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Chapter 15: Problem 1

Phases of the interstellar medium include (choose all that apply a. hot, low-density gas. b. cold, high-density gas. c. hot, high-density gas. d. cold, low-density gas.

Short Answer

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a, b

Step by step solution

01

Understand the Phases of the Interstellar Medium

The interstellar medium (ISM) is composed of different phases characterized by their temperatures and densities. Typically, it includes both hot, low-density and cold, high-density components.
02

Cold, High-Density Gas

One of the common phases is cold, high-density gas. This phase consists of molecular clouds where new stars often form. Therefore, statement b is correct.
03

Hot, Low-Density Gas

Another common phase includes hot, low-density gas, often found in the form of ionized regions and hot coronal gas. Therefore, statement a is correct.
04

Eliminate Incorrect Options

Hot, high-density gas (option c) is not a typical phase in the ISM. Similarly, cold, low-density gas (option d) is not a common phase.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Interstellar Medium
The Interstellar Medium (ISM) refers to the matter that exists in the space between the stars within a galaxy. The ISM is made up of gas in ionic, atomic, and molecular forms, as well as dust and cosmic rays. These components occupy the seemingly empty space and make up the raw material for star formation.

The ISM is not uniform; it has various phases, each with different temperatures and densities. Understanding the ISM helps us learn how stars and planetary systems develop. There are primarily three main phases of the ISM:
  • Cold, dense molecular clouds.
  • Hot, ionized regions.
  • Hot coronal gas.

Each phase plays a critical role in the cycle of matter and energy through the galaxy.
Molecular Clouds
Molecular clouds, often referred to as stellar nurseries, are cold and dense regions within the ISM. They consist primarily of molecular hydrogen (H2) and can be several light-years across. These clouds have temperatures ranging from 10 to 30 Kelvin, some of the coldest places in the universe.

Within molecular clouds, the density is high enough to allow molecules to form. They are the primary sites of star formation, as the high density enables the gas to clump together, eventually forming new stars and planetary systems. Key features include:
  • High density, often with particles reaching up to 10,000 per cubic centimeter.
  • Cold temperatures, which allow for the formation of molecules.
  • The presence of dust, which shields the molecules from starlight and keeps the clouds cool.

Examples of well-known molecular clouds are the Orion Nebula and the Eagle Nebula.
Ionized Regions
Ionized regions of the ISM are where the gas is hot and remains ionized, meaning the atoms have lost one or more electrons. These regions are also known as H II regions, where H stands for hydrogen and II denotes that the hydrogen is ionized.

Ionized regions are typically found near hot, young stars that emit enough ultraviolet light to ionize the surrounding hydrogen gas. The temperature in these regions is around 10,000 Kelvin. Some characteristics of ionized regions are:
  • They are often found around newly formed stars.
  • Contain hot, ionized gas which emits light that can be seen in nebulae.
  • Play a critical role in the lifecycle of stars and the ISM by redistributing energy.

Examples of ionized regions include the Orion Nebula and the Trifid Nebula.
Hot Coronal Gas
Hot coronal gas is a diffuse, extremely hot component of the ISM. This gas is often found in regions where supernova explosions or powerful stellar winds from massive stars have heated the surrounding material to temperatures of a million degrees Kelvin or more.

Although it is very hot, this gas has a low density which means the atoms are widely spaced. Characteristics of hot coronal gas include:
  • Very high temperatures, often exceeding millions of Kelvin.
  • Low density, with particles spread out more thinly compared to other phases.
  • Associated with supernova remnants and energetic events in the galaxy.

Detailed studies of hot coronal gas are crucial for understanding the dynamics and evolution of galaxies.

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