Question: Who realized the true nature of galaxies?
About the article "Nebula", section "Abstract".

Candidate passages:
Corvus contains no Messier objects.
The Antennae peculiar galaxy, NGC 4038 and 4039, consists of two interacting galaxies that appear to have a heart shape as seen from Earth.
The name originates from the huge tidal tails that come off the ends of the two galaxies, formed because of the spiral galaxies' original rotation.
Both original galaxies were spiral galaxies and are now experiencing extensive star formation due to the interaction of gas clouds.
The galaxies are 45 million light-years from Earth and each has multiple ultraluminous X-ray sources, the source of which is unknown.
Astronomers theorize that they may be a rare type of x-ray emitting binary stars or intermediate-mass black holes.
The Antennae galaxies appear in a telescope at the 10th magnitude.
The center of Corvus is home to a planetary nebula NGC 4361.
The nebula itself resembles a small elliptical galaxy, but the magnitude 13 star at its centre gives away its true nature. 
Measurement difficulties and techniques.
Lenticular galaxies share kinematic properties with both spiral and elliptical galaxies.
This is due to the significant bulge and disk nature of lenticulars.
The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion.
This situation is analogous to a balloon, where the motions of the air particles (stars in a bulge's case) are dominated by random motions.
However, the kinematics of lenticular galaxies are dominated by the rotationally supported disk.
Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy.
Thus kinematics are often used to distinguish lenticular galaxies from elliptical or spiral galaxies.
The distinction between spiral galaxies and lenticular galaxies is often simpler to analyze as the dividing line is often set at a particular bulge-to-disk ratio.
Determining the distinction between elliptical galaxies and lenticular galaxies often relies on the measurements of velocity dispersion (σ), rotational velocity (v), and ellipticity (ε).
In order to differentiate between lenticulars and ellipticals, one typically looks at the v/σ ratio for a fixed ε.
For example, a rough criterion for distinguishing between lenticular and elliptical galaxies is that elliptical galaxies have v/σ < 0.5 for ε = 0.3.
The motivation behind this criterion is that lenticular galaxies do have prominent bulge and disk components whereas elliptical galaxies have no disk structure.
Thus, lenticulars have much larger v/σ ratios than ellipticals due to their non-negligible rotational velocities (due to the disk component) in addition to not having as prominent of a bulge component compared to elliptical galaxies.
The kinematics of disk galaxies are usually determined by Hα or 21-cm emission lines, which are typically not present in lenticular galaxies due to their general lack of cool gas.
Thus kinematic information and rough mass estimates for lenticular galaxies often comes from stellar absorption lines, which are less reliable than emission line measurements.
There is also a considerable amount of difficulty in deriving accurate rotational velocities for lenticular galaxies.
This is a combined effect from lenticulars having difficult inclination measurements, projection effects in the bulge-disk interface region, and the random motions of stars affecting the true rotational velocities.
These effects make kinematic measurements of lenticular galaxies considerably more difficult compared to normal disk galaxies. 
Bob has published over two hundred scientific papers on topics ranging from the outer atmospheres of stars, the nature of quasars and active galaxies to the physics of forming galaxies in the most distant reaches of the Universe. 
The following is a list of notable galaxies.
There are about 50 galaxies in our Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in our Local Supercluster and an estimated number of about 170 billion in all of the observable universe.
The discovery of the nature of galaxies as distinct from other nebulae (interstellar clouds) was made in the 1920s.
The first attempts at systematic catalogues of galaxies were made in the 1960s the Catalogue of Galaxies and Clusters of Galaxies listing 29,418 galaxies and galaxy clusters, and with the Morphological Catalogue of Galaxies, a putatively complete list of galaxies with photographic magnitude above 15 comprising 30,642 items.
In the 1980s, the Lyons Groups of Galaxies listed 485 galaxy groups with 3,933 member galaxies.
Galaxy Zoo is a project aiming at a more comprehensive list; launched in 2007 it catalogued some 50 million galaxies by 2011 (some 0.03% of the estimated total).
There is no universal naming convention for galaxies, as they are mostly catalogued before it is established whether the object is or isn't a galaxy.
Mostly they are identified by their celestial coordinates together with the name of the observing project (HUDF, SDSS, 3C, CFHQS, NGC/IC, etc.)
The Catalogue of Galaxies and of Clusters of Galaxies (pr CGCG) was compiled by Fritz Zwicky in 1961–68.
It contains 29,418 galaxies and 9,134 galaxy clusters.