The Local Group: Includes M31, the Milky Way galaxy, their satellites, and other small galaxies, for a total of about 30-40 [including And XII, falling into the Local Group at 556 km/s]; It may be evolving into an elliptical galaxy, after the future collision between the Milky Way and M31.

  The Milky Way Neighborhood

  • Does the Universe end outside the Milky Way? To answer, we need to measure the distances to suspicious objects like the Andromeda Nebula and the Magellanic Clouds; To do that, we need to see objects of known luminosity, "standard candles".
  • Finding distances: Out to 30 Mpc or so, use variable stars, mainly Cepheids (10,000 times the Sun's luminosity, see M100); The LMC (50 kpc away) and SMC [with their Magellanic Stream], and some others are in our immediate surroundings; The closest one is the Sagittarius dwarf [which we may have ripped apart from the LMC].
  • The Andromeda galaxy: M31, about 2 Mly away, is the closest large galaxy to us; It is similar in structure and size (somewhat larger), and is also surrounded by small galaxies and leftover clouds of gas.

Out to a Few Hundred Mpc

  • Finding distances: We need brighter standard candles than Cepheid variables; Out to a few hundred Mpc, supernovas (10 billion times the Sun's luminosity) do the job [or the Tully-Fischer relation between luminosity and rotation rate]; after that we use another method, although measurements on supernovas are improving a lot...
  • Beyond the Local Group: More groups [like the Sculptor Group, at only 10 Mly] and larger clusters (the nearest large one is the Virgo Cluster at 16-20 Mpc, with about 2500 galaxies!), pulled together by gravity.
  • Local (or Virgo) Supercluster: Contains about 160 galaxy groups, including the Local Group and the Virgo Cluster; Flattened, 10 Mly thick and 100 Mly in diameter.
  • Anything between the galaxies? A thinner, hot gas (blowing like wind through our galaxy), and even a few hostless stars and globular clusters.

Types of Galaxies

  • Spirals: They have a thin disk, whitish/blue spiral arms (with gas & dust, and star formation), bulge, halo; The most famous one is M31, the Andromeda galaxy; Others are M51, the Whirlpool Galaxy, and M104, the Sombrero Galaxy; Varieties differ in how tight they are; most have elongated structures called bars near their center (the MW probably does), some are lenticular (no spiral arms), even warped or tangled! The largest known one, Malin 1, is 650,000 ly across.
  • How do we find the mass? From the speed of stars and Kepler's 3rd law, as usual; The most massive one has four times the mass of the MW. But the values we get always show that there is a lot of matter out there that we don't see!
  • Ellipticals: They have no disk (with some exceptions), no gas/dust, just old stars in random motions (all halo); They range from giants like M87 (1012 stars) to dwarfs like the nearby M32 (108 stars).
  • Irregulars: No definite shape; Star formation, gas & dust; They are less common, at least now; Examples include the Magellanic Clouds.
  • Ultra-compact dwarfs: UCD's were first discovered in 2000 but may be very common; They can pack millions of stars in regions 120 light years across!
  • Why are there different types? It is not clear, but we have hints; Not so much evolution of single galaxies, but interactions in groups or clusters, which can lead to new star formation (ring galaxies), distort galaxies (warped disks), and may determine overall shape; Giant ellipticals may be "fossil galaxies" from mergers of groups of galaxies.

  Overall Distribution

  • Beyond the Local Supercluster: The Great Attractor [Hydra-Centaurus Cluster]; more galaxies...; Most are in small groups, some in larger clusters and superclusters; Most galaxies are small, faint, and there are many cloudlike protogalaxies.
  • Galaxy surveys: Close to 100 billion galaxies may be observable, based on sample portions of the sky like the Hubble Deep Field; Various projects have conducte extensive surveys [such as 2dF and SDSS, which has obtained spectra for over 3 million galaxies as of 2013]; Many groups and clusters seem to be arranged in filaments and walls, with up to 100 Mpc voids; Fractal structure? At Gly scales, the universe looks homogeneous.
  • How far can we see? Out to billions of light years, but there we cannot rely even on supernovae, and we have to use a fact discovered by Hubble about how galaxies move; Plus, what we see is not what the galaxies look like "now"...

 The next questions are: Are these galaxies moving? Why do some of them not look normal?

page by luca bombelli <bombelli at>, modified 18 aug 2013