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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 19: Extreme Stars
White Dwarfs & Neutron Stars

Readings: Readings: Ch 22, section 22-4; Ch 23, sections 22-1 thru 5

Key Ideas

White Dwarfs:
Remnants of low-mass stars
Supported by Electron Degeneracy Pressure
Maximum Mass ~1.4 Msun (Chandrasekhar Mass)

Neutron Stars:
Remnants of some post-supernova massive stars
Supported by Neutron Degeneracy Pressure
Pulsar = rapidly spinning magnetized neutron star

The Stellar Graveyard

Question:
What happens to the cores of dead stars?

Answer:

They continue to collapse until either:

All of these are seen as the remnants of stellar evolution.

Degenerate Gas Law

At high density, a new gas law takes over:

Result is a "Degenerate Gas":

This means that the objects could, in principle, be very cold but still have enough pressure to maintain a state of Hydrostatic Equilibrium.

Can such objects exist?


White Dwarfs

These are the remnant cores of stars with M < 8 Msun.

Properties:

No nuclear fusion or gravitational contraction. It shines by residual heat.
White Dwarf comparison to the Earth
Comparison of a White Dwarf Star and the Earth.

Chandrasekhar Mass

Mass-Radius Relation for White Dwarfs:
Larger Mass = Smaller Radius

Chadrasekhar Mass:

This prediction of a maximum white dwarf mass is upheld by observations. So far, all white dwarfs we have seen in binary stars have masses below the Chandrasekhar Mass.


Evolution of White Dwarfs

White dwarfs shine by leftover heat.

Ultimate State: A "Black Dwarf":

Note: Be careful not to confuse Black Dwarfs (old, cold remnant cores of low-mass stars) with "Black Holes" (the extremely collapsed cores of very massive stars).


White Dwarfs: The Other Supernovae

Question:
What would happen if you added enough matter to a White Dwarf to exceed its Chandrasekhar Mass?
Above the Chandrasekhar Mass, the internal electron degeneracy pressure is no longer able to balance Gravity, and it will begin to collapse... The effect is a runaway nuclear explosion:

Type Ia Supernovae leave no remnant behind, and may be responsible for the production of much of the Iron in the Universe.


Neutron Stars

Remnant cores of massive stars:

Held up by Neutron Degeneracy Pressure:

No nuclear fusion or gravitational contraction. It shines by residual heat.
Neutron Star compared to Manhattan
Comparison of a Neutron Star and the Island of Manhattan.

Structure of a Neutron Star

At densities > 2x1014 g/cc:

Surface is cooler, forming a solid crystalline crust!

Inside is exotic matter: superfluid neutrons, superconducting protons, and stranger subatomic particle matter:

Inside a Neutron Star

Schematic of the interior of a neutron star

Accidental Discovery

1967:
Jocelyn Bell (Cambridge grad student) & Anthony Hewish (her adviser) discover rapidly pulsating radio sources while looking for something else.

They called these new, otherwise unknown objects

"Pulsars" = Pulsating Radio Sources

Pulsars emitted sharp, 1 millisecond-long pulses every second at an extremely repeatable rate (most accurate clocks known at the time).

Anthony Hewish shared the 1974 Nobel Prize for Physics for his role in the discovery of pulsars. Jocelyn Bell, however, did not.


Pulsars

Rapidly spinning, magnetized neutron stars.

Lighthouse Model:

Result:

Twin beams of radiation shining out of the magnetic poles:

Schematic of a Pulsar

If the magentic axis is mis-aligned with the rotation axis of the neutron star (as shown), the star's rotation sweeps the beams over us as it rotates, and we will see regular, sharp pulses of light (optical, radio, X-ray, etc.)


Pulsar Evolution

Pulsars spin slower as they age.

Young neutron stars:

Old neutron stars: cold and hard to find.

[But some neutron stars that are not also pulsars have been finally seen. Hubble Space Telescope detection of a lone neutron star.]


Over the Top?

What if the remnant core is very massive?
"Very Massive" = Mcore > 2-3 Msun
(original star would have had a mass M > 18 Msun)

The core becomes a Black Hole.

We will meet these exotic objects in the next lecture.


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Updated: 2006 January 30
Copyright Richard W. Pogge, All Rights Reserved.