Exam #3 Study Guide
Astronomy 101
Luminosity and Brightness
• Luminosity of a star:
– total energy
gy output
p
– independent of distance
• Apparent brightness of a star:
– depends on the distance by the inversesquare law of brightness.
– measured quantity from photometry.
ASTR111 Lecture 9
Stellar Masses and Radii
• Types of binary stars
– Visual
– Spectroscopic
– Eclipsing
• Only way to measure stellar masses:
– Only roughly a few hundred stars
• Radii are measured for
f few
f
stars.
ASTR111 Lecture 9
Stellar Colors and Spectra
• Color of a star depends on its
Temperature
– Red Stars are Cooler
– Blue Stars are Hotter
• Spectral Classification
– Classify stars by their spectral lines
– Spectral differences mostly due to
Temperature
• S
Spectral
t l Sequence
S
(T
(Temperature
t
Sequence)
• O B A F G K M L T
ASTR111 Lecture 9
HR Diagram and ML Relation
• The Hertzsprung-Russell (H-R) Diagram
– Plot of Luminosity
y vs. Temperature
p
for stars.
• Features:
– Main Sequence
q
((most stars))
– Giant & Supergiant Branches
– White Dwarfs
• Luminosity Classification
Mass-Luminosity
Luminosity Relationship
• Mass
ASTR111 Lecture 9
Physics of Stars
• Observational Clues to Stellar Structure:
–H
H-R
R Diagram
– Mass-Luminosity Relationship
– The Main Sequence is a sequence of Mass
ASTR111 Lecture 9
Main Sequences stars
• Main Sequence stars burn H into He in
their cores.
• The Main Sequence is a Mass
Sequence.
– Lower M-S: p-p chain, radiative cores &
convective envelopes
– Upper M-S: CNO cycle, convective cores &
radiative envelopes
• Larger Mass = Shorter Lifetime
ASTR111 Lecture 9
ASTR111 Lecture 9
Energy Sources
•Stage:
•Energy Source:
•Main Sequence
•Red Giant
•Horizontal Branch
•Asymptotic
y p
Giant
•White Dwarf
•H Burning Core
•H Burning Shell
•He Core + H Shell
•He Shell + H Shell
•None!
ASTR111 Lecture 9
End Point of Evolution
• End of the Life of a Massive Star:
– Burn H through Si in successive cores
– Finally build a massive Iron core
• Iron core collapse & core bounce
• Supernova explosion:
– Explosive
E l i envelope
l
ejection
j ti
– Main sources of heavy elements
ASTR111 Lecture 9
ASTR111 Lecture 9
End Points of Evolution
• White Dwarf:
– Remnant of a star <8 Msun
– Held up by Electron Degeneracy Pressure
– Maximum Mass ~1.4 Msun
• Neutron Star:
– Remnant of a star < 18 Msun
– Held up by Neutron Degeneracy Pressure
– Pulsar = rapidly spinning neutron star
ASTR111 Lecture 9
Clusters
• H-R Diagrams of Star Clusters
• Ages from the Main
Main-Sequence
Sequence Turn-off
Turn off
• Open Clusters
– Young clusters of few 1000 stars
– Blue Main-Sequence stars & few giants
• Globular Clusters
– Old clusters of a few 100,000 stars
– No blue Main-Sequence
Main Sequence stars & many
giants
ASTR111 Lecture 9
Milky Way
• The Milky Way is our Galaxy
– Diffuse band of light crossing the sky
– Galileo: Milky Way consists of many faint
stars
• The nature of the Milky Way
– Philosophical speculations: Wright & Kant
– Star counts: Herschels & Kapteyn
– Globular cluster distribution: Shapley
ASTR111 Lecture 9
Stellar Populations in Milky Way
• Disk & spheroid structure of the Galaxy
• Pop I Stars:
– Young, metal-rich, disk stars
– Ordered,, nearlyy circular orbits in the disk
• Pop II Stars:
– Old,
Old metal
metal-poor
poor, spheroid stars
– Disordered, elliptical orbits in all directions
• Gives clues to the formation of the Galaxy
ASTR111 Lecture 9