Download White Dwarfs and the age of the Universe

White Dwarfs and the age of the Universe
The Ring Nebula
The collapsing
core becomes
a White Dwarf
The Hourglass Nebula
Cat’s Eye Nebula
Twin Jet Nebula
How old are these baked potatoes?
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
baked potato physics
•  Cools off: 210 F to 70 F
•  Time required: 2 hours, 20 min
•  Size of potato: did not change
change in temperature
-140 degrees
in 140 minutes
rate of change
-1 degree per minute
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?]
change in temperature from starting point
-60 degrees
rate of change
-1 degree per minute
cooling age
60 minutes
Now you can tell me the ‘age’ of any baked potato!
White Dwarfs are the baked potatoes
of the astrophysical world:
•  No heat source (cool off)
•  Don’t change size as they cool
Canis
Minor
Orion
Canis
Major
Sirius
and
Canis
Major
1840s: Friedrich Bessell discovers odd motion of Sirius
1862: Alvan Clark discovers Sirius B
Sirius and Sirius B
How small can a star be?
X-ray image
visible light image
Hot and faint
1915: Walter Adams measures temperature and size of Sirius B
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
what factors control the settings at which your pot
of water will boil?
from which burner (all on ‘high’) do you get the most energy?
Temperature and surface area determine total energy output
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)
The Stellar Womb
Stars are born deep in dark clouds:
–  very cold (- 400° F)
–  dark because visible light
cannot penetrate
The Orion Nebula
The Trapezium Cluster
Orion
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
Stellar Gestation
•  protostar
shrinks
•  heats up
•  radiates away
the excess heat
•  gets smaller,
denser, hotter
M16
When will protostars stop
shrinking?
Hubble’s nebula
McNeil’s nebula
A Star is Born
Pleiades
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)
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
WD Cooling Curve
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
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
Ages of WDs in NGC 6397: 11.5 billion years
Ages of WDs in M4: 12.1 billion years
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