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.

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 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.

As we just calculated the sun converts mass

to Energy

Sun L = 3.9 x 1026 W.

Star Power

Luminosity (W) depends on:

- Surface Area- Temperature- Which equation relates

power to A & T?

- L – Watts J/s- A surface A m2

- T Kelvin = 5.67 x 10-8W/ m2 K4.

- L = AT4.- L = 4r2T4.

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 m2.

Calculation of Apparent Brightness (b):

24 d

Lb

L = luminosity in Wd = distance to Earth mb = 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.

• b4d2 = L

• (6.4 x 108 W/m2) (4)(4.73 x 1017 m)2.

24 d

Lb

• 1.8 x 10 45 W

• d = (9.46 x 10 15 m/LY)(50 LY) = 4.73 x 1017 m

Finding Star TemperatureRemember Black Bodies?

Wein’s Displacement Law relates peak & surface temp for black body.

mKT

x 3109.2

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 17 000 K and L = 6.1 x 10 29 W.

a. What is the peak ?b. Find its radius.

mxmKT

x 73

107.1109.2

mxT

Lr

TrTAL

94

424

102.34

4

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 movingaway from us show dark line shift.

• Find v, direction by shift of line spectra.

Blue Shift – moving toward usAmount 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 1026 W. If star Z has T = 4000 K, &L = 5.2 x 10 28 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 Rsun.

Early Star Classification

• Spectral Class

• Color Temperature Composition.

Sun

Stellar spectra

• http://www.youtube.com/watch?v=jjmjEDYqbCk

• 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

• http://www.youtube.com/watch?v=zoekfYomfjI

Why is there a larger dip in intensity for 1 position?

Brighter/hotter star blocked bigger dip in light curve.

Spectroscopic Binarytoo close to distinguish eclipse but

can see doppler shift

Red & Blue Shifted w/motion

Binary Star Types4 min.

• http://www.youtube.com/watch?v=1kFFwHkxBiI

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 Diagramstart at 1:24

http://www.youtube.com/watch?v=yX0HWr9xQ6M

Black body radiation 12 min

• https://www.youtube.com/watch?v=TiOpUAI_9mk&autoplay=1&app=desktop