By the time you have completed the 2.11. Introduction & Exploration Activities, you should be able to:
Understand what a galaxy is and differentiate between spiral, irregular, and elliptical galaxies.
Describe the ‘clumpy’ nature of matter in the Universe (how matter is concentrated in nebulae, stars, and planetary bodies; how these objects are concentrated in galaxies; and how galaxies are concentrated in clusters and along filaments and sheets—which surround vast regions of nearly empty space).
Understand how the properties of galaxies (i.e., galaxy type, size, composition, and complexity) have evolved through time.
Differentiating Galaxies
Answer the questions below.
What is a galaxy?
Answer
A galaxy is a massive, glowing aggregate of nebulae, planetary systems, and normal, giant, & mini stars held together by gravity (spacetime curvature).
The components form a relatively thin, pinwheel-shaped disk composed of objects organized into spiral arms that orbit a central bulge comprised of mostly older stars. The central bulge of spiral galaxies comes in two shapes. The bulge is spherical in normal spiral galaxies and shaped like a stubby cigar in barred spiral galaxies. Figure 2.11.3 shows typical normal and barred spiral galaxies. Our galaxy, the Milky Way, is a barred spiral galaxy. Most galaxies appear to begin as spirals.
Example:
Answer:
Term:
Answer: Spiral Galaxy
Irregular Galaxy
Definition:
Large galaxies grow by ‘consuming’ smaller galaxies. These mergers typically start at a distance. As the larger galaxy pulls the smaller galaxy towards it, it distorts the smaller galaxy—forming an irregular galaxy. In addition to affecting shape, gravitational interactions during collisions can compress nebulae and trigger enormous bursts of star formation—which concentrates gas in stars.
Example:
Answer:
Term:
Answer: Spiral Galaxy
Elliptical Galaxy
Definition:
Galaxy collisions are more like cloud mergers than automobile crashes, because the distances
between objects in galaxies are so large. Most collisions eventually produce elliptical galaxies.
But galaxies can grow without significantly changing their shape. For example, small galaxies
rarely affect the overall shape of the larger galaxies with which they merge. Our own galaxy has ‘consumed’ many such small galaxies ...and will ‘soon’ consume two more—the nearby Small
and Large Magellanic Clouds. Stars orbit the galactic center in all directions, forming a spherical or football shape (instead of a flat disk).
Example:
Answer:
Nature and Matter of Galaxies
Answer the questions below.
In the space below, describe what would happen if our Universe were filled with diffuse gas. In other words, describe what would happen to the gas and identify the natural process(es) that would cause the change.
Answer
If our Universe were filled with diffuse gas, gravity would begin to draw the gas in, to concentrate it in specific locations. This ‘clumping’ would leave other areas of the Universe essentially empty, devoid of gas. Concentrating matter under the influence of gravity can happen even where the initial distribution of matter is perfectly uniform. As we’ll show you later, the Universe started out more or less like this—filled with approximately uniform energy … then, later, matter … and later still, stars and galaxies. Today, of course, our Universe is astonishingly clumpy.
In the space below, describe a process that prevents the matter in the Universe from becoming concentrated in one spot.
Answer
The natural process that counteracts gravity’s inward tug is, of course, motion. Objects in motion resist the influence of gravity. As such, instead of one big ‘continent’ of matter, our Universe consists of many ‘islands’ of matter. As you know, this pattern exists at many scales, not only is it true for galaxies in the Universe, but it also exists inside galaxies, solar systems, …, and atoms (though processes other than gravity produce concentrations of matter in the atomic world). In addition to motion, there are other processes, which we’ll encounter later in this Unit, which counteract the concentrating effect of gravity.
In the space below, describe why galaxies exist.
Answer
Unsurprisingly, galaxies—like the stars and planets we’ve already investigated—result from interactions between gravity and motion. Have our investigations caused you to think about natural systems such as galaxies, stars, and planets a little differently now? Isn’t it cool to be able to see the lawful hand of God moving in majesty and power as you contemplate these interesting aspects of nature?! Do you find yourself interacting with nature from a somewhat different perspective? For example, has your interest in nature been rekindled? Do you feel more connected to the world around you? Is nature beginning to be ‘unmasked’ for you; that is, are you beginning to see governing processes producing lawful interactions between elements in the systems that you encounter every day? We hope so!
