Novae and Supernovae (lecture 14)

Novae and Supernovae

Nova Eridani 2009

  • Observations of Nova Eridani 2009 reveal intensities:

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    • 15.4>

    • 10.2>

White Dwarfs in Binary Systems

  • In a binary system:

    • Mass can be exchanged between stars.

    • If one member is a white dwarf, the following occurs:

Accretion Disk Components

  • Terms related to white dwarf interactions:

    • Accretion Disc: Structure formed by gas falling onto the white dwarf.

    • Disc Wind: Outflow from the accretion disk.

    • Jet: Beam of particles emitted from the system.

    • Hot Spot: Area of increased temperature due to accretion.

    • X-ray Heating: Process where the accretion disk produces X-rays that heat nearby materials.

    • Accretion Stream: Stream of material flowing from the companion star to the white dwarf.

    • Companion Star: The star paired with the white dwarf in the binary system.

Nova Characteristics

  • Survival of the White Dwarf:

    • After a nova event, the white dwarf remains intact.

    • The outer portions such as the accretion disk and parts of the companion star's corona are expelled.

Historical Novae Examples

  • Nova Cygni 1992 and Nova Herculis 1934 also produced remnant shells observed:

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Requirements for a Nova

  • A nova event requires:

    1. A white dwarf star.

    2. A companion star capable of supplying hydrogen.

Frequency of Nova Occurrences

  • Estimated occurrences: 40 to 60 novae per year.

  • Observed occurrences: approximately 10 per year.

T CrB (Recurrent Nova)

  • Characteristics of T CrB:

    • Composed of a red giant and a white dwarf.

    • It is categorized as a recurrent nova.

    • Last eruption observed in 1946.

  • Observational trend: Dimming observed shortly before nova activity.

    • Anticipated nova event expected around mid-2024, give or take a few months per AAVSO data.

Magnitude Data

  • Data showing stellar magnitudes over Julian Days (from 2021 to 2024) for U Gem (Dwarf Nova):

    • Points of measurement at various days for detailed tracking.

Subrahmanyan Chandrasekhar (1910-1995)

  • Key concepts concerning the Chandrasekhar Limit:

    • The mass limit is 1.4 solar masses (1.4M☉1.4 M_{☉}).

    • If a white dwarf exceeds this limit, it undergoes collapse and begins runaway nuclear fusion.

    • This reaction results in a catastrophic event known as a Carbon Detonation Supernova.

Prominent Scientists

  • Wilhelm Walter Baade (1893-1960)

    • Discovered two distinct stellar populations

      • population 1

      • populartion 2

  • Fritz Zwicky (1898-1974)

Crab Nebula (M1)

  • Observational data presented through comparative images showcasing the expansion over 30 years.comparison

Characteristics of Supernovae

  • Supernovae can be categorized based on discovery order:

    • Named systematically, starting from A, B, C, etc. (excluding I) then aa, ab, ac, etc. (excluding i).

    • Example: SN 2003 CG denotes the 82nd supernova discovered in 2003.

Dying Stages of Massive Stars

  • Carbon Detonation Supernovae are not the most common:

    • Stars with a core mass exceeding 1.4 solar masses will collapse.

    • Collapse generates explosive energy responsible for the supernova event.

Core Collapse Mechanisms

  • Atoms must disassociate to compress past the Chandrasekhar limit producing neutrons through Inverse Beta Decay: p+e−→n+up + e⁻ → n + u

    • The collapse halts due to degenerate neutron pressure.

  • A neutron star has a diameter of about 30 km for a mass around 1 solar mass (M☉M_{☉}).

Stellar Evolution of Massive Stars

  • Stages of a 25-Mass Star are highlighted in a table:

    • **Core Temperatures and Densities for Various Fusion Stages: **

    • Hydrogen fusion: Temperature of 7imes107K7 imes 10^7 K and density of 103kg/m310^3 kg/m^3.

    • Helium fusion: Temperature of 2imes108K2 imes 10^8 K and density of 2imes106kg/m32 imes 10^6 kg/m^3.

    • Carbon fusion: Temperature of 8imes108K8 imes 10^8 K and density of 109kg/m310^9 kg/m^3.

    • Neon fusion: Temperature of 1.6imes109K1.6 imes 10^9 K and density of 1010kg/m310^{10} kg/m^3.

    • Oxygen fusion: Temperature of 1.8imes109K1.8 imes 10^9 K and density of 1010kg/m310^{10} kg/m^3.

    • Silicon fusion: Temperature of 2.5imes109K2.5 imes 10^9 K and density of 1011kg/m310^{11} kg/m^3.

    • Core collapse temperature at 1010K10^{10} K and density at 1013kg/m310^{13} kg/m^3.

