Interstellar Medium (ISM)• Read Your Textbook: Foundations of Astronomy

– Chapter 11, 12

• Homework Problems Chapter 11– Review Questions: 1, 2, 5, 6– Review Problems: 2-6– Web Inquiries: 1

• Homework Problems Chapter 12– Review Questions: 2, 3, 8, 9– Review Problems: 3, 7-9 (old 1, 4-7)– Web Inquiries: 1

A Galaxy Rich in Gas and Dust

ISMThe bright regions are congregations of innumerable stars.

However, the dark areas are not simply "holes" in the stellar

distribution. They are regions of space where interstellar

matter obscures the light from stars beyond.

Their very darkness means that they

cannot be easily studied by optical

methods used for stellar matter.

There is, quite simply, very little to

see in visible light!

Gas and DustIdentical regions of space in the optical and infrared.

NebulosityStar formation regions are rich in gas and dust.

AbsorptionAbsorption

of starlight

by the ISM

causes

interstellar

reddening.

ExtinctionProbing the ISM by

examining absorption

lines in stellar spectra

caused by intervening

clouds.

ISM Characteristics

21-cm Radio Emission

21-cm Radio Energy Transition

21-cm Milky Way

ISM CompositionAlso popular cometary chemical composition list.

Gravitational CollapseStar formation begins when gravity begins to dominate over

heat, causing a cloud to lose its equilibrium and start to

contract.

Only after the cloud has undergone radical changes in its

internal structure is equilibrium finally restored.

Protostar Disk

Orion Nursery

Star Formation Regions

Cloud ShockWave

Cloud FragmentationStar formation usually involves creating “clusters” of stars.

Open Cluster• Thousands of stars, sometimes gravitationally bound

• Contain the youngest, hottest galactic stars

• Associated with star formation regions in the galaxy.

Globular Cluster• Hundreds of thousands of stars, gravitationally bound and

spherically symmetric.

• Contain the oldest galactic stars

• Symmetrically distributed

about the galaxy.

ProtoStar Tracks

Protostar Stages

Hyashi Tracks

Gravity Versus Pressure

Stellar Structure Laws