Properties of Stars. “All men have the stars,” he answered, “but they are not the same things for different people. For some, who are travelers, the stars.

Properties of Stars

“All men have the stars,” he answered, “but they are not the same things for different people. For some, who are travelers, the stars are guides. For others they are no more than little lights in the sky. For others, who are scholars, they are problems. For my businessman they were wealth. But all these stars are silent. You – you alone – will have the stars as no one else has them.”

Antoine de Saint-Exupery (1900 – 1944)from The Little Prince

WHAT DO YOU THINK?1. How near to us is the closest star other than the Sun?

2. What colors are stars, and why do they have these colors?

3. How luminous is the Sun compared with other stars?

4. Are brighter stars hotter than dimmer stars?

5. Compared to the Sun, what sizes are other stars?

6. Are most stars similar to the Sun, one star with planets, or in multiple-star groups?

A Snapshot of the Heavens

• How can we learn about the lives of stars, which last tens of millions to hundreds of billions of years?

• we will never observe a particular star evolve from birth to death

• so how can we study stellar evolution?

How can we study the “Life Cycles” of stars?

• Key: All stars were NOT born at the same time.• stars we see today are at different stages in their lives

• we observe only a brief moment in any one star’s life

• by studying large numbers of stars, we get a “snapshot” of one moment in the history of the stellar community

• we can draw conclusions just like we would with human census data…we do stellar demographics!

A Snapshot of the Heavens

• What two basic physical properties do astronomers use to classify stars?

• What does that classification tell us?

Classification of Stars

• Stars were originally classified based on:

• their brightness

• their location in the sky

• This classification is still reflected in names of the brightest stars…those we can see with our eyes:

Orionis

Geminorum

Order of brightness within a constellation

Latin Genitive of the constellation

Classification of Stars

• The old classification scheme told us little about a star’s true (physical) nature.

• a star could be very bright because is was very close to us; not because it was truly bright

• two stars in the same constellation might not be close to each other; one could be much farther away

Classification of Stars

• In 20th Century, astronomers developed a more appropriate classification system based on:• a star’s luminosity• a star’s surface temperature

These properties turn out to depend on: a star’s mass and its stage in life

measuring these=> reconstruct stellar life cycles

Classification of Stars

WHAT DO YOU THINK?1. How near to us is the closest star other than the Sun?

2. What colors are stars, and why do they have these colors?

3. How luminous is the Sun compared with other stars?

4. Are brighter stars hotter than dimmer stars?

5. Compared to the Sun, what sizes are other stars?

6. Are most stars similar to the Sun, one star with planets, or in multiple-star groups?

WHAT DO YOU THINK?

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

Step 1: Distance!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

PARALLAX

•Determines distance based on Earth’s Orbit around Sun.

•SMALL angular shift!

•Even closest stars (4.3 light years) show shift less than 1/4000th of a degree!

PARALLAX

•Example http://www.solstation.com/stars/61cygni2.htm

•Animation http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/otherstars.htm

•Current telescopes can measure angles as small as 1/400,000th of a degree (400+ light years)

Step 2: TEMPERATURES!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

COLORS of stars

lead to Surface Temperatures!

Pickering’s “Harem” of women computers at Harvard College

Annie Cannon

Spectral Type Classification System

O B A F G K M

Oh Be A Fine Girl/Guy, Kiss Me!

50,000 K 3,000 K Temperature

(L)

Peak of radiation is at shorter wavelengths for hotter stars

MUCH more intensity at every wavelength for hotter stars

Starlight Follows 2 Key Radiation Laws

Step 3: BRIGHTNESS!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

How LUMINOUS are stars?

Bright because they are nearby…

or…

Bright because they are really, truly bright?

Sirius is bright because it is CLOSE!

Sirius is bright because it is CLOSE!

Rigel is bright because it is REALLY bright!

Apparent Magnitudes

Ancient method for measuring stellar brightness from Greek astronomer Hipparchus (c. 190 – 120 B.C.)