    • The duration of each fusion stage varies from 10710^7 years (Hydrogen) to just 1/41/4 second for supernova.

Core Collapse in Stars

  • For stars of around 9-10 solar masses:

    • The core contains a degenerate mix of Oxygen and Magnesium, possibly collapsing through electron capture events leading to a supernova.

    • Notably, supernova event SN 2018zd reported by astronomers in 2021.

Processes Leading to a Supernova

  • Pre-supernova events include:

    • Neutrinos emitted as the core collapses.

    • Interaction of the shock with the collapsing envelope leading to light emissions, observable as a pulsar.

    • A brightening effect by extru108ext{ru } 10^8 times due to explosive envelope ejection.

Types of Supernovae

  • Type I Supernova Characteristics:

    • H is absent in the spectrum, often brighter than Type II supernovae.

  • Type II Supernova Characteristics:

    • H is present in the spectrum, usually dimmer than Type I supernovae.

Specific Types of Supernovae

  • Type Ia Supernova:

    • Spectrometry shows no hydrogen or helium, but strong ionized silicon absorption (Si II).

    • Caused by runaway carbon fusion in a white dwarf from a close binary.

  • Type II Supernova:

    • Spectrometry reveals significant hydrogen lines, indicative of a core collapse in a massive star where outer layers remain intact.

Stages of Massive Star Collapse

  1. As evolution nears its end there is an onion-layer structure to the star.

    • The outer regions consist of Hydrogen, Helium, Carbon, Oxygen, Silicon, and Iron.

  2. Core collapse occurs rapidly within seconds:

    • Neutrinos and shockwaves are emitted during the transformation, blowing apart the star.

Alternative Types of Supernovae

  • Type Ib Supernova:

    • Hydrogen lines are absent but un-ionized helium (He I) lines are present, produced by core collapse of massive stars that lost hydrogen layers.

  • Type Ic Supernova:

    • No hydrogen or helium lines, occurring under similar conditions to Type Ib, simply with a further loss of outer layers.

Supernova Nucleosynthesis

  • Supernova events contribute to nucleosynthesis, creating various isotopes:

    • Neutrons produced capture various elements contributing to stable isotopes creation,

    • Elements beyond carbon are synthesized in these explosive events.

Rarity of Supernovae

  • Roughly one or two supernova events occur every century in galaxies like the Milky Way.

  • The last observed naked-eye supernova was SN 1604 (Kepler’s Supernova) until SN 1987.

Supernova 1987A

  • Discovered by Ian Shelton on February 24, 1987, at the Las Campanas Observatory.

  • Located in the Large Magellanic Cloud, approximately 51 kiloparsecs distant.

  • Progenitor Star:

    • Sanduleak -69º 202a, classified as a B3 blue supergiant, did not reach the expected brightness upon explosion.

  • Various hypotheses explain unusualities in the progenitor's characteristics, especially relating to metallicity and evolutionary speed.

Neutrino Observations of SN 1987A

  • Detection of neutrinos by three neutrino detectors occurred about 3 hours prior to visual detection, emitting around 105810^{58} neutrinos in total.

  • This accounted for over 90% of the supernova’s energy release.

Observations and Effects of SN 1993J

  • SN 1993J originated from another blue supergiant progenitor star.

  • Additional imaging from the Hubble Space Telescope shows the evolution of observations over time for SN 1987A.

The Impact of Cosmic Rays

  • Definition and historical context of cosmic rays:

    • Cosmic rays are high-energy radiation detected from space, termed by Robert Millikan in the 1920s.

  • Types categorized by energy levels:

    • Low Energy: Solar Cosmic Rays.

    • High Energy: Galactic Cosmic Rays.

    • Intermediate Energy: Cosmic rays at the solar system boundary.

Observations of Cosmic Ray Showers

  • Cosmic ray showers originate from primary cosmic rays colliding with Earth's atmosphere, resulting in cascades of secondary particles.

    • Maximal radiation is detected at about 40,000 feet altitude.

Cosmic Ray Effects and Studies

  • Historical peaks in cosmic ray events:

    • Highest recorded energy of a cosmic ray was 3x10¹⁴ MeV, nicknamed the “Oh My God Particle,” traveling nearly at the speed of light.

  • Analysis of biological effects reveals impacts from cosmic rays such as:

    • Hair changes, increased cancer risk, and other health implications.

Stellar Evolution and End States of Stars

  • Detailed classification of stars based on their mass and resulting end states:

    • Types of stars include low-mass, mid-mass, high-mass, supermassive, black holes, and neutron stars, categorized based on final supernova types and implications of metallicity for evolution.

    • Specific end states vary significantly, with types of supernova categorized based on their progenitor stars and core collapse criteria.