Here is a diagram of a spiral galaxy, in particular, our [[Galaxies - General#The Milky Way|Milky Way]] ![[galaxy_diagram.png|align:center]] [[galaxy_diagram.png|(make image bigger)]] > [!space] Distribution of Matter > **Stars:** > - Old, metal-poor ([[Stellar Populations#Population II (Pop. II)]]) > - In the [[#Galactic Bulge]] and the [[#Stellar Halo]] > - Young, metal-rich stars ([[Stellar Populations#Population I (Pop. I)]]) > - In the star-forming regions of the [[#Galactic Disk]] (mostly the [[#Spiral Arms]]) > - [[Stellar Clusters#Globular Cluster]] ($\sim 10^{6}$ stars each) > - In the [[#Galactic Bulge]] and the [[#Stellar Halo]] > - Remnants of minor satellite galaxy mergers > - In the [[#Stellar Halo]] > > **Gas:** > - Primarily in the star-forming regions of the [[#Galactic Disk]] (mostly the [[#Spiral Arms]]) > > **Dust:** > - Primarily in the [[#Galactic Disk]] > > **Dark Matter:** > - Quasi-spherical distribution in the [[#Dark Matter Halo]] > - The dominant component at large scales ## Galactic Center The rotational center and the barycenter of a galaxy. This is where it's presumed that many galaxies contain a [[Black Hole#Supermassive Black Hole (SMBH)]] as their galactic nucleus, surrounded by the [[#Galactic Bulge]]. ## Galactic Bulge A galactic bulge is a tightly packed group of stars within the [[#Galactic Center]] containing the galactic nucleus. Some have a spheroidal structure and some have an oblate sub-structure called the **Galactic Bar** within the bulge. > [!measure] Typical Parameters > - Mass: $M_{\rm bulge} \simeq 1.067 \times 10^{10} \; {\rm M_{\odot}}$ > - Scale Radius: $b \sim 0.3 \;{\rm kpc}$ **Composition:** - Old, metal-poor stars ([[Stellar Populations#Population II (Pop. II)]]) - Metallicities of $z\sim 10^{-2}-10^{-3}$ - Less orderly Orbits (e.g. not in same direction as disk, more eccentric, etc.) - [[Stellar Clusters#Globular Cluster|Globular Clusters]] ($\sim 10^6$ stars each) - The Galactic Nucleus, which maybe a [[Black Hole#Supermassive Black Hole (SMBH)|SMBH]] and/or may act as an [[Black Hole#Active Galactic Nuclei|Active Galactic Nuclei (AGN)|AGN]] - For the [[Galaxies - General#The Milky Way|Milky Way]], the galactic nucleus is [[Black Hole#Sagittarius A*]]. ## Galactic Disk The galactic disk can be broken down into two regions - the inner [[#Thin Disk]] and the outer [[#Thick Disk]] - in which the spiral arms extends (in a [[Galaxy Classification#Spiral Galaxy]]). ### Thin Disk > [!measure] Typical Parameters > - Mass: $M_{\rm disk} = M_{\rm thin} + M_{\rm thick} \simeq 3.944 \times 10^{10} \; {\rm M_{\odot}}$ > - Scale Radius: $r_{\rm thin\ disk} \simeq 5.3 \; {\rm kpc}$ > - Scale Height: $z_{\rm thin\ disk} \simeq 0.25 \; {\rm kpc}$ **Structure:** - Contains a majority of the Milky Way's stars, gas, and dust, and can exhibit spiral structure (except in [[Galaxy Classification#Lenticular Galaxy|lenticular galaxies]]) and/or barred structure (hypothesized to be a recurrent dynamical feature while in the disk phase). **Age:** - Stellar population is typically younger (less than 10 billion years old) and more metal-rich. ([[Stellar Populations#Population I (Pop. I)]]) **Motion:** - More circular stellar orbits (mostly) confined to the plane of the disk. **Origin:** - Believed to be formed from the collapse of a gas cloud that was enriched with heavy elements from previous generations of stars. ### Thick Disk > [!measure] Typical Parameters > - Mass: $M_{\rm disk} = M_{\rm thin} + M_{\rm thick} \simeq 3.944 \times 10^{10} \; {\rm M_{\odot}}$ > - Scale Radius: $r_{\rm thin\ disk} \simeq 2.6 \; {\rm kpc}$ > - Scale Height: $z_{\rm thin\ disk} \simeq 0.8 \; {\rm kpc}$ **Structure:** - Thicker and more spheroidal in shape. Diameter is about half that of the thin disk. **Age:** - Stellar population is typically older (more than 10 billion years old) and more metal-poor. ([[Stellar Populations#Population II (Pop. II)]]) **Motion:** - More eccentric and inclined stellar orbits that are tilted with respect to the thin disk. **Origin:** - Believed to be formed from the heating and thickening of the [[#thin disk]] due to interactions with other galaxies, mergers, or accretion events. ### Spiral Arms The spiral arms are concentrations of the young stars in the galactic disk. The spiral arms form from a density wave moving through the disk *(see [[Question 114]])*. Due to the high mass density and the high rate of star formation, theses regions tend to be very bright and luminous. *(Also see [[Galaxy Classification#Spiral Galaxy]])* ## Stellar Halo The stellar halo is the luminous region surrounding the [[#Galactic Disk]], and is usually the region that defines the size of a galaxy, since we cannot observationally put good constraints on the dark matter halo size. > [!measure] Typical Parameters > - Mass: $M_{\rm stellar\ halo} \sim 10^{9} M_{\odot}$ > - Scale Radius: $R \sim 100 \; {\rm kpc}$ **Composition:** - Old, metal poor stars ([[Stellar Populations#Population III (Pop. III)]]) - Less orderly Orbits (not in same direction as disk, more eccentric) - Contains remnants of minor satellite galaxy mergers (which has been used as a test of [[Dark Matter#LCDM]]) ## Dark Matter Halo A quasi-spherical distribution of dark matter, which can explain the flat rotation curves (circular velocity of stars as a function of radius) seen in galaxies (amongst other things, see [[Dark matter]] and [[#101|DM evidence]]). The dark matter halo is the dominant structure component for the galaxy at large scales. > [!measure] Typical Parameters > - Mass: $M_{200c}^{\rm DM} \sim 10^{10} - 10^{13} \; {\rm M_{\odot}}$ > - Concentration: $C_{200c}^{\rm DM} \sim 0.1 - 50$ > - Approx. Radius: $\sim 200 \; {\rm kpc}$ **Composition:** - Non-visible halo composed of dark matter.