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 ~ 10 7 K, for nuclei to overcome Coulomb force & fuse. Interior of the star is so hot it is plasma.

BE of He higher than BE 4 H. H isotopes. He - 4.

Excess E is carried away by  photons & neutrinos. 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 10 9 kg/s. At what rate does the sun emit energy? Assuming the mass is converted to E. E = mc 2. (4.26 x 10 9 kg/s)(3 x 10 8 m/s) x J each second.

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 W. Star Power

Luminosity (W) depends on: -Surface Area - Temperature -Which equation relates power to A & T?

-L – Watts J/s -A surface A m 2 -T Kelvin  = 5.67 x W/ m 2 K 4. -L =  AT 4. -L =  4  r 2 T 4. Stars are regarded as black bodies

Apparent Brightness (b): how bright stars appear. What we see from Earth depends on L & distance from Earth

Def. Apparent brightness radiation from star that is incident on the Earth per m 2.

Calculation of Apparent Brightness (b): L = luminosity in W d = distance to Earth m b = apparent brightness W/m 2. Intensity

Ex 2: The apparent brightness of a star is 6.4 x 10 8 W/m 2. If its distance to Earth is 50 LY, find its luminosity.

b4  d 2 = L (6.4 x 10 8 W/m 2 ) (4  )(4.73 x m) x W d = (9.46 x m/LY)(50 LY) = 4.73 x m

Finding Star Temperature Remember Black Bodies?

Wein’s Displacement Law relates peak & surface temp for black body. Star’s spectra similar to black body. T in Kelvin in meters

as T inc. Tot intensity increase for all  Peak changes to shorter higher f.

Ex 3: A star has a surface temp of K and L = 6.1 x W. a. What is the peak ? b. Find its radius.

Use Stephen Boltzmann to find R.

Solar Spectrum Some radiation  absorbed by outer layers. Can identify elements in outer layers. If H is present, H will absorb = to dif between Bohr orbit levels. Form black lines.

Motion & Speed of Stars Doppler Effect/Red or Blue shift gives info. Absorption lines shift toward longer or shorter, 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 (color)

Ex 4: Our sun has T = 6000 K and L = 3.9 x W. If star Z has T = 4000 K, &L = 5.2 x W would expect: It to be larger or smaller to our sun? Calculate its radius in terms of our sun’s radius. Larger 26 x R sun.

Early Star Classification Spectral Class Color Temperature Composition.

Sun

Stellar spectra qbCkhttp:// qbCk From 4:48

Star Types

Types of Stars Single – not bound to another. Sun. Binary – 2 stars appear close. Most bound together by grav. Cepheid – varies in brightness on regular cycle of days – changing size. Red Giant – Old star. H burning is over. Low surface T. High L, lg area. Supergiant – very heavy star fuses elements beyond carbon. White dwarf – solar mass but planetary size no more fusion.

Binary Stars – Optical binary – appear together but not physically near each other.

Visual binaries orbit together around center of mass. Can be distinguished visually. Mass can be determined from period of revolution & separation.

Eclipsing Binary – Cannot see separate stars but 1 passes in front of the other so observed brightness varies with regular period.

Animation of eclipsing binary mfjIhttp:// mfjI

Why is there a larger dip in intensity for 1 position? Brighter/hotter star blocked bigger dip in light curve.

Spectroscopic Binary too close to distinguish eclipse but can see doppler shift

Red & Blue Shifted w/motion

Binary Star Types 4 min. kxBiIhttp:// kxBiI

Star Classification

Spectral Classes. Stars characterized by temperature, absorption lines & color. 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 MS Low Mass MS High Mass Small, Hot Cool, Large Cool, Super-Large Fast Burners Long Lives

HR Diagram start at 1:24

Black body radiation 12 min AI_9mk&autoplay=1&app=desktophttps:// AI_9mk&autoplay=1&app=desktop