Half of stars lurk outside galaxies
nature.com
nature.com
I'm not sure if there is already a sub-reddit for story ids.
Had this been known at the time, she could have simply dispensed with the light from the nearest galaxy altogether.
Similarly, the planet Krikkit, as described in Douglas Adams' Hitchhiker's Guide "Trilogy", would not necessarily be all that unique or unusual.
At the end of this submission, they mention that they are planning follow-up experiment which should get more data.
I first heard about this on the Skeptics Guide podcast (http://www.theskepticsguide.org/), and I was surprised because it's counter to how I think about the universe.
Presumably stars outside of galaxies are at much lower density, so they wouldn't see hardly any points of light in the sky. If most such stars are still within the Galactic groups, then they would probably just see faint smudges of other galaxies like the very dim Magellanic Clouds we can see. (Note that these are still pretty big, though, in terms of angular size. Several moons wide.) On the other hand, planets around lone stars in voids away from Galaxy clusters would presumably have nothing visible to the naked eye from their surface.
(Someone please correct me if I'm wrong. I'm not an astronomer.)
So if Andromeda is already difficult to see by us, that gives some indication of what's going to be visible from a planet that is much further away from anything, than even that.
Our Local Group has a diameter of ~10 megalight-years . Andromeda is 2.54 MLy away.
Thus, given the premise, and assuming that our own cluster is "typical", it's unlikely that a renegade star is all that much further from a galaxy like ours or Andromeda than we are from Andromeda.
* 7,943 visible stars
* 1,078 (14%) beyond 1,000 light years;
* 34 beyond 10,000 l.y. (these must be supergiants!)
If I calculated right, a +6.5 magnitude star would demand an absolute magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years. Looking this up on the Hertzsprung–Russell chart [4], this would include only very bright giants, and supergiants, respectively.
[0] https://en.wikipedia.org/wiki/Hipparcos
[1] ftp://cdsarc.u-strasbg.fr/pub/cats/I/311/
(hip2.dat.gz for data; intro.pdf for field descriptions)
[2] Hp magnitude is a 340-850 nm visible band, which diverges slightly from V band magnitude,
http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html
[3] https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_no...
[4] https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_di...
Yes, Andromeda (M31) is visible to the naked eye, but it's pretty huge and quite close to us.
Orbiting a lone star, the naked eye at night would see something between absolutely nothing, and maybe a few faint patches of light. And that's in perfect conditions - middle of desert or ocean, zero light pollution.
If half a dozen is "many", then sure. But you probably live far from any cities then.
http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_gala...
(The list includes objects that are very difficult to see, and some only "reportedly" seen naked-eye.)
> They would appear even brighter without a local galaxy to wash them out.
There's no reduction in contrast in the direction perpendicular to the galactic plane. And yet the naked eye doesn't perceive the sky there "totally covered in fuzzy dots".
The naked eye has a lot of trouble seeing anything beyond the Local Group - and these are a handful of close-knit galaxies.
You need telescopes for that, with plenty of aperture. Once the aperture is comparable to the diameter of your car's wheels, or bigger, then yes, you get the "my god, it's full of galaxies" effect.
So most of what we see are constructions inside our own galaxy? I see what you mean - the sky would be a very lonely place for the intergalactic skygazer.
Every single star you can see with your naked eye is inside our galaxy. Even with telescopes - you'll need a very big telescope, by amateur standards, to resolve stars in other galaxies.
As for non-stellar objects:
Naked-eye galaxies are very few - they're all on that short list linked above. And only the first half are easily visible, and then half of those are in the southern hemisphere.
Omega Centauri actually almost looks like a fuzzy star - almost. It's pretty small.
You can see some nebulae (like the Great Orion Nebula) and star clusters (like M13) naked-eye, but they are only a few, and you'd have to drive far into the middle of the desert for most if not all.
So why hasn't anyone ever noticed this extra-galactic population? They'd show up as anomalous field stars, probably with odd proper motions.
If you think of it in terms of the Local Group, every galaxy should have a "halo" of these free stars around it in a radius comparable to half the distance between galaxies. Admittedly that's pretty low density, but is it really low enough that no one has ever seen one of these objects?
Or are they only present at very high redshift (and if so, what happened to them?)
- the population density of extra-galactic stars is super low
- outside our galaxy we're only discerning a fraction of the biggest nearest stars anyway
You're looking for the odd ball at the remote end of a huge concert hall packed with regular balls.
