Glowing ball of gas in space which generates energy through nuclear fusion in its core Closest star to Earth is the Sun
Most of the universe is empty space even though it appears it is full of stars Stars are very far apart Light-year Distance that light travels in a vacuum in a year Approximately 9.5 trillion kilometers Closest star to the sun is about 4.3 light-years away
Scientists cannot measure distances to stars directly Apparent change in position of an object with respect to a distant background is called a parallax Astronomers measure the parallax of nearby stars to determine their distance from earth Accurately measures stars that are within a few hundred light-years
Color Size Brightness Other important properties Chemical composition Mass
Estimate temperature by color Hottest stars (temperatures above 30,000 K) Blue Relatively cool stars (temperature ~ 3000 K) Red Temperature 5000 to 6000 K Yellow
Brightness of a star does not depend on closeness to earth Brighter stars may be farther away than stars that appear dim Apparent brightness Brightness of a star as it appears from Earth Apparent brightness decreases as its distance from you increases Absolute brightness How bright a star really is
Estimate the diameter and calculate the volume No direct way of finding the mass of an isolated star Calculate the mass by observing gravitational interactions of stars in pairs
Spectrograph Instrument that spreads light from a hot glowing object into a spectrum Each star has its own spectrum Elements within stars absorb light at different wavelengths Absorption lines Set of dark lines that show where light has been absorbed Observations have shown that stars have fairly similar compositions
H-R diagram Used to estimate the sizes of stars and their distances and to infer how stars change over time
A major grouping of stars that forms a narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung- Russell diagram 90% of all stars are found here
ClassTemperatureColor O20, ,000 KBlue B10,000 – 30,000 KBlue-white A7,500 – 10,000 KWhite F6,000 – 7,500 KYellow-white G5,000 – 6,000 KYellow K3,500 – 5,000 KOrange M2,000 – 3,500 KRed
Begin their lives as clouds of dust and gas called nebulae Gravity may cause the nebula to contract Matter in the gas cloud will begin to condense into a dense region called a protostar The protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins. Begins the main sequence phase of the star Most of its life is in this phase
Life span of a star depends on its size. Very large, massive stars burn their fuel much faster than smaller stars Their main sequence may last only a few hundred thousand years Smaller stars will live on for billions of years because they burn their fuel much more slowly Eventually, the star's fuel will begin to run out.
It will expand into what is known as a red giant Massive stars will become red supergiants This phase will last until the star exhausts its remaining fuel At this point the star will collapse
Most average stars will blow away their outer atmospheres to form a planetary nebula Their cores will remain behind and burn as a white dwarf until they cool down What will be left is a dark ball of matter known as a black dwarf
If the star is massive enough, the collapse will trigger a violent explosion known as a supernova If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons.
If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole
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