George's Random Astronomical Object

Object 50: IC 10

Podcast release date: 28 June 2021

Right ascension: 00:20:23.2

Declination: +59:17:35

Epoch: J2000

Constellation: Cassiopeia

Corresponding Earth location: Just offshore of Karmoy, Norway

IC 10 (that's the letter I, the letter C, and the number 10, and not the sentence "I see ten" as in I am viewing ten of something or the adjective "icy" as in covered in frozen water followed by the number 10) is a dwarf irregular galaxy in the constellation Cassiopeia that was discovered in the year 1887 by Lewis Swift [1]. From Earth, it is seen through the Milky Way's disk, which means that, when astronomers want to look at IC 10, they have to look through all of the interstellar gas and dust and other garbage in the plane of our galaxy. The interstellar dust in particular creates a lot of problems. It functions in a way like smoke in that it can obscure other objects, including IC 10. This makes it challenging to study the galaxy, but astronomers are still very interested in it nonetheless.

One of the reasons why astronomers are so interested in IC 10 is that it is within the Local Group, which is the gravitationally-bound group of galaxies that includes our galaxy. It's actually quite close in astronomy terms; it's located at only a distance of 2.4 million light years (740 kpc) [2]. Now, the Local Group contains a lot of dwarf galaxies. The Milky Way, the Andromeda Galaxy, and Messier 33 (also called the Triangulum Galaxy) are the only three spiral galaxies within the group. Everything else is small and is primarily influenced by the gravity of the bigger spiral galaxies. What makes IC 10 stand out in particular is that it is considered to be the only starburst galaxy within the Local Group.

At this point, it's worth discussing what a starburst is, and I will not be making any jokes about the chewy fruit-flavored candies in large part because this is not a commercially-sponsored podcast about food (although it would almost be cool to have an astronomy podcast sponsored by the Mars Corporation). Anyway, in astronomy, a starburst is a galaxy that is forming stars at an abnormally high rate. These rates are usually measured in terms of the total amount of mass of interstellar gas, usually described in terms of the mass of the Sun, that is converted into stars within a year.

For IC 10, the star formation rate is between 0.05 and 0.2 solar masses per year [3]. This is not to say that, every 5 to 20 years, astronomers will be able to see a new star within the galaxy. That's because it could take an individual star hundreds of thousands to tens of millions of years to form out of an interstellar gas cloud. However, if you wait 10 million years or so, you should be able to see somewhere between 50000 and 200000 new stars in IC 10.

So, if IC 10 has a star formation rate between 0.05 and 0.2 solar masses per year, it might sound like it's going to create a lot of new stars if we give it enough time. However, this star formation rate is actaully kind of small. For comparison, the Milky Way Galaxy has a star formation rate of 1.65 solar masses per year [4], which is roughly 8 to 30 times higher than the star formation rate in IC 10. Despite this, astronomers still think that IC 10 is forming stars at a relatively high rate, but to understand why, we need to put these star formation rates into perspective.

First of all, the Milky Way Galaxy is much larger than IC 10. While the Milky Way has a mass that is 1.5 trillion times the mass of the Sun [5], IC 10 only has a mass that is 1.7 billion times the mass of the Sun [6]. That's a differece in mass of a factor of 1000. However, despite the fact that IC 10 is 1/1000 times smaller than the Milky Way, it's forming stars at a rate only about roughly 20 times slower than the Milky Way. If you divide the star formation rate by the the total mass of each galaxy, you would find that IC 10 is producing stars about 50 times more efficiently than the Milky Way.

So, IC 10 is small but very effective at forming stars. As a result of this, the stellar population in IC 10 tends to look younger than the stellar population in a typical galaxy. Since big, bright, blue stars have relatively short lifespans of only a few million years or so while Sun-like stars and red dwarfs have lifespans of billions of years or more, places where stars have formed recently tend to have a relatively high number of blue stars. This means that IC 10 looks kind of bluish. In fact, IC 10 is often referred to as the closest example of a class of galaxies called blue compact dwarfs [7], which, as the name indicates, are small, are compact, and look blue. Like IC 10, other blue compact dwarf galaxies are also forming stars at a very high rate (when you take their sizes into account of course).

