were discovered orbiting around it, enabling calculating its mass; you are correct, if the planet is in a region with high star density, it would get significantly more starlight than on earth likely not nearly as bright as the sun, life could adapt
jmarina
CC BY-SA 4.0
2023-10-12T14:44:25.753Z
@dtn it is very interesting that you pointed out the curve looks like oscillations in a spring, let's say harmonics 3, 5, 7 and 9; first, these curves look different for each galaxy - but all indicate higher orbital velocity than would be expected for Keplerian orbits; secondly, dark matter was added to the equation to try to explain this, but we don't actually know the cause; the idea is that if the orbital speed is too high, the stars should fly out of the galaxy; so "dark matter" was added, to create gravity to keep them in; but why is the orbital speed so high?
jmarina
CC BY-SA 4.0
2023-10-12T14:57:23.130Z
absolutely right, in addition to the well-known problem of orbital velocities, I was also interested in oscillations on orbital velocity curves
ayr
CC BY-SA 4.0
2023-10-13T12:31:49.390Z
Maybe you have a different definition of unstable? There are plenty of halo objects that have orbited the Galaxy for more than 10 billion years. The reference you cite is about disks of objects.
ProfRob
CC BY-SA 4.0
2023-10-11T11:04:03.083Z
2023-10-12T08:07:37.187Z
2023-10-12T08:07:37.187Z
jmarina
<p>To a very rough approximation the habitable zone of a star is <span class="math-container">$0.7\sqrt{L/L_\odot}$</span> to <span class="math-container">$1.4\sqrt{L/L_\odot}$</span> AU. So if we want a habitable zone that extends to <span class="math-container">$r$</span> AU the luminosity needs to be about <span class="math-container">$(r/1.4)^2$</span> to <span class="math-container">$(r/0.7)^2$</span> solar luminosities. So for an extragalactic planet <span class="math-container">$r$</span> will be several billion AU, requiring a total luminosity from the galaxy on the order of <span class="math-container">$10^{18}$</span> solar luminosities. This is way above even the largest galaxies (and they would of course require a much wider orbit).</p>

CC BY-SA 4.0
2023-10-11T12:07:01.790Z
2023-10-11T12:07:01.790Z
Anders Sandberg
<p>I think that the closest you could get to this is the black hole TON 618. Due to its immense mass (forty billion Suns' worth of mass) it has an accretion disk that kicks out 1.4 x 10^14 times as much energy as our Sun.</p>
<p>Basically, you could be roughly 11.83 <em>million</em> astronomical units away from TON 618 and its apparent brightness would be the same as we experience from our Sun, one astronomical unit away.</p>
<p>11.83 million AUs works out to about 187 light years, much smaller than our own galaxy, however. Someone with a serious education in astronomy would have to tell you whether a galaxy that small could have a black hole of that mass at its center, but as a layman I rather doubt it.</p>

CC BY-SA 4.0
2023-10-13T12:37:48.320Z
+1 I think this is ok - your planet orbits 200 light years from the black hole. But the caveats might be (I) the spectrum of the radiation and (ii) whether the high luminosity quasar phase will last very long.
ProfRob
CC BY-SA 4.0
2023-10-12T05:17:03.840Z
2023-10-12T05:17:03.840Z
EvilSnack
<p>Our star orbits around a galaxy, which you can see if you look up at night: The Milky Way. As you can see, it's not bright enough to really call a sun.</p>
<p>The problem is basically luminance density, or you can say candelas per cubic meter, or total flux over the smallest blob that envelops the whole galaxy. It needs to be a lot higher for what you say.</p>
<p>Making stars a lot brighter doesn't make much sense. However, you could pack them tighter. Most galaxies are not that packed, so yours would be unusual and perhaps you may need to explain that, but hey, it's science <em>fiction</em>.</p>
<p>Nothing obviously bad or weird would happen if the stars were a bit closer to each other.</p>
<p>One estimate would be to copy paste 20 solar systems, all at 40 AU from the Sun (orbits of Uranus touching). That gives you 20/1600=1/80 the light of the Sun from the first shell. The second shell, at 80 AU, would have 4x as many, so you get 80/6400=1/80. Shell 80 would be 3200 AU away, would have 6400*20=128000 stars, so the whole system is about 250k. For comparison, our galaxy has ~250 billion, a million times more. Of course, if you pack solar systems <em>that close</em>, something bad or weird probably would happen - the Uranii might collide, the Oort clouds, the orbits of the Suns... But you could space them out more and compensate by having more stars in the whole galaxy, so it seems plausible. Keep in mind that in this oversimplified calculation, the entire galaxy is only 0.1 light year across, which is <em>odd</em>, but it doesn't seem <em>impossible</em>.</p>
<p>Another way is to take the Milky Way and compress it. How much would you need to compress it, to give as much light as the Sun? Taking the galaxy at -5 and the Sun at -26, we it needs to be about 10^8 times brighter. That means things need to be 10^4 times closer. It's about 87k light years across, and instead would need to be 8.7 light years. Again, certainly <em>odd</em>, but perhaps not obviously impossible, and luckily &quot;sort of&quot; close to the other estimate.</p>
<p>One problem I see is that while the Sun makes a lot of nice visible light, a lot of objects in the galaxy make all sorts nasty stuff like X-rays. By compressing them and making them 10^8 stronger, you're also increasing the galactic radiation by a similar degree. That doesn't sound like good news for the people living there.</p>

