26 3 Life Cycles of Stars Massive Star

26. 3 Life Cycles of Stars Massive Star Life Cycle Video Lecture Red Giant Video Lecture White and Black Dwarfs Video Lecture

26. 3 Life Cycles of Stars You can often predict how a baby will look as an adult by looking at other family members. Astronomers observe stars of different ages to infer how stars evolve.

26. 3 Life Cycles of Stars How Stars Form A nebula is a large cloud of gas and dust spread out over a large volume of space.

26. 3 Life Cycles of Stars form in the densest regions of nebulae. Gravity pulls a nebula’s dust and gas into a denser cloud. As the nebula contracts, it heats up.

26. 3 Life Cycles of Stars A contracting cloud of gas and dust with enough mass to form a star is called a protostar.

26. 3 Life Cycles of Stars A group of bright young stars can be seen in the hollowedout center of the Rosette Nebula.

26. 3 Life Cycles of Stars Adult Stars spend about 90 percent of their lives on the main sequence. The amount of gas and dust available when a star forms determines the mass of each young star. A star’s mass determines the star’s place on the main sequence and how long it will stay there.

26. 3 Life Cycles of Stars High mass stars -have large cores -produce the most energy -bluest and brightest main-sequence stars. -about 300, 000 times brighter than the sun. -use up their fuel relatively quickly -last only a few million years.

26. 3 Life Cycles of Stars Middle mass stars -similar to the sun -occupy middle of the main sequence. -Surface temperature of about 6000 K -will remain stable on the main sequence for about 10 billion years.

26. 3 Life Cycles of Stars Small mass stars -small, cool stars that are long-lived. -Surface temperature of about 3500 K, -may stay on the main sequence for more than 100 billion years.

26. 3 Life Cycles of Stars The Death of a Star What happens to a star when it runs out of fuel? The dwindling supply of fuel in a star’s core ultimately leads to the star’s death as a white dwarf, neutron star, or black hole.

26. 3 Life Cycles of Stars The Death of a Star When a star’s core begins to run out of hydrogen, gravity gains the upper hand over pressure, and the core starts to shrink. • The energy flowing outward increases, causing the outer regions of the star to expand. The expanding atmosphere moves farther from the hot core and cools to red. • The star becomes a red giant.

26. 3 Life Cycles of Stars The mass of a star determines the path of its evolution.

26. 3 Life Cycles of Stars Low- and Medium-Mass Stars • Stars remain in the giant stage until their hydrogen and helium supplies dwindle and there are no other elements to fuse. • The energy coming from the star’s interior decreases. • With less outward pressure, the star collapses.

26. 3 Life Cycles of Stars • The dying star is surrounded by a glowing cloud of gas, called a planetary nebula. • As the dying star blows off much of its mass, only its hot core remains. • This dense core is a white dwarf.

26. 3 Life Cycles of Stars A. Planetary nebulas, such as the Hourglass Nebula, are clouds of gas that surround a collapsing red giant.

26. 3 Life Cycles of Stars B. The Crab Nebula is the remnant of a supernova explosion that was observed on Earth in A. D. 1054. The supernova was so bright that people could see it in the daytime.

26. 3 Life Cycles of Stars High-Mass Stars The life cycle of high-mass stars is very different from the life cycle of lower-mass stars. • A high-mass star dies quickly because it consumes fuel very rapidly.

26. 3 Life Cycles of Stars • As fusion slows in a high-mass star, pressure decreases and gravity leads to a dramatic collapse of the star’s outer layers. • This collapse produces a supernova, an explosion so violent that the dying star becomes more brilliant than an entire galaxy.

26. 3 Life Cycles of Stars Supernovas produce enough energy to create elements heavier than iron. • As a supernova spews material into space, its core continues to collapse.

26. 3 Life Cycles of Stars If the remaining core of a supernova has a mass less than about three times the sun’s mass, it will become a neutron star, the dense remnant of a high-mass star that has exploded as a supernova.

26. 3 Life Cycles of Stars A neutron star spins more and more rapidly as it contracts. Some neutron stars spin hundreds of turns per second! • A spinning neutron star that appears to give off strong pulses of radio waves is called a pulsar.

26. 3 Life Cycles of Stars Pulsars emit steady beams of radiation that appear to pulse when the spinning beam sweeps across Earth.

26. 3 Life Cycles of Stars If a star’s core after a supernova explosion is more than about three times the sun’s mass, its gravitational pull is very strong. The core collapses beyond the neutron-star stage to become a black hole. A black hole is an object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot escape from it.

26. 3 Life Cycles of Stars Assessment Questions 1. As a protostar contracts, what happens to its pressure and temperature? a. They stay constant. b. The temperature increases while the pressure stays constant. c. They both decrease. d. They both increase.

26. 3 Life Cycles of Stars Assessment Questions 1. As a protostar contracts, what happens to its pressure and temperature? a. They stay constant. b. The temperature increases while the pressure stays constant. c. They both decrease. d. They both increase. ANS: D

26. 3 Life Cycles of Stars Assessment Questions 2. Which type of main sequence star would be likely to remain in the main sequence for about 100 billion years? a. b. c. d. red yellow white blue

26. 3 Life Cycles of Stars Assessment Questions 2. Which type of main sequence star would be likely to remain in the main sequence for about 100 billion years? a. b. c. d. red yellow white blue ANS: A

26. 3 Life Cycles of Stars Assessment Questions 3. Based on its position on the H-R diagram, what will the sun become when it finally runs out of fuel? a. b. c. d. nebula supernova neutron star black dwarf

26. 3 Life Cycles of Stars Assessment Questions 3. Based on its position on the H-R diagram, what will the sun become when it finally runs out of fuel? a. b. c. d. nebula supernova neutron star black dwarf ANS: D

26. 3 Life Cycles of Stars Star Life Cycle video