Star density, outside our galaxy

[narrator]

Hi,

Is there a fairly even star (or galaxy) density in every direction we look, ignoring our own galaxy? Or are there some directions where our OU seem denser than others?

 

[Chronos]

As best as we can tell the universe is evenly populated with galaxies in every direction on large scales. There are occasional voids and overdense regions, but, it is homongenous on the whole.

 

[narrator]

Thanks Chronos.. followup questions.. and these may seem silly but I'll ask anyway. Everywhere we look in the night sky there are stars. Do all the stars we see only belong to the Milky Way galaxy? Are there stars that are not attached to galaxies, off on their own in the space between galaxies?

 

[Cosmo Novice]

Thanks Chronos.. followup questions.. and these may seem silly but I'll ask anyway. Everywhere we look in the night sky there are stars. Do all the stars we see only belong to the Milky Way galaxy? Are there stars that are not attached to galaxies, off on their own in the space between galaxies?

With the naked eye I believe it is possible to see other galaxies but not individual stars from these galaxies.

There is a fantastic deep field image here: http://en.wikipedia.org/wiki/Hubble_Deep_Field that is well worth a look :)

I think that all solar masses are in gravity bound systems - as galaxies forming subsets of larger clusters.

I think it may be better asking a question is it possible for stars to become non gravity bound due to any known phenomena. ie; for a star to somehow be "thrown" clear of its gravitationally bound neighbour.

 

[zhermes]

Thanks Chronos.. followup questions.. and these may seem silly but I'll ask anyway. Everywhere we look in the night sky there are stars. Do all the stars we see only belong to the Milky Way galaxy? Are there stars that are not attached to galaxies, off on their own in the space between galaxies?

Its a good question narrator.

All of the stars we see are only in the milky way. Actually, most of them are only in the local region of the milky way as-well; we really can't see very far. As Cosmo Novice said, we can see some galaxies with the naked eye, and more with telescopes---we can almost-never distinguish individual stars however.

There are some "intergalactic" stars---but they are very rare (almost negligible). Essentially all stars form within galaxies, and at relatively small velocities, so that they stay bound to the galaxy. Through galaxy mergers, multi-body stellar interactions, and supernovae kicks - stars can occasionally be ejected from entire galaxies (especially neutron stars and black-holes, we believe), but again, this is a very very small fraction of the total number.

 

[narrator]

Thanks Cosmo and zhermes,

It's an odd thing for us novices - we look up into the night sky and think we're seeing the universe, when in fact we're seeing almost exclusively only a part of our own galaxy.

As you say, with the naked eye it's possible to see a couple of other galaxies, but for most lay people, we wouldn't know it or distinguish it. (I've been lucky enough to have one pointed out to me through a relatively small telescope.) But knowing this puts a whole different light on things.

 

[Drakkith]

I recently read that 95% of the visible stars in the night sky are MORE luminous than the sun, but the sun is MORE luminous that 95% of all stars in the Milky Way.

 

[Vanadium 50]

As Cosmo Novice said, we can see some galaxies with the naked eye, and more with telescopes---we can almost-never distinguish individual stars however.

With telescopes we can see individual stars in nearby galaxies. The HST has seen Cepheid variables in M100, which is something like 16 or 17 Mpc away. As pointed out, with the naked eye we have enough trouble seeing stars in our own galaxy.

 

[zhermes]

It's an odd thing for us novices - we look up into the night sky and think we're seeing the universe, when in fact we're seeing almost exclusively only a part of our own galaxy.

As a historical aside: its interesting to note that only in the last 100 years did astronomers realize there were other galaxies at all. Until then, it was believed that our galaxy was the entire universe. The 'nebulae' (which are now known to be other galaxies) were simply anomalous objects at comparable distances to the stars.

 

[Chronos]

Stars capable of escaping a galaxy are called hypervelocity stars. About 16 are presently known to exist in the milky way. The first was discovered in 2005 traveling at an amazing 2,000,000 miles per hour [about 1400 km/sec] and is believed to have been ejected by the supermassive black hole at the center of the milky way. By comparison, the sun orbits the milky way at around 220 km/sec. At our location in the galaxy the escape velocity for a star is about 1000 km/sec. It is estimated there are about 1000 hypervolcity stars in the galaxy - a very modest number given there are over 100 billion stars estimated to reside in the milky way.

 

[turbo]

Hi,

Is there a fairly even star (or galaxy) density in every direction we look, ignoring our own galaxy? Or are there some directions where our OU seem denser than others?

Direction is not a clear variable in star density. However, there are severe disparities between some huge intergalactic voids, and the densities found in the galaxies that populate filaments and clusters, so direction AND density are correlated in that sense (mapping). It would be challenging to cosmology if we could show that direction of observation had any effect on the apparent stellar density that we observe.

 

[narrator]

Direction is not a clear variable in star density. However, there are severe disparities between some huge intergalactic voids, and the densities found in the galaxies that populate filaments and clusters, so direction AND density are correlated in that sense (mapping). It would be challenging to cosmology if we could show that direction of observation had any effect on the apparent stellar density that we observe.

Are there any elements or particles known/thought to live in these intergalactic voids? Or are their only inhabitants the transient photons and the like? I also wonder how spacetime is affected by distinctly large voids.