Chapter 11Surveying the Stars

I. Parallax and distance.

II. Luminosity and brightnessApparent Brightness (ignore “magnitude system” in book)

Absolute Brightness or Luminosity Inverse-Square Law

III. Stellar TemperaturesColor, Spectral lines, Spectral Classification:OBAFGKM

IV. Stellar sizes (radius)

V. Stellar Masses

Outline of Chapter 11 Part I: Properties of StarsNot exactly like book

Properties of Stars

Our Goals for Learning

• How far away are stars?

• How luminous are stars?

• How hot are stars?

• How massive are stars?

• How large (radius) are stars?

I. Parallax and distance.p = parallax angle

in arcseconds

d (in parsecs) = 1/p

1parsec= 3.26 light years

I. Parallax and distance.

Nearest Star: Alpha Centauri d = 4.3 light years

(since 1 parsec = 3.26 light years)

distance in parsecs = 4.3/3.26 = 1.32

What is the parallax of this star?

d=1/p hence p=1/d

p for nearest star is

I. Parallax and distance.

Nearest Star: Alpha Centauri d = 4.3 light years

(since 1 parsec = 3.26 light years)

distance in parsecs = 4.3/3.26 = 1.32

What is the parallax of this star?

d=1/p hence p=1/d

p for nearest star is 0.76 arcseconds

All other stars will have a parallax angle smaller

than 0.76 arcseconds

1.      The distance of a star whose parallax is 0.25 arc seconds is

Question 1

1.      The distance of a star whose parallax is 0.25 arc seconds is

A. 4 parsecs

B. 40 light-years

C. 100 astronomical units

D. 0.25 parsec

Question 1

1.      The distance of a star whose parallax is 0.25 arc seconds is

A. 4 parsecs

B. 40 light-years

C. 100 astronomical units

D. 0.25 parsec

Question 1

1. Apparent Brightness (how bright it looks in the sky)

2. Absolute Brightness or Luminosity (energy/sec)

3. Inverse-Square Law

II. Luminosity and Brightness

Energy passing through each sphere is the same

The further the observer the lower the apparent brightness proportional to 1/d2

Energy passing through each sphere is the same

The further the observer the lower the apparent brightness proportional to 1/d2

How many times fainter will the Sun seem from Jupiter (5AU) than from Earth?

Energy passing through each sphere is the same

The further the observer the lower the apparent brightness proportional to 1/d2

How many times fainter will the Sun seem from Jupiter (5AU) than from Earth? 25 times

1. Apparent Brightness (how bright it looks in the sky)

2. Absolute Brightness or Luminosity (energy/sec)

3. Inverse-Square Law apparent brightness=(absolute

brightness)/d2

4. Examples: light bulbs at different distances

II. Luminosity and Brightness

1. Apparent Brightness (how bright it looks in the sky)

2. Absolute Brightness or Luminosity (energy/sec)

3. Inverse-Square Law apparent brightness=(absolute

brightness)/d2

4. Examples of absolute brightness and apparent brightness: light bulbs at different distances

a) 10W, 1 meter awayb) 100W, 10 meters awayc) 20W, 2 meters awayd) 90W, 3 meters away

II. Luminosity and Brightness

light bulbs at different distancesa) 10W, 1 meter awayb) 100W, 10 meters awayc) 20W, 2 meters awayd) 90W, 3 meters away

Use formula: apparent brightness=(absolute brightness)/d2

Which one is faintest? Brightest?

II. Luminosity and Brightness

light bulbs at different distancesa) 10W, 1 meter awayb) 100W, 10 meters awayc) 20W, 2 meters awayd) 90W, 3 meters away

Which one is faintest? b Brightest? a and d

Watts is a unit of energy/sec (power)

II. Luminosity and Brightness

Review of distance, apparent brightness, absolute (intrinsic) brightness and luminosityDistance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears)Apparent brightness: how bright a star looks in the skyThe inverse-square Law: light from stars gets fainter as the inverse square of the distance (apparent brightness is proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2

Luminosity (energy/sec) is equivalent to absolute brightness (analogy with light bulbs: Watts)

1.      Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one?