CC BY-SA 4.0
2023-10-12T03:22:59.337Z
2023-10-12T03:22:59.337Z
gomennathan
<blockquote>
<p>A <a href="https://en.wikipedia.org/wiki/Blanet" rel="nofollow noreferrer">blanet</a> is a member of a hypothetical class of exoplanets that directly orbit black holes.</p>
</blockquote>
<blockquote>
<p>Blanets are fundamentally similar to planets; they have enough mass to be rounded by their own gravity, but are not massive enough to start thermonuclear fusion, just like planets that orbit stars. In 2019, a team of astronomers and exoplanetologists showed that there is a safe zone around a supermassive black hole that could harbor thousands of blanets in orbit around it.</p>
</blockquote>
<hr />
<blockquote>
<p>Is there an habitable zone for black holes?</p>
</blockquote>
<blockquote>
<p>So planets could potentially form around black holes, but that's no guarantee that they offer a life-friendly environment. On Earth, living things are hugely dependent on the light and warmth from the Sun to survive. Without the glow of a star, life around a black hole would likely need an alternative source of energy. <a href="https://www.sciencefocus.com/space/can-life-exist-around-a-black-hole" rel="nofollow noreferrer">https://www.sciencefocus.com/space/can-life-exist-around-a-black-hole</a></p>
</blockquote>

CC BY-SA 4.0
2023-10-13T02:19:50.590Z
2023-10-13T02:19:50.590Z
Mazura
<p>Apart from being in a dark, lonely place: Galaxies are, relative to their own size, quite close to each other.</p>
<p><a href="https://imagine.gsfc.nasa.gov/features/cosmic/nearest_galaxy_info.html" rel="nofollow noreferrer">The nearest galaxies to the Milky Way are so close that they experience tidal forces</a>, i.e. they are deformed or torn apart. A star so far outside the Milky Way that it does not interact much with its single stars would eventually interact with neighboring galaxies. There are probably no stable orbits.</p>

CC BY-SA 4.0
2023-10-11T19:55:41.870Z
2023-10-11T19:55:41.870Z
Peter - Reinstate Monica
<p>For the purpose of science fiction, could an entire galaxy essentially be a planet's equivalent to our Sun for Earth?</p>
<p>If a planet were far enough away from other objects in the galaxy but still orbited the galaxy, especially one that was very bright, perhaps mostly composed of stars, could this planet feel much like Earth?</p>
<p>It would receive heat and light, and its inhabitants might view the entire galaxy as a brilliant point in the sky.</p>

CC BY-SA 4.0
2023-10-11T10:01:58.840Z
Galaxies and stellar systems are completely different entities. In stellar systems most of the mass is concentrated in the star, whereas in a galaxy most of the mass sits in the disc. The 'nucleus' of a galaxy does not determine how things are orbiting the galaxy, nor does the nucleus heat its surroundings...
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-10-11T18:37:48.420Z
We are *in* a galaxy, what do you see at night?
Lamar Latrell
CC BY-SA 4.0
2023-10-11T22:41:23.330Z
This is kind of like asking if you could have an entire city be your next door neighbour. Too far and nothing is your neighbour, too close and only the people right next door are your neighbours, never the entire city.
DKNguyen
CC BY-SA 4.0
2023-10-13T17:21:43.117Z
I'm not in a state to want to do the math right now, but my guess would be that the amount of energy a galaxy would have to put off for a planet to receive the equivalent amount of energy as the Earth does from the Sun (and be the same relative size in the sky), that galaxy would basically have to be a galaxy-sized star in its own right, and I shudder to think of what a star that big would do to our poor unsuspecting laws of physics.
Abion47
<galaxy>
Could a galaxy be the sun of a planet?
Gatoyu
# Could a galaxy be the sun of a planet?

