Object 155: Messier 101

Podcast release date: 04 August 2025

Right ascension: 14:03:12.6

Declination:+54:20:56

Epoch: ICRS

Constellation: Ursa Major

Corresponding Earth location: About 640 km south of Seward, Alaska, in the Pacific Ocean.

This episode's coordinates have hit Messier 101, also called the Pinwheel Galaxy or NGC 5457, but no one calls it Henry. M101 an especially spectacular grand design spiral galaxy with a rather complex yet well-defined spiral structure radiating from the small central bulge, including one spiral arm with lots of gaseous star-forming nebulae that seems to straighten as it extends outwards from the galaxy. It's also one of the largest face-on spiral galaxies in the Earth's sky in terms of apparent size; it's close to the size of the Moon in terms of diameter [1]. At a distance of 22.7 million light years (6.95 Mpc) in the constellation Ursa Major [2], it's possible to observe and study the stellar and gaseous structures within the galaxy on very fine scales, and it's even possible to identify individual bright stars within the galaxy.

As I've mentioned before, the Messier Catalogue was originally created by eighteenth century French astronomer and comet hunter Charles Messier to keep track of fuzzy things in the sky that were not comets. M101 was one of a series of objects in the catalog that were discovered not by Charles Messier himself but by his junior associate Pierre Mechain. Mechain discovered M101 on March 27, 1781 [3], and soon after this, Messier compiled a final version of his catalog [4]. The catalog was originally going to include just 100 objects that Messier himself had all personally observed, but he decided to include Messier 101, 102, and 103 without checking them first [4]. This turned out to be a mistake, because M102, which was also discovered by Mechain, may have been a duplicate observation of M101, and a letter from Pierre Mechain was later found even indicating that M102 was an accidental duplicate of M101 [4]. Despite this, a few people think that M102 is actually a different galaxy that is now referred to as NGC 5866 and that Pierre Mechain is actually mistaken about being mistaken [4].

So having discussed the rather complicated history of the discovery of Messier 101, let's dive into what makes this object scientifically interesting. A lot of people, including me, have worked with M101 because it is a face-on spiral galaxy that is both relatively nearby and relatively large in terms of apparent size. One of the popular things that people like to do in this galaxy is measure the variations in the relative abundances of various elements within the galaxy [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21], with a particular emphasis on the ratio of oxygen to hydrogen. The variations among the abundances of various elements in general is related to how those elements are created within stars and then expelled into the interstellar medium when the stars die. The oxygen to hydrogen ratio specifically is a very popular ratio to study because it gives an indication of the ratio of all heavier elements to hydrogen within galaxies. (Also, astronomers like measuring this ratio because it's really easy to detect oxygen and hydrogen in space using spectroscopy.) Anyway, it takes time for stars to die and eject oxygen and other heavy elements into the galaxies' interstellar mediums, so places with more of these heavy elements, like the center of M101 and the centers of other spiral galaxies, most likely formed before places with fewer heavy elements, like the outer parts of M101 and the outer parts of other galaxies.

However, this is not the only type of research done with M101. I easily found various papers within the scientific literature using M101 to study things like magnetic fields [22, 23], novae [24], supernovae [25, 26], X-ray emission from diffuse gas [27, 28], X-ray emission from unusual binary star systems [29], interstellar dust [30], infrared emission from interstellar molecules called polycyclic aomatic hydrocarbons [31], molecular gas [32], and the relation of molecular gas to the formation of stars [33]. This is not a comprehesive list, so please don't send me hate mail if I left out something you yourself have studied in M101.

I myself have published a couple of papers that included M101 in the analysis [34, 35]. The most notable result was in a paper published in 2015, where I demonstrated that M101 is one of a few spiral galaxies where it's possible to identify the presence of a substantial amount of interstellar dust heated by the general population of stars within the galaxy, including lots of old red giants and supergiants [34]. Some people's expectations have been that most interstellar dust in galaxies is heated primarily by the hot blue stars with short lifespans found within the regions where stars are forming, so finding dust not heated by those hot blue stars may seem a little unusual to some people, and I have had at least one person at a conference tell me in effect that my results defied everything that we understood about interstellar dust within galaxies. On the other hand, other people have told me that they have expected this type of result since the 1980s and that my results were completely boring. Go figure.

