quiz question: binary stars

In which type of binary star system is the plane of the orbit in our line of sight?

A)Visual binaryB)Eclipsing binaryC)Spectroscopic binary

Quiz Question: Binary Stars

In which type of binary star system is the plane of the orbit in our line of sight?

A)Visual binary—we can see each star distinctlyB)Eclipsing binary—plane of orbit in our line of sightC)Spectroscopic binary—Doppler shift evident in the

spectral lines

Quiz Question: Binary Stars

•What determines the main-sequence lifetime of a star?•What are star clusters and how do we determine their ages?•How will the Sun evolve?

Questions—Stellar Evolution

Fig. 16.10

Types of Stars•Main sequence•Giants•Supergiants•White dwarfs

Stellar radii

The Hertzprung-Russell Diagram

Fig. 16.11Stellar lifetimes are determined by stellar masses:

Empirically luminosity is proportional to mass cubed: L∝M3

Lifetime = (Amount of fuel)/(Rate of burning)

Algebraicallyτ = M/L= M/M3

⇒ τ = M-2

Stellar Lifetimes

Lifetime = Mass-2

τ = M-2

The main sequence lifetime of the Sun is going to be 1010

(10 billion) years (yikes, we’re half way there!). What is the main sequence lifetime of a 10 solar mass star?

A)108 yearsB)1010 yearsC)1012 years

Concept Question: Stellar Lifetimes

Lifetime ∝ Mass-2

τ ∝ M-2

The main sequence lifetime of the Sun is going to be 1010

(10 billion) years (yikes, we’re half way there!). What is the main sequence lifetime of a 10 solar mass star (“star x”)?

A)108 yearsB)1010 yearsC)1012 years

τ∝ M-2

τ(sun) ∝ M-2(sun)τ(x) ∝ M-2(x)

τ(x)/τ(sun) = M-2(x)/M-2(sun)τ(x)/τ(sun) = [M(sun)/M(x)]2τ(x)/τ(sun) = [1/10]2 = 0.01τ(x) = 0.01τ(sun) = 108 years

Concept Question: Stellar Lifetimes

There are two types of star clusters: open clusters, which are small (<1000 stars), young, and unbound, and globular clusters, which are large (up to 106 stars), old, and bound.

The Pleiades (108 years old)Fig. 16.14 M80 (>1.2x1010 years old)

Fig. 16.15

Star Clusters

We measure the ages of clusters from their main sequence turnoff points.

Fig. 16.17

Star Cluster Ages

•Over time, the H in the core of the Sun is converted into He.•The core becomes denser, enhancing gravity and hence the pressure & temperature.•Eventually, all the H in the core is used up (converting ~30% of it’s mass to He).•The Sun will then turn off the main sequence and become a subgiant.

Fig. 17.9

Turning Off the MS: The Sun

As the Sun ages on the main sequence, which will it become?

A)Less luminousB) It will stay the same luminosityC)More luminous

Concept Question

As the Sun ages on the main sequence, which will it become?

A)Less luminousB) It will stay the same luminosityC)More luminous—density, pressure, and temperature in

the core increase, increasing the rate of fusion

Concept Question

Fig. 17.10

•The Sun will expand•The photosphere will be cooler

⇒The Sun will be on it’s way to becoming a red giant

Turning Off the MS: The Sun

Main Sequence•H→He in core•Duration 1010 years•Luminosity doubles over lifetime

The Post Main-Sequence Evolution of the Sun

Main Sequence•H→He in core•Duration 1010 years•Luminosity doubles over lifetime

Red Giant •He core contracts→rapid H fusion in shell•Star expands to 100 Rsolar (Mercury’s orbit!)•He core degenerate•Duration 108 years

The Post Main-Sequence Evolution of the Sun

Main Sequence•H→He in core•Duration 1010 years•Luminosity doubles over lifetime

Red Giant •He core contracts→rapid H fusion in shell•Star expands to 100 Rsolar (Mercury’s orbit!)•He core degenerate•Duration 108 years

He Core Burning •“He flash” when core reaches 108 K•3 x 4He → 12C + energy; strong wind•Duration 5x107 years

The Post Main-Sequence Evolution of the Sun

Double Shell Burning Red Giant•C core, He burning inner shell, H burning outer shell•Degenerate core very dense: 1 cm3 has 1000 kg mass!•Small amount of oxygen created

The Post Main-Sequence Evolution of the Sun

Double Shell Burning Red Giant•C core, He burning inner shell, H burning outer shell•Degenerate core very dense: 1 cm3 has 1000 kg mass!•Small amount of oxygen created

Unstable Fusion in Shells•Envelope blown off creating a planetary nebula, leaving a white dwarf behind

The Post Main-Sequence Evolution of the Sun

Fig. 17.13

The Post Main-Sequence Evolution of the Sun on the H-R Diagram

Fig. 17.14a

Planetary Nebulae

Fig.

17.

15A

Evolution of The Sun’s Luminosity

Fig.

17.

15B

Evolution of the Sun’s Radius