The Interstellar Medium (ISM) is matter such as Dust and gas between stars. (Matter between entire galaxies is called the Intergalactic Medium.) The gas may be ionized, neutral, or molecular. Density varies from as little as 1 per 10,000 cm³ to 1,000,000 per cm³. The interstellar medium was first deduced through observed Absorption Lines.

Types of ISM regions that have been detected:

• Molecular Clouds, 10-20K
• cold neutral medium (CNM), 50-100K
• warm neutral medium (WNM), 6000-10000K
• warm ionized medium (WIM), 8000K
• HII Regions, 8000K
• Coronal gas/Hot Ionized Medium (HIM), 1000-3000k