(Asked by: username_0 on 2023-10-11)

For the purpose of science fiction, could an entire galaxy essentially be a planet's equivalent to our Sun for Earth?

If a planet were far enough away from other objects in the galaxy but still orbited the galaxy, especially one that was very bright, perhaps mostly composed of stars, could this planet feel much like Earth?

It would receive heat and light, and its inhabitants might view the entire galaxy as a brilliant point in the sky.

**Tags:** galaxy
> **Comments:**
> **username_1**: Galaxies and stellar systems are completely different entities. In stellar systems most of the mass is concentrated in the star, whereas in a galaxy most of the mass sits in the disc. The 'nucleus' of a galaxy does not determine how things are orbiting the galaxy, nor does the nucleus heat its surroundings...  
> **username_2**: We are *in* a galaxy, what do you see at night?  
> **username_3**: This is kind of like asking if you could have an entire city be your next door neighbour. Too far and nothing is your neighbour, too close and only the people right next door are your neighbours, never the entire city.  
> **username_4**: I'm not in a state to want to do the math right now, but my guess would be that the amount of energy a galaxy would have to put off for a planet to receive the equivalent amount of energy as the Earth does from the Sun (and be the same relative size in the sky), that galaxy would basically have to be a galaxy-sized star in its own right, and I shudder to think of what a star that big would do to our poor unsuspecting laws of physics.  

---

## 7 answers

### Answer by username_5 on 2023-10-11. Score: 27

We are IN a Galaxy. It doesn't feel very warm to me when I go outside at night, compared with direct sunlight!

Your basic problem here is the enormous distance between stars compared with the distance between the Earth and the Sun, combined with the inverse square law of how much flux (power per unit area) a planet receives as a function of distance from a luminous source.

Another way of thinking about it is that the Sun occupied about 0.004% of the sky (when it is above the horizon), but the distant stars in a Galaxy, even though numerous, occupy a much smaller fraction because they are so far away and separated from each other by distances much larger than their individual diameters.

> **Comments:**
> **username_6**: My understanding is there is a distance from the galactic core yielding a life zone; except for the fact you're fried by radiation without some truly exotic solution.  

---

### Answer by username_10 on 2023-10-11. Score: 11

To a very rough approximation the habitable zone of a star is $0.7\sqrt{L/L\_\odot}$ to $1.4\sqrt{L/L\_\odot}$ AU. So if we want a habitable zone that extends to $r$ AU the luminosity needs to be about $(r/1.4)^2$ to $(r/0.7)^2$ solar luminosities. So for an extragalactic planet $r$ will be several billion AU, requiring a total luminosity from the galaxy on the order of $10^{18}$ solar luminosities. This is way above even the largest galaxies (and they would of course require a much wider orbit).


---

### Answer by username_7 on 2023-10-11. Score: 9

Even if the primary (galaxy around which the planet is orbiting, in this case) has an [active galactic nucleus](https://fermi.gsfc.nasa.gov/science/eteu/agn/), so the galaxy is very bright, the interstellar or intergalactic planet would be very cold due to the inverse square law which says how light/energy from a point source decreases with distance: the light/energy is [inversely proportional](http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html) to the square of distance, so at distance 1 you get 1 unit of energy, at distance 2 you get 1/4 and at distance 3 you get 1/9;

The planet would have to orbit in the galactic disk for its orbit to be stable enough for a while, though it could easily be disturbed by a passing star; if the planet would have a high inclination, orbiting above and below the galactic disk, then the gravitational forces the planet would be subjected to would vary along its orbital path, which would change its orbit significantly during a single cycle.

An important thing to consider is that the [orbital speed](https://sites.ualberta.ca/%7Epogosyan/teaching/ASTRO_122/lect24/lecture24.html) for planets around a star decreases with distance from the star - [Keplerian orbits](https://en.wikipedia.org/wiki/Orbital_elements);

![keplerian vs galactic orbit speed](https://i.sstatic.net/rHKGG.jpg)

In the case of galaxies, the orbital speed of stars (and therefore, of the interstellar planet in question) around the galactic center stays quite high (over 200km/sec) as distance increases from the center, and this also varies from galaxy to galaxy. Compare that to the [orbital speed of the Earth](https://nssdc.gsfc.nasa.gov/planetary/factsheet/) around the Sun which is about 30km/sec, whereas Pluto orbits at about 4.7km/sec.

