27

Yes, stars can form outside galaxies if the conditions are right. An impressive example is D100, a galaxy that is moving through a cluster so fast that the ram pressure from the ambient gas forces galactic gas out of it leaving a long tail. That tail is dense and cold enough to allow star formation, and there are newly formed clusters in it. In principle ...


22

Here's a rough sketch of the Milky Way and the Andromeda galaxy,* showing their approximate sizes and distance from each other to scale:   What the picture (hopefully) illustrates is the incredibly vast gap of empty space — around 2.5 million light years, to be exact — between the galaxies, each of which has a diameter of only(!) around ...


12

There can be stars (and small star systems). Stars need not be found only in galaxies. There can be gas clouds. Most nebulae are in intergalactic space. Indeed, for a while, both galaxies and nebulae were termed as "nebulae" until the differing nature of galaxies was discovered. Almost any (small) structure that can be present in a galaxy can be present ...


11

You can find all kind of objects, like stars, planetary nebulæ etc. But it is mostly a hot, ionized void. How void? The density of the intergalactic medium is about 1 to 100 particles per cubic meter (you can compare it to the mean galactic density, of about a million particles per cubic meter, or that of Earth's atmosphere, of about 10^26 particles cubic ...


11

Let me see if I can answer at least some of this. Yes, there is dark matter between galaxies. This is demonstrated by the fact that in galaxy groups and clusters, you need more dark matter than is found in the galaxies themselves to explain what's going on: in terms of why the groups or clusters are gravitationally bound in spite of the extreme velocities ...


10

It's quite possible for stars to form outside of galaxies, typically in environments where large amounts of gas have been stripped from a galaxy. This usually involves either a tidal interaction with another galaxy or the intracluster medium (ICM). In the latter group are a set of peculiar galaxies sometimes dubbed "jellyfish galaxies". Gas, dust and stars ...


10

Since Andromeda is already visible to the naked eye, to a civilization located at half the distance from the Milky Way, Andromeda would be still be visible. Its total brightness would be four times higher, but since its area would grow by the same factor, its surface brightness would stay constant. The Milky is less bright by a factor of ~2.5, but also ...


8

Planets, if any in that star system, would be visible to the naked eye, the way you can see Mercury, Venus, Mars, Jupiter, Saturn and Uranus in our system. On a virtually empty sky, they would draw that much more attention. Of course, if you had a Moon (or several moons), that would be visible too. Very rarely and briefly, asteroids passing by very close ...


7

The short answer is: probably nothing much, because galaxies are very fuzzy objects without "edges". If you look at the stellar disk, it just fades out in density, with no evidence for a sharp cutoff (this is true of disk galaxies in general). There may or may not be a "break" at some radius, but this just marks the beginning of a steeper falloff in density,...


7

The "local bubble" is a region of the galaxy where the density of interstellar gas is lower than average. It has about 50 thousand atoms per cubic metre, compared with 500 thousand averaged over the whole of the milky way. It was probably formed by one or more supernova explosions (10-20 million years ago) physically pushing gas away. There are many such ...


6

You can see the Andromeda galaxy with a naked eye, even with some level of light pollution. So if the star you are asking about would be even closer to Andromeda, you would see at least that galaxy. There are some other galaxies that can be seen from Earth with a naked eye or with binoculars, so sky of such a lonely planet would still have some night lights ...


6

The short answer to your question is no, there is not dark matter between galaxies, at least at any appreciable level. There is ordinary matter between galaxies, called the intergalactic medium (IGM). The IGM is an extremely dilute, hot gas that pervades the space between galaxies. The typical density of the IGM is about one hydrogen atom per cubic meter ...


6

Could dark energy (the mysterious accelerating expansion of the universe) be explained by "negative gravity"? But it already is "negative gravity". In general relativity, the stress-energy tensor $T_{\mu\nu}$ describes the energy, momentum, and stress of matter in spacetime. Through the Einstein field equation, it is connected with Ricci curvature $R_{\mu\...


6

It is just a poor choice of words. The comment refers to a discovery paper by Ferguson et al. (1998), where they used a very deep image in the Virgo cluster to establish that there were an excess of distant stars compared with a control field outside of the cluster. This excess of about 630 stars were all in one tiny HST field of view, so form a "group" in ...


