Life and Evolution of Stars Chapter 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 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)Active_Figure_12 center of mass = balance point of the system Both masses equal => center of mass is in the middle, r A = r B 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)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)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) Time Typical sequence of spectra from a spectroscopic binary system

Eclipsing Binaries (Animation)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 Stars Chi 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.hydrostatic equilibrium Two Types of Variable Stars: –Pulsating stars: stars that appear to pulsate and change brightness. Examples are: Cepheid variables 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 TauriNovaSupernova

Nova outburst (Active_Figure_27)

III. Spectral Types of Stars

Spectral Classification of Stars (1) Temperature Different types of stars show different characteristic sets of absorption lines.

Spectral Classification of Stars (2) Mnemonics to remember the spectral sequence: OhOhOhOhOnly BeBeBoy,Bad AAnAnAstronomers FineFForget Girl/GuyGradeGenerally KissKillsKnown MeMeMeMeMnemonics

Stellar Spectra O B A F G K M Surface temperature

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) Luminosity Temperature Spectral type: O B A F G K M Hertzsprung-Russell Diagram or Absolute mag.

The Hertzsprung-Russell Diagram (Simulation)Simulation Most stars are found along the Main Sequence

The Hertzsprung-Russell Diagram (2) Stars spend most of their active life time on the Main Sequence (MS). Same temperature, but much brighter than MS stars

L α R 2 x T 4, where, L = Luminosity of star R = Radius of star T = surface temperature of the star.

The Brightest Stars The 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 (t life ~ 1/M 2.5 ) 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)

Nova Herculis 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 النبّاض الإشعاعي Pulsar ( Video1 ) Video1

Crab Pulsar نباض السرطان 30 نبضة في الثانية

Do we see black holes? M87

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

Life Story of a Star: Summary (Simulation) (Simulation)

Properties of WD, NS, and BH (Simulation) (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) (Simulation) –Mass: 1 – ……. Solar mass