STARS Properties Birth Death

MAIN STELLAR PROPERTIES Temperature Brightness Size Mass

MEASURING THE TEMPERATURE OF STARS A star’s color reveals its surface temperature. Wein’s Law - The hotter the object, the shorter the wavelength that is emitted most intensely Formula: λ max = 3 x 10 6 T Hotter the star, the brighter it shines

1. APPARENT MAGNITUDE - How bright a star looks from Earth Depends on the star’s temperature, size, and distance from Earth The smaller the number the brighter the object. Our eye can see a star as dim as +6.0 MEASURING THE BRIGHTNESS OF STARS 3 WAYS TO DESCRIBING STELLAR BRIGHTNESS

A Closer Look at Apparent Magnitude, (m)

2. ABSOLUTE MAGNITUDE Brightness stars if all were the same distance from Earth. 10 parsecs(pc) (or ~32 ly away) It depends only on the star’s temperature and size

3. LUMINOSITY - the total amount of energy released by a star. The more energy released, the brighter the star. 2 Things influence the luminosity of a star: 1. Its size 2. Its temperature

Magnitudes and Distances For Some Well-Known Stars * StarApp.Mag.Distance (pc)Abs.Mag.Luminosity/Sun Sun × Sirius Arcturus Vega Spica Deneb ,000 Barnard's Star /2310 COMPARISON OF SELECTED STARS

CLASSIFYING STARS 1. BY SIZE The Hertzsprung-Russell (H-R) Diagram Shows relationship between star’s luminosity (or absolute magnitude) and surface temperature ( or spectral class)

H-R DIAGRAM

Major Types of Stars (based on the H-R diagram) 1. Main Sequence stars Includes 91% of stars near our solar system. Mature stars. 90% of star’s life Very dim to very bright Very small to very large Very cool to very hot Mass increases with increasing brightness Main Sequence High Mass Low Mass

2. Giant stars Size: times the radius of the Sun. Bright to very bright stars Temperature: 2,000 to 20,000 K Coolest temperature stars in the group are called Red Giants. Giants

3. Supergiants Bigger and more luminous than typical giants Brightest stars Size: up to 1000 times the Sun’s radius Temperature: 3,000-40,000+ K 1 % of the stars in our viewing area. Supergiants

4. Dwarfs (all other stars) Red Dwarfs Main Sequence star Very cool Very dim White Dwarfs Very Hot 8,000 to over 25,000 K Very dim Very small. About Earth-size 8% of local stars. White Dwarfs

Assignment Text Book Appendix A-11(Table E-5) A-12(Table E-4) Plot the 20 Brightest & 20 Nearest Stars on your H-R Diagram You will plot Absolute visual Magnitude vs. Spectral Type(G2, M5 etc).

2. BY TEMPERATURE/SPECTRAL EMISSIONS Spectral Classes are determined by the lines in its spectrum, which is determined by the star’s surface temperature

Spectral Class TemperatureStar Color (visible light)MassRadiusLuminosity O30, ,000 KBluish60151,400,000 B10, ,000 KBluish18720,000 A7, ,000 KWhite bluish tinge F6, ,500 KWhite G5, ,000 KYellowish white K3, ,000 KYellow-orange M2, ,500 KOrange-red Spectral Class of Stars The sun has a surface temperature of 5700 K. It spectral class is G2

H-R DIAGRAM

Supergiants Main Sequence White Dwarfs Stars of same temperature can have different luminosities. How can astronomers know if a star is a dwarf, giant or supergiant?

Luminosity Class Star Type IaVery luminous supergiants IbLess luminous supergiants IILuminous giants IIIGiants IVSubgiants VMain sequence stars (dwarf stars) 3. BY LUMINOSITY Luminosity Class

The luminosity classes as they would appear on the H-R Diagram

Astronomers commonly describe a star by its combine spectral type and its luminosity class. For example: The Sun is called a G2V star. G2 refers to its Spectral Class and V refers to its Luminosity Class. This notation supplies a great deal of information about a star. star’s temperature, size, luminosity, and possibly mass.

Pollux is the brightest star in Gemini. Its spectral type is K0III. What does it say about the star? Polaris is classes as a F7Ib What does it say about the star? Sirius in Canis Major is a A1V. What does it say about the star?

