A gravitationally bound collection of (typically) 50 or fewer galaxies, each at least as luminous as the Milky Way (~$10^{10}\, L_\odot$). Galaxy groups are smaller than a [[#Galaxy cluster|galactic cluster]]. [[Galaxies - General#The Milky Way]] is part of a galaxy group called the [[#Local Group]]. > [!measure] Typical Parameters > - Number of Galaxies: $\lesssim 50$ > - Mass: $M \sim 10^{13} \; {\rm M_{\odot}}$ > - Diameter: $D \sim 1-2 \; {\rm Mpc}$ **Subtypes of Groups:** - Compact Groups - a compact group consisting of a small number of galaxies ($\sim 5$) in close proximity and relatively isolated from other galaxies and formations. - Fossil Groups - the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. - Proto-groups - groups that are in the process of formation, and basically a smaller form of protoclusters. The first discovered galaxy group was Stephan's Quintet: ![[stefans_quintet.png|align:center|400]] ## Local Group The [[Galaxy Group|galaxy group]] containing about 100 galaxies, including our own [[Galaxies - General#The Milky Way|Milky Way]]. ![[Local_Group.jpg|align:center|500]] > [!measure] Properties > - Number of Galaxies: $N \lesssim 100$ (mostly faint [[Galaxy Classification#Dwarf Galaxy|dwarf galaxies]]) > - Mass: $M \sim 2 \times 10^{12} \; {\rm M_{\odot}}$ (mostly in MW and M31) > - Diameter: $D \sim 3 \; {\rm Mpc}$ ($\sim 750 \; {\rm kpc}$ between MW and M31) The **major components** of the Local Group include (by order of mass)... 1. [[Galaxies - General#Andromeda|Andromeda (M31)]] - Future merger with Milky Way in $\sim 4 \; {\rm Gyr}$ - Moving towards each other at $v_{\rm rel} \sim 120 \; {\rm km/s}$ 2. [[Galaxies - General#The Milky Way]] 3. Triangulum (M33) - satellite of Andromeda 4. The [[Galaxies - General#Large Magellanic Cloud|LMC]] and [[Galaxies - General#Small Magellanic Cloud|SMC]] 5. Many [[Galaxy Classification#Dwarf Galaxy|Dwarf Galaxies]] ![[Local_Group_Galaxies_Comparison.png|align:center|600]]