with the stars
Mizar,
Alcor and
Alkaid. M101 is a large galaxy, with a diameter of 252,000 light-years. By comparison, the
Milky Way has a diameter of 87,400 light-years. It has around a trillion stars. It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses. Its characteristics can be compared to those of
Andromeda Galaxy. M101 has a high population of
H II regions, many of which are very large and bright. H II regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where
stars form. H II regions are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hot
superbubbles. In a 1990 study, 1,264 H II regions were cataloged in the galaxy. Three are prominent enough to receive
New General Catalogue numbers—NGC 5461, NGC 5462, and NGC 5471. M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress
interstellar hydrogen gas, which then triggers strong
star formation activity in M101's spiral arms that can be detected in ultraviolet images. In 2001, the X-ray source P98, located in M101, was identified as an
ultra-luminous X-ray source—a source more powerful than any single star but less powerful than a whole galaxy—using the
Chandra X-ray Observatory. It received the designation M101 ULX-1. In 2005,
Hubble and
XMM-Newton observations showed the presence of an optical counterpart, strongly indicating that M101 ULX-1 is an
X-ray binary. Further observations showed that the system deviated from expected models—the black hole is just 20 to 30
solar masses, and consumes material (including captured stellar wind) at a higher rate than theory suggests. It is estimated that M101 has about 150
globular clusters, the same as the number of the
Milky Way's globular clusters. == Companion galaxies ==