Imagine then if the stars velocity was greater than the galactic escape velocity.

How unfortunite for any intelligent beings to be faced with that, no nearby stars, you could be thousands of lights years away from the nearest star.

The sky at night would dark except for the huge galaxy in the sky, kinda like the end of the second star wars film.

Anyway thats what I think of, when someone mentions runaway stars.

When we speak of dense gas clouds what does it mean?

I know that density is defined as weight or mass per unit volume but I don't understand why they never say what the relative density is.

They'll always say how massive an object is in terms of so many solar masses or Jupiter masses.

They'll tell you how fast an object is moving, how large it is, how bright, how distant, how energetic, etc. and always with some standard of measurement or scale.

Most articles that discuss interactions between large structures of gas and dust with stars, galaxies, galaxy clusters and so forth refer to the density of the gas clouds or dust but they neglect to say what it means relative to the vacuum of space.

Heck, Earth's atmosphere is "super dense" compared to a vacuum!

The only time they define it is when they say that a teaspoon of black hole matter would weigh as much as the earth, and then it's mostly for the wow factor.

Oh well, it wasn't meant to be a rant. I'd appreciate if someone could give me a frame of reference for the "super dense" gas near the galactic core and maybe a link to a primer on the subject.

Thanks

Simplistic? Ah, well… see, physics is only a hobby for me.

The two (or more points) are typically triangulated from other known objects/locations in order to increase accuracy. Sounds complicated, but it really isn't…. unless the object is picking up speed or bleeding it off; a key it may be orbiting something… That is another lesson entirely!

Keep in mind most speeds are not absolute, but approximations, (calculating distance between points when they are far away isn't easy or perfect; but does get better with longer observations) and rounded off.

There re many references to what you want to know.
I'd suggest you look at and article at the NASA/IPAC Extragalactic Database. Here you find a nicely written article on some of the questions you ask. This is John Hurcha of the Harvard-Smithsonian Centre of Astrophysics (how absolutely appropriate). This can be found at;
Extragalactic Redshifts
Slightly confusing is the various distances calculations, as what is relative to what is not very easy to state simply. You can see this yourself at; NASA/IPAC EXTRAGALACTIC DATABASE : Search for Objects by Object Name
Input an NGC galaxy. I.e. NGC 253 in Sculptor and see the list yourself.

Hope this helps.

Relative to _what_ do astronomer measure the speed at which they tell us stars and galaxies move through space? Is it the centre of our own galaxy? If somebody knows – I'm curious…

As for " One possible scenario is that a star in a binary system exploded as a supernova and the partner got kicked out. Another possibility is a collision between two binary star systems or a binary system and a third star. One or more of these stars could have picked up energy from the interaction and escaped the cluster."

Of course, another possibility theory is that the stars were once in a very young trapezia or quadruple multiple star system, (like theta 1 orionis in the heart of the Orion Nebula), and one was then ejected from the tussle for gravitational dominance. This creates a "run away" star and leaves a stable triple star – like a closer binary with a fainter/ smaller distance companion. This has the bonus of happening near a star formation region, and the improved possibility of interstellar material to interact with.

Again, thanks for the interesting report from the AAS conference.