Aside from the galaxy itself, people are also interested in M101 because it is at the center of a group of galaxies called the M101 Group. In a 2023 paper, Valentina Karachentseva (listed as having a Ukranian affiliation), Igor Karachentsev (listed as having a Russian affiliation), and two other people (also listed with Russian affiliations) counted a total of 25 galaxies within the group including M101 itself [36], but most of the other galaxies are much smaller than M101, and except for a faint and loosely-wound spiral galaxy named NGC 5585, all of the other galaxies are irregular or spheroidal in shape. However, among these galaxies, NGC 5474 is particularly notable because it seems to be gravitationally interacting with M101 [37, 38, 39]. As a consequence of that interaction, the spiral pattern in M101 looks a bit asymmetric, while NGC 5474 looks like a fried egg where the yolk (or in this case something that looks like a bulge of stars) has shifted to one side.

In an even bigger context, it looks like the M101 Group itself is part of a nearby extragalactic filamentary structure that also includes two more groups of galaxies: the M51 Group and the M63 Group [36]. In the early universe, this filament would have consisted of nothing but primordial hydrogen gas, but eventually, that filament would have break up into a series of clumps that would go on to form M51, M63, and M101 as well as the smaller galaxies within each of these groups.

Now, if you want to observe this galaxy, it actually is rather straightforward to spot. Messier 101 lies to north of the "handle" end of the Big Dipper and forms an approximate equilateral triangle with the two stars at the end of the handle, Eta and Zeta Ursa Majoris. The galaxy has a magnitude of about 7.9 [40], which makes it a little too faint to see with the naked eye but actually makes it bright enough to see with binoculars or with a small telescope, although it will only look like a fuzzy disc-like thing [3, 41, 42]. In a 20 cm (8 inch) telescope, it's possible to distinguish the small central bulge from the disk and to begin to see structure within the disk [41, 42, 43], although a slightly bigger telescope is needed to distinguish the spiral structure of the object [41, 42]. Quite honestly, this is not a galaxy that I have seen through an amateur telescope but one that I want to go find the next time I get the chance.

References

[1] de Vaucouleurs, Gerard et al., Third Reference Catalogue of Bright Galaxies, 1991

[2] Anand, Gagandeep S. et al., The Extragalactic Distance Database: The Color-Magnitude Diagrams/Tip of the Red Giant Branch Distance Catalog, 2021, Astronomical Journal, 162, 80

[3] O'Meara, Stephen James, Deep-Sky Companions: The Messier Objects, 2014

[4] O'Meara, Stephen James, Messier’s 102nd entry is known as the lost Messier object. Has there been any agreement on what this M object might be?, 2015, Astronomy

[5] Searle, Leonard, Evidence for Composition Gradients across the Disks of Spiral Galaxies, 1971, Astrophysical Journal, 168, 327

[6] Smith, H. E., Spectrophotometric observations of ionized hydrogen regions in nearby spiral and irregular galaxies., 1975, Astrophysical Journal, 199, 591

[7] Shields, G. A. and Searle, L., The composition gradient across M101., 1978, Astrophysical Journal, 222, 821

[8] Sedwick, K. E. and Aller, L. H., Spectrophotometry of H II Regions in the Spiral Galaxy M101, 1981, Proceedings of the National Academy of Science, 78, 1994

[9] Rayo, J. F. et al., Gradient in the physical conditions of M 101 and the pregalactic helium abundance., 1982, Astrophysical Journal, 255, 1

[10] Torres-Peimbert, S. et al., Physical Conditions of H II Regions in M101 and the Pregalactic Helium Abundance, 1989, Astrophysical Journal, 345, 186

[11] Kinkel, U. and Rosa, M. R., How metal-rich are metal-rich HII regions., 1994, Astronomy & Astrophysics, 282, L37

[12] Garnett, Donald R. and Kennicutt, Jr., Robert C., A Very Metal poor H II Region in the Outer Disk of M101, 1994, Astrophysical Journal, 426, 123

[13] Kennicutt, Jr., Robert C. et al., The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale, 2003, Astrophysical Journal, 591, 801

[14] Bresolin, Fabio, The Oxygen Abundance in the Inner H II Regions of M101: Implications for the Calibration of Strong-Line Metallicity Indicators, 2007, Astrophysical Journal, 656, 186

[15] Li, Yanxia et al., Testing for Azimuthal Abundance Gradients in M101, 2013, Astrophysical Journal, 766, 17

[16] Croxall, Kevin V. et al., CHAOS III: Gas-phase Abundances in NGC 5457, 2016, Astrophysical Journal, 830, 4

[17] Hu, Ning et al., M101: Spectral Observations of H II Regions and Their Physical Properties, 2018, Astrophysical Journal, 854, 68

