Earth Science 25.2A : Stellar Evolution

1. Earth Science 25.2A : Stellar Evolution The Birth and Evolution of Stars

2. Earth Science 25.2 : Stellar Evolution • Determining how stars are born, age and die can be difficult because the life of a star can span billions of years. • However, by studying stars of different ages astronomers have been able to piece together the life and evolution of a star.

3. Earth Science 25.2 : Stellar Evolution The Birth of a Star: • The birthplaces of stars are dark, cool interstellar clouds. • These nebulae are made up of dust and gases. • In the Milky Way, nebulae consist of 92% hydrogen, 7% helium, and less than 1 % of the remaining heavier elements. • For some reason not fully understood by scientists, some nebulae become dense enough that they begin to contract, growing smaller yet denser as they shrink.

4. Earth Science 25.2 : Stellar Evolution • A shock wave from another nearby star may trigger this contraction. • Once this process begins to occur, gravity squeezes particles in the nebulae pulling each and every particle toward the center. • As the nebulae shrinks and grows denser in it’s center, this gravitational energy increases and is converted into heat energy.

5. Earth Science 25.2 : Stellar Evolution Protostar Stage: • The initial contraction of gases spans a million years time. • As time passes, the temperature of this gas body slowly rises enough to radiate energy from it’s surface in the form of long-wavelength red light. • This large red object is called a protostar; a developing star that is not yet hot enough to attain nuclear fusion.

6. Earth Science 25.2 : Stellar Evolution • During the protostar stage, gravitational contraction continues: slowly at first than more rapidly. • This collapse causes the core of the protostar to heat much more intensely than the outer layer. • When the core of a protostarhas reached about 10 million degrees K, pressure within it is so great that the nuclear fusion of hydrogen begins, and a star is born.

7. Earth Science 25.2 : Stellar Evolution • Heat from hydrogen fusion causes the gases to increase their motion. • This in turn causes an increase in outwardgas pressure. • At some point, this outward pressure exactly balances the inward pull of gravity. • When this balance is reached, the star becomes a stable main sequence star. • A stable main-sequence star is balanced between two forces: gravity, which is trying to squeeze it into a smaller space, and gas pressure, which is trying to expand it.

8. Earth Science 25.2 : Stellar Evolution Main Sequence Stage: • From this point on in the evolution of a main-sequence star until it’s death, the internal gas pressure struggles to offset the unyielding force of gravity. • Typically, the hydrogen fusion continues for a few billion years and provides the outward pressure required to support the star from gravitational collapse.

9. Earth Science 25.2 : Stellar Evolution • Different stars age at different rates. • Hot, massive, blue stars radiate energy at such an enormous rate that they deplete their hydrogen fuel in only a few million years. • By contrast, the least massive main-sequence stars may remain stable for hundreds of billions of years. • A yellow star, such as our sun, remains a main-sequence star for about 10 billion years.

10. Earth Science 25.2 : Stellar Evolution • An average star spends 90 percent of it’s life as a hydrogen burning main-sequence star. • Once the hydrogen fuel in the star’s core is depleted, it evolves rapidly and dies. • However, with the exception of the least massive red stars, a star can delay it’s death by fusing heavier elements and becoming a giant.

11. Earth Science 25.2 : Stellar Evolution Red Giant Stage: • The red giant stage occurs because the zone of hydrogen fusion continually moves outward, leaving behind a helium core. • Eventually, all the hydrogen in the star’s core is consumed. • While hydrogen fusion is still occurring in the star’s outer shell, no fusion is taking place in the inner core. • Without a source of energy, the core no longer has enough pressure to support itself against the inward force of gravity and the core thus begins to contract.

12. Earth Science 25.2 : Stellar Evolution • As the core contracts, it grows hotter by converting gravitational heat into heat energy. • Some of this energy is radiated outward, increasing hydrogen fusion in the star’s outer shell. This energy in turn heats and expands the star’s outer shell. • The result is a giant body hundreds to thousands of times bigger than the original main-sequence size.

13. Earth Science 25.2 : Stellar Evolution • As the star expands, it’s surface cools, which explains the star’s reddish appearance. • During expansion, the core continues to collapse and heat until it reaches 100 million K. • At this temperature, it is hot enough to convert helium to carbon. • So a red giant consumes both helium and hydrogen to produce energy.

14. Earth Science 25.2 : Stellar Evolution • Eventually, all the usable nuclear fuel in these giants will be consumed. • The sun, for example, will spend less than a billion years as a giant. • More massive stars will pass through this stage even more rapidly. • The force of gravity will again control the star’s destiny as it squeezes the star into a much smaller, denser piece of matter.

15. Earth Science 25.2 : Stellar Evolution Burnout and Death of a Star: • Most of the events of stellar evolution we have looked at so far are well documented. What we will look at now is based more on theory. • We do know that all stars, regardless of their size, eventually run out of fuel and collapse due to gravity. • With this in mind, let us look at the final stages of a variety of stars of different sizes and mass.

16. Earth Science 25.2 : Stellar Evolution Death of a low-mass star: • As we see in the figure at right, stars less than one half the mass of our sun consume their fuel at a low rate and live a very long time. • Consequently, these small, cool, red stars may remain on the main sequence for up to 100 billion years.

17. Earth Science 25.2 : Stellar Evolution • Because the interior of a low-mass star never reaches high enough temperatures and pressures to fuse helium, it’s only energy source is hydrogen. • So low-mass stars never evolve into red giants. • Instead, they remain as stable main-sequence stars until they consume their hydrogen fuel and collapse into a white dwarf.

18. Earth Science 25.2 : Stellar Evolution Death of medium-Mass Stars: • Our star, the sun, is a medium mass star. As we see in the table at right, all stars with masses similar to our sun evolve in much the same way. • During their red giant phase, sun-like stars consume hydrogen and helium fuel at a fast rate. • Once this fuel is exhausted, these stars also collapse into white dwarfs.

19. Earth Science 25.2 : Stellar Evolution • During their collapse from red giants to white dwarfs, medium-mass stars are thought to cast off their bloated outer shell, creating an expanding round cloud of gas. • The remaining hot, central white dwarf heats the gas cloud, causing it to glow. • These often beautiful gleaming spherical clouds are called planetary nebulae.

20. Earth Science 25.2 : Stellar Evolution Death of Massive Stars: • In contrast to sun-like stars, stars with masses 4 times that of our sun have relatively short life spans. • These stars end in a brilliant explosion called a supernova. • During a supernova, a star becomes millions of times brighter than it’s prenova stage. • If one of the stars nearest to Earth produced a supernova, it would be brighter than our sun.

21. Earth Science 25.2 : Stellar Evolution • Supernovas are very rare. • None have been observed in our galaxy since the invention of the telescope, although Tycho Brahe and Galileo each recorded one about 30 years apart. • An even larger supernova was recorded in 1054 by ancient Chinese astronomers. • Today, the remnant of this great supernova is thought to be the Crab nebulae.

22. Earth Science 25.2 : Stellar Evolution • A supernova event is thought to be triggered when a massive star consumes most of it’s nuclear food. • Without a heat engine to generate the gas pressure required to maintain it’s balance, it’s immense gravitational field causes the star to collapse inward. • This implosion, or bursting inward, is huge, resulting in a shock wave that moves outward from the star’s interior. • This shock wave destroys the star and blasts the outer shell into space, generating the supernova event.

23. Earth Science 25.2 : Stellar Evolution • Click on the following link to view a short animation showing the life of our sun • http://janus.astro.umd.edu/astro/stars/SunsLife.html • Part B: continued tomorrow as 25.2B