Transcript RMH_Stellar_Evolution_Ast2001_09_29_09

Evolution of Low Mass Stars -- like the Sun < 2 – 3 Msun
Post Main Sequence Evolution -- the Red Giant Stage
The Sun as a red giant
Second Red Giant Stage -- the Asymptotic Giant Branch (AGB)
Post AGB, Planetary Nebulae and White Dwarfs -- final stage for the Sun
The Sizes and Masses of the stars – what do the large numbers mean?
Measurement of Sizes and Masses
Direct:
Masses -- stars in binary systems
Sizes
-- binary systems
-- interferometry, measure angular size, must know distance
Indirect: -- must know distance
Luminosity – depends on surface area (size) and temperature
(Stefan-Boltzman Law)
Mass -- with luminosity + physics , mass – luminosity relation
Sun, R = 7 x 105 km , > 100 x Earth
And putting the Sun in perspective with the size of other stars
Sirius – visually brightest star,
Arcturus – a red giant, one of oldest can see
Evolution of the Most Massive Stars -- >/= 10 Msun
He-burning begins as a red
supergiant, no electron degenerate
core
He -> C,O , C,O ->heavier elements up to
Fe, as a red supergiant or successive
transits across HR diagram
The Supergiants - - largest and most luminous for their temperatures
Rigel ~ 150 x Sun,
Betelgeuse ~ 600 x Sun
Research at Minnesota on Evolved Massive Stars
The evidence for episodic high mass loss events
One of the Largest -- The Extreme Red Supergiant VY CMa
10 arcsec
Distance -- ~ 1.5 kpc
Luminosity ~ 500,000 L sun
Initial Mass ~ 30 -- 40 M sun
Temperature ~ 3500 K
Size
~ 2000 Rsun, ~ 10 A.U. (Saturn’s orbit!)
1” = 1500 AU
What does it mean to say a star is as big as Saturn’s orbit?
Earth – Sun distance
Betelgeuse 600 x Sun
150 x 106 km
= 3 A.U.
1 astronomical unit A.U.
VY CMa 2000 Rsun
= 10 A.U. = Saturn’s orbit
Complex structure in ejecta
Second epoch HST images
Prominent arcs, numerous filaments and
Measure transverse velocities combined
clumps of knots, strong maser source, mass with radial velocities (VR) long slit spectra
(Keck) using the Doppler effect
loss rate 5 x 10-4
NW Arc
Arc 2
Arc 1
S Knots
SW Knots
2nd Epoch images with HST/WFPC2
Measured the transverse motions VT - shift in x and y positions between
the two images. 66 positions -- pushed the limits of HST ~ 0.02 arcsec
Combining VT + VR --> VTot
Total velocity relative to star
Orientation
Direction of motion
Age or time since ejected
The 3D Morphology of VY CMa
Feature
NW arc
Arc 1
Vel.
Orientation
km/s relative to sky
46
22 degrees
68
-33
Arc 2
SW knots
S knots
SE loop
64
36
42
65
-17
-25
-27
-21
Direction Age (yrs)
of motion
~ west
500
SW
800
~ south
~ west
SSE
SE
460
250
157
320
Discrete structures, -- arcs, knots, filaments – ejected at different
times, from different regions on star -- localized
Large-scale convective activity
 Starspots  Magnetic Fields
Comparison with the Sun -- prominences, sunspots and coronal mass
ejections:
Solar wind
109 kg/sec
CME
VY CMa
2 x 1019 kg/sec
discrete ejecta
1010 kg/sec
~ 1020 kg/sec
The (brightest) Most Luminous and Most Massive Star known –
(in our region of the Milky Way galaxy)
Eta Carinae
-- in the southern hemisphere
h Car :
Distance
-- 2.3 kpc
Luminosity -- 5 x 106 Lsun
Initial Mass ~ 150 – 200 Msun
Temperature ~ 20,000 K
Size
~ 0.4 A.U ( Mercury’s orbit)
h Car and the Homunculus Nebula
Historical light curve
Great Eruption – 1837 – 1858
Reached 3 x 107 Lsun
Expelled ~ 20Msun, 1 Msun/yr
Total luminous energy ~ 1050 ergs