1. Stellar NurseriesStages of Star Birth

2. The interstellar medium The space between the stars is not empty

3. Light from distant stars passing through dark clouds of gas and dust has an absorption spectrum imprinted on it. This tells us what is in the interstellar medium It is made up of 70% hydrogen, 28% helium, and 2% heavier atoms and molecules.

4. The interstellar medium consists of 70% H, 28% He, and 2% heavier elements (by mass). Many of the dust and gas clouds are hot – thermal pressure holds them up against gravity. They have to cool down (10K – 30K) before they are able to collapse: molecular clouds. Radio image of CO emission – it is too cold for molecular H to emit

5. These molecular clouds also contain dust These dust grains are usually less than 0.5 micrometers across This makes them very good at scattering visible light: “Interstellar reddening” “Reddening” is different from Doppler shift

6. How can we tell if a star’s light has been reddened by interstellar dust rather than the star just being red (very cool)? A) The lines in the spectrum won’t be Doppler shifted if it’s interstellar reddening. B) The spectral type still can be determined to tell us the surface temperature. C) The only way to tell is if there are other nearby stars that also look red. D) There is no way to tell for sure.

7. We can use the fact that infrared light gets through the molecular clouds to see inside star forming regions…

8. Gravity has to overcome several obstacles to collapse: Thermal pressure Turbulence Rotation Magnetic fields: To overcome these obstacles, the molecular cloud generally needs a mass that is a couple hundred times that of our Sun… Observations of starlight passing through molecular clouds show that the light is often polarized, indicating magnetic fields imbedded in the cloud.

9. This graphical display of a computer simulation shows how turbulent motion of the gas in the cloud causes it to get lumpy: some regions are denser than others. Each lump can go on to form one or more stars: This is “clustered star formation”

10. Smaller clouds of gas and dust can form individual stars as long as they are unusually cold and dense.

11. First generation stars were made of H and He only – there were no heavier elements yet. This means the clouds of gas were not as cold as the molecular clouds of today: molecular hydrogen cannot radiate thermal energy away below about 100 K. To overcome these higher temperatures, the clouds had to be much more massive to collapse  First generation stars: Were very massive Lived very brief lives (none exist today) Sent the heavier elements they created back into space

12. Why were the first stars that formed in our universe necessarily so large? A) Because the universe was smaller then, the gas was much denser everywhere. B) The universe was much hotter in the first billion years, so the gas clouds had to be more massive to overcome the warmer temperature. C) Hydrogen can’t radiate away heat below about 100 K, so the gas clouds had to be more massive to overcome the warmer temperature. D) The heavier elements available today weren’t available then, so the gas clouds had to be more massive to generate enough gravity.

13. Protostellar disk The disk slows down the rotation of the protostar due to the protostar’s magnetic field lines dragging through the disk. Stages of Star Birth Protostellar jets Are probably the magnetic field transferring angular momentum to material in the disk, but we don’t know for sure. But they do happen … !

15. The protostar reaches the main sequence when sustained hydrogen fusion begins in the core (“Now I’m a star!”)

16. Protostellar cocoons?