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Explore the life cycle of a star from birth in nebulae to death as a white dwarf or supernova, through stages like protostar and red giant. Understand stellar evolution with concepts like hydrogen fusion and neutron stars.
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Warm Up 6/6/08 • If star A is farther from Earth than star B, but both stars have the same absolute magnitude, what is true about their apparent magnitude? a. Star B has the greater apparent magnitude. b. Both stars have the same apparent magnitude. c. Star A has the greater apparent magnitude. d. Apparent magnitude is not related to distance. • A Hertzsprung-Russell (H-R) diagram shows the relationship between ____. a. temperature and absolute magnitude b. apparent magnitude and parallax c. absolute magnitude and apparent magnitude d. parallax and temperature • Which of the following refers to the change in wavelength that occurs when an object moves toward or away from a source? a. spectroscopy c. wave theory of light b. Doppler effect d. chromatic aberration Answers: 1) a. 2) a. 3) b.
Stellar Evolution Chapter 25, Section 2
The Birth of a Star • The birthplaces are dark, cool interstellar clouds (nebulae) • The initial contraction of the nebula can be triggered by the shock wave from an explosion of a nearby star • Once this begins, gravity squeezes the particles, pulling every particle toward the center
The Birth of a Star The Orion Nebula in normal color and infrared
Protostar Stage • The initial contraction can span millions of years • The temperature of the nebula slowly rises until it is hot enough to radiate energy from its surface • Protostar – a developing star not yet hot enough to engage in nuclear fusion • When the core of a protostar has reached about 10 million K, pressure within is so great that nuclear fusion of hydrogen begins, and a star is born • Heat from hydrogen fusion causes the gases to increase their motion
Protostar Protostars in the Horsehead Nebula are circled
Concept Check • What is a protostar? • A protostar is a developing star not yet hot enough to engage in nuclear fusion.
Main-Sequence Stage • Main-Sequence Stage – From the moment of birth until the star’s death • The internal gas pressure struggles to offset the unyielding force of gravity • Hydrogen fusion will last for a few billion years and provides the outward pressure to keep the star from collapsing • The more massive a main-sequence star, the shorter its life span • A yellow star, like our sun, can remain in the main-sequence for approximately 10 billion years • Once the hydrogen fuel of the star’s core is depleted, it evolves rapidly and dies
Red-Giant Stage • Occurs because the zone of hydrogen fusion continually moves outward • When all the hydrogen is consumed, the core no longer has outward pressure supporting it, and will contract • The core will grow hotter, the heat is radiated outward and expands the surface • As the surface expands, it cools down, producing its reddish color • The core will eventually reach a temperature which allows Helium to Carbon fusion • Eventually all the fusion fuel will be consumed • The Sun will spend less than 1 billion years as a Red-Giant
Red-Giant Stage Globular Star Cluster, some of the oldest stars in the universe
Concept Check • What causes a star to die? • A star runs out of fuel and collapses due to gravity.
Burnout and Death • We do not know that all stars, regardless of their size, eventually run out of fuel and collapse due to gravity • Low Mass Stars – consume fuel at a slow rate, may remain on main-sequence for up to 100 billion years, end up collapsing into white dwarfs • Medium Mass Stars – go into red-giant stage, followed by collapse to white dwarf by blowing out their outer layer, and eventually light up planetary nebulae • Massive Stars – these have relatively short lifetimes, end with a large supernova (brighter than the sun if near Earth), this huge explosion blasts apart the star • Supernova – an exploding star that increases in brightness many thousands of times
Concept Check • What is a supernova? • A supernova is the brilliant explosion that marks the end of a massive star.
White Dwarfs • White Dwarf – remains of low and medium mass stars, extremely small stars with densities greater than anything on Earth • The sun begins as a nebula, spends much of its life as a main-sequence star, becomes a red-giant, planetary nebula, white dwarf, and finally, black dwarf
Neutron Stars • The smaller white dwarfs are actually a result of the more massive stars • Neutron Stars – remnants of supernova events, stars that are smaller and more massive than white dwarfs • Electrons are forced to combine with protons to form neutrons, because of how closely packed the matter is
Supernovae • The outer layer of a star is ejected, while the core condenses to form a very hot neutron star • As the star collapses, it rotates faster, and it generates very strong radio waves situated at its magnetic poles creating pulses • Pulsar – a variable radio source of small size that emits radio pulses in very regular periods
Black Holes • Black Hole – A massive star that has collapsed to such a small volume that its gravity prevents the escape of everything, including light • How does astronomer find a black hole? • They look for material that is being gravitationally swept up by a location that we cannot see • Material that is swept in should be very hot and emits large amounts of X-rays
Assignment • Read Chapter 25, Section 2 (pg. 707-714) • Do Chapter 25 Assessment #1-31 (pg. 725-726)
