WHERE STARS ARE BORN The Interstellar Medium ISM

WHERE STARS ARE BORN

The Interstellar Medium (ISM) The space between the stars looks empty. . . But it’s not!

Gas * Mainly hydrogen + helium * Avg density 1 – 10 atoms/cm 3 * 99% of ISM Dust * Tiny grains (‘smoke’) ~ 0. 0001 cm * Silicates, carbon, ice (? ) * Avg density 1 grain/million m 3

‘Whirlpool’ Galaxy

Looking at a galaxy from the inside.

Looking for Gas

Emission Nebulae * Ultraviolet photons from hot stars are absorbed by gas atoms – cause gas to glow. Rosette Nebula

Great Orion Nebula

North America Nebula Trifid Nebula

H atoms Earth UV Hot Stars

Hydrogen atom UV Electron ejected Electron recaptured Photon emitted

Red Balmer emission Line

Molecules – mainly hydrogen (H 2). Carbon monoxide (CO) in Orion

Looking for Dust

Reflection Nebulae * Glows due to scattered (reflected) starlight. Pleiades Star Cluster * Dust scatters blue light more efficiently than red light.

Dark Nebulae Molecules abundant here

Emission nebula ‘Horsehead’ Nebula Reflection nebula

Star Formation

* Results from collapse of a molecular cloud. • Cloud collisions • Supernova blast wave • Expanding emission nebula • Galactic density wave

p. 271

Molecular Cloud Shrink & heat ‘protostars’ Central temp 10 million K: Hydrogen fusion ignites (on main sequence)

Star birth in the Eagle Nebula

Star birth in the Trifid Nebula Forming star?

* Accretion disk may form around young stars. . . Bipolar Flow

Bipolar Flow p. 273 Disk

Protostellar Disks

‘Evolutionary tracks’ on the HR diagram: Time required for contraction to main sequence depends on mass. pgs. 274 -5

Limits to Star Formation

Protostar mass < 0. 08 M : No hydrogen ignition: ‘Brown Dwarf’

M > 100 M Star disrupted by the pressure of photons. The ‘Pistol’ Star