Starship Asterisk*

Anthony Barreiro wrote:

bay area john wrote:

neufer wrote:...
However, the Andromeda Galaxy is approaching the Milky Way at about 100 to 140 km/s.
This exceeds the dark energy expansion fractional component of the ~58 km/s hypothetical Hubble expansion.

Thanks, that's interesting but somehow unsatisfying as to explain how galaxies expanding from a common origin can be crossing paths at such speeds billions of years later. I'm being simplistic in the extreme, but imagining an infinity large pool table with a hellava break. With no bumpers to direct the balls back "in", they scatter, and keep scattering, and never change course to cross each other's paths, especially long after the break. Saying that "gravity is stronger" may be true but gravity would not change the direction of a single moving object, only interacting objects... so how do they START to cross paths?

Hi John. I'll take a shot at your pool table analogy. Each individual pool ball is an entire galaxy cluster. Within each pool ball are galaxies, stars, dust clouds, planets, life forms, poached eggs on toast, atoms, and subatomic particles. The eight ball (and all the other balls) absorbed some energy from the break (nice shot, by the way!), but the force of gravity is much greater over the short distances within the eight ball than over the longer (and expanding) distances between the different pool balls, and thus gravity holds the innards of the pool ball together even as all the balls are scattering over the table. How does that work for you?

bay area john wrote:

neufer wrote:...
However, the Andromeda Galaxy is approaching the Milky Way at about 100 to 140 km/s.
This exceeds the dark energy expansion fractional component of the ~58 km/s hypothetical Hubble expansion.

Thanks, that's interesting but somehow unsatisfying as to explain how galaxies expanding from a common origin can be crossing paths at such speeds billions of years later. I'm being simplistic in the extreme, but imagining an infinity large pool table with a hellava break. With no bumpers to direct the balls back "in", they scatter, and keep scattering, and never change course to cross each other's paths, especially long after the break. Saying that "gravity is stronger" may be true but gravity would not change the direction of a single moving object, only interacting objects... so how do they START to cross paths?

old guy wrote:could there be something like this preventing us from seeing beyond the 'edge' of the known universe?

starsurfer wrote:

Wadsworth wrote:

APOD Robot wrote: Explanation: Described as a "dusty curtain" or "ghostly apparition", mysterious reflection nebula VdB 152 really is very faint.

When I read the first sentence of this explanation I thought to myself, from our vantage point this nebula seems to be blocking much more light than it is reflecting. I would say it is more of a 'dark nebula' than a 'reflection nebula'. At least a 90-10 ratio..
Though I suppose one could call it either.

vdB152 only refers to the small blue reflection nebula. The dusty dark nebula is catalogued as B175 and was discovered in a photographic plate nearly 100 years ago by the famous astronomer Edward Emerson Barnard, who published a well known catalogue of dark nebulae. The whole catalogue is available online here: http://www.library.gatech.edu/barnard/

neufer wrote:...
However, the Andromeda Galaxy is approaching the Milky Way at about 100 to 140 km/s.
This exceeds the dark energy expansion fractional component of the ~58 km/s hypothetical Hubble expansion.

Wadsworth wrote:

APOD Robot wrote: Explanation: Described as a "dusty curtain" or "ghostly apparition", mysterious reflection nebula VdB 152 really is very faint.

When I read the first sentence of this explanation I thought to myself, from our vantage point this nebula seems to be blocking much more light than it is reflecting. I would say it is more of a 'dark nebula' than a 'reflection nebula'. At least a 90-10 ratio..
Though I suppose one could call it either.

bay area john wrote:
This item brings back a question I've asked before but never got an answer that didn't seem contrived or avoiding the issue.

We are told we are in an expanding universe, with all parts moving away from all other parts, an effect of the Big Bang.
So how do stars move though dust clouds going different directions and speeds? How can galaxies collide (and there are so many that do...) if we are all moving apart, driven by the same starting influence? Collisions should be nearly impossible, it would seem. What am I missing?

APOD Robot wrote: Explanation: Described as a "dusty curtain" or "ghostly apparition", mysterious reflection nebula VdB 152 really is very faint.

Beyond wrote:BOO!! It's that time of year again. Apparently in space, also.

Ann wrote:

bay area john wrote:This item brings back a question I've asked before but never got an answer that didn't seem contrived or avoiding the issue.

We are told we are in an expanding universe, with all parts moving away from all other parts, an effect of the Big Bang.
So how do stars move though dust clouds going different directions and speeds? How can galaxies collide (and there are so many that do...) if we are all moving apart, driven by the same starting influence? Collisions should be nearly impossible, it would seem. What am I missing?

Dark energy isn't strong enough to make galaxies fly apart. Inside galaxies, gravity dominates over dark energy. Between galaxies that are close together, such as the Milky Way and Andromeda (~2 million light-years), gravity also dominates over dark energy. Between galaxy clusters, such as the Local Group (where we belong) and the Virgo Cluster (~60 million light-years) dark energy usually dominates over gravity, and always if the clusters are not extremely massive and are, say, 100 light-years apart.

Inside galaxies, various "currents" and "eddies" form, which make objects move in different directions.

Ann

bay area john wrote:This item brings back a question I've asked before but never got an answer that didn't seem contrived or avoiding the issue.

We are told we are in an expanding universe, with all parts moving away from all other parts, an effect of the Big Bang.
So how do stars move though dust clouds going different directions and speeds? How can galaxies collide (and there are so many that do...) if we are all moving apart, driven by the same starting influence? Collisions should be nearly impossible, it would seem. What am I missing?