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The H-R DiagramThe H-R Diagram

I bet my dwarf can beat your giant.

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GoalsGoals

• Look at parts of H-R diagram.• Lifetimes?• Sizes?• Clusters?

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Main Sequence & Thermal Radiation

• The Main Sequence makes sense!

• Hotter stars are bluer.

• Hotter stars are brighter

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The Main Sequence• Stars characterized

by what holds them up.

• 90% held up by heat of Hydrogen fusion

4H He + Energy

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M.S. Lifetime

• More Massive Hotter

• Hotter More luminous

• More luminous shorter life

SunSun

SunLife T

LL

MMassT

)(

)(

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Clusters

• Groups of stars all at the same distance and age.

• Watch evolution along the H-R diagram.

• Get age of cluster.

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Open Clusters

• These are the new stars.

• Small groups of young stars.

• Slowly drifting apart.

Jewel Box – copyright MichaelBessell

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Angular Size

• Linear size = how big something really is– Meters, inches, light years, feet

• Angular size = how big something looks– Degrees, arcminutes, arcseconds,

milliarcseconds

Circle = 360 degrees1 degree = 60 arcmin1 arcmin = 60 arcsec1 arcsec = 1000 mas

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Angular vs. Real Size

Distance

Dia

mete

r

D

R2

Distance

Diametertan

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Stellar Radii

How big are stars?• We see stars have

different luminosities and different temperatures.

• Stars have different sizes.• If you know:

– Distance– Angular size

• Learn real size.

50 mas

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Angular versus Real

Supergiants, Giants and Dwarfs

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Betelgeuse

• Angular size = 50 mas• Parallax = 7.6 mas = 0.0076 arcsec• Apparent mag = 0.6

• Distance = 1/parallax = 132 pc• True size = distance * angular size =

1400 Rsol

• Absolute Mag = m – 5log(d/10pc) = -5– Our sun M ~5, Betelgeuse = 10,000x

luminosity

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Globular Cluster

12 billion years old

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M13

M13 – Natalie Redfield ‘06

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Luminous Giants

Why are Red Giants more luminous than main sequence stars?– Hotter things are brighter per square inch.– The larger the star, the more square

inches.– Luminosity is total light given off.

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Concept TestWhich star is:

1. Hottest?2. Most luminous?3. Least likely to

be found in an open cluster?

4. Least likely to be found in a globular cluster?

Of Main Seq. Stars?1. Most massive?2. Longest Life

span?

Star Spec Type

m M

A B0 V 0.0 0.0

B G2 V 10.0 4.4

C K5 III 0.0 -2.0

D F7 I -1.0 -5.0

E K3 V 5.0 6.5

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Homework #10Homework #10

• For Oct 13:• Reread B17.1 – B17.2, • Do B16 Problem 20, and B17 Problem

1, 18