Assignments for Thurs. 20 Feb

• Review secs. 1-5 of DocOnotes– practice Quick Questions at end of sections

• Do HW1 – due 5 p.m. Thursday• submit to my office 205 Sharp Lab• or my mailbox in 217 Sharp Lab

1

A. 10

B. 5

C. 1

D. 0

E. -52

Star 1 has an apparent brightness that is 100 times that of Star 2. What is the difference in their apparent magnitude m2-m1

Stellar magnitudes

3

Apparent magnitude:

A. 10

B. 5

C. 1

D. 0

E. -54

Two stars have the same Luminosity but star 2 is 10 times further away than star 1. What is their apparent magnitude difference m2-m1?

Stellar magnitudes

5

Apparent magnitude:

The relationship between apparent brightness (a.k.a. “flux”) and luminosity depends on distance:

Luminosity Flux = 4π (distance)2

We can determine a star’s luminosity if we can measure its distance and apparent brightness:

Luminosity = 4π(distance)2 ✕ (flux)

L = 4πd2F

F = L4πd2

A. 10

B. 5

C. 1

D. 0

E. -57

Two stars at the same distance have a luminosity ratio L2/L1= 100. What is their apparent magnitude difference m2-m1?

A. 10

B. 5

C. 1

D. 0

E. -58

Two stars have a luminosity ratio L2/L1= 100. What is their absolute magnitude difference, M2-M1?

Stellar magnitudes

9

Apparent magnitude:

Absolute magnitude:

Distance modulus:

Surface Brightness

10

11

Light• c = λν– speed = wavelength frequency

• E = hν– Photon energy = Planck constant x frequency

12

A. 1

B. 2

C. 1/2

D. 1/4

E. It depends on the amplitude of the associated light.13

Two photons have a wavelength ratio λ2/λ1= 2. What is energy ratio E2/E1?

Planck Blackbody function

14

Bυ =2hυ3 / c2

ehυ /kT −1Bλ =

2hc2 / λ5

ehc/λkT −1Bλdλ = Bυdυ

Peak wavelength

15

λ peak ; 500nmTeT

A. 500 nm

B. 250 nm

C. 1000 nm

D. 125 nm

E. It depends on the stellar radius.16

For a star with a surface temperature of 12,000 K, at about what wavelength λpeak does the spectrum have it peak flux?

Stefan-Boltzmann law

17!!

LL

⊙

=TT

⊙

⎛

⎝⎜

⎞

⎠⎟

4RR

⊙

⎛

⎝⎜

⎞

⎠⎟

2

Bλ dλ0

∞

∫ = Bυ dυ0

∞

∫ =σ sbT

4

πL

4πR2= F = πB =σ sbT

4

A. Lsun/4

B. Lsun/2

C. Lsun

A. 2 Lsun

B. 4 Lsun18

What is the luminosity of a star with a surface temperature of 12,000 K and a radius of Rsun/2?

19

20

21

UBV Color Filters

22

23

A. 0.25 Rsun

B. 0.5 Rsun

C. 1 Rsun

A. 2.5 Rsun

B. 4 Rsun 24

What is the radius of a star with parallax 0.01arcsec, peak wavelength 250 nm, and apparent magnitude m=+5?

Assignments for Thurs. 20 Feb

• Review secs. 1-5 of DocOnotes– practice Quick Questions at end of sections

• Do HW1 – due 5 p.m. Thursday• submit to my office 205 Sharp Lab• or my mailbox in 217 Sharp Lab

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