ISP205 Lecture #23, April 3, 2001
- Announcements:
- New Homework Assignment: Set 8
Due: April 10, 2001
- Previous assignment: Set 7
Due: April 10, 2001
- Review: Extrasolar Planets
- Observations of planetary disks around newly formed
stars
(picture)
- Disks have "doughnut" shape for older proto
stars
Possible explanation: formation of a giant planet took place
(picture)
- Detection of extrasolar planets: detect orbital motion
of the star !
- Method 1: Detect "wiggle" directly fom
changes in stars position
- Method 2: Detect motion with doppler effect
(example for planet discoveries)
- First extrasolar planet discovered by M. Mayor and D.
Queloz 1995 orbiting 51 Pegasi (40 Ly away)
- So far: ~30 planets found around other stars
- Surprise: Hot Jupiters
Giant planets very close to the star (most easy to detect !)
Maybe they formed further away and moved then closer
(breaking due to dust)
- Review: Protostars on the HR diagram (picture
20.12)
- ZAMS: Zero Age Main Sequence: the position in the
HR diagram
where a newborn star starts fusing hydrogen into helium and begins
its life on the main sequence.
- Brown Dwarfs:
Stars with less than 0.08 solar masses that cannot ignite hydrogen.
- Life beyond the Main Sequence
- Main sequence lifetimes:
Mass in solar masses |
Main sequence lifetime |
0.4 |
200 billion years |
0.8 |
14 billion years |
1.1 |
9 billion years |
1.7 |
2.7 billion years |
3.3 |
500 million years |
16 |
10 million years |
40 |
1 million years |
- Red Giant Phase
- Contracting helium core and hydrogen shell burning
- Hydrogen burns faster - more energy is generated
- outer layers of the star expand but cool
- star becomes bigger and redder
- Red Giants can become several AU's large
Sun: will become a red giant in 5 billion years and
will incorporate earth
Betelgeuse (picture)
- Red Giants on the HR diagram (picture)
- Star Clusters:
- Types of star clusters:
- Globular clusters (picture)
- Very old: Formed before disk of the Galaxy
- 100,000 - million stars
- Open clusters (picture)
- 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
(pictures)
- Life of a low mass star (less than 8 solar masses)
- Helium burning (picture)
(Sun: only 100 Million years)
- Carbon and Oxygen core
- Planetary nebula:
No further burning possible - CO core contracts
and the generated energy triggers a wind that
blows away the remaining outer layers in a
"Planetary nebula" (pictures)
- White dwarf: (picture, example to be seen in sky)
CO core contracts until electrons get so densely packed
that they are not allowed to move closer anymore
(Quantum mechanics - Pauli principle)
- White dwarf properties:
- Size: ~ earth
- Mass: up to 1.4 solar masses
- High density: 1 Tsp weighs 15 tons ! (truck)
- Doesn't do much except for cooling
- Life of a massive star (more than 8 solar masses)
- After helium burning it survives burning the
products of the
previous burning stage successively:
Example: 25 solar mass star, core burning stages
Stage |
Duration |
Product Name |
Product Z |
Product N |
Hydrogen burning |
7 Mio yr |
Helium |
2 |
2 |
Helium burning |
700,000 yr |
Carbon,Oxygen |
6,8 |
6,8 |
Carbon burning |
400 yr |
Oxygen, Neon |
8,10 |
8,10 |
Neon burning |
1 yr |
Oxygen, Magnesium, Silicon |
8,12,14 |
8,12,14 |
Oxygen burning |
8 month |
Silicon, Sulfur |
14,16 |
14,16 |
Silicon burning |
1 day |
~Iron, Nickel |
~24-28 |
~24-28 |
Z = number of protons in nucleus
N = number of neutrons in nucleus
- Iron is the most stable nucleus -
fusion of two iron nuclei does not generate energy
anymore
- Structure of the star after silicon burning: the
onion
(picture)
- Death: Supernova explosion
- Explosion shines brighter than a galaxy
- Example: 1987 A (pictures AAST, story)
- Supernova remnants (1987A rings, older remnants
pictures)
- How does a supernova work (Movie)
- Iron core collapses (electrons cannot hold it)
- Collapse stops when protons and neutrons get
too densely packed
(same as for electrons stabilizing the white dwarf)
- Rebounce leads to explosion (Demo)
- Neutrino wind helps
- Light generated by decay of radioactive nuclei
(light curve picture)
- Such a supernova is called type II Supernova
(lots of hydrogen lines)
- Collapsed core forms Neutron star or Black hole
- Neutron Star:
- Size: ~6 miles
- Mass: 1.4 - 3 (?) solar masses
- Very dense: 1 Tsp = 700 Million tons !
(= 7000 air craft carriers)
- New born neutron stars are pulsars
- Discovered by Jocelyn Bell as grad student,
Cambridge
1967 (her advisor, Antony Hewish got the Nobel Prize)
(picture)
- Rapidly rotating neutron star (periods ...)
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
- Black holes: next lecture
- Summary: Lifecycle of stars and Nucleosynthesis
- Reviving neutron stars and white dwarfs:
- Mass transfer (picture)
Neutron stars and white dwarfs in binary systems orbit another star.
They can suck matter from the companion and shine again !
- Novae (picture)
- 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 (picture)
- 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