What is a star? A body of gases that gives off tremendous amounts of energy in the form of light & heat. What star is closest to the earth?

Stars

Answer: The SUN It s about 150,000,000 km from earth = 1 astronomical unit (AU) = 93 million miles AU = unit to measure distance in astronomy

Are all stars the same? Answer: NO!

How are they different? Differ in color Vary in size & mass Differ in composition Differ in temperature Distance from earth different Differ in brightness

If stars are so far away, how do we know what they are made of? Spectrum Analysis of visible light Use a SPECTROSCOPE separates light into different colors and wavelengths

Continued... Stars are composed of gaseous elements & each element emits a different color The two most common elements are H and He

How hot are stars? Temp indicated by color of star Blue stars = very hot Red stars = very cool Yellow stars = medium Red-Yellow What is our star? or 5,500 o C

Star Brightness Measure by Apparent Magnitude Scale 1-6 (1 = bright; 6 = faint) measure brightness as seen from earth; doesn t measure luminosity (actual true brightness)

Continued... Absolute Magnitude express the luminosity of stars as if they were seen from same distance Example: Sirius apparent magnitude is 10X greater than Antares. Yet Antares has a luminosity 250 x greater than Sirius

Motion of A Star Can move in 3 ways: Rotate on own axis Revolve around another star (not all do this) Move toward or away from earth

Continue... Most galaxies appear to have stars that are moving away from earth How do we know? The Doppler Effect

The Doppler Effect Apparent shift in wavelength of light Blue shift occurs because the light waves from star appear to be shorter as star moves toward the earth

The Doppler Effect Apparent shift in wavelength of light Red shift occurs because the light waves from the star appear to be longer as the star moves away from the earth

How is distance of star measured? By Light Years What is a light year? The distance light travels in one year. How fast is that? 386,000 ft/sec

Distance continued... Light travels approximately 9.5 trillion km in one year It takes light from the sun about 8 minutes to reach earth or 8 light minutes The closest star system to earth is Alpha Centauri

Stellar Evolution Chapter 24, pgs 667-674

Stellar Evolution Stars are formed from huge clouds of dust & gas called nebulae Composed of Hydrogen gas gravitational pull causes nebula to shrink inward and begin to spin

Nebula cloud of of interstellar gas and dust Protostar a disk shaped object with a hot condensed center Brown Dwarf Small (almost planet-sized) gaseous body too small for nuclear fusion to to begin Supergiant Fuses successively heavier elements in the core until iron core develops. Main Sequence Star Fuses hydrogen to to helium in in core; stable and longlived Red Giant Fuses hydrogen to to helium in in shell around core; later, begins fusing helium into carbon/oxygen.

Supernova The explosion caused when a massive star dies and collapses Planetary Nebula the shell of gas driven off by a dying star Neutron Star Extremely small, dim, and dense stellar remnant, composed of neutrons Black Hole Remains of a star that has exploded and collapsed in on itself White Dwarf Dim "star" of helium, carbon, or oxygen shining due to leftover heat no nuclear reactions

Basic Structure of Stars The mass and composition of a star determines nearly all of its properties: temperature, luminosity, even its diameter Temperature inside a star governs the rate of nuclear reactions Also, the more massive a star is, the greater the gravity pressing inward on it, and the hotter and denser it must be inside to balance gravity

Basic Structure of Stars Hydrostatic Equilibrium: the balance between gravity pushing inward and pressure from nuclear fusion and radiation pushing outward

Fusion Stars on the Main Sequence fuse hydrogen into helium, like our Sun Stars that are NOT on the Main Sequence fuse different elements in their cores, or do not fuse at all Other reactions can produce even heavier elements He C C + He O Other elements: Ne, Mg, Si

Star Formation Nebula: cloud of interstellar gas and dust Protostar: a disk shaped object with a hot condensed center The nebula collapses in on itself as a result of its own gravity and as the cloud contracts it takes on the disk like shape called a protostar. The condensed object becomes a new star.

= concentric cloud of dust and gas that spin & shrink takes several million years when temp = 15,000,000 o C it causes H fusion to begin & will eventually become a main - sequence star.

