Astronomy Final Review
There’s 5 math questions
Since the last test
The sun: big ball of plasma
o Hydrogen and helium
o Discovered helium by looking at sun
o Helium has less mass then proton and that is what gives the mass defect
o Sun: struggle between gravity holding it down and pressure holding it up
o Social thermostat: the sun will adjust its own temperature
A very balanced thing
Sun structure:
o Core, outer layer
o Core: where we find the fusion, the driving forces of the sun
o Surface: all the stuff that we see, sunspots, solar wheather, phases of the
sun spots (11 years) constant outpouring of articles, solar winds
o Sun is in it’s mid life right now
o The sun will die in 5 billion years
The sun: life and death of the sun
o The sun lives midway in the hr diagram
o Sun will die from inert helium in core
This is when we get red giant 10 million kelvin
Eventually core gets so hot, it becomes helium burning 100 million
kelvin. And it will still have a shell of hydrogen around it. At this
point, it won’t be a red giant anymore, this is the phase where the
sun starts to look “normal” again
Then it will be inert carbon. It will get really hot. And the outer
layer starts blowing away by solar wind. It beomes planet nebula
earth will be impacted by death of sun
high mass star will go through different phases
o it will get hot enough to burn the carbon
o goes through many layers, and phases
o this is only for high mass stars – 25x the mass of the sun
o will make onion like structure that will burn faster and faster and faster.
Iron will not burn. Because iron cannot burn to create energy.
Because nothing can be less mass per particle than iron
This is when you get a supernova. The supernova comes from high mass star
dying. And from iron not being able to burn. So what happens is when it collapses
so you get a supernova distance is a big problem in astronomy
if u measure the parallax in arcseconds
o d =1/p
you can measure how bright it looks: apparent magnitude
what we want to figure out is absolute magnitude: how bright does the star look if
it was 10 parsecs away
o use M= m +5 5log(d)
memorize the distance formula!
as for the actual magnitude system
o the smaller numbers are actually brighter
o negative numbers are very bright
o very faint stars at the very large positive numbers
absolute magnitude gets a little closer to what we really want to know… and what
we really want to know is LUMINOSITY
o LUMINOSITY: how much energy the star puts out every second
o If you know the physical property of the star:
L= 4piRsquaredxconstantxT4
lay down the stars in the HR diagram
o where everything is
• white dwarfs
• supergiants
• giants
• main sequence
o also have masses on this.
Low mass star is towards the right lower corner
Higher mass star is towards the left upper corner of the hR diagram
y axis luminosity. Log scaled
xaxis: temperature:
o hot stars are on the left. Cool stars are on the right
o the spectral system : OH BE A FINE GUY KISS ME.
The sun is a gstar
Spectral type break down how hot the star is
Spectroscopic parallax
o Living stars are on main sequence
o Parallax, is if u wait 6 month, u can see the star move a bit o Now we can do spectroscopic parallax;
How bright it looks and how bright it is
If you know the temperature, you can figure our absolute
magnitude
• And vice versa
Variable stars
o Cepheids stars are the main stars we talked about
o Lives in the instability strip
o Why does it vary:
HELIUM LAYERS
Some layers can catch heat so it grows. And then release heat. So
this causes a big change in brightness
• If you look at this, you can look at its period versus how
bright it is. Then you look at the graph and tell u what
period it is, and then you know the absolute magnitude and
what type of cepheids
Cepheids are very useful
• We just sit there and time them so we can know how far
away they are
o Half of all stars are binary
Ex: Sirius a and b
But with telescopes we can see that they sort of move around each
other
If a star comes in front o
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