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Stellar Explosions

Stellar Explosions

Novae

  • Definition: The term "novae" refers to a "new" object that is bright and not seen before in celestial observations.

Luminosity

  • Measured in solar units, the luminosity of novae can vary significantly.

  • Luminosity Values:

    • 0.01 - Low luminosity

    • 10.000 - Medium luminosity

    • Varies around several points including 0, 50, 100, 150 (units unspecified in the transcript).

Timeline of Nova Appearance

  • During the initial phases, brightness will vary as observed in nova events, represented in days on a timeline from -5 to +10.

Example: Nova Persei (1901)

  • Observed characteristics of Nova Persei showcased the evolution of novae, with significant developments noted as seen 100 years later.

Characteristics of Novae

  • Novae generally involve the following components:

    • Main Sequence Companion: A binary star system where one of the stars is a white dwarf.

    • Rotation and Lagrangian Points: The gravitational balance points in a system that relate to mass distribution.

    • Roche Lobe: The region surrounding a star in a binary system where material is gravitationally bound to that star, prominent in both the white dwarf and its companion.

    • Mass Transfer Stream: Matter is transferred from the companion star to the white dwarf.

    • Accretion Disk and "Hot Spot": As gas falls onto the white dwarf, it forms an accretion disk, radiating X-rays and leading to fusion reactions on the star's surface when conditions allow.

Summary of the Nova Process

  • Core Elements:

    • Presence of a white dwarf in a close binary configuration.

    • Mass Transfer: Slow accumulation of mass from the companion star to the white dwarf.

    • Formation of an accretion disk from infalling gas.

    • Hydrogen on the white dwarf is heated gravitationally until it reaches temperatures of 10^7 ext{ K} which ignites fusion at the surface.

    • This leads to a significant increase in brightness and the expulsion of a nova shell.

    • The white dwarf survives the process, and can potentially repeat the cycle following a nova event.

Supernova

  • Noted on February 23-24, 1987, a significant event in astrophysical studies.

Luminosity of Supernovae

  • Measured in Solar Units:

    • Typical values span wide ranges:

    • 10^{10} - High luminosity (Type I)

    • 10^{9} - Lower luminosity (Type II and beyond)

    • Absolute magnitude ranges extend from -20 to -10, depicted across a timeline from -15 to 200 days.

Types of Supernovae

Type I Supernovae

  • Characteristics:

    • Involves a binary system featuring a white dwarf star.

    • The mass of the white dwarf increases beyond the Chandrasekhar limit of 1.4 M_{ ext{☉}} (solar mass).

  • Mass Growth Causes:

    • Transfer of mass from its companion star.

    • Merger with another white dwarf.

  • Result: The increased mass leads to collapse and ignition of fusion across the star, resulting in a cataclysmic explosion.

Type II Supernovae

  • Characteristics:

    • Occurs in massive stars (masses greater than 8 solar masses) that might not be in binary systems.

    • The iron core of the star collapses within seconds, during which temperatures reach above 10^{10} ext{ K}.

    • The collapse produces heavier nuclei, but also leads to fragmentation of most nuclei.

  • Physical Process:

    • Energy absorption occurs due to the absence of supportive forces against gravity.

    • Following the core collapse, it remains to be understood what occurs next (left as a question for further study).

Gravity vs. Pressure

  • A theoretical inquiry posed in the context of supernova events, exploring the balance between gravitational collapse and pressure exerted by nuclear forces:

    • Complete Victory for Pressure: Seen in Type I Supernovae.

    • Truce with Degenerate Electrons: Observed in states of white dwarfs, where electron degeneracy pressure counteracts gravitational collapse.

    • Truce with Degenerate Nuclei: A yet-defined state of balance wondered upon in the context of gravity.

    • Complete Victory for Gravity?: The implication remains open-ended and warrants further exploration in understanding stellar dynamics in supernovae.