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Introduction to Astronomy

 

Lecture 19: The Death of High-Mass Stars

 

The bigger they come, the harder they fall.

-- "Gentleman Jim" Corbett

19.1 The Acceleration of Fusion

(Discovering the Universe, 5th ed., §12.4)
  • Carbon-Oxygen-Neon CoreAs we saw in Lecture 17, all stars will eventually expand to become supergiants.

    If the star has a high mass, larger than about eight solar masses, it will have a carbon-oxygen-neon inner core that is not burning, surrounded by a shell of burning helium, and finally an outer shell of burning hydrogen.

    As with other stars, the core is buried deep inside the outer layers of the star, which consist of nonburning H and He.

     
  • With no energy source in the inner core to balance the gravity, the star is out of mechanical equilibrium.

    A high-mass star can squeeze its core more than a low-mass star due to its larger gravity.

    So, the core is again compressed, and its pressure and temperature will rise.

    The higher temperatures provide the energy for other nuclei to fuse into new elements, provide a new source of energy in the core, and restore equilibrium.

    The most important of these reactions are detailed below, though there are others that also provide energy and produce other elements.

    In addition, many other reactions that require energy to occur are possible at these high temperatures.

    In this way every heavy element in nature is created, in proportions that are predictable from our understanding of nuclear fusion.

     
    Carbon Fusion 
  • When the temperature of the inner core reaches 600 MK, carbon fusion will begin to produce neon and helium:



    With this new source of energy in the inner core, the star's mechanical equilibrium is again restored.

    However, this balance is short-lived; in a 25-solar mass star, this process (and others) will use up most of the carbon in the core in only 600 years!

     
    Oxygen-Neon Core
  • So carbon fusion soon dies off, and the inner core becomes primarily oxygen and neon.

    Without an energy source in the inner core, the compression-shell burning process occurs once again.

    Fresh carbon produced in the upper layers of the core is pushed deeper, and the nonburning inner core becomes surrounded by a shell of burning carbon.

     
    Neon Fusion
  • Then the compression- heating- fusion- balance process repeats itself in the inner core.

    When the temperature of the inner core reaches 1200 MK = 1.2 GK, neon fusion will begin to produce magnesium and oxygen:



    With this new source of energy in the inner core, the star's mechanical equilibrium is again restored.

    However, this balance is even shorter-lived; in a 25-solar mass star, this process (and others) will use up most of the neon in the core in only one year!

     
    Oxygen Core
  • So neon fusion quickly dies off, and the inner core now becomes primarily oxygen.

    Without an energy source in the inner core, the compression-shell burning process occurs yet again.

    Fresh neon produced in the upper layers of the core is pushed deeper, and the nonburning inner core becomes surrounded by a shell of burning neon.

     
    Oxygen Fusion
  • Then the compression- heating- fusion- balance process repeats itself in the inner core.

    When the temperature of the inner core reaches 1500 MK = 1.5 GK, oxygen fusion will begin to produce silicon and helium:



    With this new source of energy in the inner core, the star's mechanical equilibrium is again restored.

    However, this balance is again very short-lived; in a 25-solar mass star, this process (and others) will use up most of the oxygen in the core in only half a year!

     
    Silicon Core
  • So oxygen fusion quickly dies off, and the inner core now becomes primarily silicon.

    Without an energy source in the inner core, the compression-shell burning process occurs once again.

    Fresh oxygen produced in the upper layers of the core is pushed deeper, and the nonburning inner core becomes surrounded by a shell of burning oxygen.

     
    Silicon Fusion
  • Then the compression- heating- fusion- balance process repeats itself one last time in the inner core.

    When the temperature of the inner core reaches 2700 MK = 2.7 GK, silicon fusion will begin to produce nickel:



    (Silicon fusion actually occurs through a series of reactions, but the result is the same.)

    With this new source of energy in the inner core, the star's mechanical equilibrium is again restored.

    But this balance is also ephemeral; in a 25-solar mass star, this process will use up the silicon in the core in only one day!

     
    Neutronization
  • Finally, the nickel rapidly decays into iron by neutronization, in which protons "capture" electrons to produce a neutron and a neutrino:



    Question: where have you seen a similar process? How did it differ?

    Neutronization requires energy to occur, but such a large nucleus can provide it from the resulting reduction in the number of its protons.

    Question: how does reducing the number of protons provide energy?

    Nickel Decay to Iron

     
    Iron Core
  • So silicon fusion ends almost as soon as it began, and the inner core now becomes primarily iron.

    Without an energy source in the inner core, the compression-shell burning process occurs once again.

    Fresh silicon produced in the upper layers of the core is pushed deeper, and the nonburning inner core becomes surrounded by a shell of burning silicon.

19.2 Supergiants

(Discovering the Universe, 5th ed., §12.4)
  • The star's core now resembles an onion, with layer on top of layer of burning gas.

    As each new layer is added, the luminosity increases and the star expands, becoming a luminous supergiant (class Ia), with a brightness of ~105 LSun.

