Galactic "comets".

In spite of a strong interest in astronomy, I heard something on NPR last evening that was news to me.

Our Galaxy is large enough that it apparently has around two dozen smaller galaxies orbiting it. The large and small Magellanic clouds visible in southern latitudes being examples.

Question 1: How long has this been known?
Question 2: How well established is the number of satellite galaxies?

Question 3: As the story didn’t mention Andromeda, I am guessing it is massive enough that it is a peer of the Milky Way rather than a satellite. Is this correct? and if so, do the Milky Way Satellites also orbit Andromeda?

Apparently there is evidence that at least one of these galaxies is on a highly elliptic orbit (like a comet in our solar system) such that at perigee it passes INTO our galaxy, even closer to the center of the Milky Way than our own sun! This galaxy has not been directly observed and is thought to be currently hidden behind the dust of our own galaxy. The evidence for it’s existence was discribed as “ripples” or “waves” in our own galaxy.

Question 4: How strong is the evidence of this “comet” galaxy.

Question 5: What might the orbital period of the galaxy be? (order of magnitude answer would be fine)

  1. Quite a while, but less than a century.

  2. The number is always growing. It’s tough to distinguish some of the satellite galaxies from our own because we have to look through our own and they’re very cosmically close…some are closer to us than the far side of our own galaxy.

  3. Andromeda is part of our galactic group but both Milky Way and Andromeda are the big galaxies in the group - its back and forth as to which is actually bigger. - Andromeda has its own family of satellite galaxies.

  4. The evidence is strong enough to be accepted by most astronomers. At the edge, the Milky way is very sparse. Some of the Satellite galaxies pass through this fringe and eventually will become part of it.

  5. Orbital periods of galaxies are in the hundreds of thousands of years to several million years.

More on how long this has been known:
The other local galaxies were known objects for a while, but were not defined as galaxies until the early 20th century. That they actually orbited ours was not discovered until later.

In the last 20 years or so the number of known satellite galaxies has increased from a handful to over a dozen and the number is still growing.

The satellite galaxy that might be most interesting to the OP would be Canis Major Dwarf Galaxy. It seems to fit most of the questions of “is this really true” in the OP.

The Milky way being about 100K LY in diameter, those orbits had better be in the 10’s to hundreds of millions of year category. Anything traveling 314,000 light years in 100,000 years is breaking the speed limit.

If I recall correctly from Monty Python, the solar system takes 200 million years to orbit the galaxy, so any object farther out than the sun would have to take longer than that.

And there are laws about that!

The dynamics of the galaxy system are complex. The sheer time scale is a problem. We haven’t had even one full year of Pluto’s orbit since it was discovered to get a complete set of data and that’s only around 250 years. Now consider the difficulty of making predictions on a dozen objects with billion year orbits based on even less data.

We do know that galaxies collide all the time. Andromeda and the Milky Way will almost certainly collide in several billion years. “Collide” is a terrible way of describing the event, however. Both galaxies outside their cores are mostly empty space. The average distance between stars is on the order of several light years. They can intersect for essentially geologic epics without causing much change other than a brightening of the night sky.

Even a dwarf galaxy hitting the center of the Milky Way would be a spectacular event, though. The problem is I can’t find any mention of this in a search. Do you have any more information that might provide a clue?

ETA: The sun is essentially circling the core. Anything on a highly elliptical orbit could reach the core much sooner, even if the total length of the orbit was longer. Think comets vs. Pluto.

‘Almost Certainly’ is a highly debatable term. The possibility is certainly there, and many do think it’s a probability, but “Almost Certainly” is overstating the current theories on this. The calculations regarding this have such a margin of error that it cannot be said one way or the other definitively.

Besides galaxies are made up mostly of nothing. A collision between them is expected to result in very few stellar collisions and a great deal of stellar birth due to gravitational compression of interstellar gas.

Basically, all we really know is that the two galaxies are currently getting closer to each other: That much is abundantly clear from the Doppler shift. But it’s extremely difficult to determine the tangential component of the velocity, so we essentially have no way of knowing whether we’ll collide, or just swing past each other.

I say they’re going to collide. And if they don’t you can meet me back here in 5,000,000,000 years and I’ll pay you a dollar.

Yeah, but is that a 2009 dollar, or a 5000002009 dollar? You’ve got to keep up with inflation, you know.

Thanks for all the info and interesting discussion.

Just want to note that if the Galaxy and Andromeda just swing past each other, then they aren’t gravitationally bound. That is, they aren’t orbiting each other and are just going to go zipping by. If they are orbiting each other, they’re going to collide.

How do you figure? They could perfectly well be in an eccentric elliptical orbit, in which case they would swing past each other and still remain bound. Plenty of things orbit each other without colliding.

Because they aren’t point objects and aren’t small enough to be approximated as such. If they are in an eccentric elliptical orbit, when they get close to each other, they will raise tides in each other which will rob them of orbital velocity. So even if they don’t crash right away, they will a couple billion years later.

Cite: The Great Milky-Way Andromeda Collision (warning: pdf)

I did not read the whole thing but I am dubious of this cite. It describes this (or uses language) to suggest this will be some apocalyptic catastrophe. IIRC the collision will be pretty unspectacular. The galaxies are mostly empty space. The two galaxies will merge or pass through or something and they will be warped out of shape but no colossal boom. Even if our sun were ejected into intergalactic space I do not see how that would affect our solar system much. Our local solar system is pretty tightly bound gravitationally. Even if we toddled off into deep space so what?

This will also be a slow motion process from our perspective.

Well, it will be for the galaxies as a whole. After the merger is complete, there will be an elliptical galaxy where there used to be two spirals. And their central black holes will eventually merge, which should make for some interesting gavitational waves, if nothing else. But other things will be happening in the meantime.

It’s true the chances of collison of two stars is unlikely (and it says so in the article). But there will be a burst of star formation and things like tidal tails (long curved streams of stars pulled out of the galaxies). These things make colliding galaxies look interesting when viewed from afar. (Note that this article was written for amateur astronomers who like to look at things like colliding galaxies.)

The sun may instead be plunged right through the center of the galaxy. Perhaps getting close to one or both central black holes. That would certainly be exciting.

Well yes. It will take a long time (perhaps a billion years) for the collision to run to completion.

Anyway, I thought it an interesting article and not unduly sensational.

While galaxies are mostly empty space, there is one thing that could case big trouble for Earth.

When two galaxies pass through each other, your local star density is going to roughly double. And, the stars that are members of one galaxy are probably going to be going a signifcantly different direction and speed than stars that are members of the other galaxy.

Seems to me that has a high potential of greatly disturbing all those millions/billions/trillions? of cometary bodies way out in the Kuiper Belt and Oort Cloud.

So, it may result in the number of comets passing through the inner solar system at any given time greatly increasing. So, rather than big impacts on earth being rare and long inbetween, for a time they become much more frequent, with unpleasant results.

And if its a grazing pass of another star, you may get the other stars Kuiper Belt and Oort cloud being swept through the inner solar system.

Well, the collision will be in about 3 billion years which is getting around the time our sun will make our planet uncomfortably warm anyway so perhaps a few snowballs dropping in will be welcome. :wink: