M101 is the brightest of a group of at least 9 galaxies, called the M101 Group, that includes NGC 5474, 5585, 5204 and 5477 plus Holmberg IV.The distance of M101 has been determined by the measurement of Cepheid variables with the Hubble Space Telescope in 1994/95 to be about 24 +/- 2 million light years, by the HST H0 Key Project Team (paper III, 1996).

The linear diameter is over 170,000 light years making M101 among the largest of the disk galaxies, and its total apparent visual brightness of 7.9 magnitude corresponds to an absolute brightness of -21.6 magnitude, or a luminosity of about 30 billion times that of our sun.

With a diameter of 170,000 light-years, M101 is getting on for twice the size of the Milky Way. Less is known about the mass of M101. A frequently cited number is an equivalent mass of about 16 billion solar masses. That value is almost certainly too low, and probably stems from M101's low surface brightness. New insights in its HII regions and rotational velocities have put the number between 100 billion and 1 trillion suns.

Three supernovae have been discovered in M101 - the first, SN 1909A, appeared on January 26th 1909. It was of peculiar type and reached magnitude 12.1. The second supernova, 1951H, was of type II and occurred in September 1951 reaching magnitude 17.5. The third, SN 1970G, also type II, was discovered on July 30th 1970 and reached magnitude 11.5.

Another remarkable property of this galaxy are its huge and extremely bright HII regions, of which a total of about 3000 can be seen on photographs. HII regions are places in galaxies that contain enormous clouds of high density hydrogen gas contracting under its own gravitational force. Eventually, when the localized hydrogen contracts enough for fusion processes to begin, stars are born. Consequently, HII regions are places that often contain large numbers of extremely bright and hot young stars giving them their characteristic blue color.

On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the past M101 may have undergone a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplifies the density waves in the spiral arms of M101. The amplification of these waves also leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

M102, in the constellation of Draco, is in some dispute because there was no object at the location given by Messier and during modern times various people have "assigned" NGC 5866 to M102.

From original NASA press release:
This is a unique view of the disk galaxy NGC 5866 tilted nearly edge-on to our line-of-sight. Hubble's sharp vision reveals a crisp dust lane dividing the galaxy into two halves. The image highlights the galaxy's structure: a subtle, reddish bulge surrounding a bright nucleus, a blue disk of stars running parallel to the dust lane, and a transparent outer halo. NGC 5866 is a disk galaxy of type "S0" (pronounced s-zero). Viewed face on, it would look like a smooth, flat disk with little spiral structure. It remains in the spiral category because of the flatness of the main disk of stars as opposed to the more spherically rotund (or ellipsoidal) class of galaxies called "ellipticals." Such S0 galaxies, with disks like spirals and large bulges like ellipticals, are called 'lenticular' galaxies. NGC 5866 lies in the Northern constellation Draco, at a distance of 44 million light-years. It has a diameter of roughly 60,000 light-years only two-thirds the diameter of the Milky Way, although its mass is similar to our galaxy. This Hubble image of NGC 5866 is a combination of blue, green and red observations taken with the Advanced Camera for Surveys in February 2006.

M103, within the Milky Way, is one of the more remote open clusters in Messier's catalogue.

The appearance of the cluster is dominated by the non-member binary Struve 131 (ADS 1209; components A, 7.3 magnitude, Sp B3, and B, 9.9 magnitude). However, John Herschel refers to this double star in context of the cluster. The two brightest cluster members, of about magnitude 10.5, are a B5 Ib supergiant and a B2 III giant. M103 also contains one red giant star, of spectral type M6 III, or gM6, and magnitude 10.8. The main sequence stars indicate a wide range of ages between 9 and 22 million years,

At a distance of 8,500 light years, M103's angular diameter of 6 arc minutes corresponds to 15 light years linear extension, which is approaching us at 37 km/sec.

In binoculars, M103 is fairly easy to find and identify, and visible as a nebulous fan-shaped patch. A 10x40 finder resolves the cluster into stars under good viewing conditions. Although telescopes will show up fainter member stars, the object is not so easy to identify in telescopes because it is quite loose and poor, and may be confused with other star groups or clusters in the vicinity.

The striking spiral galaxy M104 is famous for its nearly edge-on profile featuring a broad ring of obscuring dust. Viewed from 6 degrees south of its equitorial plane, M104 is seen in silhouette against a bright bulge of stars, the swath of cosmic dust lanes lends a hat-like appearance to the galaxy in optical images suggesting its popular moniker, The Sombrero Galaxy. The visibility of some details are lost in overwhelming glare. If you look carefully, features of the galaxy's dust lanes can be followed well into the bright central region.

This galaxy is of type Sa-Sb, with both a big bright core, and as one can see in shorter exposures, also well-defined spiral arms. It also has an unusually pronounced bulge with an extended and richly populated globular cluster system - several hundred can be counted in long exposures from big telescopes. Recent very deep photographs from the Anglo-Australian Observatory show that this galaxy has a very extended faint halo.

M104 is the dominating member of a small group of galaxies, the M104 group or NGC 4594 group of galaxies.

Dust ring:
As noted, this galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy. The dust lane is actually a symmetric ring that encloses the bulge of the galaxy. Most of the cold atomic hydrogen gas and the dust lie within this ring. The ring might also contain most of the Sombrero Galaxy's cold molecular gas, although this is an inference based on observations with low resolution and weak detections. 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.

Nucleus:
The nucleus of the Sombrero galaxy is classified as a low ionization nuclear emission region. These are nuclear regions where ionized gas is present, but the ions are only weakly ionized (i.e. the atoms are missing relatively few electrons). The source of energy for ionizing the gas in LINERs has been debated extensively. Some LINER nuclei may be powered by hot, young stars found in star formation regions, whereas other LINER nuclei may be powered by active galactic nuclei (highly energetic regions that contain supermassive black holes). Infrared spectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation activity. However, a supermassive black hole has been identified in the nucleus (as discussed in the subsection below), so this active galactic nucleus is probably the energy source that weakly ionizes the gas in the Sombrero Galaxy.

Central supermassive black hole:
In the 1990s, a research group led by John Kormendy demonstrated that a supermassive black hole is present within the Sombrero Galaxy. Using spectroscopy data from both the CFHT and the Hubble Space Telescope, the group showed that the speed of rotation of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun is present in the centre. This is among the most massive black holes measured in any nearby galaxies.

M105 is a type E1 elliptical galaxy and the brightest member of the Leo I galaxy grouping, which includes M95 and M96 plus various NGC and other galaxies.

This Hubble image of the central 5.4 arc seconds of M105 reveals striking detail of a bright pointlike nucleus and and interesting dark band-like structure. Spectroscopic HST studies have shown that the stars near M105's center are in rapid motion around this centre. Their motion is so rapid that it indicates a supermassive central black hole equal to 50 million solar masses.

The lower image shows M105 on the right, NGC 3384 above and left of centre, and NGC 3379, a more distant galaxy, at the bottom left.