Scholars Online Astronomy - Chapter 23: Galaxies

Homework

• Reading Preparation
  • Study Guide/Reading Notes
• Key Formulae to Know
• WebLecture
• Study Activity
• Chat Preparation Activities
• Chapter Quiz
• Lab Work

Reading Preparation

Reading: Astronomy, Chapter 23: Galaxies

Study Guide

• Section 1: In the mid-1700s, Charles Messier identified over 100 "nebulous" objects that he could not resolve to distinct points as stars with his telescope. Later observations by Dreyer added hundreds more. Unable to get good parallax data on these objects, it was impossible to calculate their distances, and debates, like the famous one between Heber Curtis and Harlow Shaplow, highlighted the controversy over whether these objects were part of the Milky Way or not, and whether the sun was at the center of the Milky Way or not.
• Section 2: Hubble used the period-luminosity relationship for Cepheid variables (discovered by Henrietta Swan Leavitt (chapter 19.6) to prove that spiral nebula containing these stars were far beyond the limits of the Milky Way.
• Section 3: Hubble's surveys of the galaxies (made with the 100-inch Hale Telescope at Mt. Wilson before LA's lights made dark-sky observations too difficult) identified major types and subtypes of galaxies, based on their estimated mass, luminosity, diameter, types of stars, and structure:
  • Spiral Galaxies
    • Unbarred spiral galaxies: Sa have large central bulges, wide arms; Sc have small central budges, narrow well-defined arms.
    • Barred spiral galaxies: SBa have large central bulges and bar; SBc have small central bulges, clearly defined bars, and narrow well-defined arms.
  • Elliptical Galaxies: E0 are large, spherical galaxies with isotropic random motions (different speeds in all directions); E7 are the most elliptical, with anisotropic motions (different speeds in different directions).
  • Lenticular Galaxies: S0 galaxies have central disk and bulges; SB0 galaxies have central disk, bulges and bars, but neither has arms.
  • Irregular Galaxies: IrrI galaxies have little organized structure, and contain young OB stars associated with HII regions. Irr II galaxies hare asymmetrical, either due to violent nucleus activity or collisions.
• Section 4: Standard candles are objects that lie within a galaxy and have characteristics that allow us to determine their distance, such as an absolute magnitude depending on variable period (Cepheid variables). For distances beyond 30Mpc, astronomers use Type Ia supernovae peak brightness and fade rates to determine the supernovae absolute magnitude and distance. The Tully-Fisher relationship uses the breadth of the hydrogen-21 line to determine galaxy luminosity for rotating galaxies like spirals. The Davis-Djorgovsky analysis of elliptical galaxies' fundamental plane luminosity distributions can be used for galaxies beyond 100 Mpc. The set of distance calculation methods are collectively called the "cosmological distance ladder". Each of these methods uses an intrinsic value (such as luminosity) that varies with distance to calculate distance from observed values. A new method independent of distance itself uses molecular water clouds for rotational doppler shifts to determine distance.
• Section 5: Spectra from galaxies are compared to lab values for elemental and molecular "stationary" spectra to determine doppler shifts and rate of motion toward or way from earth. Distant galaxies appear to be moving away more rapidly than nearby galaxies. (See red shift and Hubble Law equations below). The Hubble constant H₀ is a rate describing how much faster a galaxy appears to recede for each Mpc it is distant from Earth. Because distances to galaxies are hard to determine, the value of H₀ is difficult to pin down. Initially, using Type Ia supernovae distances, H₀ varies between 40-65 km/s / Mpc, but using the Tully-Fisher relationship gives values between 80-100 km/s / Mpc. Better data from the Hubble telescope has allowed for recalculation; the current accepted value is 73 km/s / Mpc.
• Section 6: Galaxies appear to be grouped together in clusters, not randomly scattered in space. Rich galactic clusters contain many galaxies interacting gravitationally within the same region; poor galactic clusters contain fewer galaxies and are often considered "groups" rather than "clusters". The Local Group to which the Milky Wa belongs has about 50 known galaxies; the Virgo cluster has more than 2000. Some clusters (rich or poor) are definitely spherical in shape and therefore regular; those not conforming to a spherical shape are irregular. The most massive cluster, the Great Attractor appears to be drawing all the galaxies of the local supercluster, including the Milky Way, toward it. Clusters of galaxies may themselves belong to supercluster organizations of galaxies, but these clusters are not bound within their supercluster by gravity; instead, they appear to be drifting away from each other. Galaxies appear to be distributed on the surfaces of large bubbles, like soap bubble clusters, with clusters separated from each other by spherical voids, and dense clusters like the Sloan Great Wall in the niches where several bubbles share a common interface.
• Section 7: Colliding galaxies form different structures depending on the speed of their collision. High-speed collisions strip the colliding galaxies of their interstellar dust and gas, which gets left behind. Lower speed collisions produce density waves which form stellar nurseries, becoming starburst galaxies. Tidal forces during collisions deform galaxies and may cause them to merge (if they are the same size) or a larger galaxy may strip stars from a smaller galaxy (cannibalism) while retaining much of its own structure. The largest elliptical galaxies may be the result of cannibalistic collisions.
• Section 8: Stars are held within galaxies by gravitational attraction, but the observable luminous matter in most galaxies is insufficient to produce the gravitational forces required. Astronomers posit the existence of non-light-interacting dark matter to account for the mass required to produce sufficient gravitational force. Rotational dynamics for the luminous matter of galaxies like the Milky Way indicates that galaxies must have mass that extends far beyond their observable luminous-matter boundaries. Since massive galaxies bend space, we can measure their masses using gravitational lensing effects. To explain dark matter effects, physicists propose new types of mass particles that do not react with light or electrical or magnetic fields, but only with gravity. Evidence from some colliding galaxies where dark matter mass distributions do not align with luminous matter support this definition of dark matter.
• Section 9: Computer models allow us to predict formation of galaxies of different kinds through gravitational collapse or through collisions.

Key Formulae to Know

Web Lecture

Read the following weblecture before chat: Galaxies

Study Activity

Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.

• Lab instructions: Galaxy Classification

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