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Overview
This lecture covers the types of nebulae observed by the Hubble Space Telescope, their formation, characteristics, and the role they play in the life cycles of stars.
What Are Nebulae?
- Nebulae are clouds of gas (mainly hydrogen and helium) and cosmic dust in space.
- They form part of the interstellar medium, which is extremely low-density material between stars.
- Nebulae occur where gas and dust become dense, often due to gravity or the death of stars.
- They are sites of both star formation and destruction and can exist in large complexes.
Types of Nebulae
Emission Nebulae
- Emit their own light due to ionized gas energized by ultraviolet radiation from nearby stars.
- Characterized by glowing gas as electrons rejoin atoms, producing visible light.
- Example: The Orion Nebula contains young, massive stars shaping the gas and triggering more starbirth.
Reflection Nebulae
- Do not emit light, but reflect the light of nearby stars, making them glow.
- Often appear bluish due to the scattering of light by dust particles.
- Example: NGC 1999 and the nebula around the Pleiades cluster.
Planetary Nebulae
- Created when medium- or low-mass stars die and expel their outer layers.
- The exposed core becomes a white dwarf, whose UV radiation makes the ejected gas glow.
- Planetary nebulae can have various shapes (rings, hourglasses) and are not related to planets.
- Example: The Helix Nebula.
Supernova Remnants
- Result from the explosive death (supernova) of massive stars.
- Debris from the explosion expands rapidly and glows from shock heating and synchrotron radiation.
- Example: The Crab Nebula, powered by a central pulsar (rapidly spinning neutron star).
Absorption (Dark) Nebulae
- Dense clouds of gas and dust that block light from behind, appearing as dark patches.
- Include Bok globules, which may be sites of future star formation.
- Example: Bok globules in the NGC 281 region.
Key Terms & Definitions
- Nebula — A cloud of gas and dust in space, often a site of star formation or death.
- Interstellar Medium — The sparse material (gas and dust) found between stars.
- Emission Nebula — Nebula that emits its own light due to ionization by nearby stars.
- Reflection Nebula — Nebula that shines by reflecting starlight, not by emitting its own.
- Planetary Nebula — Gas shell ejected by a dying star, illuminated by a hot white dwarf core.
- Supernova Remnant — The expanding gas and dust left after a massive star explodes.
- Absorption Nebula/Bok Globule — Dark cloud that absorbs background light, often a stellar nursery.
- Ionization — The process of removing electrons from atoms, usually by UV light.
- White Dwarf — The dense, hot core left after a star expels its outer layers.
Action Items / Next Steps
- Review images and diagrams of different nebula types for identification practice.
- Prepare examples of nebulae for each category for discussion or further research.