For this reason, besides being cold and dark, such an interstellar planet would have an unstable orbit, and it could be either captured or thrown out of the galaxy by [gravity assist](https://en.wikipedia.org/wiki/Gravity_assist) of a passing star.

![agn](https://i.sstatic.net/tcKAN.jpg)

The only places where the planet might get enough light/radiation would be in the jet stream of an active galactic nucleus (AGN), except that is in the galactic north and south pole directions, and an orbit above or below the galactic disk would be unstable, and passing through the jet stream from the poles would happen once in a few hundred million years(the night), depending on the orbit, and would last probably a few hundred years(the day), if such an orbit were stable, which it is not.

> **Comments:**
> **username_8**: I became interested.  Is the blue curve an observation or a calculation?  Why does it have such an oscillatory character?  
> **username_7**: @dtn the blue curve represents orbital speeds of stars around the galactic center at different radiuses, so it is observation data; at the [source](https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect24/lecture24.html) I put in the post, you can see the above diagram, and further below, another one, showing the orbital speeds of stars in 4 separate galaxies, and you will see they are all different, so this curve varies from galaxy to galaxy  
> **username_8**: Yes, I understand the meaning of this graph, but I still do not understand why the speed of rotation of stars around the center of the galaxy has such an oscillatory character.  
> **username_7**: @dtn this is an unsolved problem in astronomy/astrophysics - [the galaxy rotation curve](https://en.wikipedia.org/wiki/Galaxy_rotation_curve) difference between expected and actual orbital speed as distance increases from the center is why dark matter was invented, to try to explain why galaxies rotate so fast  
> **username_9**: Why the planet should necessarily be interstellar? Why it can't orbit non-luminous source? like a brown dwarf. neutron star of even a black hole. at the same time it could be illuminated by the jet, as you propose. Moreover, the density of stars per unit of volume is different in different parts of galaxy/universe., so why jet, why central halo can't illuminate such a planet?  
> **username_8**: I know about this problem, it is mentioned in the article about dark matter on the wiki and there is also an example of such a curve: https://en.wikipedia.org/wiki/Dark_matter#/media/File:GalacticRotation2.svg But it was about another aspect (in the figure from your answer).  Let me clarify by attaching my drawing: https://i.sstatic.net/htRDi.png So in the picture I noticed a strange oscillation (like “oscillations of a spring”) and did not quite understand what it was? Error of measuring instruments or the uneven distribution of this dark matter in the galaxy?  
> **username_5**: I can't see why you are arguing that various orbits are "unstable" - we see stars in such orbits. They aren't unstable.  
> **username_7**: [orbital instability due to high inclination](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6677160/) for Keplerian orbits (inside a solar system) causes the orbits to change, and therefore they are unstable; in the case of interstellar orbits, this is likely different anyway, and not necessarily undesirable in this case;  
> **username_7**: @dEmigOd the question was about a planet orbiting a galactic center, so that would be an interstellar planet; you are correct that it could orbit a non-luminous source; this is in fact how we know the mass of the central black hole in our galaxy to be 4M solar masses although we cannot see it - because the star S2 and then several other stars were discovered orbiting around it, enabling calculating its mass; you are correct, if the planet is in a region with high star density, it would get significantly more starlight than on earth likely not nearly as bright as the sun, life could adapt  
> **username_7**: @dtn it is very interesting that you pointed out the curve looks like oscillations in a spring, let's say harmonics 3, 5, 7 and 9; first, these curves look different for each galaxy - but all indicate higher orbital velocity than would be expected for Keplerian orbits; secondly, dark matter was added to the equation to try to explain this, but we don't actually know the cause; the idea is that if the orbital speed is too high, the stars should fly out of the galaxy; so "dark matter" was added, to create gravity to keep them in; but why is the orbital speed so high?  
> **username_8**: absolutely right, in addition to the well-known problem of orbital velocities, I was also interested in oscillations on orbital velocity curves  
> **username_5**: Maybe you have a different definition of unstable? There are plenty of halo objects that have orbited the Galaxy for more than 10 billion years. The reference you cite is about disks of objects.  

