Object 114: Messier 33

Podcast release date: 25 December 2023

Right ascension: 01:33:50.9


Epoch: ICRS

Constellation: Triangulum

Corresponding Earth location: An area in the desert in eastern Libya slightly more than 160 km south southwest of Tobruk

This episode's coordinates point to the galaxy Messier 33, which is often abbreviated as just M33. This galaxy is also known as the Triangulum Galaxy, because it is located in the constellation Triangulum as seen from Earth, and as NGC 598, because it is the 598th object in the New General Catalogue (NGC), and as a bunch of other names, such as 2E 409, MCG+05-04-069, and RBS 214, but no one ever uses those names.

Anyway, Messier 33 is a spiral galaxy. As viewed from Earth, the galaxy is seen nearly face-on, although it looks slightly tilted. Still, we have a good view of the entire disk. Typically, spiral galaxies are described as consisting of a disk of stars and gas that contains the spiral arms and a spherical bulge of stars that sits in the center of the galaxy. However, Messier 33 is actually one of many spiral galaxies that contains no bulge, although the disk gets brighter in the center. Also, the structure of Messier 33's spiral arms is referred to as flocculent [1]. Most people will think of a spiral galaxy as having two really well-defined spiral arms that form a very distinct spiral pattern like what is seen in the Whirlpool Galaxy, but the spiral arms in Messier 33 look sort of wispy. This is actually the case for many other spiral galaxies as well, but not all of them. Those wispy arms contain a lot of individual bright nebula that are the sites where stars are forming. I'll discuss those later.

Messier 33 was potentially discovered by the Italian astronomer Giovanni Battista Hodierna in the seventeenth century, who documented the existence of the object as well several star clusters and nebulae in 1654 [2]. However, people apparently did not pay attention to Hodierna's works. The galaxy was then re-discovered in 1764 by Charles Messier [3], an eighteenth century French astronomer who spent his career looking for comets, which in eighteenth century telescopes looked like fuzzy things that move across the sky. Messier was quite annoyed when he found fuzzy things in the sky that did not move, so he created a catalog of them so that he could keep track of them and otherwise ignore them, and Messier 33 was the 33rd object to be listed in his catalog of definitely-uninteresting non-comets. That catalog of non-comets are among the most famous objects in astronomy, and are much more famous than any of the comets discovered by Charles Messier. The galaxy Messier 33 is actually, according to the SIMBAD Astronomical Database, the eleventh most heavily studied astronomical object outside the Solar System.

The most notable thing about Messier 33 is that it is the third largest galaxy within the Local Group, which is the gravitationally bound group of galaxies that contains our galaxy and the Andromeda Galaxy. Consequently, at a distance of about 2.7 million light years (0.84 Mpc) [4], Messier 33 is also the second closest spiral galaxy to the Milky Way. However, even though Messier 33 is a spirial galaxy and even though it is the third largest in the Local Group, it is actually not that large. The Andromeda Galaxy is the largest of the three spiral galaxies with a mass of 3 trillion times the mass of the Sun [5]. Multiple measurements of the mass of the Milky Way place it at about 1 trillion times the mass of the Sun [6]. Messier 33 is only half a trillion times the mass of the Sun [7].

Given that Messier 33 is so close, it's possible with modern telescopes to study various individual objects within the galaxy. In fact, some of those objects are so interesting and so scientifically important that it would be possible for me to create several podcast episodes based on those individual objects. I'll just try to give some summary information.

Messier 33 contains quite a few bright X-ray sources, but two of them are quite notable scientifically. One of those X-ray sources is named M33 X-7, which is an incredibly boring name. Anyway, M33 X-7 contains a really large blue star 38 times the mass of the Sun and a black hole with a mass of 11.4 times the mass of the Sun that are in orbit around each other [8]. The orbital periods of the two objects in this system are a very short 3.45 days [9]. The two objects are so close that the outer layers of the really large blue star are being gravitationally stripped away by the black hole. As that gas falls into the black hole, it gets gravitationally compressed and really hot, thus producing X-ray emission. These types of star systems are called high mass X-ray binaries, and they are seen in both our galaxy and other galaxies, but what is unique about M33 X-7 is that the orbits of the two objects are arranged in such away that, as seen from Earth, the objects eclipse each other [10]. This has allowed for making a lot of measurements of the two objects in the star system that would not be possible in non-eclipsing X-ray binary systems.

The other really interesting X-ray source in Messier 33 was given the boring name of M33 X-8. This is the closest ultraluminous X-ray source to Earth [11]. An ultraluminous X-ray source is defined as an object that produces a total amount of X-ray emission exceeding 1032 W (or, to use astronomy units of measurement, 1039 erg/s) of X-ray emission. Many of these ultraluminous X-ray sources are expected to be X-ray binary star systems which are very bright because gas is falling into the black holes in these systems at extremely rapid rates, and since M33 X-8 is so close to Earth, it has been used to study this type of phenomena in more detail.

Aside from the X-ray sources in Messier 33, the other objects that are really interesting are the locations where stars are forming out of the interstellar gas within the galaxy. Messier 33 contains a lot of these star forming regions, but the location which is forming stars the most rapidly is called NGC 604. This looks like a really big nebula in the northeastern part of the disk of the galaxy. NGC 604 is, in fact, the second largest star forming region in the Local Group. Star formation appears to have started about 4 million years ago, and the region appears to have created a number of stars with a total mass of about 160 thousand times the mass of the Sun (which could be thought of as being very roughly equivalent to 160 thousand stars) [12]. The bright blue stars in this region have also ionized some of the surrounding gas and heated up the dust within the interstellar medium. This region is very well studied by people who want to understand extreme star formation.