As you’ve studied stars and planets, you have not only begun to master the lawful interactions of natural processes that create planets, stars, and galaxies, but you’ve also begun to see how order arises in nature. For example, no doubt you’ve begun to recognize important commonalities between otherwise unrelated systems in which order emerges from chaos. For example, you might have realized that these situations always seem to involve competing processes and the flow of energy through the system. As we continue to explore additional systems and as you interact with the world around you every day, we invite you to continue to search for commonalities wherever order self-assembles.
Galaxy Evolution
Answer the questions below.
In the box below, describe whether the motions of elements (e.g., stars) inside spiral galaxies is haphazard, random, or coherent/directional.
Answer
The motions of individual elements (stars & nebulae) inside spiral galaxies is coherent and highly directional; that is, most stars travel around the galactic center in roughly circular orbits, in the same direction and plane.
Using your knowledge of elliptical galaxies and the images above, describe whether the motions of elements (e.g., stars) inside elliptical galaxies is haphazard, random, or coherent/directional.
Answer
The motions of individual elements (stars & nebulae) inside elliptical galaxies is random. By this we mean that the stars orbiting the center of elliptical galaxies do not share a galactic plane. These random orbits generate the characteristic shapes of this galaxy type.
Ellipticals typically contain very little gas & dust, consist mostly of older low-mass stars, and form few new stars. In addition, they can contain a concentration of stars near their centers and are usually surrounded by abundant globular clusters. The smallest ellipticals are a few thousand light years wide and contain just 100 million stars. In contrast, the largest ellipticals—such as M87—can have diameters up to 700,000 light years and contain as many as a few trillion stars. Elliptical galaxies, which can be produced by galactic collisions, are especially abundant near the centers of galaxy clusters.
Using your knowledge of irregular galaxies and the images below, describe whether the motions of elements (e.g., stars) inside irregular galaxies is haphazard, random, or coherent/directional.
Answer
The motions of individual elements (stars & nebulae) inside irregular galaxies is… well, irregular (haphazard). As the name implies, irregulars lack the characteristic shapes of spirals and ellipticals. That does not mean that irregulars lack shape or structure; instead, it means that they display a great variety of shapes and structures. Irregular galaxies represent transitional galactic forms that result from gravitational interactions between two galaxies. Take a moment to re-examine the galaxies below. Knowing what you do now, can you see the interactions between originally separate (non-interacting) galaxies. In time, irregular galaxies can evolve into elliptical galaxies.
Using your knowledge of galaxies, identify the type of galaxy the collision will produce—shown in the ‘In 6 billion years’ image. Also, briefly explain how the collision produces this galaxy type.
Answer
The ‘In 6 billion years’ image shows an elliptical galaxy. Ellipticals can form from collisions of spiral galaxies because gravitational interactions between the galaxies randomize the orbits of the stars. Of course, it takes a long time to pass through the transition that separates the ‘two spiral galaxy’ state, shown in the first two images, from the ‘one elliptical galaxy’ state, shown in the last image.
Watch these two videos that show what it might look like as the Milky Way and Andromeda galaxies collide. One is far away, and one is close up.
Large-scale view of the Simulated Collision (75 sec):
Close-up view of the Simulated Collision (45 seconds):
Briefly describe how the interplay of gravity and motion produces a spiral galaxy, instead of a massive black hole.
Answer
As gravity pulls matter towards regions of higher density, it causes the matter to accelerate. Then, as the rapidly moving matter encounters the growing galaxy, gravity converts the straight-line motion into rotational motion, which prevents the recently acquired matter from entering the central black hole at the center of the galaxy.
Describe what happens to the size and rotation of the main galaxy through time, as it ‘consumes’ additional star clusters and smaller galaxies.
Answer
As time passes and the main spiral galaxy collides with and consumes star clusters and small galaxies, the central galaxy becomes larger and spins faster; as the galaxy evolves, the characteristic spiral shape emerges.
