Why don't we see other galaxies or even stars near us in space?

 By chance, mostly, we happen to be over 4 light years from the closest star and our galaxy isn’t that close to other galaxies. Beings on another planet around a different star in a different galaxy might have very different night skies, full of light, with little darkness in between objects from their point of view.

Even inside our galaxy, different worlds would see very different skies. A planet near the center of the Milkyway would have a sky crammed full of stars, separated by mere light weeks, instead of the average in our sector of 5 light years or so. We are much further from the center, about 2/3rds of the way out.

In our universe, galaxies are quite far apart. The light that reaches us on Earth has thinned out into very faint glows. Andromeda is pretty close to us as galaxies go. But it is still 2.4 Million light years away. If our eyes were sensitive enough we would see Andromeda clearly as the single dominant object in our sky. Here’s the Moon and Andromeda illustrated to show their relative sizes in our current sky:

Andromeda is actually way bigger than the Moon in our sky. But the light is too faint for us to really see it well. You CAN see it with your eyes if you know where to look, but it won’t look like much with just your eyes. Merely a faint smudge of light. And that is only possible if the sky is REALLY REALLY dark. The stars we see are much closer to us so their light is stronger, much stronger than the entire Andromeda galaxy from 2.4 Million miles away. Even a star 1000 light years from Earth is going to seem WAY brighter than the collection of stars in the Andromeda galaxy.

But nothing is permanent. Our sky will change dramatically over time.

The above shows a sequence of skies over Earth spanning several billion years. The merger will start in about 4.5 Billion years and the Andromeda and Milkyway will both be permanently distorted and altered. Eventually, we end up in some sort of elliptical galaxy with a sky blazing with stars.

But most of the universe is pretty uniform and spread out, with clumps of galaxies along strands of galaxy clusters and super-clusters. Because gravity clusters masses together and even causes galaxies to collide, the perception we currently have of everything being so far away won’t always be the case.

In the above Hubble image, you can see the results of two galaxies merging. The spiral arms get swept out in distorted shapes as the central regions of the galaxies try to merge into a more unified whole. This has happened countless times in our Universe and the above image is certainly not a rare case.

So regarding what we see in our sky, rest assured it won’t always be this boring. It just changes slowly, over billions of years.

I tend to think of our sky as boring. We live in the suburbs of our galaxy, nothing too interesting is clearly visible with our eyes. We need telescopes to look at the cool stuff in any detail. That said, even a modest pair of binoculars can reveal a lot of interesting objects and allow you to see way more stars than just your eyes can pick up on their own. There are a lot of stars in our sky. If we could see every star in our sky, it would look very different. But the light from more distant objects is typically weaker, so we just see black in between the brighter closer stars. But if we could just stand there, letting the weak photons from distant stars stack up in our eyeballs, we would see something like this:

The bright vertical band is the plane of our galaxy, and you can see dark dust lanes cutting through that light. More stars are visible here than you would see with your eyes. Photos like this leave the shutter open for several minutes to let the light accumulate.

So it is a matter of our perspective and the limitations of human eyes that give this impression that the sky isn’t full of light sources at night. We can reveal the truth of how full the universe really is by doing long-term exposures with cameras and by using telescopes. Of course, this is STILL a perspective. The universe is, in fact, mostly EMPTY. But if we can sample a large swath of the sky, we see a LOT of sources of light, most from objects in our galaxy.

Why can life only arise in spiral galaxies? I have heard this said but never explained.

 

Whether elliptical galaxies can host life doesn’t matter for us much now, as they are too far away, but if our biological or robotic descendants still exist billions of years from now, they will reside in an elliptical galaxy that will form after the collision of the Milky Way with Andromeda.

In spiral galaxies, stars can have well-defined orbits that keep them at roughly the same distance from the center. This provides a stable environment for the evolution of life, characterized by a lower likelihood of nearby supernova explosions or radiation from high-density stars in the center of galaxies, as well as the emission of radiation from supermassive black holes’ accretion disks or relativistic jets.

In elliptical galaxies stars have radomly oriented radial orbits that can take them for millions of years to the densest parts where there is a high chance of something dramatic to happen, like a nearby supernova explosion or active galactic nucleus when supermassive black holes feed for a while on more matter than usually, releasing a lot of deadly radiation or even relativistic jets that can sterilize planets like powerful lasers.

There are also other types of galaxies besides elliptical and spiral. Irregular ones typically retain this shape for a while after colliding with another galaxy, and living plants could survive with some luck until the situation in the galaxy stabilizes. Some satellite galaxies assume this shape due to the tidal effects of the massive host galaxy and nearby other satellite galaxies. Such irregular galaxies can only make a few orbits before they are absorbed, though, and the survival of life on planets then depends on where they end up in the galaxy after their system is absorbed.

Dwarf non-spiral galaxies can be too small to have safe areas where life could emerge uninterrupted without nearby supernova explosions. A supernova that detonates anywhere in a tiny galaxy can affect a large part of the galaxy.

All of this suggests that spiral galaxies, like the Milky Way, are thought to be more likely to host life and technological civilizations than other types of galaxies.

The question was: Why can life only arise in spiral galaxies? I have heard this said but never explained.

What is a satellite galaxy?

 

We live in the Milky Way Galaxy, which is above average in size, mass, and number of stars. Satellite galaxies are smaller in these three measures. They used to be independent, but they found themselves attracted to the gravity of the Milky Way and became its satellite. Galaxies are strongly affected by tidal effects and cannot orbit their hosts for as many orbits as planets can. Each such revolution can take hundreds of millions of years, and their ultimate fate is to be absorbed; their stars to become part of their master galaxy.

Some that are already close to the Milky Way and orbit at an angle can pierce through the disk and come out of the other side on their last few revolutions.

Until about a year ago, we knew about 59 satellite galaxies of the Milky Way, which contains 200 to 400 billion stars. The most massive is the Large Magellanic Cloud, which looks quite misformed. It used to be a pretty spiral galaxy, but now its interaction with the Milky Way and other satellite galaxies like the Small Magellanic Cloud has made it look wonky. It contains about 30 billion stars and might be 10 to 20% as massive as the Milky Way.

The least massive satellite galaxies can have millions of stars, but we still discover new ones. A recent research paper proposed that galaxies like ours should have up to hundreds of satellite galaxies and found an additional tens on top of 59 already known.

Some are difficult to spot because they are faint, and some we cannot see well because they are behind the bright central bulge and thick disk of our galaxy. The brightness of stars, their density, and the presence of dust make it difficult to see what’s behind. One of the objectives of the space mission of the European Space Agency, Gaia, was to find more satellite galaxies via peering through the central bulge and the disk, and it found some indeed.