Outlines •  Chapter 15 “Surveying the Stars”

– What kinds of star out there? – How can we measure their properties?

•  Chapter 16 “Star Birth” – How do they form?

•  Chapter 17 “Star Stuff” – How do stars live/evolve?

•  Chapter 18 “The Stellar Graveyard” – How do stars die?

Chapter 15

Surveying the Stars

Insert TCP 5e Chapter 15 Opener

Sun

Let’s look at some pictures of stars

A variety of colors.

Star Evolves (Change with time) High mass star

Low mass star Evolution depends on mass.

Planetary Nebula (singular) Planetary Nebulae (plural)

The end stage of most stars.

Nova -- Novae Small explosion on stellar surface causes nova (star suddenly becomes bright).

The explosion is caused by the material falling from a companion star.

Binary star

Two stars orbiting around each other.

Prof. Fred Walter is an expert of Novae.

Supernova – Supernovae Two types of suparnova (Type I and Type II)

Type II Supernova

Type I Supernova Computer simulation (By Prof. Alan Calder at SBU)

Stellar Clusters Open Clusters

Cluster in Formation

M45 (Pleiades)

Open clusters are mostly in the Milky Way.

Stellar Clusters Globular Clusters

Concentrations of stars. All stars in stellar clusters formed almost at the same time (hence, all stars have the same age).

Mostly outside the Milky Way (MW) disk, but still inside the MW.

15.1 Properties of Stars

Our goals for learning: •  How do we measure stellar luminosities? •  How do we measure stellar temperatures? •  How do we measure stellar masses?

In science, we measure stuffs and understand them. But how?

Remember: In case of Sun

•  We know the mass and luminosity (amount of lights per second) of the Sun.

•  From those numbers, we made the model of the Sun.

•  For other stars, we have to measure those numbers first, and then make models (understand it!).

How do we measure stellar luminosities?

What is the intrinsic brightness of stars?

How bright this Bulb is? How much watt??

The brightness of a star depends on both distance and luminosity

Luminosity:

Amount of power a star radiates

(energy per second = watts) Apparent brightness:

Amount of starlight that reaches Earth

(energy per second per square meter)

Insert TCP 5e Figure 15.1

Thought Question

These two stars have about the same luminosity -- which one appears brighter?

A. Alpha Centauri B. The Sun

Luminosity passing through each sphere is the same Area of sphere: 4π (radius)2 Divide luminosity by area to get brightness

The same amount of energy passes through these three squares. The area of the squares increases with distance à energy per unit area decreases.

The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4π (distance)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4π (distance)2 x (Brightness)

Thought Question

How would the apparent brightness of Alpha Centauri change if it were three times farther away?

A. It would be only 1/3 as bright B. It would be only 1/6 as bright C. It would be only 1/9 as bright D. It would be three times brighter

So how far are these stars?

To calculate luminosity from apparent brightness, we need distance.

Parallax is the apparent shift in position of a nearby object against a background of more distant objects

Parallax angle

Because the Earth orbits around the Sun every year, the direction of a star changes with season.

Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun

Parallax angle depends on distance

Angle=1 arcsecond

Because the Earth orbits around the Sun every year, the direction of a star changes with season.

Distance = 1 parsec

Parallax and Distance

p = parallax angle

d (in parsecs) = 1p (in arcseconds)

d (in light-years) = 3.26 × 1p (in arcseconds)

Parallax angle gives us distance.

Now you know •  Apparent brightness

• how much energy you receive on Earth •  Distance

Measuring Luminosity

You can calculate Luminosity

Most luminous stars: 106 LSun Least luminous stars: 10-4 LSun (LSun is luminosity of Sun)

The Magnitude Scale

€

m = apparent magnitude , M = absolute magnitude

apparent brightness of Star 1apparent brightness of Star 2

= (1001/ 5)m1−m2

luminosity of Star 1luminosity of Star 2

= (1001/ 5)M 1−M 2

A 1st magnitude star is 100 times brighter than 6 magnitude star.

How do we measure stellar temperatures?

Thermal Radiation

•  Nearly all objects emit thermal radiation, including stars, planets, you…

•  An object’s thermal radiation spectrum depends

on only one property: its temperature

Properties of Thermal Radiation Hotter objects 1.  emit more light at all frequencies per unit area. 2.  emit photons with a higher average energy. 3.  are bluer (whiter).

Bluer Redder

Color of Stars

Star’s color depends on its surface temperature. More massive stars have higher temperature, therefore are bluer (whiter) than less massive stars.