I've actually made a few images of the infrared emission from the interstellar dust in Messier 33, and NGC 604 is the brightest infrared source within the galaxy. However, I have actually spent much, much more time working on observations from the Atacama Large Millimeter/submillimeter Array (ALMA) of submillimeter wavelength emission from carbon monoxide (CO) from interstellar molecular gas in NGC 604 as well as a couple of other star forming complexes within the galaxy. About once a year, I have a data processing project as part of my professional work that, for one reason or another, takes up an inordinate amount of time and effort which I think of as my annual data processing project from hell. The ALMA observations of NGC 604 as well as the other two star forming regions in Messier 33 were my data processing project from hell for the year 2020.

Processing the ALMA data took me a couple of months for multiple reasons. First of all, I needed to make really large image cubes for each field at three different frequencies, each of which covered emission from CO at a different frequency. Just to explain what image cubes are, ALMA and telescopes like it don't just observe at one single frequency but at multiple frequencies at once, so it is possible to stack the images from each frequency to make cubes where two dimensions correspond top dimensions on the plane of the sky and the third corresponds to frequency. The image cubes I made had dimensions of either 900×900×750 or 1000×1000×750 pixels, which took a lot of time and computing power to make. Second, the imaging step requires identifying where all of the real astronomical emission is coming from to remove the noise from the other regions. Since I was looking at small locations within the disk of a very nearby galaxy, the emission from the interstellar molecular gas was all over the place, and the automatic source identification didn't work very well, so I needed to do this step manually, and that meant doing it 750 times for the 750 frequencies in each image cube. Third, I tried combining data from two different subarrays within ALMA, one of which imaged the small-scale emission and one of which imaged the large scale emission, but the imaged areas covered different regions, so I ended up with weird effects when I tried to combine the data from the two subarrays. Molecular clouds that looked skinny closer to the center of the final images would look really fat at the edges of the images. I eventually gave up trying to do this. Anyway, with all of these problems, you can understand why it took me a couple of months to create the final image cubes.

So, just to wrap things up, here are some tips on how you yourself can observe Messier 33 without going through two months of data processing hell. Messier 33 lies in the constellation Triangulum, but the easiest way to find it would be to first find the constellation Andromeda, which to me looks like a cone-shaped thing attached to one end of the Great Square in the constellation Pegasus. The stars Beta Andromedae, Mu Andromedae, and Nu Andromedae form a line of stars spanning from one side of the cone to the other, and the Andromeda Galaxy lines right next to Nu Andromedae. Estimate the distance from Beta Andromedae to Nu Andromedae and then go the same distance in the opposite direction from Beta Andromedae to find the approximate location of Messier 33.

Messier 33 is identified by some references as the most distant thing that can be seen without the telescope, although it will be barely visible as a very faint fuzzy object [3]. With a pair of binoculars or a small telescope, the galaxy will look like a faint oval with a size slightly more than twice the diameter of the Moon [13]. Using a telescope with a large aperture and a low magnification eyepiece, it is possible to begin to see the spiral structure within the galaxy [14]. Since the galaxy looks relatively wide but faint, don't try observing the galaxy using high magnifications; the galaxy will look too spread out, and it will be difficult to see anything.


[1] Elmegreen, Bruce G. et al., A Turbulent Origin for Flocculent Spiral Structure in Galaxies. II. Observations and Models of M33, 2003, Astrophysical Journal, 593, 333

[2] Fodera-Serio, G. et al., G.B. Hodierna's Observations of Nebulae and his Cosmology, 1985, Journal for the History of Astronomy, 16, 1

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

[4] Bhardwaj, Anupam et al., Large Magellanic Cloud Near-infrared Synoptic Survey. II. The Wesenheit Relations and Their Application to the Distance Scale, 2016, Astronomical Journal, 151, 88

[5] Patel, Ekta et al., Evidence for a Massive Andromeda Galaxy Using Satellite Galaxy Proper Motions, 2023, Astrophysical Journal, 948, 104

[6] Bobylev, V. V. and Baykova, A. T., Modern Estimates of the Mass of the Milky Way, 2023, Astronomy Reports, 67, 812

[7] Kam, S. Z. et al., H I Kinematics and Mass Distribution of Messier 33, 2017, Astronomical Journal, 154, 41

[8] Ramachandran, V. et al., Phase-resolved spectroscopic analysis of the eclipsing black hole X-ray binary M33 X-7: System properties, accretion, and evolution, 2022, Astronomy & Astrophysics, 667, A77

[9] Orosz, Jerome A. et al., A 15.65-solar-mass black hole in an eclipsing binary in the nearby spiral galaxy M 33, 2007, Nature, 449, 872

[10] Pietsch, W. et al., M33 X-7: ChASeM33 Reveals the First Eclipsing Black Hole X-Ray Binary, 2006, Astrophysical Journal, 646, 420

[11] Dubus, G. et al., High resolution Chandra X-ray imaging of the nucleus of M 33, 2004, Astronomy & Astrophysics, 425, 95

[12] Martínez-Galarza, J. R. et al., Ongoing Massive Star Formation in NGC 604, 2012, Astrophysical Journal, 761, 3

[13] Ridpath, Ian, Astronomy: A Visual Guide, 2022

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


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: balloonhead, ivolipa, jameswrowles, josefpres, Kostrava, maciejadach, metrostock99, PoemaBeats, and Xulie at The Freesound Project

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