## Nebula A nebula is a distinct luminescent part of interstellar medium where gas, dust, and other materials "clump" together to form a relatively denser region. These regions tend to be star-forming regions. **Consist of**: - [[Interstellar Medium#Ionized Hydrogen|ionized]], [[Interstellar Medium#Atomic Hydrogen|neutral]] or [[Interstellar Medium#Molecular Hydrogen|molecular]] hydrogen - cosmic [[Interstellar Medium#Dust|dust]] - Particle density ~$10^{4}\,\pu{cm^{-3}}$ (Earth at sea level has ~$10^{19}\,\pu{cm^{-3}}$) **Examples**: - Eagle Nebula (star-forming Pillars of Creation within) - Crab Nebula - Orion Nebula (brightest nebula) - Carina Nebula ## Emission Nebula Circumstellar junk. A [[#Nebula|nebula]] formed of ionized gas emitting in various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. **Types**: - H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons - [[#Planetary nebula|Planetary nebulae]] ## Planetary Nebula A **planetary nebula** is an [[Nebulae#Emission Nebula|emission nebula]] created from the irradiated outer layers of a dying, intermediate mass star ($0.8 M_\odot \lesssim M \lesssim 8 M_\odot$) along the [[Hertzsprung-Russell Diagram#Post-Asymptotic Giant Branch (Post-AGB)|post-AGB track]] ![[planetaryNebula_SouthernRing.jpeg|align:center|500]] > [!measure] Typical Parameters > - Diameter: $\sim 0.3 \; {\rm pc}$ > - Particle Density: $10^{2} - 10^{4} \; {\rm cm^{-3}}$ *(decreases over expansion)* > - Mass: $0.1 - 1 \; {\rm M_{\odot}}$ **Formation:** - During the [[Hertzsprung-Russell Diagram#Post-Asymptotic Giant Branch (Post-AGB)|post-AGB]] track, the outer layers of the stellar envelope are radiated away through strong stellar winds from the hot [[Stellar Classes#White Dwarf|white dwarf]] core. - This creates a distinct, denser cloud in the [[Interstellar Medium|interstellar medium]]. - Once ejected, the medium is irradiated with strong [[Electromagnetic Spectrum|UV]] radiation, ionizing the gas (sometimes, into a HII region). - $T_{\rm remnant} \sim 3 \times 10^{4} \; {\rm K}$ - The ionized gas can then recombine, de-excite, and emit spectra that we observe - The shape of the nebula can be spherical ($\sim20\%$) or have more complex morphologies ([[Stellar Classes#Bipolar Flows]], multipolar structures, etc.) **Example:** Southern Ring Nebula > [!note] Historical Note > A **planetary nebula** is not actually associated with planetary bodies. They are named this because in the 19th century astronomers had low-resolution telescopes, and they thought these spherical gas clouds looked like gas planets at the time. ## Reflection Nebula ![[reflectionNebula.jpg|align:center|300]] Clouds of [[Interstellar Medium#Dust|interstellar dust]] which reflect the light of a nearby star, in which the energy of the light is insufficient to ionize the material and create an [[#Emission nebula|emission nebula]], so the light is instead scattered by the dust, making it visible. Shown is [[Catalogs#IC|IC]] 2631 ## Dark Nebula A [[Interstellar Medium#Molecular Clouds|molecular cloud]] for which enough [[Interstellar Medium#Dust|dust]] is present and the medium is dense enough to block visible light from background sources like stars, [[Nebulae#Emission nebula|emission nebulae]] or [[Nebulae#Reflection nebula|reflection nebulae]]. > [!image] GAIA Milky Way Data > > ![[GAIAMilkyWay.jpg|align:center]] > > Dark nebulae in black and stellar clouds ([[Stellar Clusters#Globular Cluster|globular clusters]] and [[Stellar Clusters#Open Cluster|open clusters]]) in white ## Pulsar Wind Nebula [[Stellar Explosions#Supernova|Supernova]] remnant blown around by a [[Neutron Star|neutron star]] like the crab nebula. ![[Crab_Nebula.jpg|align:center|500]]