Announcements• Exam Grades Wednesday March 31• Angel Grade update Friday April 2• Star Assignment 6,

due Wednesday March 31Do Angel quiz,

Objectives

• Plot an Hertzsprung-Russell Diagram• Locate the regions of giant, main

sequence and white dwarf stars. • Describe the relations between

surface temperature, luminosity, mass and size for these types of stars.

• Describe the evolutionary state of the 16 brightest northern-hemisphere stars.

What should we do with our information?

• Luminosities

• Surface Temperatures

• Masses

Number of Stars vs Luminosity

LOWHIGH Luminosity1100 0.01 0.0001

Stellar Masses

What should we do with our information?

• Luminosities

• Surface Temperatures

• Masses

Look for Relationships

No Relationship

Luminosity - Mass Relation

L~M3.5

HRDiagram:Luminosity

vs.Surface

Temperature

Luminosity - Temperature

Relation: Hertzsprung-

Russell DiagramL

umin

osit

y ->

Temperature

Lu

min

osi

tyRecall:

Hotter -> BrighterHotter ->

BluerBigger -> Brighter

Then cool+

Luminous-> Large

Temperature

Lu

min

osi

tyRecall:

Hotter -> BrighterHotter ->

BluerBigger -> Brighter

Then small Luminosity

+ Hot -> small

Stellar Size, Mass

& HR

Diagram

Stars with low temperature and high luminosity must have large radius

Large radius

Small radius

Stars with low temperature and high luminosity must have large radius

GIANTS

SUPERGIANTS

WHITEDWARFS

Low-Mass Stars

High-Mass Stars Most Low Mass stars have

SmallLuminosity

Most Large Massstars have

LargeLuminosity

Regions of the H-R Diagram

Activity

• Answer the questions on your scantron sheet

Temperature

Lu

min

osi

ty

1. Which star is the hottest?

A

BC

D

Temperature

Lu

min

osi

ty

1. Which star is the hottest?

A

BC

AD

Temperature

Lu

min

osi

ty

2. Which star is the most luminous?

A

BC

D

Temperature

Lu

min

osi

ty

2. Which star is the most luminous?

A

BC

D C

Temperature

Lu

min

osi

ty

3. Which star is a main-sequence star?

A

BC

D

Temperature

Lu

min

osi

ty

3. Which star is a main-sequence star?

A

BC

D

D

Temperature

Lu

min

osi

ty

4. Which star has the largest radius?

A

BC

D

Temperature

Lu

min

osi

ty

4. Which star has the largest radius?

A

BC

DC

Temperature

Lu

min

osi

ty

5. Which star is most like our Sun?

A

B

C

D

Temperature

Lu

min

osi

ty

5. Which star is most like our Sun?

A

B

C

D

B

HOT COOL

BRIGHT

FAINT

HRDiagram

Astronomy Place Questions

6. Spica is a Main Sequence star, having a surface temperature of 26,000K. In order for a star with a surface temperature of only 6,000K to have the same Luminosity as Spica, its radius must be?

Luminosity = 4R2T4

If T ~ 4 x smaller, then R ~ 42 x larger = 16x

7. The rate of fusion (number of He nuclei produced per second) in the core of the star Vega (a main-sequence star with a temperature of about 10,000K is roughly?

Lvega = 60 Lsun, so rate of energy generation

= rate of energy loss is 60 x larger than Sun’s

8. A star having the same radius as the Sun, but a surface temperature 5 times higher than the Sun, would have a luminosity?

L ~ T4

54 = 625

9. A main-sequence star that is twice as massive as the Sun will have a Luminosity?

L ~ M3. 5

23 = 8, 24 = 16, so 23.5 in between

Lifetime on the Main Sequence

How long will it be before MS stars run out of fuel? i.e. Hydrogen?

How much fuel is there? M

How fast is it consumed? L M3.5

How long before it is used up?

M/L = M/M3.5 = M-2.5

Main SequenceLifetimes

Lifetime on the Main Sequence

• O & B Dwarfs burn fuel like a bus!

• M Dwarfs burn fuel like a compact car!

• Our Sun will last 1010 years on the Main Sequence

• MS Lifetime = 1010 yrs / M2.5

Stars occur alone

or in clusters

another type of cluster

Star Clusters

• Stars approximately the SAME AGE

• Stars approximately the SAME DISTANCE

Cluster HR

Diagrams

Same DistanceSame Age

Pleiades now has no stars with life expectancy less than around 100 million years

Main-sequenceturnoff

Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old

ClusterAges

Activity

• Answer the questions on your scantron sheet

Temperature

Lu

min

osi

ty

6. Which of these stars will have changed the least 10 billion years from now?

A

B

C

D

Temperature

Lu

min

osi

ty

6. Which of these stars will have changed the least 10 billion years from now?

A

B

C

D

C

Temperature

Lu

min

osi

ty

7. Which of these stars can be no more than 10 million years old?

A

B

C

D

Temperature

Lu

min

osi

ty

7. Which of these stars can be no more than 10 million years old?

A

B

C

D

A

A stars properties depend mostly on

1. Mass2. Age

NEXT TIME

• Evolution of Stars

• READ chapter 17