ASTR 2020
Space Astronomy
Week 7
Stellar masses
Variability
Evolution
Supernovae
Announcements:
- Meet in Fiske on Wednesday
- Midterm #1 date change:
Friday 11 to Monday 14 March
Nuclear Reactions in Stars
On the main sequence, stars burn hydrogen (H) into
Helium (He). Consider a 30 Solar mass star. Which type
of nuclear reaction dominates energy generation?
a) P-P chain
b) Deuterium Burning
c) CNO-cycle
d) Neutron decay
Nuclear Reactions in Stars
Which fundamental force is most responsible for
converting protons into neutrons in the core of a star
which burns hydrogen (H) into Helium (He)?
a) gravity
b) electromagnetism
c) weak nuclear force
d) strong nuclear force
~1 Solar mass stars: Post main-sequence evolution
MS = main sequence
H => He
RG = red giant
He => C
HB = horizontal branch
C => O
AG = asymptotic giant branch =>
planetary nebula (PN) => White dwarf
Masses of Stars:
Not easy to measure!
Find stars in a binary system
Determine the distance (parallax)
Measure the Orbit radius (from distance and
apparent separation of stars on the sky)
Measure apparent orbit velocity
(Doppler shift of spectral lines, or
“proper motion” of stars on images taken years apart)
Apply Newton’s laws of gravity and orbit speed
M = Rorbit V2orbit / G
A double - double star
Epsilon Lyra
Evolution of
A Star:
• Birth
• Main-sequence
life
- fusion: H=>He
 ~ 0.007 Mc2
• Death
> 60 M:
black hole
8 – 60 M:
neutron star
< 8 M:
white dwarf
The Sun ….
Massive
(10 - 100 Mo)
short-lived
< 107 yrs
(10 Myr)
Main Sequence
(~90% of stars)
Low Mass,
(1 - 0.1 Mo)
long-lived
>1010 yrs
(10 Gyr)
Variable Stars:
Temperature and radius oscillate !
As radius grows, photosphere cools
“ “ shrinks,
“
heats
RR Lyra stars (periodic)
Low to moderate luminosity ~ 1 to 1,000 Lo
Period of a few days
Change in magnitude ~ 0.1 to few magnitudes
Cepheid variables (periodic)
High Luminosity (103 to 104 Lo)
Periods of days to month
Change in magnitude ~ 0.1 to few magnitudes
Mira variable (periodic)
High Luminosity (103 to 104 Lo)
Periods of months to years
Change in magnitude ~ 1 to 10 magnitudes!
T-Tauri stars (irregular)
Low mass (0.2 to 2 Mo), young (< 10 Myr) stars
Change in magnitude ~ 0.1 to 2 magnitudes
Period-Luminosity
Relation:
 Get L from Period P
 Measure Flux: F = L / 4D2
 D ~ (L / 4pF)1/2
8 Solar mass stars: Post main-sequence evolution
Shell burning. Red supergiant (RSG), blue supergiant (BSG),
Wolf-Rayet (WR) star,
luminous blue variable (LBV)=>
Type II Supernova
ASTR 2020
Space Astronomy
Week 7
Stellar masses
Variability
Evolution
Supernovae
Announcements:
- Midterm #1 date change:
Friday 14 to Monday 14 March
Supernovae:
- Type I: No hydrogen,
Type 1a: Nuclear detonation of white dwarf
accreting from a binary companion
Old stellar populations
Thermonuclear detonation of a 1.4 Mo
Carbon-Oxygen white dwarf.
Type 1 b/c: Core collapse of a massive (> 8 Mo)star
which has lost its hydrogen envelope
in a stellar wind, exposing a Helium,
Carbon, or Oxygen-rich core.
Young populations
Role of (compact) Binary stars in
Stellar Evolution
Type 1a:
Nuclear detonation of
A 1.4 Mo white dwarf
accreting from a
binary companion
Burn 1.4 Mo of He, C
to iron. E~ 0.001 Mc2
~ 1051 erg
“standard candle”
Supernovae:
- Type II:
Hydrogen dominated
Core collapse of a massive (> 8 Mo)star
still dominated by a hydrogen envelope
Type IIn:
Narrow-line. Blast interacts with shells
shed during RSG, BSG, LBV, or WR phases
Type IIs leave behind neutron
pulsars
stars or black holes
gamma ray bursts
Supernova 1987a in the LMC:
Which Space Telescope?
The shock-wave from a supernova explosion generated plasma
with a temperature of about 106 Kelvin. Which of the following
space-craft are best suited for its study.
a) Chandra X-Ray Observatory (CXO)
b) Hubble Space Telescope (HST)
c) Spitzer Space Observatory (Spitzer)
d) Planck
Crab Nebula (M1) AN 1054:
X-ray
Visual
Pulsar: rotating, r ~ 10 km neutron star
Spin period ~ 0.001 to 10 sec,
Magnetic field: B ~ 1010 - 1015 gauss
Pulsar Audio files:
http://www.naic.edu/~pfreire/47Tuc/
http://www.naic.edu/~pfreire/47Tuc/
http://www.jb.man.ac.uk/~pulsar/Education/Sounds/47tuc2-8000.wav
10 Gyr old Globular Cluster (M13 in Hercules)
23
Millisecond
pulsars
in
globular
cluster
47 Tucanae
Cass A
SNR (CXO)
Simeis 147 A
H