I.) Properties of Stars • Color and Temperature • A star’s color indicates that temperature of its surface • Hottest stars (surface temps above 30,000K) appear blue • Milder stars (surface temps between 5,000-6,000K) appear yellow • Relatively cool stars (surface temps ~3,000K) appear red
Hertzsprung-Russell Diagram: a graph of the surface temperature (or color) and absolute brightness of a sample of stars • H-R diagrams are used to estimate the sizes of stars and their distances, and to infer how stars change over time • Horizontal axis=surface temperature • Vertical axis=absolute brightness
Main-Sequence Stars • Main sequence: A line or sequence in an H-R diagram where most stars spend 90% of their life. • A diagonal band running from the bright, hot stars on the upper left to the dim, cool stars on the lower right • Example: The Sun lies in the main sequence • The sun is a yellow star (medium sized) that does fusion of hydrogen to helium
Giants and Dwarfs a. Very bright/cool stars in upper right of H-R diagram are known as supergiants • Very large (100 to 1000 times the diameter of the sun) • Just below the supergiants are the giants • Below the main sequence are the white dwarfs (hot/dim), which are the small, dense remains of a low- or medium-mass star
Energy From Stars • Energy moves through the layers of a star by combination of convection, conduction, and radiation • Convection: rising hot gas moves upward, away from the star’s center, and cooler, denser gas sinks toward the center • Conduction occurs when energy is transferred from individual atom to individual atom
Radiation occurs when energy radiates out from the star in the form of light • Travels at the speed of light • Light from Sun takes 8 minutes to reach Earth
II.) Life Cycle of Stars • How Stars Form • Nebula: a large cloud of dust & gas in interstellar space • Composed of mainly hydrogen & helium • Gravity pulls the small pieces of dust to clump together & form a protostar
When a protostar is dense & hot enough it will begin fusion. Nuclear fusion begins to take control over the gravity and the star begins to stabilize. • Fusion reactions in the core produce an outward force that balances the inward force of gravity
Adult Stars • Most stars spend 90% of their life in main sequence • Once achieving nuclear fusion, stars radiate energy into space C. The brightness of a star depends on its distance from earth, size, and temperature
The Death of a Star • When the hydrogen fuel has run out & fusion can no longer continue, the star collapses because the forces are no longer balanced • Core collapses because of its own gravity & then rebounds with a shock wave that violently blows the star’s outer layers away from the core
Results in huge, bright explosion called a supernova • Supernova produce elements heavier than iron (like gold or lead)
Some Definitions • Universe- the sum of all space, matter, & energy that exists, that has existed in the past, & that will exist in the future • -everything physical that exists in space & time • Galaxy- a collection of stars, dust, & gas bound together by gravity • Solar System- the sun & all of the planets & other bodies that travel around it.
III.) Expanding Universe • Hubble’s Law • Edwin Hubble used observations of spectral lines from other galaxies to conclude that the universe is expanding • Lines shifted toward the red end (longer wavelengths) of the visible light spectrum
Red shift: the apparent shift toward longer wavelengths of light caused when a luminous object moves away from the observer • Red shift showed that nearly all galaxies are getting farther away from Earth • Blue shift: an apparent shift toward shorter wavelengths of light caused when a luminous object moves towards the observer • The red/blue shift help us to better understand the Doppler Effect
Big Bang Theory • The Big Bang Theory states that all matter and energy in the universe was compressed into an extremely small volume that 10 to 20 billion years ago exploded and began expanding in all directions
Evidence for the Bang • Red shift, blue shift, and cosmic background radiation • Cosmic background radiation: steady, but very dim signals in the form of microwaves that are emitted all over the sky • Scientists believe that these microwaves are the remains of the radiation produced during the Big Bang
IV.) Phases of the Moon & Eclipses • The different shapes of the moon visible from Earth are called phases • Moon’s phasesare caused by changes in the relative positions of the moon, sun, and Earth as the moon revolves around the Earth [Fig 8, p.634]
The “Big Crunch” • The varying theories of the universe’s eventual fate all depend upon the universe’s current mass • The evidence that the universe is expanding makes some scientists believe that everything will collapse one day upon itself and result in the “big crunch”
Phases of the Moon • The moon is able to be illuminated due to the sun’s light reflecting off of its surface • As the moon revolves around Earth, the illuminated portion of the side of the moon facing Earth changes. • When the moon is full, the earth is between the sun and moon and the half that faces you is lit. • When the moon is new, the side that is facing you is dark, so you can’t see it.
Eclipse: an event in which the shadow of one celestial body falls on another [Fig 9, p.635] • Caused by the changes in relative positions of the moon, sun, and Earth—when all three are lined up in a certain order • Solar Eclipse: Moon blocks the sun • Lunar Eclipse: Earth’s shadow blocks the moon
Solar Eclipse • Solar Eclipse: Moon blocks the sun and causes a shadow on the earth As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially covers the Sun as viewed from some location on Earth. The sky will turn dark as the moon blocks out the sun.
Lunar Eclipse A lunar eclipse occurs when the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse & it passes into the Earth’s shadow. All observers on the night side of earth can see the full moon darken • Lunar Eclipse: Earth’s • shadow blocks the moon
Tides • Caused by the differences in the pull of the moon’s gravity on different areas of the Earth • Coastal areas have two high tides and two low tides each day • Times of the tides will vary because the moon is orbiting around the Earth