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Stars and Galaxies BIG Idea: ►The life cycle of every star is determined by its mass, luminosity, magnitude, temperature, and composition. ►Much of our information about our galaxy and the universe comes from ground-based observations Hubble Space Telescope: ► In orbit since 1990. ► Takes extremely sharp images from space. ► Hubble observations have led to numerous scientific breakthroughs, including rate of expansion of the universe. I. Characteristics of Stars A. Composition and Temperature ► What are stars made of? ► mostly hydrogen (H)…about 73% of a star’s mass, ► approximately 25% helium (He), ► and the other elements in small amounts: oxygen (O), carbon (C), nitrogen (N), silicon (Si), magnesium (Mg), neon (Ne), iron (Fe), sulfur (S) Spectral Types: ►Spectroscope: instrument used to determine chemical composition by separating light into different colors (wavelengths). Which stars are the hottest stars? • Blue stars are the hottest • Red stars are the coolest B. Motion and Distance to the Stars ► Doppler Effect: shift in wavelength of light source moving toward or away from an observer. Blue Shift: shorter wavelengths, stars moving towards Earth Red Shift: longer wavelengths, stars moving away from Earth Distances between stars and Earth are measured in light-years. Light Year = distance a light wave travels in one year (one light year = 236,750,151 times around the Earth) C. Stellar Magnitude 1. Apparent Magnitude: how bright a star appears from Earth (depends on light emitted and distance from Earth). LOWER NUMBER = BRIGHTER STAR 2. Absolute Magnitude: true brightness of a star. How bright a star would appear if seen from the same distance (32.6 light years) Most stars fall between -5 and +15 Our sun is +5: middle of the range 3. Luminosity: energy output from the surface of a star per second; measured in watts. ► An Astronomer must know both the star’s apparent magnitude and how far away the star is. ► The brightness depends on both a star’s luminosity and distance from Earth. D. Classification of Stars ► H-R Diagram – shows relationship between absolute magnitude and surface temperature of star the brighter the star, the hotter it is ►Stars have a finite lifetime and evolve over time ►The mass of a star controls its evolution, length of lifetime, and ultimate fate ►As stars evolve, their positions on the Hertzsprung-Russell diagram move… II. Stellar Evolution “theory” ► 1. Nebula – stars start out as clouds of gas and dust 70 % hydrogen 28 % helium 2% heavier elements ►Nebula in the Constellation Orion ►Star Formation ► Gravity pulls particles together, forming a sphere ► As density increases, gravitational attraction increases ► Gravitational forces cause denser regions of nebula to shrink ► As regions become smaller, they spin more rapidly ►Think of an ice skater… …as he/she pulls his or her arms in closer, what happens to the rate of the spin? Protostar – flattened disc of matter with a central concentration (caused by shrinking, spinning region) ►Pressure and density build within the protostar’s center, causing temperature to rise ►Gas is so hot it becomes plasma (a fourth state of matter) ►Temperature continues to increase until it reaches 10,000,000ºC ►At this temperature nuclear fusion begins ►A star is born ! 2. Main Sequence Stars – ► The second and longest stage in the life of a star ► Most stars fall within the main sequence band ► Our sun is a Main Sequence Star ►During this main sequence stage, energy is generated in the core of the star as hydrogen atoms fuse to become helium atoms ►Fusion releases huge amounts of radiant energy 3. Giants – very large cool bright star ► Hydrogen starts to run out and the star expands greatly. ► Super Giants are very big Giants ► Our sun is 5 billion years old and has only converted 5 % of its hydrogen to helium. A Giant is 10x bigger than the Sun and a Supergiant is 100x bigger 4. White Dwarf – final stage of a star ► Planetary Nebula – expanding shell of gases shed by a dying star ►Gravity causes the last of the matter in the star to collapse inward ►What remains is a hot, dense core of matter…a WHITE DWARF ►White dwarfs shine for billions of years before they cool completely ►As white dwarfs cool they become fainter and fainter… ►When they no longer emit energy, they become a black dwarf, a dead star Black dwarfs probably do not yet exist… WHY? Novas – explosions that occur as a white dwarf cools ► Supernova – star that has such a tremendous explosion that it blows itself apart. Neutron Stars – small but incredibly dense ball of neutrons, formed from the collapsed core of a supernova. ► One teaspoon of material from a neutron star would weigh 100 million tons on Earth. Black Holes – hole left by the collapse of a supernova. ► The