Life and Evolution of StarsChapter 9

Outline

I. Masses of Stars: Binary Stars

II. Variable Stars

III. Spectral Types of Stars

IV. H-R Diagram

V. The Source of Stellar Energy

VI. Life Story of a Star

Video Trailer:Birth of Stars

I. Masses of Stars:Binary Stars

1. Binary Stars

More than 50 % of all stars in our Milky Way

are not single stars, but belong to binaries:

Pairs or multiple systems of stars which

orbit their common center of mass.

If we can measure and understand their orbital

motion, we can

estimate the stellar masses.

The Center of Mass (Active_Figure_12)

center of mass = balance point of the

systemBoth masses equal => center of mass is in the middle, rA = rB

The more unequal the masses are, the more it shifts toward the more massive star.

1. Visual Binaries

2. Spectroscopic Binaries

3. Eclipsing Binaries

Types of Binaries:

Visual Binaries (Video_Visual_Binaries)

The ideal case:

Both stars can be seen directly, and

their separation and relative motion can be followed directly.

Sirius A and Siruis B (white dwarf)

Spectroscopic Binaries (Video_Spectr_Binaries)

Usually, binary separation “a” can not be measured directly

because the stars are too close to each other.

A limit on the separation and thus the masses can

be inferred in the most common case:

Spectroscopic Binaries

Spectroscopic Binaries (2)

The approaching star produces blue shifted lines; the receding star produces red shifted lines

in the spectrum.

Doppler shift Measurement of radial velocities

Estimate of separation “a”

Estimate of masses

Spectroscopic Binaries (3)T

ime

Typical sequence of spectra from a spectroscopic binary system

Eclipsing Binaries (Animation)

Usually, the inclination angle of binary systems is unknown uncertainty in

mass estimates

Special case:

Eclipsing Binaries

Here, we know that we are looking at the

system edge-on!

Eclipsing Binaries (2)

Peculiar “double-dip” light curve

Example: VW Cephei

Eclipsing Binaries (3)

From the light curve of Algol, we can infer that the system contains two stars of very different surface

temperature, orbiting in a

slightly inclined plane.

Example:

Algol in the constellation of Perseus

The Light Curve of Algol

II. Variable Stars

Video Trailer: Variable StarsChi Cygni expands and dims, and then contracts

and brightens over 408 days

Variable Stars• A variable star is a star that has lost its hydrostatic

equilibrium. The brightness and size of a variable star change with time as it evolves.

• Two Types of Variable Stars:– Pulsating stars: stars that appear to pulsate and change

brightness. Examples are:

Cepheid variables – RR Lyrae – Neutron stars

(1 to 60 days - About 1 day - A couple of seconds)

– Exploding stars: stars that show extreme brightness variability. Examples are:

Nova – Supernova – T Tauri

Nova outburst (Active_Figure_27)

III. Spectral Types of Stars

Spectral Classification of Stars (1)

Tem

pera

ture

Different types of stars show different characteristic sets of absorption lines.

Spectral Classification of Stars (2)

Mnemonics to remember the spectral sequence:

Oh Oh Only

Be Boy, Bad

A An Astronomers

Fine F Forget

Girl/Guy Grade Generally

Kiss Kills Known

Me Me Mnemonics

Stellar Spectra

OB

A

F

GKM

Surface tem

perature

The Composition of Stars

From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars.

IV. H-R Diagram

Organizing the Family of Stars: The Hertzsprung-Russell Diagram

We know:

Stars have different temperatures, different luminosities, and different sizes.

To bring some order into that zoo of different types of stars: organize them in a diagram of

Luminosity versus Temperature (or spectral type)

Lum

inos

ity

Temperature

Spectral type: O B A F G K M

Hertzsprung-Russell Diagram

orA

bsol

ute

mag

.

The Hertzsprung-Russell Diagram (Simulation)

Most stars are found along the

Main Sequence

The Hertzsprung-Russell Diagram (2)

Stars spend most of their

active life time on the M

ain

Sequence (MS).

Same temperature,

but much brighter than

MS stars

L α R2 x T4 ,where,

L = Luminosity of star

R = Radius of star

T = surface temperature of the star.

The Brightest StarsThe open star cluster M39

The brightest stars are either blue (=> unusually hot) or red (=> unusually cold). (Is this a contradiction?)

The Radii of Stars in the Hertzsprung-Russell Diagram

10,000 times the

sun’s radius

100 times the

sun’s radius

As large as the sun

Rigel Betelgeuse

Sun

Polaris

The Relative Sizes of Stars in the HR Diagram

V. The Source of Stellar Energy

Energy of Stars • All stars are considered as huge balls of gases

where nuclear fusion in their cores produces most of their energies.

• It is possible to calculate an approximate star’s lifetime by determining its mass (tlife ~ 1/M2.5)

• Cold (red ones) stars have longer lifetime than hot stars:– O star: ~ 1 million years– G star (Sun): ~ 10 billion years– M star : ~ 5,000 billion years

• First stage: all stars start fusing hydrogen (H) to make helium (He)

• This stage is considered to be the longest stage in a star’s lifetime ( 90% of its total lifespan)

• Second stage: Fusing of helium (He) to make carbon (C)

• The life of some stars (like our Sun) stops after this stage, but others will continue processing heavier and heavier elements than carbon in their cores.

• For the massive stars (more than 8 solar masses), iron will be the last element that a star can form in its core.

• Stars start their lifetime with a light element core (H) and end up with a heavy element core.

Simulation

VI. Life Story of a Star

Life Story of a Star• Stars are born inside huge interstellar clouds following three

stages:– Giant molecular cloud– Dense cores– Protostar T Tauri star

• Stars are divided into two main groups:– Stars with masses less than 8 solar mass– Stars with masses larger than 8 solar mass

• Stars with mass less than 8 solar mass– Giant molecular cloud– Dense core– T-Tauri star– Main-sequence star: fusing H to make He– Giant star: fusing He to make C– Planetary Nebulae

– White Dwarf (with mass less than 1.4 solar mass)

A T-tauri stage of a star: fast stellar winds

While on the main sequence, a star is in “hydrostatic equilibrium”: inward pressure due to gravity balances the outward pressure due to heat.

Simulation

Do we see white dwarfs?

A special binary system: a white dwarf and a regular star

Outcome: a nova (or Supernova type Ia)

NovaHerculis

1934

March 1935 May 1935

• Stars with mass larger than 8 solar mass– Giant molecular cloud– Dense core– T-Tauri star– Main-sequence star: fusing H to make He– Supergiant star: fusing He to make C, O, Ne, Mg,

Si, ..Fe– Supernova explosion– Neutron star (with mass less than 3 solar

mass), black hole (with different masses..)

Betelgeuse: a supergiant star

Do we see neutron stars?

Neutron star: size no bigger than a city (10-15 km)

Pulsar: Lighthouse Model

�اض النّباإلشعاعي

Pulsar(Video1)

Crab Pulsarالسرطان نّباض

الثانية 30 في نّبضة

Do we see black holes? M87

Another special binary system: a black hole and a regular star

Life Story of a Star: Summary (Simulation)

Properties of WD, NS, and BH (Simulation)

• White dwarfs (WD):– Size: Earth’s size– Mass: less than 1.4 solar mass (Chandrasekhar limit)

• Neutron stars (NS):– Size: 10 to 15 km– Mass: less than 3 solar mass

• Black holes (BH):– Size: depends on the mass (3 - ….. Km) (Simulation)

– Mass: 1 – ……. Solar mass