2.  Glowing ball of gas in space which generates energy through nuclear fusion in its core  Closest star to Earth is the Sun

3.  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

5.  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

7.  Color  Size  Brightness  Other important properties  Chemical composition  Mass

8.  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

10.  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

11.  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

12.  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

14.  H-R diagram  Used to estimate the sizes of stars and their distances and to infer how stars change over time

15.  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

16. ClassTemperatureColor O20,000- 60,000 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

17. http://www.answers.com/topic/stellar-classification

18. http://hea- www.cfa.harvard.edu/CHAMP/EDUCATION/PUBLIC/ICONS/life_cycles.jpg

19.  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

20. 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.

21.  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

22.  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

23.  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.

24.  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

25. http://www.seasky.org/cosmic/sky7a01.ht ml