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

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

Main-Sequence Stage

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

Life Cycle of a Sun-like Star

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

Possible Function of a Binary Pair

Planetary Nebula

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

White Dwarfs

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

Neutron Stars

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

Supernovae

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

Black Holes

Stellar Evolution

Assignment Read Chapter 25, Section 2 (pg. 707-714) Do Chapter 25 Assessment #1-31 (pg. 725-726)