Stars and Their Characteristics 28.2

Constellations Constellation- groups of stars that appear to form patterns –88 constellations can be seen from n. and s. hemispheres –So far away that only after thousands of years might the motions be observed –Big Dipper- asterism (small-star grouping) Part of Ursa Major- Great Bear

Distance to Stars Light year- distance light travels in a year –Used instead of kilometers/astronomical units Parallax- basic way to measure star distance The nearest stars have the largest parallax angles, while those of distant stars are too small to measure

Elements in Stars sphere of mostly hydrogen and helium gases –small percentage may be heavier elements (oxygen, carbon, nitrogen, etc..) no two stars contain exactly the same elements in the same proportions –wavelengths depend on both composition and temperature

Mass, Size, and Temperature of Stars mass can be determined by the inertial properties of the body or by its gravitational effect on the bodies around it –more mass = more gravitational effect stars vary more in size than they do in mass; and even more in density range of colors a star emits depends on its surface temperature –Blue= hottest; red = coolest

Luminosity and Absolute Magnitude Luminosity (apparent magnitude)- brightness of a star at its current distance from Earth –depends on its size and temperature –Bigger stars tend to be brighter –Bluer stars tend to be brighter absolute magnitude- measure of how bright the star would be if all stars were at the same distance from Earth –The more negative the number, the brighter the star Objectmvmv MvMv Sun Sirius Vega Betelgeuse Polaris1.99-3

Hertzsprung-Russell Diagram Diagram that plots the luminosity of stars against their surface temperatures

H-R Diagram Most stars (90%) are in a band that runs from the upper left (high, high) to the lower right (low, low) –Called main sequence stars Stars vary in surface temperature and absolute magnitudes commonality: actively fusing hydrogen into helium

H-R Diagram cont… giant stars- great luminosity and diameter; x greater than sun supergiants- higher luminosity; diameter > 100x than sun white dwarfs- stars near end of life –once red giants that lost atmosphere

Stellar Evolution Birth of a Star (any size) begins as a nebula- cloud of dust and gas (99% hydrogen) nebula may condense when an outside force acts upon it particles move closer together under gravity increase density = increase temperature if nebula glows, called protostar center will become hotter until fusion takes place and a star is born

Nebula and Protostar

Stellar Evolution Death of a Star (size of the sun) remain same size (main sequence) for billions of years because energy produced through fusion equals gravitational pull hydrogen is used up so gravity takes over, which then produces heat by contraction entire star expands (Red Giant) core temperature rises enough for helium to fuse into heavier elements, producing a carbon-oxygen core surface gases are blown away, leaving core (white dwarf) planetary nebula- glowing halo of gases –fades as gases dissipate into space leaving white dwarf behind –once all fuel is used up, the core will no longer glow, becomes a black dwarf

Main Sequence and Red Giant

White Dwarf, Planetary Nebula, and Black Dwarf

Stellar Evolution Death of a Massive Star fusion process will continue until iron nuclei are formed –absorbs energy, so iron core quickly collapses supernova- explosion that produces a brilliant burst of light –produces many elements: copper, uranium, silver, and lead Remnants of Massive Stars neutron star-dense mass of neutrons formed through gravity pulsar- a distant neutron star that emits rapid pulses of light and radio waves instead of steady radiation black hole- remnant of a star at least 15 times as massive than the sun –gravitational force is so strong that light cannot escape

Supernova and Neutron Star

Pulsars and Black Holes