The Construction of Galaxies

Some of the most important insights into the process of galaxy formation have come from computer simulations. Using computer models programmed to simulate the laws of physics, scientists can experiment with varying initial conditions—different distributions of density and temperature, for instance—to see what conditions are required for stars and galaxies to form. In this way, hypotheses about galaxy formation can be tested experimentally, albeit in a computer-simulated universe rather than the real world. These simulations also help us to visualize how different types of galaxies (spiral galaxies, elliptical galaxies, etc.) might be constructed.

Despite the evidence from both observational data and computer simulations, however, the process of galaxy formation is not fully understood. One of the deepest mysteries involves the influence of an unknown substance called “dark matter,” which we’ll discuss later in this chapter. No theory explains all the data perfectly, but some aspects of galaxy formation have been worked out. For now, I’ll leave aside the complexities and provide a brief overview of the processes by which various types of galaxies are thought to form.

The leading theories of galaxy formation begin with the Big Bang model. According to the Big Bang model, the early universe was full of hydrogen gas (with small amounts of helium and lithium), as explained earlier in this chapter. The distribution of matter was not perfectly uniform, however. Some regions were denser than others. This is evident in the cosmic microwave background. Subtle variations in the CMB show that some regions of space contained greater concentrations of matter. Over time, gravity pulled more and more matter toward the densest, heaviest regions, forming huge clouds called nebulae with empty space between them. These nebulae consisted mostly of hydrogen gas, which would eventually form stars through a process that we’ll discuss on the next page.

Meanwhile, as these clouds of gas continued to collapse they began to swirl rapidly. Swirling gas spins faster when it is compressed, for the same reason that a twirling figure skater spins slower when her arms are outstretched and faster when she tucks them in.See chapter 2 for further discussion of the conservation of momentum. Rotating vortices of gas flattened into disks like spinning pizza dough, and clusters of newly-forming stars were swept along in the swirling winds. Over billions of years, smaller nebulae and star clusters merged into large, spiral galaxies, pulled together by their mutual gravitational attraction. This process is illustrated in the NASA supercomputer simulation shown below.

Supercomputer Simulation of a Spiral Galaxy Forming

This simulation, created by a NASA supercomputer,According to NASA’s website, this simulation “ran on the Pleiades supercomputer at NASA's Ames Research Center in Moffett Field, Calif., and required about 1 million CPU hours.” begins with the collapse of a huge nebula, which forms numerous small galaxies that eventually merge into one large spiral galaxy over a simulated period of 13.5 billion years.

Credit: NASA.

Elliptical galaxies probably began to form later, perhaps a few billion years or so after the Big Bang. The most successful models suggest that elliptical galaxies form when old galaxies collide. As a spiral galaxy ages, the process of star formation gradually consumes the interstellar gas which had shaped the galaxy into its disk-like structure. So, when mature galaxies collide with each other, there isn’t enough rotating gas left to reshape the merging galaxies into a disk. As a result, the collision disrupts the galaxies and scatters their stars into randomly-oriented orbits around the center of mass, forming an elliptical galaxy. This may happen to our own Milky Way when it collides with the Andromeda Galaxy, which is hurtling toward us at 250,000 miles per hour. But don’t worry! Even at that alarming speed, Andromeda won’t get here for another four billion years. God might not allow the earth to persist that long anyway,The Earth is not our permanent home, as Jesus explained (John 14), and there is some scriptural evidence that even the physical universe itself may not last forever. See for instance Isaiah 34:4 and Revelation 6:14. and the space between stars is so vast that chances of any disturbance to our solar system are slim. (The chance of the earth being destroyed by an asteroid within our solar system is much higher.) Check out this NASA page for images and a video animation depicting the collision.

Lenticular galaxies, which have both spiral and elliptical features, may have formed through some combination of the processes explained above. And most irregular galaxies are probably just galaxies that recently collided and haven’t yet stabilized into a recognizable structure.

Those are the basic ideas behind the prevailing theories of galaxy formation. As mentioned above, however, some aspects of galaxy formation remain mysterious. It’s anyone’s guess whether the current theories will hold up to scrutiny as new evidence comes in from more powerful telescopes and more advanced computer simulations. The process of star formation, on the other hand, is well-understood. Star formation is easier to study, because it is still happening within our own galaxy! We’ll talk about that on the next page.