On the other hand, who knows how much science and telescope-making would have been set back without there being anything to look up at. Without other stars (and our rather extraordinary moon), it would take a lot longer than 1500 years to break out of the Ptolemaic mindset.
If this continues, the universe is going to end up being nothing like we expected only a few years ago.
Unless you mean the ex-planets themselves would end up alone intergalactic space. Apparently, the vocabulary used for such objects is, "sub-brown dwarf."[1]
No, that is demonstrably false. You can try this with any particle gravity simulator - Place a bunch of particles, and watch how many get ejected. Star systems behave similarly.
The Jeans mass depends inversely on the square root of the density of the cloud. The main problem in forming very low mass objects is that you need very large densities. For a Jupiter mass object you need a density of ~5 x 10^4 / cm^3. Even in molecular clouds it is difficult to achieve densities that large. There are ways around it --- turbulence can produce small regions of very high density, for instance. But like I said, they're not well understood. In general it's much easier to create a low mass object in the vicinity of something bigger because then it can form by fragmenting out of the disk that surrounds the larger protostar.
In other words, could those unseen before stars explain a part of the dark matter problem?
There is, however, a problem called the "missing baryons problem." Measurements of the CMB predicts a certain amount of ordinary matter in the universe, but the amount of matter observed is too small by a factor of about 2. It is possible that these stars could contribute to the missing matter in this problems. (It probably would not be the stars directly, but a tenuous gas from which the stars formed.)
The first good indication that dark matter must exist was in fact that the rotation rates of the observable galaxies were too large for the amount of visible mater they contain. Of course the first explanation was that this matter was ordinary gas, dust, rocks, planets (which are just larger dust particles, when compared with the size of even a small galaxy.), etc that wasn't emitting any light. However, there would have to be so much of it that it would occlude the distant galaxies, something that obviously isn't happening. Thus the modern theory is that dark matter is a form of matter that interacts gravitationally with ordinary matter, but has no interaction with the electromagnetic force (light).
On the other hand, maybe galaxies are just naturally inimical to life, and extragalactic stars are the only places where long-term biological survival is possible.
Gamma ray bursts would be more likely in a galaxy and those are inimical to life.
But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If 1/2 of stars are outside of feasible colonization distances that changes the Fermi paradox formula quite a bit.
Assuming acceleration of 1g, from here to Alpha Centauri, it would take 6 years objective, 3.5 subjective. To the center of the galaxy, it would take 27,000 years objective, but only 20 years subjective. Magellanic clouds? 162k years objective, but only 23 years subjective! 28 years to Andromeda.
Fuel may be in short supply, but relativity really works for you - ignoring abrasion and impacts, if you can get to one galaxy, you can get to most others in your neighborhood.
(All calculations derived from http://spacetravel.nathangeffen.webfactional.com/spacetravel..., because I'm too lazy to do it by hand.)
Even if you protect yourself from relativistic atoms, they still create a drag. And that happens before you reach 0.99c.
Unedited original post follows:
> to actually stop yourself you need an exponential amount of fuel, because you need to accelerate half of it just to stop yourself
Hang on, I don't understand this part. First, you're right, I didn't think about deceleration - but that only doubles the trip length at most, since you have to accelerate halfway, then decelerate halfway. And probably not even exactly that if you're carrying your fuel, since deceleration will be a little bit easier - you've burned some gas, so there's less mass to push around.
And you're right, I didn't think about the drag - but that actually works out better for extragalactic visitors! There isn't as much stuff there to stop them when they're taking off, and once they hit a galaxy, it actually helps them decelerate. (Again, my dreamy eyes are ignoring the practical hazards of this "help" which might just turn them into a fast moving gas cloud.)
That is what equations tell you: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h... ( scroll to How much fuel is needed? )
I wonder if you could accelerate the whole way in a giant ship, and only slow down a tiny capsule at your 'destination' - maybe just a tiny self-replicating robot factory and some data storage. Decelerate that, land it, have it build you a new body, some tools, etc., etc.
The first is the technology to support their culture for a very long time without the support of a nearby star. In that case, there is no particular reason for them to visit galaxies at all. They simply pick a direction and go. Galaxies would be of no particular interest to a civilization that doesn't even need one star, nor would they need to travel at particularly high accelerations.
