Astronomy/colliding galaxies


Tom, I just read your answer to a question about relative movement. I have a related question. From what I understand of the Big Bang theory, all galaxies are moving away from each other and the farther away from us they are, the faster they are moving away.

If this is true, how is it possible for any two galaxies to collide as we know happens? I have heard that our neighbor galaxy, Andromeda, will in the very distant future collide with the Milky Way. So within a local group of galaxies does the "moving away from each other" rule not apply? If so, how is this contradiction possible?

Hi Clif,
Yes, the Hubble constant of space expanding (about 71 Km per second or about 42 miles per second, PER MEGAPARSEC), is in effect everywhere.  A megaparsec is equal to 3.2 million light years and most galaxy cluster members are within about 3 - 10 million light years of each other.
So gravity in the galactic cluster (and therefore collision closure speed)... easily "over-rides" the Hubble spacial expansion rate at those "very short" distances.  In the case of M-31 and Milky Way, the closure speed is something like 180 miles per second, MINUS the ~40 miles per second of the spacial expansion at a distance of about 2.9 million light years (distance to Andromeda Galaxy).  So this bottom line result equals a closure speed of about 140 miles per second for us and M-31.  And that's why we see many many galaxies in collision as their "closeness" to each other in a galaxy group, their combined massive gravity's, easily over-rides the Hubble spacial expansion rate.  

At least that's true today; since the spacial expansion is accelerating (1998 - a study of Type Ia supernovae), no one knows for sure what the distant future will hold. That's why that instead of a future "Big Crunch" of all matter coming back together, today you read about the future "BIG RIP" as space expands faster and faster and faster... accelerating, and not decelerating. This led to the introduction of anti-gravity "dark energy" in our entire Universe, but that's another story.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA  


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Tom Whiting


Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.


Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at: and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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