Features:

  1. HCG 87 is a Hickson Compact Group of galaxies. Hickson Compact Groups are compact galaxy groups.

  2. HCG 87 is ∼ 130 Mpc away.

  3. The sources with points and probably many of the faint sources are foreground stars in the Milky Way.

    The points are part of the diffraction patterns of the brighest stars. They show up because of strong overexposure. The brightest stars are much brighter than the other astronomical objects in the image.

    There are 4 points for each diffraction pattern because the CCD camera is held in front of the primary mirror by 4 arms. The 4 arms give a 4-fold symmetry that communicates itself to the diffraction patterns.

    Galaxies are actually faint by comparison to bright foreground stars. So to image the galaxies well, you ineluctably overexpose the bright foreground stars.

  4. The HCG 87 galaxies:
    1. HGC 87a: low member in the image: Nearly edge-on spiral galaxy, Hubble sequence type Sb spiral, redshift velocity 8694 km/s, distance 117.54 Mpc.
    2. HGC 87b: right member in the image: Hubble sequence type Lenticular S0 (but it looks like an elliptical galaxy to the eye), redshift velocity 8972 km/s, distance 121.65 Mpc.
    3. HGC 87c: high member in the image: Hubble sequence type Sb spiral, redshift velocity 8920 km/s, distance 104.71 Mpc.
    4. HGC 87d: middle member in the image: Hubble sequence type Sd spiral, redshift velocity 10200 km/s, distance ???. Actually, this spiral galaxy is probably distinctly more remote than the other spiral galaxies since its apparent size is so small compared to them. Normal spiral galaxies usually do NOT vary in size by more than a factor of 2??? or so. Thus, it is probably NOT really a member of the galaxy group: i.e., it is probably NOT gravitationally bound to the galaxy group and it's probably a background astronomical object only coincidentally aligned with galaxy group.

  5. The two main classes of galaxies are illustrated in the image:

    1. Spiral galaxies which have galactic disks and spiral arms.

      In enhanced true-color images, the spiral arms are a complex mix of blue (from hot young OB stars), pink (from H II regions emitting the atomic hydrogen line Hα), and brown/black (from obscuration by interstellar dust).

        "Enhanced true-color" means the light of that color is really there and has important implications, but it has been artificially increased relative to what the human eye would see in order to be obvious. It is actually hard to know how true-color any astonomical image is since all images are digital nowadays and the makers can make the color anything they like.

        Some images though are clearly false color.

      The bulges of spiral galaxies are yellow since they contain mostly older Population I stars and Population II stars which are yellow to red in color.

      The galactic halos of spirals are faint and often have populations of globular clusters.

      Seen face-on or obliquely, spirals are easily recognized.

      Seen edge-on, they can still be easily recognized from the colors of the spiral arms even if the spiral arms CANNOT be seen as spirals themselves.

    2. Elliptical galaxies are less interesting to look at than spirals.

      The are just spherical or elliptical blobs that in true color are almost always just yellow with the brightness increasing toward the center.

      Ellipticals have little interstellar dust and little star formation in the modern observable universe.

      This accounts for their yellow color. They have NO or very few blue hot young OB stars and contain mainly older older Population I stars and Population II stars which, as aforesaid, are yellow to red in color.

      Actually, there are NO ellipticals in HCG 87, but lenticular (S0) galaxies (like HGC 87b) look a lot like ellipticals since they have no spiral arms. HGC 87b, in particular, looks like an elliptical to casual inspection, and so can represent ellipticals.

  6. A key point about galaxies is that we can only see them from one orientation.

    For spiral galaxies, we can infer to some degree there appearance from other orientations because their galaxy types can be recognized independent of orientation.

    For elliptical galaxies, situation is a bit harder because their 3-dimensional shapes CANNOT easily be inferred from their 2-dimensional projections on the sky.

    Yours truly thinks of galaxies as constituting a giant mobile in space---but one we can only see from point of view---our Earth.