gravity of a black hole is so great that not even light can escape from it. III. Star Groups A. Constellations: patterns of stars in the sky there are 88 different patterns of stars recognized. ► ► ► B. Galaxies ►A galaxy is a large group of stars bound by gravity. typically 100,000 light-years wide contain billions of stars Types of Galaxies 1. Spiral: central mass (nucleus) of bright stars with flat arms that spiral around it arms contain millions of young stars, gas, and dust. Top View Side View 2. Elliptical: nearly spherical with very bright centers; no spiral arms No young stars, dust, or gas 3. Irregular: no particular shape. small and faint, with little gravitational attraction to organize it into a shape may also be unorganized due to the collision with another galaxy What type of galaxy do we live in? The Milky Way -Spiral galaxy in which our solar system is located -100,000 light-years wide - The sun is 30,000 lightyears from the center - The sun revolves around the center at 250 km/sec - It takes 200 million years to make one revolution. IV. FORMATION OF THE UNIVERSE ► The Big Bang Theory: most widely accepted theory explaining the formation of the universe. All matter and energy in the universe was once concentrated in a very small, very hot, very dense volume 12-15 billion years ago, the “big bang” occurred Matter and energy were propelled outward in all directions The universe began to E X P A N D… As matter and energy moved outward, the force of gravity had an effect Matter began to condense, forming the galaxies The galaxies continued to move outward, as they continue to do today… V. The Sun ► There is nothing special about the Sun… It’s just close enough to Earth to give us light and warmth The Sun is similar to most other stars in our galaxy: ►A large ball of gas made mostly of hydrogen and helium held together by gravity. ► The Sun is 300,000 times larger than Earth A. Structure of the Sun 1. Core The very hot (15,000,000ºC) gaseous center This is where nuclear fusion occurs FUSION is how the Sun makes its energy! Hydrogen Helium Hydrogen 2. The Inner Zones a. Radiative Zone ►Zone surrounding the core ►Heat energy moves by radiation b. Convective Zone ►Around the radiative zone ►Heat energy moves by convection 3. The Sun’s Atmosphere (uppermost region of solar gases) a. Photosphere: “light sphere” ►visible surface of the Sun ►what we see b. Chromosphere: “color sphere” thin layer seems to glow with reddish light ► c. Corona: “crown” outermost layer of Sun’s atmosphere cannot be seen with naked eye, except during TOTAL solar eclipse huge cloud of gas that keeps the atomic particles from the surface from escaping into space ►Solar winds the corona does not have a defined edge gas flows outward from the corona at high speeds, forming solar winds electrically charged atomic particles stream into space through holes in the corona flow to distant parts of solar system B. Solar Activity 1. Sunspots: cooled regions within the photosphere ►Appear darker than the areas around them 2. Prominences: disturbances in solar atmosphere great clouds of glowing gases huge arches that reach high above Sun’s surface 3. Solar Flares: sudden violent eruptions of electrically charged atomic particles. 4. Auroras (aka “Northern Lights” or “Southern Lights”): ► Magnetic storms in the Earth’s upper atmosphere ► Solar winds attracted to Earth’s magnetic poles by the Earth’s magnetosphere (space around Earth that contains a magnetic field) ► Electrically charged particles strike gas molecules in the upper atmosphere ► Green, red, blue, or violet sheets of light are produced ► Visible about 5 times per year, usually in polar regions ► VI. Our Solar System ► How many stars do we have in our Solar System? ► What bodies make up our Solar System? Formation of the Solar System ► Solar system = the Sun and the planets and other bodies that travel around the Sun ► Solar Nebular Theory: our best current idea for the origin of the solar system… ► Big Bang spread matter throughout universe ► Some matter gathered into clouds of dust and gas ► Cloud of gas and dust that eventually developed into our solar system is called the ‘SOLAR NEBULA’ ► Solar nebula was larger than our solar system is now ► Heat and pressure from force of gravity caused center of solar nebula to become hotter and denser ► While the Sun was forming in the center of the solar nebula, planets were forming in the outer regions ► Planetesimals (small bodies of matter within the solar nebula) joined together through collisions and the force of gravity to form larger bodies called protoplanets ► Protoplanets then condensed into existing planets and moons… ►The four protoplanets closest to the Sun became: ►Mercury ►Venus ►Earth ►Mars ►The next four protoplanets became: ►Jupiter ►Saturn ►Uranus ►Neptune