6

While near encounters with supermassive black holes are bad for the stability of solar systems [citation needed?], many intergalactic stars become intergalactic without a close encounter. They are part of streams and tidal tails that emerge when galaxies or globular clusters get tidally disrupted by another galaxy. Still, even while encounters with black ...


6

It should probably be added that the article includes a glaring error of the type you often see when the science writer apparently did not take an elementary astronomy class (this is why we have such classes!). When the article states that the "lost matter exists as filaments of oxygen gas", you can be sure that Michael Shull never said any such thing, ...


5

The answer is almost certainly magnetic fields. A collisionless shock occurs when you try to propagate an increase/density in pressure through a sparse plasma at faster than the sound speed. Whilst the ions and electrons don't collide (very frequently), the magnetic fields that thread the plasma do accelerate the charged particles. In a collisionless shock ...


5

Usually the distance data doesn't take the expansion of the universe fully into account. That's not important for a distance of 200 million lightyears, but as we get close to the border of the visible universe, it makes a relevant difference. Therefore astronomers prefer to talk of redshift instead of distance. To be a little more precise: The light ...


5

Update for clarity: For the visible part of a standalone galaxy, the stars can all be measured to orbit that galaxy's core. So if you wanted to measure the furthest extent at this simplified level, it would be very easy. The problem is that there is a lot of mass which is not stars, and which is dark matter. Some of it is so far out, it may be impossible to ...


4

The dipole in the microwave background indicates motion of the Milky Way and thus of the whole Local Group, at least, at about 600 km/s in a certain direction. The straightforward explanation is that the density irregularities nearby from superclusters and voids result in a net gravitational acceleration that, over the age of the universe, resulted in this ...


4

The Wikipedia article on voids is pretty good (though IMO unusually awkwardly written.) The key thing is that voids are not empty, they are just large volumes which have a lower density (typically around 10% of average) compared with the rest of the universe. These low density areas still contain stars and galaxies, just fewer of them and the galaxies they ...


3

I can address the part of the question that states "have 'few or no' galaxies, but I can't find much else". In recent years there has been a lot of work published on void galaxies. Identifying void galaxies is not easy: you first have to find voids and then you have to be able to establish whether a galaxy is or is not located in one of those voids. One ...


3

As barrycarter already pointed out, it is due to the huge time it takes for the milky way to rotate. For once, as he pointed out, the stars in an orbit similar like the sun take about 250 million years for one full revolution around the center of the galaxy. If you put that in a relation to an earth year and take our civilization of roughy 6000 years of ...


3

While the Solar wind is pretty much homogenous in all directions, the galaxy puffs out stellar winds from discrete and temporary star forming regions. Protostars and giant stars which go supernova young, punch "chimneys" through the interstellar medium in the galactic disk here and there. The Local Fluff is btw our region inside the Local Chimney, a ...


3

Different types of observations have been made: hypervelocity stars: there are stars in the halo of our Galaxy (it is kind of the suburbs of the Milky Way) with such velocity that they are escaping it, and aiming to the intergalactic medium (IGM). There not yet fully in the IGM, but the're on their way. (Reference) molecular clouds in the IGM: there are ...


3

It is argued that the edge of the galaxy has the halo of dark matter at around 100,000 light years, but there are arguments that extend the galaxy from anywhere between it being 120,000 to 200,000. The major factor though for putting the edge at around 100,000 light years is that there is a major star density fall off for no known reason. Similarly the ...


3

OK, so I don't think the article is claiming that it is a place "without matter", but that it is a large region of space where the density of matter is lower. The original research, published in 2007, suggests that a lack of radio sources seen in a particular direction coincides with a "cold-spot" that is seen in the cosmic microwave background by WMAP (...


2

As already answered, the definition of the size of a galaxy must always to some extend be arbitrary. In astronomy, several definitions are used, according to the context in which it's used, e.g.: $R_{\mathrm{vir}}$ (the virial radius): Used when considering the galaxy's dynamics; defined by $GM_{\mathrm{vir}}/R_{\mathrm{vir}} \sim V^2$, where $G$ is the ...


2

The local environment is very important to star formation because it requires material as well as something to perturb the nebula to start collapsing and forming star(s). Therefore, both the type of galaxy as well as the location in the galaxy are very important to creating stars. However, once a star moves from protostar onto the main sequence, external ...


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