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

Orion

Canis Minor

Canis Major

and

Canis Major

Sirius

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

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

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