Lecture 7: The HR-Diagram- They Might Be Giants?

"The whole of science is nothing more than the refinement of everyday thinking."
A. Einstein, Out of My Later Years

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Date: February 14, 1995
Reading Assignment: pp. 401-414

Description : discussion of the HR diagram its meaning

Objectives

be able to describe what an HR diagram is and why it is important

be able to describe what is plotted on an HR diagram

be able to draw a simple HR diagram

be able to describe and locate the Main Sequence

be able to locate the Giant Branches and White Dwarfs

be able to describe how the masses of stars are determined

be able to describe how the mass of a star influences its luminosity and its lifetime

Lecture Outline

Slide # 1: Measuring Stars - II Slide # 2: Measuring Stars - I

distance

luminosity and brightness

motion- radial and transverse velocity

diameters

color index

Spectral Types

Slide # 3: Measuring Stars -II

Spectral Classes

the HR diagram

stellar masses

luminosity classes

Slide # 4: Luminosity and Size (GRAPHICS)

the total luminosity is related to temperature and surface area

Slide # 5: Luminosity and Brightness (GRAPHICS)

concept map

Slide # 6: Large Baselines : Earth's Orbit (GRAPHICS)

A very large baseline is need to measure the parallax of stars

Slide # 7: Parsecs and Arcseconds

Parsec

206265 AU's

about 3.26 light years

Arcsecond

1/3600 degrees

1/60 arcminutes

Slide # 8: Parallax (GRAPHICS)

the formula

Slide # 9: Brightness of Stars

luminosity or absolute brightness

amount of energy created by the star

independent of distance

brightness or apparent brightness

how a star appears from Earth

depends on luminosity and distance

Slide # 10: Facts about Magnitudes

small is bright

5 magnitudes = 100 in brightness

magnitudes add - brightness multiplies

Slide # 11: Absolute vs Apparent

Apparent magnitude

what you see from Earth

measures apparent brightness

Absolute magnitude

magnitude of a star IF it were 10pc away

measure of luminosity or absolute brightness

Slide # 12: Color Index

hot stars emit more blue light than cool stars

color index is the ratio of blue/red

temperature is related to color index

Slide # 13: Stellar Spectra (GRAPHICS)

red filters and blue filters

the 60,000K curve has the bluest color index and highest temperature

Slide # 14: Orion Revisited (GRAPHICS)

star have visible color differences

Slide # 15: Wien's Law (GRAPHICS)

Peak Wavelength in cm

T in Kelvin

Slide # 16: Measuring Stellar Diameters (GRAPHICS)

indirect method

measure temperature

measure distance

measure brightness

calculate luminosity from brightness and distance

calculate radius from luminosity and temperature

direct method

speckle interferometery

interferometery

Slide # 17: Line Intensity (GRAPHICS)

temperature determines line intensity

Slide # 18: Spectral Classes (GRAPHICS)

stars are classified by their spectra

line intensity changes with temperature

Slide # 19: Space Motion (GRAPHICS)

stars are moving through space

motion can be divided into radial and transverse

Slide # 20: Radial Motion (GRAPHICS)

motion directly toward or away from observer

Slide # 21: Doppler

Blue Shift

higher frequencies, shorter wavelength

the observer moves toward the source

the source moves toward the observer

Red Shift

lower frequencies, longer wavelengths

the observer moves away from the source

the source moves away the observer

Slide # 22: Doppler Effect (GRAPHICS)

the change in wavelength is proportional to velocity

Slide # 23: Radial Motion (GRAPHICS)

measured by using the Doppler shift

Slide # 24: Transverse Motion (GRAPHICS)

motion in the plane of the sky

Slide # 25: Transverse Motion

angular motion

observed change in position

transverse velocity

actual motion in the sky

related to angular motion and distance

Slide # 26: Proper Motion (GRAPHICS)

change in apparent position of stars due to their motion

Slide # 27: Measuring Transverse Velocity

measure distance to star

parallax

measure change in angular position due to motion

proper motion

calculate transverse velocity

Slide # 28: Transverse Velocity (GRAPHICS)

the Formula

Slide # 29: Constellation Corner

Constellation De Jour

Slide # 30: Taurus (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 31: Taurus (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 32: Taurus (GRAPHICS)

Feb 15 - 9pm - S - 5.0

Slide # 33: Spectral Types (GRAPHICS)

O - 30,000K

strong ionized helium

multiply ionized heavy elements

faint hydrogen lines

B - 20,000K

neutral helium

singly ionized heavy elements

moderate hydrogen lines

Rigel (B8)