Binary stars Stars that orbit another star held together by their mutual gravitational attraction. At least half of all stars have orbiting companions. Some stars have more than 1 companion. 4. BY CHANGES IN BRIGHTNESS (VARIABLE STARS)

Why are they important? A method to determine the mass of some stars. Based on its gravitational influence on nearby object such as another star or a planet

Detecting Binaries 1.Visual Binaries – pair can be seen visually with telescopes. (closer stars) Example: Albireo in the constellation Cygnus Sirius A & Sirius B (white dwarf)

Spectropical Binaries – very far stars. Study their spectral lines & by Doppler effect

Eclipsing binaries – plane of orbit is in line with our line of sight. Pass in front of one another, affecting the total apparent brightness of the pair.

BIRTH & DEATH OF STARS

2 Forces Control the life & death of stars: Gravity (tries to crush the star) Thermal Pressure (tries to explode the star) What controls these? The Star’s Mass

A.BIRTH: STAR FORMATION PROCESS (THEORETICAL) CLUMP STAGE Giant Molecular Clouds - Nebulas Large interstellar cloud of cool gas and dust Size: ly across Very cold: 10K (-440 o F) Something triggers a clump to collapse inward BIRTH STAGES (Clump stage & Protostar stage

Types of Star Forming Nebula Emission Nebula Reflection Nebula Dark Nebula

Emission Nebula Bright New stars inside heat up surrounding gases. Gases emit their own light. 3 Examples

The Eagle Nebula 7,000 ly away

“Pillars of Creation” within Eagle Nebula Bright tips show new stars forming

Orion Nebula

Middle star of sword 1600 ly away 16 ly across 300 solar masses

Infrared image Deep within Orion Nebula

Rosette Nebula 5,200 ly from Earth 10,000 solar masses Diameter: 130 ly Core temp: 6 million K

Reflection Nebula Gas and dust reflect light/energy from stars within the nebula.

Pleiades 375 ly from Earth Blue glow is reflection of light off interstellar gas and dust

Reflection Nebula IC349

Reflection Nebula M ly away In Orion

Dark Nebula Dark regions of gas and dust. Appear as dark clumps because they block light from distant stars.

Horsehead Nebula 1500 ly away Constellation Orion

Barnard ly away.5 ly across

PROTOSTAR STAGE Core region of highly compressed hydrogen. Core invisible. Surrounded by a shell of infalling gas and dust. New Size :.1-.3 ly across. (.6 – 2 trillion miles) Core Temp.: 1,500 K (2,000-3, F) “Shines” mostly in infrared & radio

MATURE STAR STAGE (MAIN SEQUENCE STARS) Core temp reaches million 0 F Nuclear Fusion begins Star is Born at this event!!!!!! Hydrostatic Equilibrium Reached Outward thermal pressure ( due to high core temp.) balances the gravitational force. Contraction stops.

THERMONUCLEAR FUSION Conversion of lighter elements to heavier elements Core of star very dense. 4 Hydrogen atoms fuse together to produce 1 helium atom. Mass of 1 H= H = Mass of 1 He = Mass Loss of.029 Converted to E-M radiation Every second 600 million tons of H fused to form 596 million tons of He. 4 million tons of matter converted to energy (E-M Radiation) Hydrogen nucleus

B. MATURITY: LIFE ON THE MAIN SEQUENCE Lifetime depends on star’s mass and luminosity Mass = amount of fuel in the core. Luminosity = rate of fuel consumption. High mass, High luminous stars live shortest lives. Low mass, low luminous stars live longest

DEATH OF STARS H Fuel in core is gone; Thermal pressure (outward) decreases; gravity(inward) takes over Star collapses.Temperature of core increases Outer gases heat up & expand. Star Becomes Red Giant or Red Supergiant Final Fate Depends on Star’s Mass (Low mass & High mass)

Death of Low Mass Stars (.8 to 8 Solar Masses) A red giant star has formed. Size: 100’s X sun Core continues to contract. Temperature: 100 million K (180 million 0 F More fusion: Helium Carbon & Oxygen

Death of Low Mass Stars as Seen on HR Diagram

Red Giant becomes a Yellow Giant Unstable. Pulsates. Total Time frame: 1 billion years

Yellow Giant becomes Red Giant again Core size decreases again, compressing and heating it. Outer shell expands. Size increases. Red Giant again

