What is the Interstellar Medium?

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  • 0:01 The Interstellar Medium
  • 0:52 The ISM's Characteristics
  • 2:04 How Do We Know the ISM…
  • 4:53 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

We'll explore something known as the interstellar medium, the birthplace of stars around our universe. You'll learn what it is made of, what interstellar dust is, and why it is there in the first place.

The Interstellar Medium

If you look carefully around your room, unless it is as messy as mine, you'll notice that while there are photos on the wall, a bed, and other furniture, most of the room is actually empty.

But this emptiness is a misconception. Even though you may flail your arms and not touch anything while running around your bedroom, you're still running through air, a mixture of gases that you just can't see.

The space between stars is also commonly thought of as empty, even though this isn't always the case. This is because in between (inter-) these stars exists the interstellar medium (ISM), a collection of gas and dust located between stars. The ISM is the birthplace of stars, so let's take a closer look at the ISM.

The ISM's Characteristics

About 99% of the ISM is made up of gas. Spectroscopy, the use of unique patterns of spectral lines to identify a chemical substance, has helped us identify the chemical composition of the interstellar medium.

It is made up of approximately 70% hydrogen, 28% helium, and 2% of other elements. These atoms join together to form molecules in dense and cold regions, which astronomers call molecular clouds. Over 150 different molecules have been found in such places. It is in these molecular clouds that gas can become dense enough to form stars, something we explore in another lesson.

At any rate, just like your room's 'empty' space has dust floating around in it, the ISM's remaining 1% contains interstellar dust, microscopic solid particles in the interstellar medium. These dust particles are made not of dead skin cells, but rather carbon, iron, ice, silicates, and organic compounds.

How Do We Know the ISM Is Even There?

So, how do we know the ISM is even there if we can't really see it all that well? I mean, you can at least see dust floating in your room if the light hits it just right, but interstellar dust and gas isn't as simple to detect.

The key to knowing it's actually there is the spectroscopy I mentioned before as well as relationships between properties like distance, spectral classes, brightness, and luminosities of stars.

There are several concepts I want to delineate for you in this respect. Concepts that help explain how we know the ISM is there.

One concept is called interstellar extinction, the dimming of light coming from a star due to its obstruction by the interstellar medium. Normally, we know that a star with a particular luminosity (a star's intrinsic brightness) should have a certain apparent brightness based on its distance away from us. But the ISM is like a fog, and it dims the starlight. It's just like with car headlights and fog. You know that a car has bright headlights, but if there's a thick fog, you may be only ten feet away and barely see them.

The other concept I want you to know is called interstellar reddening, a process where starlight is made redder by the interstellar medium. It applies to interstellar extinction in the fact that the interstellar extinction is stronger for short wavelengths of light, rather than long ones. Long wavelengths, like red, are filtered through, while short ones, like blue, are scattered by the ISM.

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