[18] Vílchez, J. M. et al., Metals and dust content across the galaxies M 101 and NGC 628, 2019, Monthly Notices of the Royal Astronomical Society, 483, 4968

[19] Esteban, C. et al., Carbon, nitrogen, and oxygen abundance gradients in M101 and M31, 2020, Monthly Notices of the Royal Astronomical Society, 491, 2137

[20] Garner, Ray et al., Deep Narrowband Photometry of the M101 Group: Strong-line Abundances of 720 H II Regions, 2022, Astrophysical Journal, 941, 182

[21] Lamarche, C. et al., Direct Far-infrared Metal Abundances (FIRA). I. M101, 2022, Astrophysical Journal, 925, 194

[22] Berkhuijsen, E. M. et al., Radio polarization and magnetic field structure in M 101, 2016, Astronomy & Astrophysics, 588, A114

[23] Weżgowiec, M. et al., Magnetic fields and hot gas in M 101, 2022, Astronomy & Astrophysics, 664, A108

[24] Coelho, E. A. et al., The Rate and Spatial Distribution of Novae in M101 (NGC 5457), 2008, Astrophysical Journal, 686, 1261

[25] Franchetti, Nicholas A. et al., Physical Structure and Nature of Supernova Remnants in M101, 2012, Astronomical Journal, 143, 85

[26] Matheson, T. et al., The Infrared Light Curve of SN 2011fe in M101 and the Distance to M101, 2012, Astrophysical Journal, 754, 19

[27] Kuntz, K. D. and Snowden, S. L., The Chandra M101 Megasecond: Diffuse Emission, 2010, Astrophysical Journal Supplement Series, 188, 46

[28] Warwick, R. S. et al., An XMM-Newton view of M101 - III. Diffuse X-ray emission, 2007, Monthly Notices of the Royal Astronomical Society, 376, 1611

[29] Chandar, Rupali et al., A New Window into the Nature of X-Ray Binaries in M101 from Their Optical Emission, 2020, Astrophysical Journal, 890, 150

[30] Pricopi, D. et al., Uncovering the truth about M101, NGC 3938, and their significant others through radiative transfer, 2025, Monthly Notices of the Royal Astronomical Society, 537, 56

[31] Gordon, Karl D. et al., The Behavior of the Aromatic Features in M101 H II Regions: Evidence for Dust Processing, 2008, Astrophysical Journal, 682, 336

[32] den Brok, Jakob S. et al., Wide-field CO isotopologue emission and the CO-to-H2 factor across the nearby spiral galaxy M101, 2023, Astronomy & Astrophysics, 676, A93

[33] Suzuki, T. et al., Kiloparsec-scale star formation law in M 81 and M 101 based on AKARI far-infrared observations, 2010, Astronomy & Astrophysics, 521, A48

[34] Bendo, G. J. et al., The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 μm surface brightness ratios, 2015, Monthly Notices of the Royal Astronomical Society, 448, 135

[35] Bendo, G. J. et al., Polycyclic aromatic hydrocarbon excitation in nearby spiral galaxies, 2020, Monthly Notices of the Royal Astronomical Society, 496, 1393

[36] Karachentseva, Valentina E. et al., The M 101 galaxy group as a node in a nearby cosmic filament, 2023, Astronomy & Astrophysics, 678, A16

[37] Bellazzini, M. et al., The strange case of the peculiar spiral galaxy NGC 5474. New pieces of a galactic puzzle, 2020, Astronomy & Astrophysics, 634, A124

[38] Pascale, R. et al., An off-centred bulge or a satellite? Hydrodynamical N-body simulations of the disc galaxy NGC 5474, 2021, Monthly Notices of the Royal Astronomical Society, 501, 2091

[39] Linden, Sean T. and Mihos, J. Christopher, A Dynamical Model of the M101/NGC 5474 Encounter, 2022, Astrophysical Journal Letters, 933, L33

[40] Gil de Paz, Armando et al., The GALEX Ultraviolet Atlas of Nearby Galaxies, 2007, Astrophysical Journal Supplement Series, 173, 185

[41] Plotner, Tammy, Messier 101, 2010, Universe Today

[42] Armstrong, Mark, Messier 101: Ursa Major’s spectacular Pinwheel Galaxy, 2023, Astronomy Now

[43] Eicher, David J., The Universe from Your Backyard, 1988

Credits

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Dalibor, Dorothy Jean Thompson, ivolipa, jameswrowles, metrostock99, modularsamples, Skyrite, and szegvari at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)