    While the star becomes almost as big as the orbit of Jupiter, the core is highly compressed, only 10-5 of that size, about the diameter of the Earth.

     
  • A familiar supergiant is the M1Ia star Betelgeuse, the left shoulder of Orion.

    Betelgeuse is ~500 ly away, and is estimated to be 18-20 solar masses.

    Betelgeuse has recently been imaged by the Hubble Space Telescope, the first star besides the Sun whose surface has been observed:

    Betelgeuse
    Picture Information

    In this false-color picture taken in the ultraviolet, Betelgeuse's surface can be seen (red), as well as a large hot spot (white).

     
  • As a star becomes a bigger and brighter supergiant, its mass loss also increases.

    As with red giants, mass loss occurs continuously, carried away by the stellar wind.

    If this material collides with surrounding interstellar material, it can be pushed into a circumstellar shell, with a typical diameter of a light year.

    The interior star may then ionize it, producing a bright nebula.

    The Bubble Nebula (NGC 7635) is a beautiful example of such a shell, surrounding the massive blue supergiant BD+602522:


     
  • A supergiant may also eject matter in brief explosive intervals, probably the result of the many changes in nuclear fusion occuring in its interior.

    Ejections of matter may be the result of overall pulsations of the star's surface, or convection cells that carry material rapidly outward (like bubbles in boiling water).

    Extra: learn more about recent observations of Betelgeuse's atmosphere that support the latter idea.

    These ejections may also surround the supergiant with a slowly expanding circumstellar shell.

     
  • Betelgeuse is estimated to be losing ~10-6 solar masses per year, and may already have lost half of its main sequence mass.

    Betelgeuse's circumstellar shell is expanding at a rate of about 10 Km/s, and is currently 1/3 ly across.

     
  • An outstanding example of mass loss in a luminous supergiant can be found deep inside the Great Nebula of Carina (NGC 3372), 9000 light years away in the southern sky:

    Great Nebula in Carina, Visible
    Picture Information

    The dark nebula covering the brightest portion is known as the Keyhole Nebula (NGC 3324).

    To the lower left of the the Keyhole lies a bright yellowish billowing cloud, which hides the luminous red supergiant Eta Carinae.

    In the near (2 µm) infrared, Eta Carinae becomes quite visible (at the lower left):

    Great Nebula in Carina, Infrared
    Picture Information

    Eta Carinae may at one time have been as much as 100 times as massive as the Sun, and appears to be very near the end of its relatively short life.

    Designated a fourth magnitude star in 1677 by Halley, Eta Carinae brightened to a magnitude of -1 by 1843, making it the second brightest star in the sky.

    Then, between 1857 and 1870, Eta Carinae dimmed and finally disappeared from naked-eye visibility.

    The Hubble Space Telescope revealed that Eta Carinae's dimming was because it had ejected a large cloud of material that had shrouded the star:

    Eta Carinae
    Picture Information

    Because of its outbursts, Eta Carinae may now only be about fifty or sixty times as massive as the Sun.

    Presumably some of the material in the Great Nebula of Carina is due to this star's earlier mass loss.

    An image from the Chandra X-Ray Observatory reveals a circumstellar ring around Eta Carinae with a diameter of about two light years:

    Eta Carinae Shell
    Picture Information

    The x-ray light results from the collision of supersonic ejecta with the surrounding interstellar gas and dust in the Carina Nebula.

19.3 Supernovae, Take II

(Discovering the Universe, 5th ed., §12.5)
  • A supergiant that is concentrating iron at its core has a problem, because iron cannot be burned to produce energy!

    In the figure at the right, the dependence of nuclear stability on atomic mass is shown (the vertical variation is due to different elements with the same atomic mass).

    Iron is the most stable of all nuclei, so its fusion to create even larger nuclei requires an input of energy.
     
  • With no way to counter the tremendous force of gravity, the supergiant continues to compress its core.

    Eventually the iron in the inner core is so tightly packed, a density of about 3 x 109 g/cm3, that it becomes electron-degenerate.

    For a brief period of time compression is halted, and electron degeneracy pressure supports the outer layers of the star.

     
  • As the inner core is compressed, the silicon fusion in the shell above adds more iron to the inner core, and it grows to more than one solar mass of material.

    Once the inner core reaches a diameter of about 3000 Km, the gravitational forces acting on it are so intense that it is forced to collapse.

    In about a tenth of a second, the electrons are pushed towards the iron nuclei, and the temperature increases dramatically, surpassing 5 GK.

    The core is suddenly hot enough to produce significant amounts of gamma radiation.

    The gamma photons have enough energy to break the iron nuclei apart into their constituent protons and neutrons, a process called photodisintegration.

 

The star chart background was produced on a Macintosh with the Voyager II program, and are ©1988-93 Carina Software, 830 Williams St., San Leandro, CA 94577, (510) 352-7328. Used under license.
 
©1996-2001 Scott R. Anderson
Last update: 2001 July 25
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