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 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 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.

Luminosity (L) = total power output of a star W or J/s. 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 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 = 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 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.

b4pd2 = L (6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2. 1.8 x 10 45 W 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. 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 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.

Use Stephen Boltzmann to find R.

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 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=jjmjEDYqbCk From 4:48

Star Classification

Spectral Classes. Stars characterized by temperature, absorption lines & color. Old Classification O – M.

Oh be a fine girl – kiss me. Then subdivided in 10 smaller groups 0-9. 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.

H-R Diagram Cool, Super-Large Cool, Large Small, Hot MS High Mass 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=TiOpUAI_9mk&autoplay=1&app=desktop