Study Points• Define luminosity and brightness.

• When you see a bright star, can you tell if it is luminous? If it is

close? Explain.

• Describe how astronomers measure the distance to a nearby star.

• Describe how to use brightness and luminosity to find the distance

to a star.

• What is an H-R diagram? Sketch one. Given the position of a star

on the H-R diagram, indicate if the star is hot or cold, luminous or

not.

• Roughly describe the luminosity, surface temperature, and size of

the following types of stars: main sequence, giant, supergiant,

white dwarf, & red dwarf. Which are the most common?

• Why are supergiant stars rare?

• Know the comparison of mass and lifetime of main sequence, giant,

supergiant, white dwarf, & red dwarf stars. Which live the shortest

lives? Which live the longest lives?

Brightness (B) and Luminosity (L)

Distance to a star (D)

Star sizes

H-R diagram

Luminosity vs Temperature

Some star types

Star Properties

Brightness, B

• *Measure of energy received from a star

– Defined by magnitude

• Brightness = Magnitude (visual scale)

– from Hipparchus: 1(bright) to 6(faint)

– Now -1.5 Sirius to faint stars at 6+

Luminosity, L

• *Measure of energy emitted by a star

– Energy per second (Watts)

• *Useful symbol, L

– Luminosity of our sun

Light bulb

• Brightness and Luminosity

• Ex – Two 40 watt bulbs.

– One here and one at Subway across the street.

• Which is more luminous?

–Energy emitted

• Which is brighter?

–Energy received

Light bulb

• Brightness and Luminosity

• Ex – Two 40 watt bulbs.

– One here and one at Subway across the street.

• Which is more luminous?

–Energy emitted

• Which is brighter?

–Energy received

• Same

• Closer one

Light bulb

• Brightness or luminosity

• Back to 40 watt bulb

• Walk farther down street with it.

• Brightness depends on distance

• Compare with other light bulbs

– Different watts

Bright stars? Magnitude? Very Luminous stars?

Bright stars are: ______________ or ______________

Bright stars are:

more distant and very luminous

ORcloser (shorter distance) and not so luminous

DL

DL

*Brightness, B depends on:

Luminosity, L

Distance, D

2D

LkB =

Sirius: B?

Sirius: B L?

Sirius: B L?

Close (8.6 LY)

Sirius: B L

Close (8.6 LY)

Average

luminosity

( 33 L

)

Sirius is BRIGHT (-1.46) because it is CLOSE, not because it

has a high luminosity.

Betelgeuse: B?

Betelgeuse: B L?

Betelgeuse: B L?

Far (429 LY)

Betelgeuse: B L

Far (429 LY)

Very Luminous!

(330,000 L

)

Betelgeuse is BRIGHT because it is VERY LUMINOUS.

It is not close.

Finding distances to stars

• *Use parallax and

trigonometry on nearby

stars to get distance

(valid for few hundred LY)

Finding distances to stars

2D

LkB =

For those same nearby stars

• Know D

• Measure B

• Then calculate L

*Finding distances to stars!!

• Type of star gives you L

• Measure B

• Calculate D

• To read more about L

and types of stars see

Henrietta Levitt

• Watch (1:00):• https://scientificwomen.net/wome

n/leavitt-henrietta-55

2D

LkB =

*H-R

DiagramHertzsprung-Russell

*Star

Luminosity

vs.

Surface

Temperature

*H-R

Diagram

*Star

Luminosity

vs.

Surface

Temperature

H-R Diagram

1. Which star is hotter?

2. Which star is more luminous?

Why?

H-R Diagram

1. Which star is hotter?

2. Which star is more luminous?

Why?

SAME (COLOR)

LARGER STAR

BECAUSE IT IS LARGER

H-R Diagram

3. Which star is hotter?

4. Which star is more luminous?

Why?

H-R Diagram

3. Which star is hotter?

4. Which star is more luminous?

Why?

WHITE STAR

SAME SIZE BUT

WHITE IS MORE LUMINOUS

LUMINOSITY DEPENDS ON

TEMPERATURE AND RADIUS

H-R Diagram

5. Which star is hotter?

6. Which star is more luminous?

Why?

H-R Diagram

5. Which star is hotter?

6. Which star is more luminous?

Why?

BLUE STAR

BLUE STAR

LUMINOSITY DEPENDS ON

TEMPERATURE AND RADIUS

H-R Diagram

Luminosity depends on…

Temperature and Size (Radius)

H-R Diagram

Which star type can be

hot but not luminous?