This scale runs backwards:

The bigger the number, the fainter the star

Brightest stars are #1, next brightest are #2, etc.

Inverse Square Law for Light (how distance relates to brightnes)

Absolute Magnitudes

•Takes into account distance of star!

•Treats all stars as if they were same distance away!

•True measure of star’s real brightness.

Relating Measurable Quantities

Measure distances of nearby stars Deduce how bright they really are Determine surface temperatures from

spectra

HOW do temperatures and brightness relate to one another?

Step 4: HR Diagram!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

Sun is brighter than most “nearby” stars

Sun is much, much dimmer than most “bright” stars

Our Star! The Sun

The Hertzsprung-Russell (HR) Diagram

HOT COOL

BRIGHT

FAINT

90% of all stars lie on the main sequence!

WHY??

Hypotheses to Explore

Stars are born like children, cool and small, and then heat up and grow brighter over time.

Stars are like candles, hot and bright, and then cool off and get dimmer over time.

Stars are born with different temps and brightnesses, and change little over 90% of their lives.

Hypotheses to Explore

IF…. Stars are born like children, cool and small, and then heat up and grow brighter over time.

THEN… new clusters of stars should all be hot O stars, and old clusters only M stars.

Hypotheses to Explore

IF…Stars are like candles, hot and bright, and then cool off and get dimmer over time.

THEN… new clusters should show only M stars, and older ones should show O stars.

So… DO the EXPERIMENT!

The Pleiades

Hypotheses to Explore IF…. Stars are born like children, cool and small,

and then heat up and grow brighter over time.

IF…Stars are like candles, hot and bright, and then cool off and get dimmer over time.

“New” clusters like the Pleiades show all kinds of stars!

Step 5: Size!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

Diameter (Size) of Stars Calculated from known values:

Luminosity Temperature

Laws of Physics Stefan-Boltzmann Law:

Luminosity ~ Surface Area x T4

PingPong balls, volleyballs, and Stars…

Bigger

Smaller

HUGE

tiny!

Determining Size with SPECTRA!

Larger stars can be less dense at edges Less dense gas changes absorption lines

Luminosity Classes of Stars

•Based on Spectral Line

Shapes & Density

•Tied to physical SIZE

Step 6: Mass!

1. How near are stars? DISTANCE

2. What colors are stars? TEMPERATURE

3. How luminous are stars? LUMINOSITY

4. Are brighter stars hotter? TEMP vs. LUMIN.

5. What sizes are stars? SIZE

6. Single or Multiple? ORBITS MASS

Masses from BINARY STARS

Masses from BINARY STARS

•Doppler Shift of Spectra Lines tells us orbital velocities

•Velocities get us Orbital Sizes

•Orbits get us Mass

Eclipsing BINARY STARS

Mass from Light Curve

Know periods from the light curve… Get orbit distances from Kepler’s Laws Get relative masses from Law of Gravity! Match mass to main sequence star type

FIND: O-stars are more massive than M stars…

Mass-Luminosity Relationship

for Main Sequence

stars

HR Diagram Mass-Luminosity

Relationship for

Main Sequence stars

So… what is going on?

• Why are only SOME stars in the supergiant regions?

•Why are 10% in the “giant” region

•What are the tiny ones in the “white dwarf” region?

Clusters of Stars as Key Tests

Look at large populations of stars Open clusters in Milky Way’s “disk” Globular Clusters around galaxy

Assume all stars *about* same age

Assume all stars *about* same distance

Open Clusters

• 100’s of stars• 106 - 109 years old• irregular shapes• gas or nebulosity is

sometimes seen

Pleiades (8 x 107 yrs)

Globular Clusters

105 stars 8 to 15 billion years

old (1010 yrs) spherical shape NO gas or

nebulosity

M 80 (1.2 x 1010 yrs)

Pleiades H-R Diagram

Globular Cluster H-R Diagram

Palomar 3

Summary of Key Ideas

Magnitude Scales Determining stellar distances from Earth is the first step to

understanding the nature of the stars. Distances to the nearer stars can be determined by stellar parallax, which is the apparent shift of a star’s location against the background stars while Earth moves along its orbit around the Sun. The distances to more remote stars are determined using spectroscopic parallax.

