1. Goal: To understand the Interstellar Medium (ISM) and how it applies to the “circle of life”. Objectives: 0) To learn about the Overall make up of the ISM 1) To learn about the Hot phase 2) To learn about the Warm phase 3) To learn about theCool phase 4) To understand How these 3 phases evolve – forming the “circle of life”. 5) To examine Other phenomenon such as bubbles and chimneys.

2. Overall makeup of the ISM Empty space isn’t completely empty. Over thousands of light years a particle here and a particle there really add up! The space in between stars is therefore called the Interstellar Medium (ISM for short). What is the ISM mostly made of? A) Hydrogen B) Helium C) Oxygen D) dust

3. Overall makeup of the ISM What is the ISM mostly made of? A) Hydrogen – 90% B) Helium – 9% C) Oxygen - trace D) dust – 1% 1% seems small, but if our atmosphere had 1% dust – you would not be able to see more than half a block down the street!

4. Hot Phase Temperature – 1 million degrees K. Density (very low) 10 -3 particles per cubic cm About half of all space is filled by the Hot Phase including our local region of space (but only 10% of the mass)! Don’t worry though, the densities are so low that this is not a danger. The Hot Phase is created from supernova remnants which expand and cool with time. Why do supernova remnants expand?

5. Warm Phase – HII region The warm phase consists of several sub parts: 1) HII regions. These are regions where Hydrogen is ionized (it has its electron removed and is basically just a proton). HII regions (not to be confused with H2) are short lived (a few million years). They form in regions where massive stars are forming. The massive stars heat up and ionize the surrounding (and dense) giant molecular clouds. The HII region then disperses very quickly (and forms some stars).

6. Warm Phase - rest 2) Warm Ionized Medium – just like HII regions, but a little cooler and less dense. 3) Warm Neutral Medium. Hydrogen is not ionized. Overall, the ranges: Temperature ranges from 1-10 thousand degrees K. Densities range from 0.01 to 10 particles per cubic cm for the Warm mediums and from 100-1000 in the HII regions. Overall the warm phase takes up about 45% of the total volume of the ISM and about 40% of the mass.

7. Cool Phase The cool phase has all the cool (dark) nebula and Giant Molecular Clouds. Half of the mass of the ISM is located in the cool phase! However, only about 5% of the volume. The cool phase has 2 components. 1) Cool Neutral Medium – which has cool atomic Hydrogen. Densities of 1-1000 particles per cubic cm and temperatures of 50-100K. 2) Giant Molecular Clouds – which has H2 (molecules). Densities are 1000 to 1 million particles per cubic cm and temperatures of 20-50K.

8. How does this affect us? It turns out that the phases change and evolve with time. This evolution is VERY important because we would not be here without it! What is this evolution?

9. Circle of life! The interstellar medium undergoes a continuous cycle much like life on the earth – the circle of life. Stars are born from Giant Molecular Clouds. The most massive stars die quickly throwing off most of their materials into the ISM and creating a supernova. This supernova creates the seed of a new Hot Phase section. The supernova remnant over the course of millions to tens of millions of years cools and becomes part of the warm phase. Over tens to hundreds of millions of years the warm phase cools to the cool phase. The cool phase cools and shrinks becoming a Giant Molecular Cloud. And the cycle repeats again!

11. Other phenomenon So far I have simplified this a bit. As we have seen though, stars form in large clusters. So, you don’t get isolated supernova remnants. Eventually these remnants will get bigger and merge together. If you get tens, hundreds, or even THOUSANDS of these remnants to merge together what will you get?

12. Bubbles (RCW 79) 17k light years away, 70 light years across We live in a bubble. Our bubble is called the Local Bubble (yeah I know really lame name). Bubbles can range from hundreds to thousands of light years across. Bubbles are basically giant hot phase areas that slowly expand with time. However, what do you think would happen if one of these bubbles broke out of the disc of our galaxy?

13. Chimneys When the bubbles burst above or below the plane you get a Chimney. Chimneys expel gasses and dust into above and below the plane of the galaxy. These materials eventually fall back to the galaxy in the form of high velocity clouds. This causes materials to be circulated through the entire galaxy!

14. Conclusion The evolution of the ISM is what keeps the formations and deaths of stars in our galaxy on an never ending cycle. There are 3 components to the ISM each with different temperatures and densities. For the hot phase, often times it creates bubbles. When those bubbles burst through the plane of our galaxy materials get deposited throughout our galaxy.