Star basics 101

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Star basics 101

• Observations
• Stars are hot and emit energy into space
• Spectroscopy tells us that stars are made of hot
  gas
• Most stars are on the main sequence (HR diagram)
• Physics
• Hot objects radiate energy into their
  surroundings
• Question
• Why are stars not simply cooling off?

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• Observations of our Sun
• Climate models, geology and the fossil record
• Suns energy output (luminosity) has been near
  constant for at least the last 3 to 4 billion
  years
• Therefore, the Sun must have tapped a good
  energy supply that has kept it hot and luminous

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• The question where does the energy of a star
  (the Sun) come from?
• Answer
• historically, many energy sources have been
  considered but we now know that stars are powered
  by nuclear fusion and the conversion of Hydrogen
  (H) into Helium (He)

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Sir Arthur Stanley Eddington (1882 - 1945)
Stars and Atoms (1920) I ask you to look both
ways. For the road to a knowledge of the stars
leads through the atom, and important knowledge
of the atom has been reached through the stars
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Nuclear fusion

• Theory
• 1938 nuclear fusion - conversion of four
  hydrogen atoms into one helium atom with the
  liberation of energy
• 4H ? He energy
• Theory built on many independent discoveries
• 1920 Francis Aston experimental result
• mass (He atom) lt mass (4 hydrogen atoms)
• mass difference 0.007 mass of 4 H atoms

Francis Aston (1877 1945)
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• Proposition
• 1926 A. Eddington suggests mass difference may
  appear as energy
• 1905 A. Einstein shows that E mc2
• Result
• Enuclear (Dm) c2

OOTETK
Energy generated per second
Mass converted into energy per second
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Some numbers

• Sun emits 4 x 1026 Watts of energy into space
• To produce Suns energy through fusion reactions,
  Einsteins equation tells us
• Dm (4 x 1026)/(3 x 108)2 4x109 kg/sec

matter converted into energy per second
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A quick calculation

• How much matter has the Sun converted in to
  energy since it formed?
• Age 4.5 billion years

Mass converted to energy by the Sun since it
formed is equivalent to
95 x
or, about 6 x 1026 kg
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• Further Result detailed numerical models confirm
• once fusion reactions begin the star has tapped
  a long-lived energy source and sits on the MS

Main sequence stars
Stars converting 4H ? He within their cores
?

• Hence Heres the reason why most stars are
• on the main sequence
• The MS phase is the longest-lived phase because
  stars
• are mostly composed of hydrogen

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Computer models reveal

• HR diagram features
• Main sequence
• Stars converting hydrogen into helium
• Giant stars
• Stars converting helium into carbon
• White dwarfs
• Cooling-off region for low mass stars that have
  exhausted helium in their cores

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Energy generation and location in HR diagram
Main sequence Hydrogen ? Helium
Bright
Red Giants
Helium ? Carbon
Planetary Nebula
Absolute magnitude
Sun
Star formation
White Dwarfs
Cooling
Dim
Spectral type
O B A F G K M
Hot
Cool
No longer generating energy in their interiors
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• Question
• how long can the Sun shine by Hydrogen fusion ?
• t (amount of fuel available)/(rate of fuel
  consumption)

t? (0.1)(0.007)M? c2 / L? (calculation
assumes 10 of Suns H is converted to He)
time during which H burning can take place in
the Sun is t? 1010 years
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All good things come to an end

• Stars have a large, but limited supply of
  hydrogen
• hence a finite main sequence life time
• tms main sequence life time
• fuel available / rate of consumption
• (constants) (M? / L?) ()

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• Recall main sequence stars obey a ML law
• L? constant M?3.5
• Combine the ML law with eqn(), then,
• Important result
• tms constant / M?2.5
• The more massive a star is, the shorter is its
  main sequence life time

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• The main sequence lifetime of a star of mass M is

?
Some example life times
tms(yrs) 6 x 1010 for M 0.5 M?
tms(yrs) 3 x 107 for M 10 M?
M 7.5 M?
Textinct(dinosaurs) 65 Myr
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See class Handout And Astronomy 202
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End gameswhere the core and the envelope part
company

• M lt 8 M?
• core ? White Dwarf star
• envelope ? Planetary Nebula (envelope material
  slowly ejected into space)
• M gt 8 M?
• core ? Neutron star (Minitial lt 40 M?)
• ? Black hole ? (Minitial gt 40 M?)
• envelope ? supernova remnant (most of star
  ejected into space at very high speed)

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The Ring Nebula (in Lyra) Composed of material
that was once the outer layers of the star
Central White Dwarf
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X-ray images
Crab SNR 1054
The Historic Supernova
LMC SNR 1987A
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To infinity and beyond

• SN are the brightest standard candles known
• Absolute magnitude M -19 (recall m? -26.75)
• Hence, with HST and mlimit 24, the 5 star
  formula shows that SN can be seen to a distance

The scale of the Universe is determined
by studying supernova in distant galaxies
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SN 1997 ck
SN 1997 cj
SN 1997 ce
D 2 x 109 pc
SN out shines Galaxy
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Cosmic recycling

• Final stellar end stages (PN and SN)
• material that has been processed by nuclear
  reactions inside of stars is returned to the
  interstellar medium
• In addition, the cosmologists tell us
• Only H and He produced in the early universe
• Hence - an incredible result
• All of the important elements (C, N, O, etc..)
  that make up the Earth and enable life as we know
  it were formed inside of stars
• We are indeed all STAR CHILDREN

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Definition Z elements (metals) all elements
other than hydrogen and helium
Big Bang
Hydrogen helium
Star formation
ISM
time
Z enhancement
End phase (PN SN)
WD, NS, BH
Result ISM becomes increasingly metal rich with
time