Which star type can be

cool but is luminous?

Graphing T vs L tells us the size of the star

H-R Diagram

Which star type can be

hot but not luminous?

Which star type can be

cool but is luminous?

Graphing T vs L tells us the size of the star

WHITE

DWARF

RED

SUPERGIANT

Sizes of stars

https://www.youtube.com/watch?v=cMWMtLGGg6o

Sizes of starshttps://en.wikipedia.org/wiki/List_of_largest_stars

*H-R

Diagram

Main Sequence

• ~80% of all stars: most common*

• Long lived - Sun ~1 X 1010 yrs

• Average, middle class stars

• ~0.1 to 10 R

• ~average masses

Main Sequence

Ex: Sun

Main Sequence

Ex: Pleiades

Main Sequence

• Ex: Sun,

Sirius A,

Procyon A,

Alpha Centauri A

Giants (Often red giants)

• Rare

• Shorter lived

• ~1 - 5 M

• ~10 to 100 R

Giants

Ex: Arcturus,

Aldebaran,

Spica

Giants

Ex: Arcturus,

Aldebaran,

Spica

Giants

Ex: Arcturus,

Aldebaran,

Spica

Red

Giants

Blue

Giants

Supergiants• Very rare

• Shortest lived

– Few exist at any given time due to “short” life*

• ~ 5 to 100 M

• ~ 10 to 1000 R

• Ex: Betelgeuse -- Orbit of Mars or Jupiter

Super-

giantsEx:

Cephei

Orbit of

Jupiter

White Dwarf

• Mass ~ M

• Size ~ Earth (0.01 R

)

• One teaspoon weighs ~one ton

• Extremely long lives

• Ex: Sirius B

Red Dwarf

• Smaller, cooler version of sun

(Not a cooler

white dwarf)

• Many

• M ~ 0.1 M

• R ~ 0.1 R

• L ~ 1/2000 L

• Ex: Proxima Centauri

• Extremely long lives

Rho Ophiuchi has 30+ Red dwarfs

Black Dwarf

• Theoretical

• None found yet

• Cold & dark (still warmer than space)

• Cooled down white dwarf

• Our universe may not be old enough yet to

have any black dwarfs.

H-R Diagram

Brown Dwarf

• Not on HR diagram

• Size ~ Jupiter (~1/10 R

)

• Mass ~ 1/100 M

• Failed star (planet?)

• 10-100 times more

massive than Jupiter

Star Life Summary*

SHORTEST LIFE and LARGEST MASS

• Supergiants – shortest lives; ~3–50 million years

• Giants – shorter lives; ~100 million years

• Main Sequence – long lives; billions of years

• White & Red Dwarfs – longest lives; hundreds of

billions of years or trillions of years (maybe to the life of a

proton 1032 or more)

LONGEST LIFE and SMALLEST MASS

KNOW THE ORDER.

Don’t need to know

numbers.

Brightness and Luminosity

Distance to star

B depends on L and D

H-R diagram (Plot of L vs T)

Types of stars (Main sequence,

Giants, Supergiants, White dwarfs,

Red dwarfs, Brown dwarfs)

Summary

Homework & Updates• Keep up with Study Points

• Astronomy Crash Course #26 about Stars:

https://www.pbs.org/video/crash-course-astronomy-26/

• D2L Quiz 9 & 10 available; Quizzes 9-13 for Test 3

• Lab Update – last lab this week on Lenses & Telescopes• Open notebook Lab Test coming up on Nov. 19 & 21

• Lab Test preparation in lab this week

• Observations:• Astrophysics Lecture Due Dec. 3 (10 pts) Write report (Lecture done 10/10)• Astronomy News Evalution Due Dec. 10 (20 pts) Evaluate astronomy news• Planetarium Due Dec. 3 (10 pts) Go to a planetarium show (Field trip 10/3)• Stargazing Due Dec. 17 (20 pts) Go stargazing & write report• Telescope Due Dec. 17 (20 pts) Look through a telescope

• Look at calendar options & weather• Moon Craters Due Dec. 17 (10 pts) Look at magnified moon craters

• Borrow binoculars from Lab room

Astronomy Crash Course #26 about

Stars (also on YouTube):

https://www.pbs.org/video/crash-

course-astronomy-26/