Properties of Stars: color, brightness, mass, temperature, size
must use indirect methods
stellar parallax - shift in the apparent position of a nearby star due to the orbital motion of the Earth. (largest parallax angel is less than 1/3600 of a degree.
Light-year: the distance light traveling at 300,000,000 meters per second travels in one year. [5.8 trillion miles]
depends on:
Apparent Magnitude how bright a star seems to be from Earth Scale extends from negative values (very bright) to 23 (very dim).
Absolute Magnitude distance must be considered and brightness at a fixed distance calculated so that stars can be compared. Most stars fall between -5 and 15
Blue: hot
Yellow: mid-range
Red: relatively cool
Binary stars two stars that orbit each other.
shows the relationship between temperature and brightness
Main-sequence stars hotter, more massive blue stars to cooler, less massive red stars
Red Giants and White Dwarfs do not fall along the main sequence. Our sun does.
nebula concentrations of dust and gases in space.
bright nebula glowing dust and gases due to a nearby blue star emission nebulae, absorb uv, emit visible light reflectin nebulae, dense "clouds" that reflect the light of nearby stars.
dark nebula interstellar material that is opaque and does not glow
Stars exist because of gravity. A star balances two forces: gravity trying to contract it and thermal energy trying to expand it.
A nebula undergoes an initial contraction that lasts about a million years.
A red protostar is formed and continues to contract and heat up
A stable main-sequence star is formed when equilibrium is reached between fusion and gravitational contraction
Hydrogen fusion lasts a few billion years. Our sun has a life span of 10 billion years and is middle-aged now
Red Giant Stage Red giants form from contraction of the core that becomes hot enough to fuse Helium
Our sun is destined to spend one billion years as a red giant.
Burnout and Death
Low-Mass Stars spend life as red main sequence stars then collapse to white dwarfs
Medium-Mass Stars (our sun) main-sequence star to red giant to planetary nebula to white dwarf
Massive Stars main sequence star to red supergiant to supernova explosion to black hole or neutron star
White Dwarfs hot (25,000 K), small (Earth sized). high density (one spoonful = tons)
Neutron Stars remnants of supernova events, electrons and rpotons combine to create neutrons, pea-sized sample=100 million tons, very strong magnetic field, pulsars
Black Holes gavitational field so dense light cannot escape it.
Countless stars that the eyes cannot resolve. 100 billion stars in our galaxy. Our sun is in one of the three spiral arms of our galaxy.
Hundreds of billions of galaxies fall into three types
Galaxies exist in clusters and superclusters
Doppler effect shift in wavelength due to motion of source or observer
Expanding Universe most galaxies are receding from us.
Hubble's Law galaxies are receding from us at a speed that is proportional to their distance.
The Universe began 20 billion years ago and will reach some kind of limit to its expansion in about 20 billion years. Whether it contracts again or not depends on the total amount of matter in the universe.