The second is a propulsion technology that does not require reaction mass (or reaction energy, as with laser propulsion). You would have to devise a way to move something without throwing something in the opposite direction. Thanks to the equations involved, trying to get a long distance away with decent speed using only chemical rockets basically means your ship will start the trip as more than 99.9% fuel by mass.
That is what prompts the imagining of alternate propulsion technologies, such as the Orion nuclear rocket, solar/laser sails, Bussard ramjets, slingshot orbits, and magnetic braking loops. Accelerating the fuel that you will later need to decelerate is a huge problem just for inter-system travel; you can't even bother with it for inter-galactic travel.
One of these problems is much more easily solved than the other. Either way, that civilization would then have no particular use for galaxies as a travel destination.
Tourism usually requires that you be alive when you finally get there. At that scale, if you chose to visit even the closest galaxy to ours, not only will you be long dead and thoroughly recycled when your vessel arrives, but the passengers that disembark to snap a group photo might not even be considered Homo sapiens any more.
That kind of commitment can only come from existential necessity. Any visitors to a galaxy that came from outside of one would undoubtedly have a technology that allows travel without actually traversing the intervening distance.
Imagine an alien lifeform with an average lifespan of several million years, perhaps the size of a mouse (not much mass) and with extremely slow metabolism (not much supplies needed for travel). For them, traveling to a nearby galaxy at relativistic speeds (only a few years from the traveler's perspective) might be seen as little more than a nice long vacation. Sure, a dozen lifespans might have passed by the time they get home, but maybe they don't care because they don't have children like we do and their civilization doesn't change much. "They released the Galaxy S9 already? That's crazy! Three new models in a billennium!"
You don't even need FTL transportation if you can afford to spend a few eons strapped to a seat.
And if they don't need to, they probably won't. If you lived in the country, and wanted to visit the city, you might do so frequently if the trip cost you 15 minutes and $10. You might never do it at all if the trip took 50 years and $100billion.
Space travel is more like the latter than the former.
If, on the other hand, travel to anywhere on Earth cost you 1 second and $0.01, you might just visit every city. That's why I say that any non-galactic visitor to a galaxy is more likely to have a kick-ass travel technology. It's a purely time-based argument, and has nothing to do with any property of the species that has it. They would simply spend far more time at their intended destinations than traveling between them.
I'm in California. If the whole Earth was this 2mm breadcrumb, then the Sun would be a soccer ball 25m away (across the street). Speed of light would be the speed of a running ant. Uranus would be a peanut 1/2km away.
And the nearest star would be another soccer ball somewhere in Greenland.
It. Just. Boggles. The. Mind.
http://florin.myip.org/blog/i-had-no-idea-just-how-big-solar...
I look up at the sky often. Those specks of light are so incredibly far away. We just need new physics, otherwise we'll never get there.
I'm an amateur astronomer and telescope maker, on a quest to see what's the biggest aperture that an amateur could build working alone. I can't go to the stars, but I can bring them a few hundred to a few thousand times closer to the eye.
But can an ant run 25m in 8 minutes? I honestly can't imagine how fast an ant runs. That would be a foot in 4.8 secs - that's a fast ant actually. But I guess close.
The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.
So 50 mm/sec seems quite possible.
Here's another comparo:
If your average galaxy was the size of a coin, the size of the observable universe would be on the order of a large town.
For travel between star systems, these assumptions do not need to be true. For deceleration, it makes more sense to transfer the original kinetic energy out of the vehicle's movement than to expend even more energy on accelerating reaction mass yet even faster in the original direction of travel. Remember that you have a whole star system of reaction mass at your destination. There may be engineering challenges, but there is no physics reason why you couldn't expend your original energy in "pushing off the sun", and then recover it by "pushing against the new sun" when you arrive. Depending upon efficiency, this could then leave you with the energy for another flight (perhaps home).
But the real problem is the stupendous amount of energy. Even with total mass conversion, you'd need a bazillion tons of mass to get to Andromeda. See the links posted below.
We need new physics.
Though I do like some scifi stories I've read where the first humans went our on generation ships and in the meantime a new physics was developed that allowed those target stars to be colonized hundreds and even a thousand years before they arrive.
But developing a new physics presumes much.
The mountain village might be the only place survivable. The center is a lot more dynamic environment. Radiation levels also increase really close to the center.
> But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If more than the resources of one stellar system are required for intergalactic travel, then they are stranded there forever. They will die with their star.