Chapter 4; Lesson 4.2
T.O.C:
Stars Change over their Life Cycles

• Amount of light & distance from Earth determines how bright it appears. If star is dimmer but closer it can seem brighter than star farther away but its not.
• Parallax(w.w)- apparent shift in the position of an object when viewed from different locations. How astronomers measure distance. Plot star from both sides of Earth’s orbit & calculate differences.
– Left eye/right eye demo.

• Hard to tell how big stars are from Earth. (even the
Sun doesn’t seem as big as it is & diameter is about
100x bigger than Earth)
• Giant & supergiant stars range from 10 to 100s of times larger than the Sun (Betelgeuse is over 600x greater in diameter than Sun & is one of the brightest stars in the sky even though it’s 522 light-years away)
• Stars smaller than Sun are called white dwarfs. About
100x smaller than Sun. Can’t be seen w/out telescope

• Some stars can vary in color. Most look white but some appear blue or red.
• Color difference is due to difference in temperature
– Heated metal demo
• Like metal, color of star tells temperature
– Coolest= red, hottest= blue/white
– Scientists can spread a stars light into a spectrum to learn about star’s composition. These colors & lines show what gases are present in star’s outer layers.

• That’s right. They die. Stars go through birth, maturity, & death. How long they live varies depending on how big they are.
– Higher mass stars develop quicker than low mass stars
& behave differently as well
• Nebula(w.w)- stars form inside a cloud of gas & dust via gravity inside a nebula
• When star dies, matter does not disappear. Some may form new nebula or move into existing nebula to create more new stars.
http://www.youtube.com/watch?v=PM9CQDlQI0A

• Stages in the Life Cycles of Stars:
• Main sequence(w.w)- when stars produce energy thru the fusion of hydrogen into helium
1. Lower-Mass Stars- can remain in main sequence for billions of years. (the Sun has been in main sequence about 4.6 billion years & will remain about another 5 billion years) When these stars run out of hydrogen; it expands to a giant star & helium turns into carbon. Over time they become a white dwarf (the dead core of a giant star) but remain hot for millions of years.
2. Higher-Mass Stars- stars more than 8x as massive as our Sun.
Much less time in main sequence stage b/c they use their fuel rapidly. After millions of years, expands to supergiant star where fusion in core produces heavier & heavier elements.
When iron core forms, fusion stops & gravity causes core to collapse. Star erupts in explosion called supernova. (for brief period, supernova can give off as much light as a galaxy!!)

• Neutron star(w.w)- collapsed core of a supergiant star can form this extremely dense star (masses are
3x that of the Sun)
– Emit little visible light. Strongly emit other forms of radiation.
• Black hole(w.w)- when supernova leaves behind core w/ mass more than 3x that of Sun. Core collapses even further, forming this invisible object.
– Gravity of black hole is so strong that no form of radiation can escape it.
• http://www.youtube.com/watch?v=8grTbzAo0PA
• http://www.youtube.com/watch?v=3pAnRKD4raY
• http://www.youtube.com/watch?v=ueCXydnpMX4

• Unlike our Sun, most stars don’t exist alone. Usually grouped w/ 1 or more companion stars.
• Important sources of info about star masses. Can measure effect of stars on one another via stars gravitational pull.
• Held together by force of gravity between them.
– Binary system- 2 stars that orbit each other
– Multiple star system- more than 2 stars
– Sometimes stars are too close together to be seen individually

1. What factors affect how bright a star appears from Earth? (2 points)
2. What unit do astronomers use to describe the distances between stars? (2 points)
3. How does a stars temperature affect its appearance? (2 points)
4. Where do stars form? (2 points)
5. What 2 space objects can form when a higher-mass star dies? (2 points)