July 30 , Lecture 9, AST201
Midterm. Chapter 17 not in midterm. Info about stars and evolution, focus on the
information on the PDF, read the textbook.
Stars do not live forever. They are “born” and they “die”.
3 different groups, supergiants, giants, white dwarfs.
Supergiants are slowly dying.
Stars have life spans. Mass can determine the lifetime of a star. The smaller will live
longer, the rate it diffuses hydrogen is lower. Blue and red star, red one is cooler,
less energy. Red star determines temperature, diffusion rate will be slower, not as
much material and energy. Blue stars die off much faster.
What colour of star would you expect to mainly see where new stars are being
made? Blue star.
Temperature, mass, age, luminosity (blue stars are brighter)
Stars are born in cool and relatively dense and clouds of gases and dusts. Obscure
the light that comes from these stars. Expect to see more blue stars just because
they are brighter. High luminosity they would dominate the light. Galaxies. Easily
identify bright blue stars. Dominated by blue colours. Not only blue stars are more
massive but shorter lifetime. Less amount of time to form high mass star than a low
mass star. You make them faster, initially they will dominate the region. Much easier
The regions where dark parts are, tell us where the dust is. Most of the light at the
spiral is dominated by blue light. Some pockets of red. We see red parts dotted by
Red nebulae hydrogen gas. Also, dust and blue stars.
Lots of blue stars lots of young stars
Lots of dust among the gas and stars
The interstellar medium is filled with microscopic dust particles and gas. Cold
enough to form molecules. Clouds will have molecules hydrogen, carbon monoxide.
Mainly silicone and icy cover.
The same galaxy seen in visible light showing hot gas and stars, and in far-infrared
light showing cold dust.
Infra-red longer wavelength, go over it. Dust absorbs visible light,
Where a lot of dust, there are a lot of red nebulae and blue stars.
The orion nebula: a nearby star-forming region.
In invisible light, we see mainly gas. In infrared light, we see that a very young star
cluster is embedded in the gas. In such nebulae, we tend to find lots of dark patches,
which are places where the gas and dust are concentrated enough that they block
out background light. Often these dark, dusty structures are studded with very
young stars. They are the places where stars are formed. Usually these nebulae are
rich in molecules, often quite complex ones. Molecular clouds. Barnard 68 or B68, dark patch. In visible light, there’s a dark nothing. In infrared
light, we can see right through it. We can measure how much light it blocks and
figure out its structure.
Dust blocks short wavelengths of visible light but lets red light and infrared light
through. Same as the sky. Blue light has short wavelength, cannot go through,
scattered. Long wavelength able to move through. Temperature in dust clouds:
doesn’t contain blue high energy light will make it cooler.
Gravity vs Pressure = hydrostatic equilibrium
Molecular clouds are cold (10-30K).
Cold cloud molecules move slowly no outward pressure. Lower temperature.
Cold cloud molecules move slowly no outward pressure. Gravity wins! Cloud
begins to contract.
If got a little cooler, pressure would fall, and contract, gravity wins. As it contracted,
conservation of angular momentum would cause it to spin faster, flatten. Angular
momentum as “tendency to spin”. Cloud contracts, spin faster, doesn’t go from one
big to one small, because of these laws, it contracts into a disk.
L = m x v x r
L is angular momentum
M is mass
V is rotation speed
R is the radius
How fast th