01:750:109 Chapter Notes - Chapter 11.1: Radiography, Hydrogen Atom, Planetary Nebula
Our Galaxy: The Milky Way
Galactic Recycling
●Stars are continually forming and dying within it through the star-gas-star cycle
○Born when gravity causes the collapse of molecular clouds within interstellar medium
○Shine for yearsssssss
○Die when exhausted fuel for fusion
○Return much of their material back into interstellar medium through a planetary nebula
(low mass star) or supernova (high mass star)
●Remaining stage of the star-gas-star cycle take place in the interstellar medium
○Hot gas bubbles: ionized gas hot enough to emit X-rays
■Takes many thousand years to cool but eventually reaches a temp of around
10^4 K (quite warm but cool enough for hydrogen atoms to remain neutral
rather than being ionized
■This warm gas known as: atomic hydrogen gas
●Distribution of atomic hydrogen gas mapping by radio observations of
spectral lines at a wavelength of 21 cm tells us it is distributed
throughout the galactic disk
○Atomic hydrogen clouds:
■Matter remains in warm atomic hydrogen stage for millions of yrs and during
this time, gravity slowly draws blobs of atomic gas together into tighter clumps,
which radiate energy and cool more efficiently as they grow denser
■Even so, interstellar medium remains a nearly perfect vacuum: on avg, each
cm^3 contains only one hydrogen atom
■Eventually gas clumps drops below 100K allowing the hydrogen atoms to pair up
into hydrogen molecules
■Cool, dense clumps become
