ISM, Page 64: [Previous Page] [Next Page] [Class Home Page] Textbook pages 368 - 374

The Four Types of Insterstellar Regions

The insterstellar medium is highly dynamic, with ionizing photons transversing it, supernova going off, and enormous molecular clouds collapsing to make new stars.

Phases of the ISM

Name	Density (cm -3)	Temperature	How Observed	Remarks
Molecular Clouds	>1000	10-80 K	Radio Telescopes	Dust has icy mantles
H I Regions	30	100	Radio Telescopes	Diffuse ISM
Hot ISM	10-3	106.5	X-rays Telescopes	Little mass high volume
H II Regions	>100	104	H-alpha, radio, other lines	Expanding, transient




Other Important points to note: Neutral Hydrogen exists mostly in the Molecular Clouds and the HI Regions. Molecular Clouds are sites of star formation. Ionized Hydrogen exists in the Hot ISM and the HII regions. HII Regions have very high pressures (large Temperature and Density), so are expanding relative to their local environment. They will die out as soon as the hot central stars ionizing the gas die, usually within 10 million years of their birth. For example, The Orion Nebula (Belt & Sword) is an HII Region which will fade in a few million years.