Discovering The Pinwheel Galaxy

The Milky Way is perhaps the most famous galaxy people know, and even preschool kids can name it. But little did we learn and hear about other galaxies and that there are hundreds of billions of them scattered in the universe. We  can see some galaxies with our naked eyes, especially under clear and good condition night sky, while others are only visible galaxies using binoculars or telescopes.

Pinwheel galaxy is one of the galaxies visible on Earth without using a telescope. The galaxy is also known as M101, a giant spiral galaxy formed from a disk of stars, dust, and gas that is 170,000 light-years across — nearly twice the diameter of the Milky Way galaxy. It is estimated that 1 trillion stars comprise M101, and the galaxy’s spiral arms are sprinkled with large regions of star-forming nebulas. These nebulas are areas of intense star formation within giant molecular hydrogen clouds. Brilliant, young clusters of hot, blue newborn stars trace out the spiral arms.

History of The Pinwheel Galaxy

French astronomer Pierre Méchain, one of Charles Messier’s colleagues, first observed and discovered the Pinwheel galaxy in 1781. He initially described the galaxy as a “nebula without a star as it is very obscure and 6’ to 7’ in diameter large, between the left hand of Böotes and the tail of the Great Bear. 

He reported his discovery to the famous colleague Messier, the author of an astronomical catalog listing 110 nebulae, faint star clusters, and other deep-sky objects, which later became known as the Messier objects. The Pinwheel galaxy discovered by Pierre Méchain then became the 101st entry in the catalog and was therefore dubbed Messier 101 or M101.

The galaxy was found 25 million light-years away from Earth in the constellation Ursa Major, with an apparent magnitude of 7.9. It can be spotted through a small telescope and is most easily observed during April. Despite its strikingly large size compared to the Milky Way, the Pinwheel galaxy is very faint due to the very far distance. The galaxy’s spiral structure was only observed in the late 19th century as the technology of optical telescopes developed and improved further.

The galaxy’s spiral structure was first documented by an Anglo-Irish astronomer Lord Rosse using a 72-inch Newtonian reflector in the second half of the 19th century. Pierre was an avid comet hunter who is credited for discovering many other celestial bodies. During 1779 and 1782, he found the Pinwheel Galaxy and many spherical bodies, and eight comets. Several prominent companion galaxies include the irregular spiral galaxy NGC 5204, the peculiar dwarf galaxy NGC 5474, the dwarf galaxy NGC 5477, the spiral galaxy NGC 5585, and the irregular galaxy Holmberg IV (UGC 8837).

Among the M101 Group, a group of galaxies in Ursa Major, the M101 or Pinwheel galaxy is the brightest. The M101 Group is one of the galaxy groups within the Virgo Supercluster, including the Local Group of galaxies and the Virgo Cluster. Experts believe the Pinwheel Galaxy underwent a near collision with another galaxy in the past, and the associated gravitational tidal forces caused it to appear asymmetrical. The collision could have also caused these bright regions to trigger even more star-forming activity, making it one of the more remarkable objects to see in space.


people sitting on the fence, people staring at the sky, myriad stars, blue night sky

The center of the galaxy makes it strange and distinguished from other spiral galaxies in the known Universe. Though it has been observed across a wide range of wavelengths, there was no evidence that the core emits strong radio or X-ray emissions. This spiral galaxy goes against the typical characteristic of rotating around one gigantic black hole because the galaxy contains a bunch of smaller black holes no bigger than 3-30 solar masses. The

Chandra X-ray Observatory detected an ultraluminous X-ray source in 2001 and was later observed in the visible-light wavelengths by the Hubble and XMM-Newton space observatories.

How To Find The Pinwheel Galaxy In The Night Sky

To find the Pinwheel galaxy reasonably easily, you need at least a 4-inch telescope when observing and hunting. It is best observed under areas with minimal to no light pollution for a clear night sky view. Springtime is also an ideal season when the Ursa Major climbs high in the northern celestial hemisphere.

The M101 creates a triangle with the two stars in the handle of the Big Dipper. The Big Dipper asterism is sometimes used interchangeably with the Ursa Major constellation, also known as the Big Bear. The handle consists of several stars, and the Pinwheel galaxy lies slightly above the line connecting the stars known as Alkaid and Mizar.

If you want to see the Pinwheel galaxy without using a telescope, you won’t be able to do it. The Andromeda galaxy is the only object visible without a telescope outside of our Milky Way. On the other hand, the Pinwheel galaxy may be seen with even a tiny and inexpensive telescope.

Facts About The Pinwheel Galaxy

Pierre François André Méchain photo, a painting of Pierre François André Méchain, a man wearing black coat, a man with white hair

1. The Pinwheel galaxy has an 85,000 light-year radius. This indicates it is larger than our Milky Way galaxy but smaller than the Andromeda galaxy in terms of size.

2. The Pinwheel galaxy is estimated to be a little less than 21 million light-years away from the Earth.

3. M101, or Messier 101, is another name for the Pinwheel galaxy. 

4. It was found by Pierre François André Méchain, a French astronomer who, together with Messier, was responsible for several discoveries in the 18th century.

5. When looking up into the sky, the Pinwheel galaxy can be spotted in the Ursa Major constellation. It’s only a short distance from the “Great Bear’s” head.

6. In terms of galaxy type, the Pinwheel is most closely related to spiral galaxies.

7. The Pinwheel galaxy is part of the M101 group of galaxies, which is named because it is the giant galaxy in the region.

8. The Pinwheel Galaxy differs from many other galaxies because it lacks a black hole in its center. Instead, it has a swarm of smaller black holes all over it.

9. Although the actual number of stars in the Pinwheel galaxy is unknown, astronomers think there are more than a trillion.

10. The Pinwheel galaxy is a little crooked and out of shape. This, according to astronomers, is due to a billion-year-old collision.

11. Planets can be found everywhere along the spiraling arms of the Pinwheel galaxy. Planets that aren’t in our solar system are known as exoplanets.

12. The Pinwheel galaxy is around 170,000 light-years across. This makes it a reasonably massive galaxy with more than a trillion stars.