LAB 07Astronomy 105 Laboratory

LAB QUIZ 7

Last week you determined the Earth’s orbital velocity using ______.1. Newton’s 2nd Law2. Wein’s Law3. the photoelectric effect4. Newton’s Law of Gravity5. the Doppler effect

If a star is moving away from Earth, the absorption lines in its spectrum will1. have wavelengths shorter than

those of an identical stationary star (blueshifted).

2. have wavelengths longer than those of an identical stationary star (redshifted).

3. be the same as those of an identical stationary star.

In this week’s lab you will make a graph (HR diagram) comparing two stellar properties. Which property is plotted on the horizontal axis?1. mass2. size3. temperature or spectral class4. luminosity or absolute

magnitude5. eccentricity

A star’s apparent magnitude is a measure of brightness as viewed from1. the Earth.2. 10 parsecs.3. 100 parsecs.4. 1000 parsecs.5. 10 light-years

A star’s absolute magnitude is a measure of brightness as viewed from1. the Earth.2. 10 parsecs.3. 100 parsecs.4. 1000 parsecs.5. 10 light-years

THE HR DIAGRAMLab 07

Apparent Brightness of Stars Stellar Luminosity -- Total amount of light

energy emitted each second Surface Area Temperature

Distance from the Earth

1 2 3 4 5 6Brighter Dimmer

Magnitude Scale

2.512

Difference in apparent brightness between each magnitude step is 2.512

(2.512)3 16

(2.512)5 = 100

Brightest stars in the night sky Faintest stars visible to naked-eye

Measuring a Star’s Brightness

(2.512)2 6.3

5 pc 10 pc 15 pc-26.5

1.3

3.3

2.0

4.2

Sun

A

B

C

D

5.06.0

0.0

2.0

Apparent Magnitude

Absolute Magnitude1 parsec (pc) = 3.26 ly

Magnitude Stellar Brightness

Apparent Magnitude (mv) - Brightness from Earth Absolute Magnitude (Mv) - Brightness from 10 pc

Absolute magnitude depends only on a star’s luminosityMagnitude Difference

Brightness Ratio (Brightness Difference)

1 (2.512)1 2.5 2 (2.512)2 6.3 3 (2.512)3 15.9 4 (2.512)4 40 5 (2.512)5 100 6 (2.512)6 251

Spectral Classification

B

OBAFGKM

he

ineirlisse

Hot 30,000 K

Cool 3,500 K

Sun (G2) 6,000 K

A0A1A2A3A4A5A6A7A8A9F0

Spectral Classification

/Guy

(a temperature scale)

The SunM = +5 G2

O B A F G K M

-10-5 0+5+10+15A

bsol

ute

Mag

nitu

de

Temperature

HR DiagramVertical Axis: Absolute Magnitude or Luminosity

Horizontal Axis: Temperature (color, Kelvin or spectral type)

HR Diagram

EE

6,000 K 12,000 K?

Rsun = Rhot starR R

The Sun A Hot Star

Luminosity1 Lsun

Temperature

16 Lsun

Luminosity: Temperature

4TELaw BoltzmannStefan

E

Rs

E

6,000 K

Small Star Big Star

Surface Area = 4r2

smallbig SAA 61S

22 4164 smallbig πRπR

smallbig RR 4

Star 2 is more luminous than Star 1: Mv1 - Mv2 = 5 - 2 = 3 magnitudesStar 2 is 16 time brighter because it has 16 times the surface area.

Star 2 is more luminous than Star 1: Mv1 - Mv2 = 5 - 2 = 3 magnitudesStar 2 is 16 time brighter because it has 16 times the surface area

Msmall = 5

Mbig = 2

Magnitude Difference

Brightness Ratio (Brightness Difference)

1 (2.512)1 2.5 2 (2.512)2 6.3 3 (2.512)3 15.9 4 (2.512)4 40 5 (2.512)5 100 6 (2.512)6 251

Big Star is more luminous than Small Star:

Big Star is 16 times brighter. Why?

Big Star has 16 times the surface area!

Msmall – Mbig = 5 - 2 = 3 magnitudes

Rb

Luminosity: Size

22 16 smallbig RR

22 16 smallbig RR

Luminosity Class Size Ia & Ib Supergiant

II Bright Giant

III Giant

IV Sub-giant

V Dwarf

The Sun’s Spectral and Luminosity Class: G2 V

THE END

E

R1

E

6,000 K

Small Star Big Star

Surface Area = 4r2

116

RR

2S

2B

14

RR

S

B

SmallBig 4RR

Star 2 is more luminous than Star 1: Mv1 - Mv2 = 5 - 2 = 3 magnitudesStar 2 is 16 time brighter because it has 16 times the surface area.

Star 2 is more luminous than Star 1: Mv1 - Mv2 = 5 - 2 = 3 magnitudesStar 2 is 16 time brighter because it has 16 times the surface area

M1 = 5

M2 = 2

Magnitude Difference

Brightness Ratio (Brightness Difference)

1 (2.512)1 2.5 2 (2.512)2 6.3 3 (2.512)3 15.9 4 (2.512)4 40 5 (2.512)5 100 6 (2.512)6 251

Big Star is more luminous than Small Star:

Big Star is 16 times brighter. Why?

Big Star has 16 times the surface area!

M1 – M2 = 5 - 2 = 3 magnitudes

R2

116

RR

R 4πR 4π

Star Small S.A.Star Big S.A.

LL

2Small

2Big

2Small

2Big

Small

Big

Luminosity: Size