Different kinds of Stars

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Different kinds of Stars
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In an Hertzsprung-Russell Diagram (or H-R diagram
or color-magnitude diagram) each star is plotted
as a point.
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The horizontal axis of the H-R diagram can be any
of several measures of surface temperture

• The surface temperature itself, often called the
  effective temperature or Teff
• The color
• The spectral class

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Spectral Class A classification based on the
taxonomy of a stars spectrum Now known to be a
sequence of surface temperture

• hot O, B, A, F, G, K, M cool
• Often split into subclasses 09, e.g., G2
• And further qualified with a roman numeral giving
  the luminosity or size, e.g., G2V (V dwarfs,
  III giants, I supergiants)
• The Sun is a G2V star
• A few other examples
• O B stars are massive and shortlived
• M dwarfs are the common low mass stars

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The stars do not lie in random locations in the
H-R diagram but lie on distinct sequences.
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• We find different types of star because
• We find stars in different evolutionary stages
• Stars have different initial masses
• While one might expect that different chemical
  composition is important, most stars have a
  composition much like that of the sun H ? 70,
  He ? 28
• The evolutionary phases after core H-burning are
  short compared to the core-H burning phase. Hence
  these stars are relatively rare.

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• At least as for the most important difference
  between stars is the initial mass.
• As M increases self gravity increases
• to maintain the balance with gravity as M
  increases the pressure P must increase.
• the increase in P requires an increase in the
  interior temperature T
• the increased T leads to increased nuclear
  burning rate and increased luminosity L
• Indeed L increases rapidly with M .

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Mass-Luminosity Relation
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Core H-burning stars of differing mass define the
main-sequence in the H-R diagram.
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More massive stars have shorter lifetimes
lifetime ?
?
Consider Sirius M 2M?
L 25L?
M? L?
lifetimeSirius ?
0.1
0.1 lifetime?
1 Gyr
? There is no intelligent life at Sirius
Indeed, no star with M 1.2 M? is a good SETI
candidate
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But
Massive stars are still important for SETI
because
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For stars more massive than 10M?, carbon and
eventually several other nuclear species
ignite. The final nuclear burning stage of such a
star is sort of an onion-like structure with many
nuclear shells. The core is usually called the
iron core although it is actually composed of
mostly of 56Ni.
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56Ni is the most strongly bound nucleus at the
immense densities found in the core. (56Fe is
more stable at Earth-like densities---hence the
confusing nomenclature.) No more energy can be
obtained from nuclear fusion---the core is the
ultimate nuclear ash. The Fe core is supported
by degeneracy pressure from electrons. Basically
it is an Fe white dwarf in center of a massive
star. However it is a white dwarf which is
increasing in mass since the nuclear burning
shells are dumping more Fe on its outer parts.
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The White Dwarf Mass Limit 1930 Chandrasekhar
worked out structure of white dwarfs. He was able
to derive a relation between the radius and mass
For WDs bigger than 1.4 M? there was no stable WD
configuration. Degeneracy pressure cannot balance
gravity White dwarf or Chandrasekhar mass limit
1.4 M? .
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If a WD grows in mass, when M 1.4 M? ?
collapse
So when the mass of the Fe core exceeds 1.4 M? it
will collapse.
Ordinarily neutrons are unstable
neutron ? proton electron anti-neutrino
n ? p e- ?
However if the pressure is high enough the
reaction can be forced to reverse. p
e- ? n ?
Don't worry about the neutrini and anti-neutrini.
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When the pressure in the WD is big enough the e-
start combining with the n in the Fe nuclei

• But the pressure holding up the WD is supplied by
  the e-,
• so when the number of e- decreases pressure
  starts to drop
• so gravity is bigger than
• contraction
• Fe core adjusts to get higher pressure
• p e- ? n ? goes even faster
• this cycles back to the beginning and we have a
  runaway.
• In about 1 second the Fe core collapses almost to
  nothing.

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When it has a radius of about 10 km the collapse
is stopped by a repulsive component of the
nuclear force degeneracy pressure from
neutrons The core bounces the exterior of the
onion which is following the core inward hits the
bouncing core and there is a huge explosion. The
exterior with all of the nuclear ashes is
blasted back into the interstellar medium. Most
of the common elements from carbon up to iron are
made in exploding stars like this. The remnant
core is a ball of neutrons called a neutron
star. These exploding massive stars are observed
as supernovae (SN) (In particular, the scenario
described is a Type II SN)
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Tychos SN
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SN1054
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SN1972 NGC5253
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SN1987a
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Vela SNR
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Part of Veil SNR
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• Ejecta of Type II Supernovae are enriched in
  heavy elemants C, O, Ne, Mg, Si, S, Ar, Ca .
• Other kinds of SN and red giants produce other
  elements.
• Almost all elements and isotopes heavier than He
  are made in stars.
• Which elements are abundant in the galaxy now is
  determined by nuclear physics
• Element building (nucleosynthesis) was very
  active in the early galaxy (Age

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• Conclusions---stars
• During the time life has evolved on Earth the
  Sun's luminosity has increased 2030.
• Stars bigger than 1.2 M? do not live long
  enough for intelligent life to evolve.
• Since we tend to see high luminosity, hence high
  M stars, most familiar naked eye stars are not
  good ETI candidates.

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• The elements of which we are made were produced
  in earlier generations of stars. Which elements
  are common is determined by the laws of nuclear
  physics.
• We are made (mostly) of the most common elements
  and such elements are common throughout the
  Galaxy (and Universe)
• First generation stars are deficient in the
  elements (C, O. Si, out of which living things
  are made. (Such stars are fairly rare.)
• The heavy elements built up very rapidly in the
  early Galaxy.There have been adequate heavy
  elements to make planets and critters throughout
  most of the life of the Galaxy. (RememberAge of
  Galaxy 15 Gyr Age of Solar System 5 Gyr)
• The other terms in the Drake Equation also affect
  the value we adopt for R?. We will estimate a new
  value later.