Stars and Galaxies BIG Idea The life cycle

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

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. ØLight Year = distance a light wave travels in one year

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, 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 10 x bigger than the Sun and a Supergiant is 100 x 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 Ø arms contain millions of young stars, gas, and dust. Top View Side View

2. Elliptical: nearly spherical with very bright centers; no spiral arms ØNo 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, 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” Øthin layer Øseems to glow with reddish light Øthe only time it is seen is during an eclipse (when moon moves between the Earth and Sun)

► c. Corona: “crown” § outermost layer of Sun’s atmosphere § only seen during an 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 to 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