PES 106        Spring 2003 Previous Lecture General Astronomy II Next Lecture

Lecture Notes:

Final States of Stars: White Dwarfs and Neutron Stars

text: Chapter 14 - Sections 1 and 2

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White Dwarfs:

exposed cores of low mass stars - no nuclear reactions any more

• size:
  • small - about Earth sized
• mass
  • less than 1.4 x mass of Sun
  • rest of mass was lost in earlier stages of life
• density
  • high - (1 tsp = 5.5 tons!)
• temperature
  • initially hot (25,000 K)
  • slowly cool
  • very uniform temperature throughout
• luminosity
  • low (small size) - (<0.01 x Sun)
  • get dimmer as they get older
  •  
• magnetic field
  • strong
  • observe from splitting of spectral lines

Very different internal structure:

white dwarf is a "degenerate gas"

pressure and temperature are not connected

Balance is no longer between gravity and hot gas pressure

Now balance of gravity and degenerate gas pressure

Increasing the mass will cause the star to shrink

stars with more than 1.4 x Mass of Sun will collapse

White Dwarfs in Binary star systems

If white dwarf and red giant are in a binary system

white dwarf can capture gas from expanding red giant (mainly hydrogen)

• small mass gain
  • nuclear reactions suddenly resume
  • star becomes very bright
    • 1 million x Sun
  • "nova"
  • Figure 14.4 shows this.
• large mass gain
  • gravity strengthened by mass gain
  • star collapses and explodes
  • "supernova"
  • star is destroyed

  Two types of supernova:

• Type I
  • from exploding white dwarfs
  • observe very little hydrogen in spectrum
  • animation (requires FLASH plug-in) showing the formation of Type I supernovas: http://www.pbs.org/wgbh/nova/universe/super1a.html
• Type II
  • from exploding high mass supergiants (as discussed in last chapter)
  • observe hydrogen in spectrum
  • animation (requires FLASH plug-in) of a type II supernova: http://www.pbs.org/wgbh/nova/universe/super2.html

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Neutron Stars:

remnant of high mass stars after a supernova

• size:
  • very small - about 20 km across
• mass
  • 1.5 - 3 x Mass of Sun
  • rest of mass was lost in earlier stages of life
• density
  • very high - (one tsp = 100 million tons!!)
• temperature
  • very hot - several million degrees
  • cools slowly
• luminosity
  • very low (very small size)
• magnetic field
  • very strong

fast rotation

if original star rotated about once per month (like Sun)

neutron star should rotate once in less than a second (Conservation of Angular Momentum)

Conservation of Angular Momentum of rotating systems

depends on

1. mass of rotating system ( more mass => more angular momentum)
2. distribution of mass in system (more distant from center => more angular momentum)
3. speed of rotation (faster => more angular momentum)

Angular momentum is conserved.

=> total amount of angular momentum in a system does not change.

optional: Picture this as a box of angular momentum divided into three parts. The amount of each part can change but the total size of the box never changes. This means that if one part changes, the other MUST also change in order to keep the total amount constant.  If one part changes, the other parts must change to make up for it.

Often, the total mass of the system is constant. Then we just have a trade-off between the distribution of the mass and the speed of rotation.

As the mass moves away from the center, the speed must slow down.

As the mass moves toward the center, the speed of rotation must increase.

This is why a figure skater can control the speed of a spin by moving his/her arms in close to the body or stretching them out.

[Demo: rotating platform, weights, bicycle wheel]

Here is a link to a description of the demonstration: http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/D1/

Pulsars

stars which appear to blink on and off regularly (10 - 1000 x per second)

rotating (not pulsating) neutron stars with beams of radiation (act like a lighthouse)

Figures 14.7 and 14.9 show these concepts.

beam is from charged particles which radiate near the magnetic poles where the field lines converge

strong in radio part of spectrum

If Earth is in the path of the beam, we see the pulsar

If not, we see nothing (neutron star is too low in luminosity)

period gradually gets longer as pulsar loses energy

Internal structure

Three layers:

• very thin "atmosphere" - about 1 mm thick
• thin solid crust - a few hundred meters thick
• core of neutrons
  • "superfluid" conditions
  • no friction as neutrons flow

Neutron stars in binary systems

not easy - companion must survive a nearby supernova !

when companion expands to giant or supergiant, neutron star "steals" gas from outer layers

Results:

• Mass could orbit the neutron star in an "accretion disk"
  • 
• Mass could fall into star at angle which causes pulsar spin to get faster (transfer angular momentum)
• Mass could fall into magnetic poles creating X-ray beams (rather than radio)
• Mass could create hot spots on surface
  • nuclear reactions start again locally
  • produce x-ray bursts
  • then cool again

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