Physical Universe Chapter 16

Interstellar Medium

The gas between the stars

Star Forming Clouds

dark clouds of dusty gas in
interstellar space

Cloud Composition (2)

• determined by gas absorption lines
  

• Milky Way Region:
  70% H 28% He 2% heavier elements

Molecular Clouds (3)

• most of the matter is the form of molecules (H2, CO, etc)

• temp = 10-30k

• density = 300 molecules per cubic centimeter

Interstellar Dust (2)

• blocks our view of stars

• made of elements like C, O, Si, and Fe.

Interstellar reddening

when interstellar dust blocks shorter-wavelength light more effectively than longer-wavelength, so stars viewed through the edges of the dust appear redder

Observing Newborn Stars

using infrared light, as most of a newborn star’s visible light is trapped within the dark gas clouds (interstellar reddening)

Glowing Dust

dust grains that absorb visible light, causing the grains to heat up and emit infrared light

Gravity vs. Pressure (2)

• gravity creates stars when it can overcome thermal pressure

• pressure buildup can be prevented by the conversion of thermal energy into infrared/photons

Mass of Star forming cloud

• typical molecular cloud (T~ 30 K, n ~ 300 particles/ cm3) must be at least a few hundred solar masses in order for gravity to overcome pressure.

Gravitational resistance

• if a cloud has other forces (ex. magnetic fields, turbulent gas) opposing gravity, the cloud must be more massive to begin gravitational contraction

Cloud Fragmentation (3)

• denser gas within cloud → stronger gravity within cloud

• meaning, in smaller, denser sections of a cloud, gravity can overcome pressure

• this leads to the cloud breaking apart into fragments, which can each go on and form a star
  

Isolated star formation

• in a small cloud, gravity can easily overcome pressure, forming a single star (like fragmentation)

First stars (2)

• more massive than today’s stars, as gravity has to overcome pressure for formation

• there was no CO in the early universe, meaning the clouds were much warmer

Trapping of Thermal Energy (3)

• contraction packs molecules and dust closer together, therefore making it harder for infrared and radio photons to escape the cloud

• this leads to a buildup of thermal pressure

• therefore, contraction slows down, and a protostar can form

Protostar

early stage of a star

Protostar Growth

matter from the surrounding cloud continues to fall into the protostar until it’s blown away

Nebula Theory of solar system (2)

• says the solar system formed from a nebula that collapsed from its own gravity, forming a disk

• illustrates the importance of rotation

Conservation of angular momentum 

cloud contraction → increased rotation speed

(smaller radius = higher velocity)

Flattening (2)

• collisions of particles within a cloud flattens it into a disks

• also reduces up and down motions

Jets

jets of matter shooting out along the rotation axis of a star, caused by rotation

protostar to main sequence (3)

• once the surrounding gas is blown away, the thermal energy in the protostar comes from gravitational contraction rather than fusion

• this contraction will continue until the core is hot enough to begin nuclear fusion

• the protostar becomes a main sequence when equilibrium between nuclear fusion and surface radiation is reached, stopping contraction
  

Life Track (2)

• determined by mass

• illustrates surface temp and luminosity at different stages

Convective Contraction (2)

• process in which energy is mainly transported through convection

• during this stage, the star shrinks, luminosity decreases, but surface temp remains constant

Radiative Contraction (2)

• thermal energy is released as radiation into space

• in this stage, luminosity remains constant

self-sustaining fusion

• core temps continue to rise until the star can maintain fusion, becoming a main-sequence star

Life-Tracks for different masses

higher-mass → faster formation

lower-mass → slower formation

Degeneracy Pressure (3)

• states particles cannot occupy the same space at the same time

• prevents stars from collapsing under their own pressure

• two types: electron and neutron

Thermal Pressure (2)

• main form of pressure in most stars

• caused by hot gas in plasma in the core from fusion

Brown Dwarfs (3)

• not massive enough to begin fusion

• emits infrared light due to leftover heat from contraction

• luminosity decreases over time as thermal energy is lost

Radiation Pressure (2)

• exerted by photons generated from fusion

• in very massive stars, this pressure can dominate gas pressure, influencing evolution and driving supernovas

Mass limits (3)

• mass is limited by radiation pressure

• upper limit - 150M sun, would blow apart

• lower limit - 0.08, less cannot sustain fusion

star demographic

• star cluster observations show that low-mass stars are more common than high-mass

Electron Degeneracy pressure (3)

• from density of electrons in core

• stops lower mass stars from collapsing, instead, they become white dwarfs supported by this pressure

• insufficient in stars reaching the Chandraskhar limit (1.44 SM)

neutron degeneracy pressure (3)

• in cores of massive stars after a supernova

• supports neutron stars from collapse

• in the most massive stars, this can be overcome, leading to a black hole