Class Announcements • Homework is due today. • Email to me the title of your choice of class project TODAY. • Friday there is a class midterm test and a homework due. • NEW HOME WORK • Complete the lecture tutorial ‘Motion of Extrasolar Planets’, hand in on Tuesday 2nd August at the start of class.
Life Stages of High-Mass Stars • By high mass stars we mean stars greater than 8Msun • Late life stages of high-mass stars are similar to those of low-mass stars: • Hydrogen core fusion (main sequence) • Hydrogen shell fusion (subgiant to supergiant) • Helium core fusion (supergiant)
Helium Capture • High core temperatures allow helium to fuse with heavier elements.
Advanced Nuclear Burning • Core temperatures in stars with >8MSun allow fusion of elements as heavy as iron.
Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells.
Iron is a dead end for fusion because nuclear reactions involving iron do not release energy (Fe has lowest mass per nuclear particle.) Fusion of nuclei heavier than iron absorb energy and would cause the core to cool and collapse).
Evidence for helium capture: Higher abundances of elements with even numbers of protons
Iron builds up in the core until electron degeneracy pressure can no longer resist gravity. The core then suddenly collapses, creating a supernova explosion. The Death Sequence of a High-Mass Star
Supernova Explosion • Core electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. • Neutrons collapse to the center, forming a neutron star.
Energy and neutrons released in a supernova explosion enable elements heavier than iron to form, including Au and U.
Supernova Remnant • Energy released by the collapse of the core drives outer layers into space. • The Crab Nebula is the remnant of the supernova seen in A.D. 1054.
Supernova 1987A • The closest supernova in the last four centuries was seen in 1987.
Role of Mass • A star’s mass determines its entire life story because it determines its core temperature. • High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions. • Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.
Life Stages of Low-Mass Star Main Sequence: H fuses to He in core Red Giant: H fuses to He in shell around He core Helium Core Fusion: He fuses to C in core while H fuses to He in shell Double Shell Fusion: H and He both fuse in shells 5. Planetary Nebula: leaves white dwarf behind Not to scale!
Reasons for Life Stages • Core shrinks and heats until it’s hot enough for fusion. • Nuclei with larger charge require higher temperature for fusion. • Core thermostat is broken while core is not hot enough for fusion (shell burning). • Core fusion can’t happen if degeneracy pressure keeps core from shrinking.
Life Stages of High-Mass Star Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around He core Helium Core Fusion: He fuses to C in core while H fuses to He in shell Multiple Shell Fusion: many elements fuse in shells 5. Supernova leaves neutron star behind and creates all elements heavier than Iron.
How are the lives of stars with close companions (close binaries) different?
Thought Question The binary star Algol consists of a 3.7MSun main- sequence star and a 0.8MSun subgiant star. What’s strange about this pairing? How did it come about?
Stars in Algol are close enough that matter can flow from the subgiant onto the main-sequence star.
The star that is now a subgiant was originally more massive. As it reached the end of its life and started to grow, it began to transfer mass to its companion (mass exchange). Now the companion star is more massive.
How does the life of a high-mass star differ from the Sun’s life? • It forms much faster. • It lives a shorter time on the main sequence. • As a red giant or supergiant, it makes elements heavier than carbon. • When it dies, it explodes in a tremendous supernova explosion. • All of the above
What is different about nuclear reactions of elements lighter than iron or heavier than iron? • Lighter elements give off energy when they fuse, heating the stars core and keeping gravity from crushing it. • Heavier elements take in energy if they fuse, taking away heat from the core and leading to a collapse. • A and B
What remnant does a supernova leave? • White dwarf • Neutron star • Black hole • B or C
Why are supernovas important to galactic ecology? • They recycle material. • They create new elements and blow them out into space, and a new generation of stars can be made from them. • They destroy elements, letting each new generation of stars begin anew.
The binary star Algol has a 3.7 solar mass mainsequence star and a 0.8 solar mass red giant. How could that be? • In this system the lower mass star must have evolved faster than the higher mass one. • The red giant might be made of some different elements, so it evolved faster. • The lower mass star used to be a more massive main sequence star, but when it became a giant some of its mass went onto the other star.
Suppose the universe contained only low-mass stars. Would elements heavier than carbon exist? • Yes, all stars create heavier elements than carbon when they become a supernova. • Yes, but there would be far fewer heavier elements because high-mass stars form elements like iron far more prolifically than low-mass stars. • No, the core temperatures of low-mass stars are too low to fuse other nuclei to carbon, so it would be the heaviest element. • No, heavy elements created at the cores of low-mass stars would be locked away for billions of years. • No, fission reactions would break down all elements heavier than carbon.
If you could look inside the Sun today, would you find that its core contains a much higher proportion of helium and a lower proportion of hydrogen than it did when the Sun was first born? • Yes, because the Sun is about halfway through its hydrogen-burning life, so it has turned about half its core hydrogen into helium. • No, the proportion of helium only increases near the end of the Sun’s life. • No, the proportion of helium in the Sun will always be the same as when it first formed. • No, the lighter helium will rise to the surface and the proportion of hydrogen in the core will remain the same.
Finish yesterday’s lecture tutorial, Luminosity Temperature and Size. • Then work on the lecture tutorial section Analyzing Spectra.