stars and their characteristics 28.2. constellations constellation- groups of stars that appear to...
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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
Stars
• Polaris- north star– Doesn’t appear to move
• Circumpolar stars- seem to move around Polaris because Earth rotates
• Position of Earth
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
Object mv Mv
Sun -26.8 4.83
Sirius -1.47 1.41
Vega 0.04 0.5
Betelgeuse 0.41 -5.6
Polaris 1.99 -3
Variable Stars
• variable stars are those stars that show regular variation of brightness – pulsating stars change brightness as they expand and
contract– Cepheid variable- yellow supergiants whose cycles
of brightness range from about 1 day to 50 days• the slower the cycle, the greater the luminosity of the star• can calculate the distances to galaxies in which they can
identify Cepheid stars
• non-pulsating star can change brightness because is part of a binary star system– results in eclipses
H-R Diagram
• Hertzsprung-Russell Diagram• diagram plots the luminosity of stars against
their surface temperatures• most stars (90%) are in a band that runs from
the upper left (high, high) to the lower right (low, low)– main sequence stars– main sequence stars vary in surface temperature and
absolute magnitudes– commonality: actively fusing hydrogen into helium
H-R Diagram cont…
• giant stars- great luminosity and diameter; 10-100x greater than sun
• supergiants- higher luminosity; diameter > 100x than sun
• white dwarfs- stars near end of life– once red giants that lost atmosphere
HR Diagram
Stellar Evolution
Birth of a Star• 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 EvolutionDeath 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