Transcript Astro 2 ppt - Red Hook Central Schools

Stars
Luminous gaseous celestial
body – spherical in shape held
by its own gravity
How do we study stars?
• Light!!
Stellar Radiation
• H fusion occurs in star’s interior
converting mass to E (mass
deficit).
• T must be ~ 107 K, for nuclei to
overcome Coulomb force & fuse.
• Interior of the star is so hot it is
plasma.
BE of He higher than BE 4H.
He - 4.
H isotopes.
Excess E is carried away by g photons & neutrinos
n. Some E gets absorbed in star heats interior more
& exerts outward pressure.
Stellar Equilibrium- outward P from
radiation balances gravity inward in
stable stars.
Stable Stars maintain size.
The sun is stable
Ex 1. The sun is losing mass at 4.26
x 109 kg/s. At what rate does the sun
emit energy?
•
•
•
•
Assuming the mass is converted to E.
E = mc2.
(4.26 x 109 kg/s)(3 x 108m/s)2.
3.83 x 10 26 J each second.
Star Power
Luminosity (L) = total
power output of a star W
or J/s.
As we just calculated
the sun converts
mass to Energy
Sun L = 3.9 x 1026 W.
Luminosity (W)
depends on:
- Surface Area
- Temperature
- Which equation
relates power to A & T?
Stars are regarded as
black bodies
- L = sAT4.
- L = s4pr2T4.

L – Watts J/s. L = Power
A surface A m2
T Kelvin
s = 5.67 x 10-8W/ m2 K4.
Apparent Brightness (b):
how bright stars appear.
What we see from Earth
depends on L &
distance from Earth
Apparent brightness
• radiation from star that is incident on the
Earth per m2. It’s like Intensity
Calculation of Apparent Brightness (b):
L
b
2
4pd
L = luminosity in W
d = distance to Earth m
b = apparent brightness W/m2.
Intensity
Ex 2: The apparent brightness of a star
is 6.4 x 108 W/m2. If its distance to Earth
is 50 LY, find its luminosity.
L
b
2
4pd
• d = (9.46 x 10 15 m/LY)(50 LY) = 4.73 x 1017 m
• b4pd2 = L
• (6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2.
• 1.8 x 10 45 W
Finding Star Temperature
Remember Black Bodies?
Wein’s Displacement Law relates peak l &
surface temp for black body.
3
2.9 x10
l
mK
T
T in Kelvin
l in meters
Star’s spectra similar to black body.
as T inc.
• Tot intensity increase for all l.
• Peak changes to shorter l higher f.
Ex 3: A star has a surface temp of 17 000 K and L =
6.1 x 10 29 W.
a. What is the peak l?
b. Find its radius.
3
2.9 x10
7
l
mK  1.7 x10 m
T
Use Stephen Boltzmann to find R.
L  AsT  4pr sT
4
2
4
L
9
r

3
.
2
x
10
m
4
s 4pT
Solar Spectrum
• Some radiation l absorbed by outer layers.
• Can identify elements in outer layers.
• If H is present, H will absorb l = to dif
between Bohr orbit levels. Form black lines.
Absorption of l
Motion & Speed of Stars
• Doppler Effect/Red or Blue shift gives
info.
• Absorption lines shift toward longer or
shorter l, depending on motion.
Red Shift Spectrum – stars moving
away from us show dark line shift.
• Find v, direction by shift of line
spectra.
Blue Shift – moving toward us
Amount of Shift relates to speed of
motion
List 3 observations we can
make using light to get
information about stars.
State what we can learn from
each type of observation.
Use Spectrum to find:
• Chemical composition surface
• (absorption spectrum)
• Motion toward or away from Earth
• Red/blue shift
• Surface temp
• Peak l (color)
Ex 4: Our sun has T = 6000 K and L = 3.9 x 1026 W.
If star Z has T = 4000 K, &L = 5.2 x 10 28 W, would
it be:
larger or smaller to our sun?
Calculate its radius in terms of our sun’s radius.
• Larger
• 26 x Rsun.
Early Star Classification
• Spectral Class
• Color Temperature Composition.
Sun
Stellar spectra
• http://www.youtube.com/watch?v=jjmjEDY
qbCk
• From 4:48
Star Classification
Spectral Classes.
• Stars characterized by temperature,
absorption lines & color.
• Old Classification O – M.
OBAFGKM
• Oh be a fine girl – kiss me.
• Then subdivided in 10 smaller groups
0-9.
• Sun – G2.
H-R diagram graphs temp
against luminosity – Not Linear
• Be able to identify general regions of star
types on the H-R diagram
• 90% Stars on Main sequence.
MS High
Mass
H-R Diagram
Cool,
Super-Large
Fast
Burners
Cool,
Large
Small,
Hot
MS Low
Mass
Long
Lives
http://www.youtube.com/watch?v=yX0HWr9xQ6M
HR Diagram
start at 1:24
Black body radiation 12 min
• https://www.youtube.com/watch?v=TiOpU
AI_9mk&autoplay=1&app=desktop