The apparent magnitude of a star, denoted m, is a measure of how bright the star appears to Earth-based observers. The absolute magnitude of a star, denoted M, is a measure of the star’s true brightness and is directly related to the star’s energy output, or luminosity.

Magnitude Scales

The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 pc. Absolute magnitudes can be calculated from the star’s apparent magnitude and distance.

The luminosity of a star is the amount of energy emitted by it each second.

The Temperatures of Stars

Stellar temperatures can be determined from stars’ colors or stellar spectra.

Stars are classified into spectral types (O, B, A, F, G, K, and M) based on their spectra or, equivalently, their surface temperatures.

Types of Stars The Hertzsprung-Russell (H-R) diagram is a graph on which

luminosities of stars are plotted against their spectral types (or, equivalently, their absolute magnitudes are plotted against surface temperatures).

The H-R diagram reveals the existence of four major groupings of stars: main-sequence stars, giants, supergiants, and white dwarfs.

The mass-luminosity relation expresses a direct correlation between a main-sequence star’s mass and the total energy it emits.

Distances to stars can be determined using their spectral types and luminosity classes.

Stellar Masses Binary stars are surprisingly common. Those that can be

resolved into two distinct star images (even if it takes a telescope to do this) are called visual binaries.

The masses of the two stars in a binary system can be computed from measurements of the orbital period and orbital dimensions of the system.

Some binaries can be detected and analyzed, even though the system may be so distant (or the two stars so close together) that the two star images cannot be resolved with a telescope.

Stellar Masses A spectroscopic binary is a system detected from the

periodic shift of its spectral lines. This shift is caused by the Doppler effect as the orbits of the stars carry them alternately toward and away from Earth.

An eclipsing binary is a system whose orbits are viewed nearly edge-on from Earth, so that one star periodically eclipses the other. Detailed information about the stars in an eclipsing binary can be obtained by studying its light curve.

Mass transfer occurs between binary stars that are close together.

Key Termsabsolute magnitudeapparent magnitudebinary starcenter of massclose binaryeclipsing binarygiant starHertzsprung-Russell (H-R) diagraminitial mass functioninverse-square law

light curveluminosityluminosity classmain sequencemain-sequence starmass-luminosity relationOBAFGKM sequenceoptical doublephotometryradial-velocity curvered giant

spectral typesspectroscopic binaryspectroscopic parallaxstellar evolutionstellar parallaxstellar spectroscopysupergiantvisual binarywhite dwarf

WHAT DID YOU THINK?

How near to us is the closest star other than the Sun?

Proxima Centauri is about 25 trillion mi (40 trillion km) away. Light from there will take about 4 years to reach Earth.

WHAT DID YOU THINK?

How luminous is the Sun compared with other stars?

The most luminous stars are about a million times brighter, and the least luminous stars are about a hundred thousand times dimmer than the Sun.

WHAT DID YOU THINK?

What colors are stars, and why do they have these colors?

Stars are found in a wide range of colors, from red through violet as well as white. They have these colors because they have different temperatures.

WHAT DID YOU THINK?

Are brighter stars hotter than dimmer stars?

Not necessarily. Many brighter stars, such as red giants, are cooler but larger, than hotter, dimmer stars, such as white dwarfs.

WHAT DID YOU THINK?

Compared to the Sun, what sizes are other stars?

Stars range from more than 1000 times the Sun’s diameter to less than 1/100 the Sun’s diameter.

WHAT DID YOU THINK?

Are most stars isolated from other stars, as the Sun is?

No. In the vicinity of the Sun, one-third of the stars are found in pairs or larger groups.