white dwarfs and the age of the universe · -140 degrees in 140 minutes rate of change -1 degree...
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White Dwarfs and the age of the Universe
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The Ring Nebula
The collapsing core becomes a White Dwarf
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The Hourglass Nebula
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Cat’s Eye Nebula
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Twin Jet Nebula
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How old are these baked potatoes?
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how to make a baked potato
• rub with oil; sprinkle with salt and pepper • bake at 450 F for 1 hour • done when internal temperature = 210 F
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baked potato physics
• Cools off: 210 F to 70 F • Time required: 2 hours, 20 min • Size of potato: did not change
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change in temperature -140 degrees
in 140 minutes
rate of change -1 degree per minute
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Another baked potato rate of change: -1 degree per minute
Starting temperature: 210 F Current temperature: 150 F
How ‘old’ is it? [how long since in oven?]
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change in temperature from starting point -60 degrees
rate of change -1 degree per minute
cooling age 60 minutes
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Now you can tell me the ‘age’ of any baked potato!
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White Dwarfs are the baked potatoes of the astrophysical world:
• No heat source (cool off) • Don’t change size as they cool
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Orion
Canis Minor
Canis Major
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and
Canis Major
Sirius
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1840s: Friedrich Bessell discovers odd motion of Sirius 1862: Alvan Clark discovers Sirius B
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Sirius and Sirius B How small can a star be?
X-ray image visible light image
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Hot and faint
Sirius • d = 2.64 parsecs • T = 9,900 K • 23 times brighter than Sun
Sirius B • d = 2.64 parsecs • T = 25,200 K • 34 times fainter than Sun
1915: Walter Adams measures temperature and size of Sirius B
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what factors control the settings at which your pot of water will boil?
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from which burner (all on ‘high’) do you get the most energy?
Temperature and surface area determine total energy output
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What is the size of Sirius B? we know temperature
we know total energy output we can figure out size
• four times hotter than the Sun (“hot”)
• 34 times fainter than the Sun (“faint”)
• 100 times smaller than the Sun (“small”)
diameter of Sirius B = 99% of the diameter of Earth (Sirius is 1.7 times bigger in diameter than Sun)
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The Stellar Womb Stars are born deep in dark clouds:
– very cold (- 400° F) – dark because visible light
cannot penetrate
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The Orion Nebula
The Trapezium Cluster
Orion
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the role of gravity • mass attracts other mass • all parts of star gravitationally attract all
other parts of star • result: all objects squeeze themselves
how does force of gravity change as star squeezes itself?
INCREASES SMALLER INCREASES
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Stellar Gestation
• protostar shrinks
• heats up
• radiates away the excess heat
• gets smaller, denser, hotter
M16
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When will protostars stop shrinking?
Hubble’s nebula
McNeil’s nebula
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A Star is Born
Pleiades
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the birth of white dwarfs
• they start off as hot (150,000 K), exposed cores of planetary nebulae
• they generate no new energy
• they cannot shrink (size determined by degeneracy pressure and mass/gravity)
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the fate of white dwarfs • WD is hot
• Emits light from surface
• surface is small, cools off slowly
• cooling rate depends only on temperature (since size can’t change)
• as WD cools off, rate of cooling goes down dramatically
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WD Cooling Curve
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properties of the oldest WDs
• the coolest and faintest ones are the oldest
• if we know the temperature and luminosity of a white dwarf, we know its size
• if we know temperature, luminosity, and size now, we know the rate at which it is cooling today
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how old are the oldest WDs?
• from the above we can ‘run the movie backwards’ and calculate how warm the white dwarf was last year and the year before and 1,000 years ago and 1 million years ago ...
• from these calculations, we can determine how long it has been a white dwarf
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Ages of WDs in NGC 6397: 11.5 billion years
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Ages of WDs in M4: 12.1 billion years
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could we have missed fainter WDs?
• newest, biggest telescopes can detect stars as faint as 70,000 times fainter than Sun
• only one has been found • WD 0346+246 • 80 light years from Sun • 30,000 – 70,000 fainter than Sun • T = 3,820 degrees (K) → age as WD = 12.7 billion years