---

### Answer by username_11 on 2023-10-12. Score: 5

I think that the closest you could get to this is the black hole TON 618. Due to its immense mass (forty billion Suns' worth of mass) it has an accretion disk that kicks out 1.4 x 10^14 times as much energy as our Sun.

Basically, you could be roughly 11.83 *million* astronomical units away from TON 618 and its apparent brightness would be the same as we experience from our Sun, one astronomical unit away.

11.83 million AUs works out to about 187 light years, much smaller than our own galaxy, however. Someone with a serious education in astronomy would have to tell you whether a galaxy that small could have a black hole of that mass at its center, but as a layman I rather doubt it.

> **Comments:**
> **username_5**: +1 I think this is ok - your planet orbits 200 light years from the black hole. But the caveats might be (I) the spectrum of the radiation and (ii) whether the high luminosity quasar phase will last very long.  

---

### Answer by username_12 on 2023-10-12. Score: 0

Our star orbits around a galaxy, which you can see if you look up at night: The Milky Way. As you can see, it's not bright enough to really call a sun.

The problem is basically luminance density, or you can say candelas per cubic meter, or total flux over the smallest blob that envelops the whole galaxy. It needs to be a lot higher for what you say.

Making stars a lot brighter doesn't make much sense. However, you could pack them tighter. Most galaxies are not that packed, so yours would be unusual and perhaps you may need to explain that, but hey, it's science *fiction*.

Nothing obviously bad or weird would happen if the stars were a bit closer to each other.

One estimate would be to copy paste 20 solar systems, all at 40 AU from the Sun (orbits of Uranus touching). That gives you 20/1600=1/80 the light of the Sun from the first shell. The second shell, at 80 AU, would have 4x as many, so you get 80/6400=1/80. Shell 80 would be 3200 AU away, would have 6400\*20=128000 stars, so the whole system is about 250k. For comparison, our galaxy has ~250 billion, a million times more. Of course, if you pack solar systems *that close*, something bad or weird probably would happen - the Uranii might collide, the Oort clouds, the orbits of the Suns... But you could space them out more and compensate by having more stars in the whole galaxy, so it seems plausible. Keep in mind that in this oversimplified calculation, the entire galaxy is only 0.1 light year across, which is *odd*, but it doesn't seem *impossible*.

Another way is to take the Milky Way and compress it. How much would you need to compress it, to give as much light as the Sun? Taking the galaxy at -5 and the Sun at -26, we it needs to be about 10^8 times brighter. That means things need to be 10^4 times closer. It's about 87k light years across, and instead would need to be 8.7 light years. Again, certainly *odd*, but perhaps not obviously impossible, and luckily "sort of" close to the other estimate.

One problem I see is that while the Sun makes a lot of nice visible light, a lot of objects in the galaxy make all sorts nasty stuff like X-rays. By compressing them and making them 10^8 stronger, you're also increasing the galactic radiation by a similar degree. That doesn't sound like good news for the people living there.


---

### Answer by username_13 on 2023-10-13. Score: 0

> A [blanet](https://en.wikipedia.org/wiki/Blanet) is a member of a hypothetical class of exoplanets that directly orbit black holes.

> Blanets are fundamentally similar to planets; they have enough mass to be rounded by their own gravity, but are not massive enough to start thermonuclear fusion, just like planets that orbit stars. In 2019, a team of astronomers and exoplanetologists showed that there is a safe zone around a supermassive black hole that could harbor thousands of blanets in orbit around it.

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> Is there an habitable zone for black holes?

> So planets could potentially form around black holes, but that's no guarantee that they offer a life-friendly environment. On Earth, living things are hugely dependent on the light and warmth from the Sun to survive. Without the glow of a star, life around a black hole would likely need an alternative source of energy. <https://www.sciencefocus.com/space/can-life-exist-around-a-black-hole>


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### Answer by username_14 on 2023-10-11. Score: 0

Apart from being in a dark, lonely place: Galaxies are, relative to their own size, quite close to each other.

[The nearest galaxies to the Milky Way are so close that they experience tidal forces](https://imagine.gsfc.nasa.gov/features/cosmic/nearest_galaxy_info.html), i.e. they are deformed or torn apart. A star so far outside the Milky Way that it does not interact much with its single stars would eventually interact with neighboring galaxies. There are probably no stable orbits.


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