ISP205 Lecture #24, April 5, 2001
- Announcements:
- New Homework Assignment: Set 8
Due date extended: April 17, 2001
- Previous assignment: Set 7
Still due: April 10, 2001
- Review: Life of stars
also on HR diagram
- Star Clusters:
- Types of star clusters:
- Globular clusters (picture)
- Very old: Formed before disk of the Galaxy
- 100,000 - million stars
- Open clusters (picture)
- Younger
- Located in galactic disk
- 50-100 stars
- OB Associations (picture)
- Very young, massive O,B stars in
starforming region
- 100-10000 stars
- Clusters are a sample of different stars born at
the same time
- HR diagrams of clusters with different ages
(transparency)
- Summary: Lifecycle of stars and Nucleosynthesis
(transparency)
- Neutron Star:
- Size: ~6 miles
- Mass: 1.4 - 3 (?) solar masses
- Very dense: 1 Tsp = 700 Million tons !
- New born neutron stars are pulsars
- Discovered by Jocelyn Bell as grad student,
Cambridge
1967 (her advisor, Anthony Hewish got the Nobel Prize)
(picture)
- Rapidly rotating neutron star (periods of
ms to seconds)
that emits a beam of radio radiation (picture)
(lighthouse effect - DEMO)
- Crab pulsar in supernova remnant (pictures)
powers the nebulas emission and slows down !
Crab: 33ms, AD1054 SN (Chinese astronomers in Taurus)
(6500 Ly away, spin down 0.01ms/year)
- Pulsars live for ~10 Million years until
too slow to
create radio radiation
(1350 known today)
- Neutron stars usually get a kick during the
supernova explosion
and move away from the remnant
- Reviving neutron stars and white dwarfs:
- Mass transfer (transparency)
Neutron stars and white dwarfs in binary systems orbit another star.
They can suck matter from the companion and shine again !
- Novae (transparency)
- White dwarfs accumulate material from the
companion
for 20 - 100,000 years
- It explodes within days as a Nova
- Burned material is ejected and white dwarf
stars
again to accumulate material - goto 1.
- Type Ia Supernova
- White dwarfs the accumulate too much and become
heavier than 1.4 solar masses explode in a supernova
powered by the fusion of carbon and oxygen
- The star is completely disrupted
- X-ray bursters and ms pulsars
- Neutron stars accumulate material from
companion
for hours - days
- Frequent explosions seen as X-ray bursts
- The accumulation (accretion) spins the neutron
star up
and it can become a pulsar again
- Pulsar periods as short as a few milliseconds
are possible
- Relativity
- The speed of light in vacuum is the highest possible
speed
(example)
This is a well established fact recognized everyday in
particle accelerators.
- The speed of light is constant (example)
- Consequences: times and lengths of events differ
depending
on the motion of the observer (Relativity)
- This is the basis of "Special Relativity"
- Equivalence principle (example)
- Consequence 1: Gravity bends space
- Consequence 2: Gravity makes time run slower
- Together this means: Gravity bends space time
- This is the basis of "General Relativity"
- Observational evidence for general relativity
- Orbit of Mercury - Perihelion moves additional 43
arcsec per century
(on top of 531 arcsec from the gravity pull of other planets)
picture
- Starlight bend by sun during eclipse (1.75 arcsec)
(picture)
- Clocks run slower on top floor than in basement
- demonstrated by Robert Pound and Glenn Rebka
1959
at the Harvard physics building using an atomic clock.
- Clocks on rocket confirm general relativity to
0.02%
- Gravitational redshift
- Flightpaths of spacecraft (need relativity to
calculate them)
- Black Holes
- Iron core more massive than ~ 3 solar masses collapses
into point
(nothing can stop collapse)
- Event horizon: an imaginary sphere around the
black hole
- Defines the "Point of no return"
Nothing inside the event horizon can escape the black hole
because space is bent backwards (that's why its black)
- Radius of event horizon ~3 km for 1 solar mass
black hole
- Black holes have the same gravitational attraction than
any
other object with the same mass (at the same distance from
center)
- Properties of black holes: Mass, Spin, Charge
Nothing else !
- Falling into a black hole - as seen from outside
- Falling into a black hole - as seen from inside
- Observational evidence for black holes
- So far not much "direct" evidence
(event horizon)
- Black hole X-ray binaries
Compact objects with masses larger than 3 solar masses
have been found as part of a binary system (2 stars orbiting
each other)
As they cannot be neutron stars, the only thing we know they
can be are black holes
- Supermassive black holes in the center of the
galaxy
(Millions of solar masses)