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 • 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)
Spectroscope: instrument used to determine chemical composition by separating light into different colors (wavelengths). Spectral Types:
Bluestars are the hottest Redstars 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 towardsEarth • 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
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
Supernova – star that has such a tremendous explosion that it blows itself apart. Novas – explosions that occur as a white dwarf cools
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