Download PHYSICS 015

White Dwarfs Exposed:
Planetary Nebulae
The ‘Ring’ Nebula
‘Planetary’ Nebulae
discovered by Herschel
Planetary Nebulae are
Shells that Look Like Rings
despite the name, they have nothing to do with planets
Soap Bubble
Shells Can
Also Look
Like Rings!
note the very
prominent edges
Some Examples
[thousands are known]
The Shell is ‘Puffed Off’Gently
In total, a considerable fraction of the star’s mass
comes off gently, at speeds of perhaps 30 km/sec.
Only part of the original stellar mass is left behind.
Note that the material in the nebula is raw star stuff –
not the products of the nuclear reactions deep
within the core! It is almost pure H + He.
The escaping material eventually merges into the
interstellar medium, and is available to be used in
other stars that may form later.
This Process Leaves Behind
Incandescently Hot Dense Cores
The Cores…
These intensely hot, luminous cores are new white
dwarf stars that will never succumb to gravity.
Thanks to their extreme heat, they emit an enormous
amount of ultraviolet light. That energetic radiation
causes the gas in the planetary nebula to glow by
fluorescence, like a celestial neon lamp.
The nebula disappears after some tens of thousands of
years as (i) the gas dissipates and (ii) the central
star cools down and becomes fainter.
Some Real Examples
Found Everywhere
Cold carbon clinkers – ‘diamonds in the
sky’
[although not like any diamond you ever encountered!
– a million times the density of water, held up by electron
degeneracy – but nevertheless full of Carbon nuclei]
There are literally tens of millions of such
stars in our galaxy.
“Like a Diamond…” (sort of!)
White Dwarfs in Plenty!
Here, in the core of the globular cluster M4
The ‘Cooling Track’
The prominent sequence to the right is the main sequence. All
the brighter hotter stars (to the upper left) have evolved away;
the bright red giants lie to the upper right of this figure, off the
plot. The star shown in yellow has been there for billions of
years, essentially unchanging.
The red spot to the left is a
recently-formed white dwarf
– still relatively hot and bright.
It will cool off and get fainter,
moving down along the cooling
track. Many other white dwarfs
have already done so.
The Sun’s Life In Summary