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A astronomy cast episode 677 the answer is always dust. Welcome to Astronomy Cast from weekly

Facts based journey through the cosmos where we help you understand not only what we know but how we know what we know

My name is Fraser Kane and the publisher of University with me is Dr. Pamela K. a senior scientist for the planetary science institute and the director of Cosmic Quest. Hey Pamela, how you doing?

I am doing well. I have to admit, dust is one of those things that likes to get involved in every aspect of our lives

and we've avoided the show for a long time but at a certain point the dust catches up with you. Is it a spring cleaning thing? Is it an allergies thing?

Something put dust into our minds. Maybe it's the dust itself choosing the show. It's like microplastics. It's in everything.

Carl Sagan said we were made of star dust. Are microplastics technically dust?

No. They probably are. Should we just stop the show right now and dig into this? No. No. We'll put a pin in that. You will wait for the helpful email.

It's true. It's being written right now. All right, whenever astronomers discover something surprising the answer often turns out to be dust. Dust obscuring our view. Dust changing the polarity.

Dust warming things up. Dust cooling things down. It's always dust until it isn't.

So I guess before we talk about how it's always dust, what is dust in astronomically speaking?

It is molecules of stuff and the molecular nature of it is kind of the key.

And the molecules can be really, really big like polycyclic aromatic hydrocarbons, PA, Hs.

They can be things like methane molecules. But dust is where you start to get collections of larger and larger molecules coming together.

This is where you start seeing amino acids and eventually as dust is bigger, you start getting grains of ice and grains of silicates.

And eventually things get big enough that we no longer refer to them as dust. But that stuff we can't generally see at a great distance.

So what would be, what would be no longer dust?

Dust is really defined at a certain level by its behavior. If you have a cloud of dust and you shine light through it, it is going to preferentially scatter out the shorter wavelengths of light.

You're going to see the blue getting sent sideways. This is how you get the really cool reflection nebulae while the longer, rather wavelengths are able to make it through.

And as the dust gets thicker and thicker, you're eventually only going to get the longest shades of infrared light coming through.

But once you start getting to things that are big enough that they're just like, hi, I am solid.

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