Question

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

Short answer

The galactic disk contains the coldest gas.

Step-by-step solution

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.

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.

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.

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.

Key concepts

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 (H₂), 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’s 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.