Hottest stars: 50,000 K Coolest stars: 3,000 K (Sun’s surface is 5,800 K)

Solid

Molecules

Neutral Gas

Ionized Gas (Plasma)

So! Spectral lines (level of ionization) also tell us star’s temperature

10 K

102 K

103 K

104 K

105 K

106 K Phase of Matter

1. Phase of matter depends on temperature.

2. Spectral line emission and absorption depends on the phase.

Absorption lines in star’s spectrum tell us ionization level

Lines in a star’s spectrum correspond to a spectral type that reveals its temperature (Hottest) O B A F G K M (Coolest)

(Hottest) O B A F G K M (Coolest)

Remembering Spectral Types

•  Oh, Be A Fine Girl, Kiss Me

•  Only Boys Accepting Feminism Get Kissed Meaningfully

Pioneers of Stellar Classification

•  Annie Jump Cannon and her collaborators at Harvard laid the foundation of modern stellar classification

How do we measure stellar masses?

Most luminous stars: 106 LSun Least luminous stars: 10-4 Lsun (LSun is luminosity of Sun)

Most massive stars: ? MSun Least luminous stars: ? Msun (MSun is mass of Sun)

Question

The orbit of a binary star system depends on strength of gravity REMEMBER: Newton’s law of gravity. Gravity (Mass) Orbital Motion

Types of Binary Star Systems

•  Visual Binary •  Eclipsing Binary •  Spectroscopic Binary

About half of all stars are in binary systems

Visual Binary

We can directly observe the orbital motions of these stars

Newton’s law of gravity is universal à Binary stars show elliptical orbits like Earth’s orbit around the Sun

SBU Prof. Mike Simon is doing this.

Eclipsing Binary

We cannot observe the orbital motion, but can measure periodic eclipses

A B

Spectroscopic Binary

Again, we cannot see the orbit, but can determine the orbit by measuring Doppler shifts

We measure mass using gravity

Direct mass measurements are possible only for stars in binary star systems

p = period

a = average separation

p2 = a3 4π2

G (M1 + M2)

Need 2 out of 3 observables to measure mass:

1)  Orbital Period (p) 2)  Orbital Separation (a or r = radius) 3)  Orbital Velocity (v)

For circular orbits, v = 2πr / p r M

v

Most massive stars: 100 MSun Least massive stars: 0.08 MSun (MSun is the mass of the Sun)

Supermassive Black Hole at the Center of Milky Way 4 million times the mass of our Sun

Gravity (Mass)

Orbital Motion

What have we learned? •  How do we measure stellar luminosities?

–  If we measure a star’s apparent brightness and distance, we can compute its luminosity with the inverse square law for light

– Parallax tells us distances to the nearest stars

•  How do we measure stellar temperatures? – A star’s color and spectral type both reflect its

temperature

What have we learned? •  How do we measure stellar masses?

– Newton’s version of Kepler’s third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a)

15.2 Patterns Among Stars

Our goals for learning: •  What is a Hertzsprung-Russell diagram? •  What is the significance of the main

sequence? •  What are giants, supergiants, and white

dwarfs? •  Why do the properties of some stars vary?

What is a Hertzsprung-Russell diagram?

Stellar Classification •  Library Classification (example in our life)

– Alphabetical indexing – Subject access (politics, economics, etc)

•  Stellar Classification

– Many types of stars – Need to categorize (classify) them before understanding. – Of course, we want to do this in a scientific way.

Temperature (Color)

Lum

inos

ity

Stellar Classification on an H-R diagram. An H-R diagram plots the luminosity and temperature (color) of stars

Hertzsprung-Russell diagram

Observe luminosity & surface temperature of stars.

Plot on the H-R diagram

Roughly, Three types: •  Giants & supergiants

• Main sequence •  White Dwarfs

Most stars fall somewhere on the main sequence of the H-R diagram Sun is a main sequence star.

Main Sequence

Stars with lower T and higher L than main-sequence stars must have larger radii: giants and supergiants

Large radius

Main Sequence

They are redder àLower Temperature à Fainter (in unit area) à Have to be bigger to be bright (higher L)

Small radius

Stars with higher T and lower L than main-sequence stars must have smaller radii: white dwarfs

Main Sequence

They are bluer (whiter) àHigher Temperature à Brighter (in unit area) à Have to be smaller to be faint (lower L)

Temperature

Lum

inos

ity

H-R diagram depicts:

Temperature

Color

Spectral Type

Luminosity

Radius

Color Blue Red

High Low

Spectral Type O B A F G K M

Small radius

Large radius

Temperature

Lum

inos

ity

Which star is the hottest?

A, B, C, or D

Quiz 1

Temperature

Lum

inos

ity

Which star is the most luminous?

A, B, C, or D

Quiz 2

Temperature

Lum

inos

ity

Which star is a main-sequence star?

A, B, C, or D

Quiz 3

Temperature

Lum

inos

ity

Which star has the largest radius?

A, B, C, or D

Quiz 4

Hertzsprung-Russell diagram

•  An H-R diagram is a good, scientific way to classify stars.