Very hot core heats expanding shell – It glows forming a planetary nebula. Size is about.25 ly across Gases expand at rate of over 45,000 mph. Core of star remains as tiny glowing ball called a white dwarf. Red Giant core becomes White Dwarf

EXAMPLES OF PLANETARY NEBULA

Helix Nebula 450 ly away In Sagittarius

Ring Nebula 2300 ly away In Lyra

Blinking Eye Nebula NGC ,200 ly away In Cygnus

Menzel ly away

Hour Glass Nebula M8 8,000 ly away In Musca (southern hemisphere)

Death of Higher Mass Stars (>8 X Solar Masses) By-pass 1 st red giant stage and go to yellow giant stage. Eventually become red supergiants. Core temp. increase greatly Nucleosynthesis occurs – process of fusion which creating successively heavier elements. Creates a layered structure in core

Nucleosynthesis in High Mass Stars

Core size: smaller than Earth Coret temp: about 2 billion K (3.8 billion 0 F) Iron signals end of life for high mass stars. Fuel gone, gravity takes over, core collapses instantly. Collapse forces protons and electrons to merge: forms neutrons. New size: ~ 6 miles in diameter. Instant collapse causes recoil effect outward in violent explosion known as a supernova

2 TYPES 0F SUPERNOVAS Type Ia Supernovas Binary stars having a white dwarf White dwarf draws matter from companion star Mass increases and explodes as supernova White dwarf is destroyed. Type II Supernova High mass main sequence stars Core becomes neutron star or black hole

Results of Supernova Explosion Explosive force able to create elements heavier than iron. Over 50-80% of star’s mass is lost into space Core becomes neutron star or black hole Supernova Remnant is formed Huge expanding, glowing cloud of debris Several light years across before it disappears

EXAMPLES OF SUPERNOVA REMNANTS

Crab Nebula (Taurus )

Vela Supernova Remnant

Supernova 1987a Supernova Remnant 1987a

The Gum Nebula (300 ly away)

THE REMAINS OF HIGH MASS STARS Neutron Stars. Core composed of neutrons. Size: 6-12 miles across Extremely dense matter Some become Pulsars Rapidly rotating neutron stars Emit strong radiation in narrow beam from its poles. Received as a pulsating raditation source Famous Pulsars: Vela pulsar in Gum Nebula Crab Pulsar in Crab Nebula

Conservation of Angular Momentum Spinning of a figure skater at the end of her performance Mass is spread over a wide radiusMass is spread over a narrow radius

Let’s Apply it to a rotating star like the sun. When the sun’s fuel runs out, it will collapse to the size of Earth. That is a reduction from 880,000 miles in diameter to 8,000 miles in diameter. According to the Law of the Conservation of Angular Momentum, what must happen to the Sun’s rotational speed? What would happen if its diameter is reduced to less than 6 miles

Emission of Radiation by Pulsating Neutron Star

The Crab Pulsar in the Crab Nebula

What would happen if a very massive star collapsed to a volume nearing 0? How strong would it gravitational influence be? Escape velocity of a stellar object is good expression of the influence of gravity on local matter. If the object’s mass was very large then the escape velocity could exceeded that of the speed of light. The object would be a BLACK HOLE

Black Holes Dead Core of the very High Mass Stars Gravity collapse stellar remnant to point with zero volume and infinity density. This is called the singularity Gravity so strong, no force to halt collapse of core Causes nearby space to close in on itself All theory on physics become invalid

Event Horizon Boundary that separate Black holes from rest of universe Not a surface. Escape velocity = speed of light No escaping once crossed

Schwarzschild Radius Distance from singularity to the event horizon To calc. the Schwarzchild Radiuis of any star: R = 2GM/c 2 c = speed of light For our sun: R is ~2 miles

What Happens as object approach a Black Hole? Spaghettification. Total destruction of matter

How Are Black Holes Detected? From effects on nearby matter Binary Black Holes Sucks matter from companion star Matter compressed, heats up and releases X- rays Cygnus X-1 – 1 st black hole detected

D. Summary of Evolution of Low and High Mass Stars Low Mass Stars Protostar (gravity supplies energy) Main Sequence Red Giant Yellow Giant Red Giant White Dwarf surrounded by Planetary Nebula SUMMARY OF STAR EVOLUTION

High Mass Stars (Greater than 8 solar masses) Protostar Main Sequence Yellow Supergiant Red Supergiant Supernova Supernova Remnant w/ Neutron Star or Black Hole