Nebula

Protostar

Fusion Begins Once the temperature inside of the protostar is high enough, fusion begins. The star is stable when hydrogen to helium fusion begins because it then has reached hydrostatic equilibrium

Main-Sequence Star Our sun is a main-seq. Star These stars are generally stable H atoms are combining to form He the gravitational pulls inward are equal to the ones pushing outward

What is a Red Giant? A very large, cooling star when star begins to run out of H the inward force becomes greater putting lots of pressure on the core causing the He to form carbon this fusion releases massive amounts of energy causing the star to enlarge to a red giant or super giant

Red Giant Once hydrogen fusion has ceased in the core, it then has a helium center However, some hydrogen will continue to react just outside of the core of the star The energy that is produced just outside of the core pushes the outer layers of the star outward The star expands and begins cooling increasing its luminosity, but decreasing its temperature

Red Giant It begins losing gases from its outer layers due to its low surface gravity The helium core begins fusion at 100 million K and creates carbon The star contracts back into its normal size during the helium-burning phase This phase will last for about one tenth of the hydrogen burning phase Once complete, the star is left with a carbon core

A Nebula Once Again A star like the Sun is not hot enough for carbon to react The outer layers expand and are driven off by pulsations Planetary Nebula: the shell of gas driven off by a dying star In the center of the planetary nebula is the former core of a star called a white dwarf made of carbon

What are dwarf stars? Form when He fusion stops outer gravity pushing inward causes star to begin to collapse still emits light of that star! no longer emits light & is the last stage A white dwarf that explodes into a black dwarf is called a NOVA

Pressure in White Dwarfs White dwarfs are stable It is supported by the resistance of electrons being squeezed close together

Smaller Stars Stars less massive than the Sun undergo a similar life cycle Helium may never fuse into carbon So the white dwarf can be either helium or carbon The main sequence is longer since lowmass stars are dim and do not use up their fuel as rapidly

Massive Stars These stars are created in the same way Nebula collapses, protostar is formed Hydrogen is used up very quickly due to higher luminosity It becomes a red giant several times throughout its life forming many different layers of reactions These are supergiants

Massive Stars Lose much of their mass during life cycle A star with 8 times the mass of the Sun can end up with a mass of less than 1.4 times the Sun s The composition of the white dwarf depends on how many different reaction phases the star went through Therefore it could be made out of neon!

Supernovae A star that begins with 8-20 times the mass of the Sun will end up with a core too massive to be supported by the electron pressure seen in white dwarfs The core of the star will collapse upon itself Protons and electrons in the core merge together and form neutrons

Supernovae Neutrons cannot be packed too tightly together The resistance to being squeezed creates pressure and stops the collapse of the core making it a Neutron Star: the imploded core of a massive star produced by a supernova explosion

Neutron Star It will form after an explosion of a super nova the core contracts into a dense ball of neutrons some will emit beams of radiation and are called pulsars like this neutron star above

Supernovae The neutron star forms quickly and the outer layers of the star are still falling inward The falling gas strikes the hard surface of the neutron star and it explodes outwards Supernova: the explosion caused when a massive star, at least 8 times the Sun's mass, dies (exhausts its fuel) and collapses This creates elements heavier than iron and enriches the universe Supernovae are 100 times brighter than a nova

Black Holes Stars 20 times the mass of the Sun cannot form neutron stars instead they form black holes Black Hole: the remains of a supermassive star that has exploded and collapsed in on itself and has a gravitational pull so great, light cannot escape

Black Holes Contain many times more mass than the Sun, but are contained in a small volume making them extremely dense Remember more mass, means greater gravitational pull!

Polaris: The North Star A three star system Extremely close to the north celestial pole

Polaris: The North Star At the North Pole, the North Star appears to be directly overhead At the Equator, the north star appears to be stationary and low on the horizon At the South Pole, it cannot be seen!

Planetary Nebula

Hubble Site

Stellar Flowchart Take a few minutes to cut out and complete the stellar flowchart. You may work with a partner (if you re quiet) and you may use your notes. GOOD LUCK!!!!!

Nebula cloud of interstellar gas and dust Protostar a disk shaped object with a hot condensed center Brown Dwarf Small (almost planet-sized) gaseous body too small for nuclear fusion to begin Supergiant Fuses successively heavier elements in the core until iron core develops. Main Sequence Star Fuses hydrogen to helium in core; stable and longlived Red Giant Fuses hydrogen to helium in shell around core; later, begins fusing helium into carbon/oxygen.