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Chapter 15: Problem 30

Which part of the galaxy contains the coldest gas? (a) the disk (b) the halo (c) the bulge.

Short Answer

Expert verified
The galactic disk contains the coldest gas.

Step by step solution

01

Understand the galaxy structure

Galaxies, like the Milky Way, are composed of three main parts: the disk, the halo, and the bulge. The disk is the flattened, rotating component which contains stars, gas, and dust, along with spiral arms. The halo surrounds the galaxy and contains older stars and globular clusters. The bulge is a dense region of stars at the galaxy's center.
02

Identify where gas is located

Gas in galaxies is primarily found in the disk. This gas forms the interstellar medium, which is cooler in comparison to other regions. While some gas exists in the halo, it is usually ionized, making it hotter.
03

Determine the temperature of gases in each area

The gas in the galactic disk is mostly in the form of cold molecular clouds, which can reach very low temperatures compared to the warmer, ionized gas in the halo. The bulge also contains warmer stellar material and lacks the cold dense molecular clouds found in the disk.
04

Conclusion

Based on the distribution and temperature of the gas in these areas, the disk contains the coldest gas due to the presence of cold molecular clouds.

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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) is the matter that fills the space between the stars in a galaxy. It's composed mainly of gas and dust, playing a crucial role in the lifecycle of stars. The ISM is found primarily in the disk of the Milky Way and consists of various forms of gas, including ionized, atomic, and molecular gas.

In the Milky Way's disk, the interstellar medium is a mix of:
  • Ionized gas, which is hot and produced by high energy events such as supernovae.
  • Neutral atomic gas, hydrogen being the most abundant element.
  • Molecular gas, usually found in large clouds.
These components of the ISM are essential for star formation. When molecular clouds within the ISM collapse, new stars are born. The balance between hot ionized gas and cold molecular clouds regulates the processes of star formation and the evolution of galaxies.
Molecular Clouds
Molecular clouds are dense regions within the interstellar medium where the coldest gas is found. These clouds are mainly composed of molecular hydrogen (H鈧�), and they can get extremely cold, with temperatures often plummeting to just 10 Kelvin above absolute zero. This makes them the chilliest regions within a galaxy's structure.

Properties of molecular clouds are:
  • Dense, with particles packed closely together.
  • Cold, enabling the formation of complex molecules including organic compounds.
  • They act as stellar nurseries, where new stars are born.
The low temperatures within molecular clouds allow atoms to bond into molecules, encouraging the development of stars and potentially planets. As these clouds collapse under their own gravity, they break into smaller regions, which can each form one or more stars.
Milky Way Disk
The Milky Way disk is a vast, flattened region that contains the majority of the galaxy's stars and its interstellar medium, filled with spiral arms. In this disk, cold molecular clouds exist alongside stars and other celestial objects. The presence of cold gas makes the disk a dynamic environment for star formation.

Key features of the Milky Way disk include:
  • Rotating structure with a distinct pattern of swirling spiral arms.
  • High density of stars, including young, middle-aged, and some older stars.
  • Containing most of the galaxy鈥檚 cold molecular clouds, which are crucial for star formation.
The interaction between stars and the interstellar medium within the disk influences the galaxy's lifecycle. Stars are born from the gas and dust of the ISM, and in turn, they affect the structure and composition of the disk as they evolve and eventually shed their outer layers.

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Most popular questions from this chapter

How do we determine the Milky Way's mass outside the Sun's orbit? (a) from the Sun's orbital velocity and its distance from the center of our galaxy (b) from the orbits of halo stars near the Sun (c) from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun.

Unenriched Stars. Suppose you discovered a star made purely of hydrogen and helium. How old do you think it would be? Explain.

Enrichment of Star Clusters. The gravitational pull of an isolated globular cluster is rather weak-a single supernova explosion can blow all the interstellar gas out of a globular cluster. How might this fact relate to observations indicating that stars ceased to form in globular clusters long ago? How might it relate to the fact that globular clusters are deficient in elements heavier than hydrogen and helium? Summarize your answers in one or two paragraphs.

High-Velocity Star. The average speed of stars in the solar neighborhood relative to the Sun is about \(20 \mathrm{km} / \mathrm{s}\). Suppose you discover a star in the solar neighborhood that is moving at a much higher speed relative to the Sun, say, \(200 \mathrm{km} / \mathrm{s}\). What kind of orbit does this star probably have around the Milky Way? In what part of the galaxy does it spend most of its time? Explain.

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer We did not understand the true size and shape of our galaxy until NASA launched satellites into the galactic halo, enabling us to see what the Milky Way looks like from the outside.
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