Colliding Galaxies: The Universe in Turmoil

by Barry Parker

I remember sitting spellbound, watching the movie When Worlds Collide. Two planets hurled through space toward Earth while scientists and engineers frantically raced to complete a rocket­ ship that would take them to safety. In the final moments the spaceship lifted off as the occupants watched the Earth bulge, crack, then literally explode as one of the planets struck it. As I left the theater I wondered if it was really possible for another world to collide with Earth. Later I learned that while many catastrophic collisions no doubt occurred early in the his­ tory of the solar system, today they are exceedingly rare. I was relieved, but in another sense I was disappointed (not that I hoped a collision of this type would actually occur). A collision of two objects in space, say, two stars, I was sure would be a spectacular event. It is quite unlikely, however, that we will ever witness the collision of two stars. The event is just too rare. But collisions of systems of stars-galaxies-oddly enough, are relatively com­ mon. In fact, we see evidence of several in the sky right now.

308 pages, Paperback

First published September 1, 1990

Reviews

[J.D. Steens · Rating 4 out of 5]

This is one of the few non-photo books I’ve come across on galaxies. This is surprising, given that they are for me the most distinctive feature out there in the cosmos.

Though dated (1990), its main theme is that galactic structures are formed by galaxies colliding with each other. A larger galaxy pulls from a smaller galaxy, following this sequence: Irregular galaxies with relatively undifferentiated gas, dust and matter; spiral galaxies, followed by elliptical galaxies, with the latter said to be “pulling” filaments from the former and consisting of older stars that constitute a dense bulge, the center of which was thought (the book was written in 1990) to consist of a black hole.

This is all a fairly standard rendition, much like Hubble’s classification from the 1920s. There are some problems, however. Parker uses the term “orbiting” to describe the circular movement of galaxies, yet he also refers to the “pulling” effect of energy-matter toward the galactic center (and its black hole). Orbiting and pulling are different phenomena. Orbiting is a balance between inertial straight-line movement and movement toward the center of gravity. In other words, it’s roughly a 45 degree balance point between these two directions of movement. Yet, Parker and the illustrations he uses indicate that energy-matter is being swept up (pulled) by the large gravitational center, hence galactic spirals are winding inward.* (The rotating galactic center creates the spiral as the energy-matter closer to the center moves faster than the energy-matter on the outside of the gravitational field, creating the delay effect that constitutes the spiral arms.)

Is it possible that Einstein’s theory of relativity applies here? Gravity is not a force. Rather, a large gravitational mass shapes space-time, creating a spherical center of gravity (the field of which varies per the inverse square law), and inertial straight-line movement from energy-matter (gas and dust) follows curved space-time toward the gravitational center. In this way, the same phenomena Parker describes can be explained differently. The galactic center does not reach out and pull energy-matter back into itself, but rather, it just sits there, inactive, and absorbs inertial straight-line movement inward. Once all of the surrounding energy-matter “is swept up” (or “eaten” by the black hole) by the galaxy, then the galactic structure is consolidated to become an elliptical (with the “barred” effect across the horizontal plane being the remnant disk) galaxy. Seen this way, galaxy formation is not limited to collision phenomena. All energy-matter moves toward the gravitational center, not just that from surrounding galaxies, which is one, albeit a significant, source of material for galactic formation.

Parker refers to jets coming out of galaxies (he says it’s often one jet, though he also suggests it’s possible that another jet is present on the opposite side, but not seen).** This part of the book was frustrating and I think it’s fair to conclude that not a lot is known about jets coming from galaxies. What is interesting is that more recent books have the jets coming out of the poles, perpendicular to the horizontal plane of the disk. Regarding this horizontal plane, this is where the spiral arms do their thing. While Parker says there are “two or three” arms in a spiral galaxy, from the photos I’ve seen, it looks like there are two large spiral arms moving toward the galactic plane from opposite directions, with side arms moving into each of the main arms. If true, then you have this phenomenon where spiral arms form a horizontal disk (where gravity’s effect is greatest) and jets coming from the vertical poles (where gravity’s effects is lowest)?

The end of the book references Einstein’s belief that the universe is curved, yet discusses the standard three future cosmic scenarios (flat and open- no curvature; closed, where cosmos collapses on itself) due to the relative presence of matter as if Einstein’s theory doesn’t apply: The big bang moves all energy-matter outward, substantially spherically, not linearly, so almost by definition the cosmos is curved, though from our limited human perspective it seems flat. Rather than being limited by these three paradigmatic scenarios, what if the big bang is the source of inertial movement outward, progressively freeing itself from the effects of the inverse square law (the open scenario), until it is totally free (the flat scenario), when it continues around cosmic curvature and returns to its beginning point (the closed scenario, but continuing around curvature, and not in a linear out-and-back way).

Parker says that stars are separated by 100 million times their diameter, which was helpful in conveying the emptiness of space, and why colliding galaxies don’t so much collide but merge (with stars, though closer, keeping some distance from each other). He also notes density waves in the spiral arms, with density of energy-matter occurring at the leading edge of the spiraling arms.

*Some descriptions of galaxies suggest that the spiral arms are unwinding, i.e. moving outward from the galactic center.

**Parker discusses large lobes at the end of the jets, which is new information for me.