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Chapter 24: Problem 20

What is the difference between a cluster and a supercluster? Are both clusters and superclusters held together by their gravity?

Short Answer

Expert verified
A cluster is a group of galaxies that are gravitationally bound together. A supercluster is a larger structure made up of several galaxy clusters or galaxy groups. While gravity does influence both clusters and superclusters, its effect differs - it holds the galaxies in a cluster together, but a supercluster isn't tightly bound by gravity and is currently expanding.

Step by step solution

01

Define a Cluster

In the context of astronomy, a cluster refers to a group of galaxies that are gravitationally bound to each other. This means that their gravity is strong enough to keep them together in spite of the expansive force of the universe.
02

Define a Supercluster

A supercluster is a large group of smaller galaxy clusters or galaxy groups. It is among the largest known structures in the Universe. A supercluster is not tightly bound by gravity. Instead, it is seen as a 'fluctuation in the cosmic web'.
03

Compare Gravity's Role in Cluster and Supercluster

Both clusters and superclusters are influenced by gravity. However, the effect is not the same. In a cluster, the gravity thereby is strong enough to hold the galaxies together, even against the expansive force of the universe. Conversely, in a supercluster, the gravity is not strong enough to hold the constituent galaxy clusters together permanently. Superclusters are currently, in the cosmic timeline, expanding and will eventually disperse, not forming a binding entity.

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Most popular questions from this chapter

In the early twentieth century, there was considerable debate about the nature of spiral nebulae and their distance from us, but the debate was resolved by improvements in technology. As a group, list three issues that we, as a culture, did not un-derstand in the past but understand today, and explain why we now have that understanding.

Hubble made his observations of Cepheids in M31 using the 100 -inch (2.5-meter) telescope on Mount Wilson. Completed in 1917 , this was the largest telescope in the world when Hubble carried out his observations in 1923. Why was it helpful to use such a large telescope?

Explain why the dark matter in galaxy clusters could not be neutral hydrogen.

Use the Starry Night Enthusiast \({ }^{\text {TM }}\) program to examine clusters of galaxies. Select Favourites \(>\) Deep Space \(>\) Virgo Cluster to center this collection of galaxies in the view, as seen from a distance of about 53 Mly from the Sun. You are looking at a three-dimensional view of the Tully Database. Open the Find pane and locate Virgo \(\mathbf{A}\), one of the galaxies examined in the previous question, which is close to the center of this cluster of galaxies. Right-click on this galaxy to open the contextual menu (Macintosh users Ctrl-click on this galaxy) and click on Highlight "GA Virgo Cluster" Filament to highlight this cluster in yellow. Click on the "up" arrow in the Viewing Location to move to about 30 Mly from the Sun. Hold down the Shift key while holding down the mouse button and move the mouse to use the location scroller to rotate this rich group of galaxies. (a) Describe the general shape of the Virgo cluster. (b) As you rotate the Virgo cluster, you should notice other groupings of galaxies. Stop this rotation at some position and make a sketch of the screen, circling what you believe are other groups on your sketch. Right-click (Macintosh users Ctrl-click) on one of the other clusters (and clouds and extensions) near to the Virgo Cluster to open the contextual menu and use the Highlight option to see how astronomers have grouped these other galaxies. Repeat this process until you have identified all of the clusters around Virgo. Outline and label these clusters on your drawing. (c) Choose three of these clusters, center on each in turn and right- click (Ctrlclick on a Macintosh) to open the contextual menu and use the Centre command. Use the Zoom facility and location scroller to move around these collections of galaxies, and describe their distribution compared to the Virgo cluster. For example, what are their shapes and relative sizes compared to Virgo and to each other? Are they rich spherical concentrations or walls of galaxies?

How might you determine what part of a galaxy's redshift is caused by the galaxy's orbital motion about the center of mass of its cluster?
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