•  The Location of a star in H-R diagram gives you an idea of characteristics of the star.

What is the significance of the main sequence?

Main-sequence stars are fusing hydrogen into helium in their cores like the Sun Luminous main-sequence stars are hot (blue) Less luminous ones are cooler (yellow or red)

Luminous and Hotter (blue)

Less luminous and Cooler

Sun is a main-sequence star.

Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones

Mass of main-sequence star Luminous and Hotter (blue) Massive

Less luminous and Cooler Less massive

The mass of a normal, hydrogen-burning star determines its luminosity and spectral type!

High-mass stars

Low-mass stars Why more massive main-sequence star is more luminous and bluer?

Two forces balance Gravity ~ Pressure

Remember: Gravitational Equilibrium

More mass à Stronger gravity à Higher core pressure à Higher core temperature à More nuclear fusion (more energy) à More luminosity Nuclear Fusion

Stellar Properties Review Luminosity: from brightness and distance

10-4 LSun - 106 LSun Temperature: from color and spectral type

3,000 K - 50,000 K Mass: from period (p) and average separation (a)

of binary-star orbit

0.08 MSun - 100 MSun

(0.08 MSun) (100 MSun)

(100 MSun) (0.08 MSun)

Mass & Lifetime Massive star

Less massive star

Guess Which star dies first? In other word, which star exhausts the fuel for nuclear fusion first?

A = Massive star B = Less massive star

Mass & Lifetime Sun’s life expectancy: 10 billion years Life expectancy of 10 MSun star:

10 times as much fuel, uses it 104 times as fast

10 million years ~ 10 billion years x 10 / 104

Life expectancy of 0.1 MSun star:

0.1 times as much fuel, uses it 0.01 times as fast

100 billion years ~ 10 billion years x 0.1 / 0.01

Until core hydrogen (10% of total) is used up

Note: the age of the Universe is 14 billion years.

Main-Sequence Star Summary High Mass:

High Luminosity Short-Lived Large Radius Blue

Low Mass:

Low Luminosity Long-Lived Small Radius Red

What are giants, supergiants, and white dwarfs?

Off the Main Sequence •  Stellar properties depend on both mass and age: those that

have finished fusing H to He in their cores are no longer on the main sequence

•  All stars become larger and redder after exhausting their

core hydrogen: giants and supergiants •  Most stars end up small and white after fusion has ceased:

white dwarfs

After the lifetime in main-sequence, a star evolves to the giants/supergiants part of an H-R diagram.

Stellar envelope (outer layer) expands à Cool down à Bigger and Redder

Main-sequence à Giants/Supergiants

Sizes of Giants and Supergiants

Planetary Nebula

An expanding outer layer of a star eventually escapes from star’s gravitational field.

Planetary nebula is an expanding envelope (outer layer) of star.

White Dwarf

At the end, The stellar envelope (outer layer) keeps expanding. The core shrink to become white dwarf.

If it’s a massive star, the core becomes even more massive, dense object (i.e., neutron star or black hole).

What have we learned? •  What is a Hertzsprung-Russell diagram?

– An H-R diagram plots stellar luminosity of stars versus surface temperature (or color or spectral type)

•  What is the significance of the main sequence? – Normal stars that fuse H to He in their cores

fall on the main sequence of an H-R diagram – A star’s mass determines its position along the

main sequence (high-mass: luminous and blue; low-mass: faint and red)

What have we learned? •  What are giants, supergiants, and white

dwarfs? – All stars become larger and redder after core

hydrogen burning is exhausted: giants and supergiants

– Most stars end up as tiny white dwarfs after fusion has ceased

•  Why do the properties of some stars vary? – Some stars fail to achieve balance between

power generated in the core and power radiated from the surface

15.3 Star Clusters

Our goals for learning: •  What are the two types of star clusters? •  How do we measure the age of a star

cluster?

What are the two types of star clusters?

Open cluster: A few thousand loosely packed stars All member stars formed almost at the same time, so they have

the same age.

Globular cluster: Up to a million or more stars in a dense ball bound together by gravity All member stars formed almost at the same time, so they have the same age.

How do we measure the age of a star cluster?

Massive blue stars die first, followed by white, yellow, orange, and red stars

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

Main-sequence turnoff Massive stars have

short lifetimes on main-sequence, and must have moved to giants/supergiants.

No star Giants Supergiants

Main-sequence turnoff point of a cluster tells us its age

4 clusters

To determine accurate ages, we compare models of stellar evolution to the cluster data

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

Born very early in the Universe.

Globular Cluster

What have we learned? •  What are the two types of star clusters?

– Open clusters are loosely packed and contain up to a few thousand stars

– Globular clusters are densely packed and contain hundreds of thousands of stars

•  How do we measure the age of a star cluster? – A star cluster’s age roughly equals the life

expectancy of its most massive stars still on the main sequence