TYPES OF GALAXY

Cartwheel Galaxy

The Cartwheel Galaxy (also known as ESO 350-40) is a lenticular galaxy about 500 million light-years away in the constellation Sculptor. It is about 150,000 light-years across. The estimated mass is 2.9–4.8 × 109 solar masses, and it is rotating at 217 km/s.

Evolution
The galaxy was discovered by Fritz Zwicky in 1941 and was once a normal spiral galaxy like the Milky Way before it underwent a head-on collision with a smaller companion approximately 200 million years ago.When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful shock wave through the galaxy, like a rock being tossed into a sandbed.
An estimation of the galaxy's span resulted in a conclusion of 150,000 light years, which is slightly larger than the Milky Way. It can be seen that the galaxy is beginning to retake the form of a normal spiral galaxy, with arms spreading out from a central core.

Whirlpool Galaxy

The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting grand-design spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. It is one of the most famous galaxies in the sky.The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars.

Discovery
What was later known as the Whirlpool Galaxy, was discovered in 1774 by Charles Messier, and is designated as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain.
Sometimes M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).
In 2005 a supernova (SN 2005cs) was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14.

Triangulum Galaxy

The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy approximately 3 million light-years away in the constellation Triangulum. The galaxy is also sometimes informally referred to as the Pinwheel Galaxy by some amateur astronomy references
With a diameter of about 50,000 light-years, it is the third largest galaxy in the Local Group, a group of galaxies which also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. Estimates indicate that Triangulum may be home to between 30 and 40 billion stars,compared to 200-400 billion for the Milky Way, and 1 trillion stars for Andromeda.

Distance
At least three techniques have been used to measure distances to M 33. Using the Cepheid variable method, an estimate of 2.77 ± 0.13 Mly (850 ± 40 kpc) was achieved in 2004.
Also 2004, the Tip of the Red Giant Branch (TRGB) method was used to derive a distance estimate of 2.59 ± 0.08 Mly (794 ± 23 kpc).
When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 3.1 ± 0.2 Mly (940 ± 70 kpc).
Averaged together, all these distance measurements give a combined distance estimate of 2.81 ± 0.09 Mly (861 ± 28 kpc).

Tadpole Galaxy

The Tadpole Galaxy is a disrupted barred spiral galaxy located 400 million light years from Earth toward the northern constellation Draco. Its most dramatic features are a trail of stars about 280 thousand light-years long and massive, bright blue star clusters.

During this close encounter, tidal forces drew out the spiral galaxy's stars, gas, and dust, forming the conspicuous tail. The intruder galaxy itself, estimated to lie about 300 thousand light-years behind the Tadpole, can be seen through foreground spiral arms at the upper left. Following its terrestrial namesake, the Tadpole Galaxy will likely lose its tail as it grows older, the tail's star clusters forming smaller satellites of the large spiral galaxy.

Sombrero Galaxy

The Sombrero Galaxy (also known as M 104 or NGC 4594 ) is an unbarred spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. The galaxy has an apparent magnitude of +9.0, making it easily visible with amateur telescopes.

History

Discovery

The Sombrero Galaxy was discovered in March of 1767 by Pierre Méchain, who described the object in a May 1767 letter to J. Bernoulli that was later published in the Berliner Astronomisches Jahrbuch.

William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disc, what is now called a dust lane. Later astronomers were able to connect Méchain's and Herschel's observations.

Designation as a Messier object

In 1921, Camille Flammarion found Messier's personal list of the Messier objects including the hand-written notes about the Sombrero Galaxy. This was identified with object 4594 in the New General Catalogue, and Flammarion declared that it should be included in the Messier Catalogue. Since this time, the Sombrero Galaxy has been known as M104.

Historical redshift measurements

In the 1910s, Vesto Slipher discovered that the spectra of several galaxies, including the Sombrero Galaxy, are redshifted. The average velocity calculated from these redshifts was 400 km/s. The redshift for the Sombrero Galaxy itself was calculated to be 1100 km/s.

Dust ring

As noted above, this galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy. This dust lane is actually a symmetric ring that encloses the bulge of the galaxy. Most of the cold atomic hydrogen gas and the dust lies within this ring.

Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring. Based on infrared spectroscopy, the dust ring is the primary site of star formation within this galaxy.