17-1 how we can measure the distances to the stars 17-2 how we measure a star’s brightness and...

17-1 How we can measure the distances to the stars

17-2 How we measure a star’s brightness and luminosity

17-3 The magnitude scale for brightness and luminosity

17-4 How a star’s color indicates its temperature

17-5 How a star’s spectrum reveals its chemical composition

17-6 How we can determine the sizes of stars

17-7 How H-R diagrams summarize our knowledge of the stars

17-8 How we can deduce a star’s size from its spectrum

Chapter 17By reading this chapter, you will learn

17-9 How we can use binary stars to measure the masses of stars

17-10 How we can learn about binary stars in very close orbits

17-11 What eclipsing binaries are and what they tell us about the sizes of stars

Parallax

D = 1 / pD = distance to a star in parsecP = parallax angle

Parallax only works for the stars that we can measure the apparent change in position. 500lys is about the upper limit!Hipparcos (89 – 93) contributed greatly for this purpose!

If too far for Parallax:Apparent brightness of the stars and inverse square law!

Apparent magnitudes of the stars in Pleiades and Orion’s surroundings

Now you are somewhat familiar with the method to measure distances, Please work on “Distant objects” worksheet (part of your homework for this week!)

Imagine that you have received pictures of children from near by planetary systems. Picture shows a child on his or her 12th birthday. The pictures were then send electronically to us on the day of the child’s birthday. (lys = light years)

• Yvone lives on a planet orbiting Garg, which is 9.5 lys away from the sun.

• RYAN lives on a planet orbiting NORDSTROM, which is 14 lys away from the sun.

• Julie lives on a planet orbiting Bray-Ali, which is 21.5 lys away from the sun.

• MALLEREY lives on a planet orbiting LEWIS, which is 6 lys away from the sun.

• RAQUEL lives on a planet orbiting VASQUEZ, which is 4.5 lys away from the sun.

• MICHELLE lives on a planet orbiting LA HUE, which is 8.2 lys away from the sun.

Stellar Motions: Proper / radial motions

Color -> Temp (recall Wien’s law)

1. If a star has parallax angle of 1/5 arc-sec, what would be the distance to the star? A)1/5, b)1. c)5, d)25 pc

2. Star A and B have same luminosity. If star A is 4 times closer to Earth then star B, then _____ to earthly viewer.: a=A is 4 x brighter, b=B is 4x brighter, c=A is 16 times brighter d=B is 16 times brighter, e=A is 64x brighter

3. Apparent magnitude is measure of: a=light output, b=light received by the observer, c=temperature of light source

4. Which color star is hotter? A=red. B=white, c=blue

Click “ enter to review the questions C, c, b, c

Color Temp (K)

O B-V 30k – 50k

B B-W 11k – 30k

A White 7500 – 11k

F Y-W 5900 – 7500

G Yellow 5200 – 5900

K Orange 3900 – 5200

M O-R 2500 – 3900

L Red 1300 – 2500

T Red T<1300

L and T are brown dwarfs

Spectral types (O, B, A, F, G, K, and M), based on the major patterns of spectral lines Williamina Fleming (standing): left, Annie Jump Cannon: right

Properties of Nearby Stars

Stefan – Boltzmann law

L = 4R2T4

L = LuminosityR = radius = constantT = temp

Inverse square law

L = 4d2bL = Luminosityb = brightness (W / m2)d = distance

Hertzsprung-Russell (H-R) diagrams

Spectroscopic Parallax

Stefan – Boltzmann law

L = 4R2T4

L = LuminosityR = radius = constantT = temp

Inverse square law

L = 4d2bL = Luminosityb = brightness (W/m2)d = distance

Use of these laws tell us distance, radius and luminosity.How about mass?

1. Williamina Fleming and Annie Jump Cannon were involved in developing: a=luminosity class, b=spectral class, c=distance measurements

2. Which type of star is hotter? a=G2V, b=A1II, c=M4III, d=O1Ia e=K7IV

3. The sun’s spectrum is perfect continuous spectrum. a=ture, b=false, c=we don’t know!

4. Which type of star is more stable? a=G2V, b=A1II, c=M4III, d=O1Ia e=K7IV

5. H-R diagram is a plot of: a=mass vs. temp., b=Hydrogen vs. radius, c=Luminosity vs. age, d=mass vs. luminosity, d=luminosity vs. temp.

Click “ enter to review the questions B,d,b,a,d

Binary Star SystemVisual binary : 2 stars can be

observed as 2 with mutual gravitySpectrum binary : appear to be one star with incongruous spectrum.Spectroscopic binary : appear to be one but shows Doppler shiftsOptical Double : Appear to be close, but just close line of sight.

Calculating mass from Binary Star System

Kepler’s 3rd law:

M total = a3 / p2

M = mass, a = semimajor axisp = orbital period

Example:Distance between two stars = 1.8AUTime to orbit each other = 0.7 years

M = (1.8)3 / (0.7)2 =

Spectroscopy in binary system

Light curves of eclipsing binaries

1. Binary system means: a=stars young, b=stars are very large, c=stars are old, d=2 stars

2. Binary system let the astronomers measure ____ for the first time: a=mass, b=temperature, c=age, d=Hydrogen content,

3. For the eclipsing binary, we can observe ____ to determine their motion. A=Doppler shift, b=Luminosity, c=apparent magnitude (brightness), d=temperature change

Click “ enter to review the questions D, a, a&c