Question 2

1.      Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one? (Hint: calculate the distance first and then estimate the apparent brightness)

Question 2

1.      Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one? (Hint: calculate the distance first and then estimate the apparent brightness)

A. 100

B. 1000

C. 10,000

D. 400

Question 2

I. Parallax and distance.

II. Luminosity and brightnessApparent Brightness Absolute Brightness or Luminosity Inverse-Square Law

III. Stellar TemperaturesColor, Spectral lines, Spectral Classification:OBAFGKM

IV. Stellar sizes (radius)

V. Stellar Masses

Outline of Chapter 11 Part I

How hot are stars?

1. Color ( hotter > bluer; cooler > redder)

2. Spectral lines

3. Spectral Classification:OBAFGKM (from hottest to coldest)

III. Stellar Temperatures

hotter brighter, cooler dimmer

hotter bluer, cooler redder

Laws of Thermal Radiation

(from Ch. 5)

Hottest stars:

blue

Coolest stars:

red

(Sun’s surface is about 6,000 K)

Lines in a star’s spectrum correspond to a spectral type that reveals its temperature: O B A F G K M (Hottest) (Coolest)

Table 11.1

Luminosity is proportional to surface area (how large) x temperature (how hot): L= 4R2T4

If we can measure the Luminosity and the

temperature of a star we can tell how large its

raduis is.

IV. Stellar sizes (radius)

Luminosity is proportional to surface area x temperature: L= 4R2T4

If we can measure the Luminosity and the

temperature of a star we can tell how large its

raduis is.

IV. Stellar sizes (radius)

Summary of Ch 11 Part IDistance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears)Apparent brightness: how bright a star looks in the skyThe inverse-square Law: light from stars gets fainter as the inverse square of the distance (brightness proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2

Luminosity (energy/sec) is equivalent to absolute brightnessL= 4R2T4

If we can measure the luminosity and the temperature of a star we can tell how large it is. Binary stars allow us to determine stellar masses

Summary of Ch 11 Part IDistance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears)Apparent brightness: how bright a star looks in the skyThe inverse-square Law: light from stars gets fainter as the inverse square of the distance (brightness proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2

Luminosity (energy/sec) is equivalent to absolute brightnessL= 4R2T4

If we can measure the luminosity and the temperature of a star we can tell how large it is. Binary stars allow us to determine stellar masses

Binary Stars• Definition• Three main types of Binary Stars

• Visual• Spectroscopic• Eclipsing

• Stellar Masses and Densities

• Definition:When two stars are in orbit around their center of mass

• Three main types of Binary Stars• Visual: orbits• Spectroscopic: Review of Doppler effect, spectral lines,

double and single lines• Eclipsing: masses and radii of stars

• Stellar Masses and Densities

Binary Stars

Visual Binary

Visual Binary

Radial Velocity Approaching stars: more

energy, Receding stars: less energy,

Doppler Effect

Approaching stars: more energy, spectral lines undergo a blue shift

Receding stars: less energy, spectral lines undergo a red shift

/ = v/c

Radial Velocity

Spectroscopic Binary

Spectroscopic Binary

We determine the orbit by measuring Doppler shifts

Eclipsing Binary

We can measure periodic eclipses

Eclipsing Binary: Masses and Radii

Radii of Stars

Stellar Masses

Stellar Densities

High

Same as water

Low

Properties of Stars

Our Goals for Learning

• How far away are stars?

• How luminous are stars?

• How hot are stars?

• How massive are stars?

• How large (radius) are stars?

I. The Hertzprung-Russell (H-R) Diagram:

Surface Temperature Luminosity Analogy: horsepower vs weight

Where Stars plot is the H-R diagram Main Sequence: 90% of all stars Giants, Supergiants, White Dwarfs

Outline of Ch 11 part 2: The H-R Diagram

Figure 11.5

Team Responsible for Stellar Classification in Late 1800s and Early 1900s