Slide # 34: Spectral Types

A - 10,000K

very faint neutral helium

singly ionized heavy elements

strong hydrogen lines

Vega (A0), Sirius (A1)

F - 8000K

singly ionized heavy elements

neutral metals

moderate hydrogen lines

Canopus (F0)

Slide # 35: Spectral Types

G - 6000K

singly-ionized heavies

neutral metals

faint hydrogen

Sun (G2), Alpha Centauri (G2)

K - 4000K

singly-ionzied heavies

neutral metals strong

faint hydrogen

Aldebaran (K5)

Slide # 36: Spectral Types

M - 3000K

neutral atoms strong

molecules moderate

hydrogen very faint

Betelgeuse (M2)

Slide # 37: Temperature of Stars

Color Index

blue light vs red light

Spectral Class

strength of spectral lines

Slide # 38: Luminosity of Stars

Absolute Magnitude

magnitude of stars at 10 pc

Energy Output

ergs/second or watts

Solar Luminosities

energy output compared to the Sun

Slide # 39: The HR Diagram

Is there a correlation between surface temperature and luminosity?

is the radius tied to surface temp?

E. Hertzsprung and H. Russell 1920's

luminosity vs surface temperature

Slide # 40: The HR Diagram (GRAPHICS)

observations

Slide # 41: The Main Sequence

90% of stars

most of these stars are Red Dwarfs

K and M stars with low luminosity

a few are Blue Giants

hot, luminous stars

a very few are Blue Supergiant

very hot, very luminous stars

Slide # 42: The Main Sequence (GRAPHICS)

the HR diagram

Slide # 43: Red Giants and White Dwarfs

Red Giants

cool, luminous stars

upper right on HR diagrams

must have large radius

White Dwarfs

hot, low luminosity stars

lower left on HR diagram

must have small radius

Slide # 44: The HR Diagram (GRAPHICS)

HR diagram

Slide # 45: Size of Red Giants

Luminosity = 100 L

Temperature = 1/2 T (= 3000K)

L = R2T4

if everything is in Solar Units

100 = R2 (1/2)4

100 = R2 (1/16)

1600 = R2

R = 40 times the Radius of the Sun

32 million km!

Slide # 46: Size of White Dwarfs

Luminosity = 0.01 L

Temperature = 2 T (= 12,000K)

L = R2T4

if everything is in Solar Units

0.01 = R2 (2)4

0.0001 = R2 (16)

0.00000625 = R2

R = 0.0025 times the Radius of the Sun

size of Earth!

Slide # 47: Spectral Subclasses

each spectral class is subdivided

10 subdivisions in each class

...A0 A1 A2... A8 A9 F0 F1 F2 ... F8 F9 G0 G1...

Slide # 48: Spectral Types

spectral classes

OBAFGKM

spectral subclasses

0123456789

Luminosity Class

I,II,III - giants

IV - subgiants

V - main sequence

can be determined using spectral analysis

Slide # 49: Spectral Types (GRAPHICS)

complete Spectral Type

Slide # 50: Spectroscopic Parallax

spectral type = luminosity and temperature

luminosity = absolute magnitude

apparent and absolute magnitude can be used to calculate distance

inverse square law

distance can be determined using spectral type

spectroscopic parallax

Slide # 51: Stellar Masses

determined in some binary star systems

Kepler's law

all stars on the MS with the same spectral type have the same mass

hot stars on the MS have higher masses than cool stars

Slide # 52: Spectral Type and Mass

B - 10 M

A - 2.5 M

G - 1.0 M

M - 0.1 M

Slide # 53: Mass and Luminosity

mass and luminosity are related for MS stars

all F stars on the MS have the same luminosity

all F stars on the MS have the same mass

therefore, all stars with the same mass on the MS have the same luminosity

Slide # 54: Stellar Lifetime

O stars are more luminous than M stars

1,000,000 brighter

O stars are more massive than M stars

100 times more massive

O stars must be burning their fuel very fast

O stars will run out of fuel before M stars

Slide # 55: Star Clusters (GRAPHICS)

formed out of same gas clouds

formed at about the same time

seen at about the same distance

IDEAL "labs" for studying stars

Slide # 56: The HR Diagram

HR diagram

Slide # 57: M13 (GRAPHICS)

a Globular Cluster in Hercules

Slide # 58: Taurus (GRAPHICS)

Feb 15 - 9pm - S - 5.0

Slide # 59: The Pleiades (GRAPHICS)

UK Schmidt Image of an Open Cluster