One of the consequences of this is that IC 10 contains a relatively high number of Wolf Rayet stars [8,9]. These are really massive stars (between 10 to 25 times the mass of the Sun) that form out of short-lived blue stars like the ones in IC 10 [10]. Before these stars became Wolf Rayet stars, they would have initially fused hydrogen into helium in their cores for a few million years, but when they ran out of hydrogen in their cores, they started to fuse helium into carbon instead, with the fusion of hydrogen into helium continuing in shells around their cores. When all the helium in the stars' cores got used up, they started to fuse the carbon in their cores into heavier elements, with the fusion of helium into carbon continuing in a shell around the cores and the fusion of hydrogen into helium continuing in shells around that. All of this intense fusion makes the stars really hot and really blue, and they will eventually blow away their outer layers of hydrogen gas. When stars reach this stage, they are considered to be Wolf Rayet stars.

Because IC 10 is forming stars at an abnormally high rate for its size, a relatively high percentage of its stars are the types of big blue stars that become Wolf Rayet stars, which is why so many Wolf Rayet stars are found in this galaxy. When the cores of these Wolf Rayet stars eventually fill up with iron, which can't be fused to make energy, then the stars will first implode and then explode as supernovae. As far as I can tell, no supernovae have been observed in IC 10, but that may be because supernovae would appear about once every 500 years in this galaxy [11]. Nonetheless, it is possible to see bubbles in the interstellar gas formed by old supernovae explosions [12, 13].

One of the final freaky things about the Wolf Rayet stars in IC 10 is that one of them appears to be in orbit around a black hole about 10 to 15 times the mass of the Sun [14]. This abnormal binary star system is called IC 10 X-1, where the X indicates that it is an X-ray source and the 1 indicates that it is the brightest X-ray source in the galaxy. In a lot of situations where astronomers see a star closely orbiting a black hole, the black hole is close enough to gravitationally strip away the outer layers of gas from the other star, with the gas falling into the black hole getting very hot from gravitational compression and producing really strong X-ray emission. IC 10 X-1, however, contains a black hole orbiting a Wolf Rayet star that is already blowing away its outer gas layers, so the black hole doesn't need to do any work stripping these layers of gas off the star. Instead, the black hole can just kick back and relax and wait for gas to fall into it, and this infalling gas will still get hot and still produce lots of X-ray emission [14].


[1] Swift, L., Catalogue No. 7 of Nebulae discovered at the Warner Observatory, 1888, Astronomische Nachrichten, 120, 33

[2] Tully, R. Brent et al., Cosmicflows-2: The Data, 2013, Astronomical Journal, 146, 86

[3] Leroy, A. et al., Molecular Gas in the Low-Metallicity, Star-forming Dwarf IC 10, 2006, Astrophysical Journal, 643, 825

[4] Licquia, Timothy C. and Newman, Jeffrey A., Improved Estimates of the Milky Way's Stellar Mass and Star Formation Rate from Hierarchical Bayesian Meta-Analysis, 2015, Astrophysical Journal, 806, 96

[5] Watkins, Laura L. et al., Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions, 2019, Astrophysical Journal, 873, 118

[6] Mateo, Mario L., Dwarf Galaxies of the Local Group, 1998, Annual Reviews of Astronomy and Astrophysics, 36, 435

[7] Buta, Ronald J. et al., The de Vaucouleurs Atlas of Galaxies, 2007

[8] Massey, Philip and Holmes, Shadrian, Wolf-Rayet Stars in IC 10: Probing the Nearest Starburst, 2002, Astrophysical Journal Letters, 580, L35

[9] Crowther, P. A. et al., Gemini observations of Wolf-Rayet stars in the Local Group starburst galaxy IC 10, 2003, Astronomy & Astrophysics, 404, 483

[10] Crowther, Paul A., Physical Properties of Wolf-Rayet Stars, 2007, Annual Reviews of Astronomy and Astrophysics, 45, 177

[11] Leitherer, Claus et al., Starburst99: Synthesis Models for Galaxies with Active Star Formation, 1999, Astrophysical Journal Supplement Series, 123, 3

[12] Lozinskaya, T. A. and Moiseev, A. V., A synchrotron superbubble in the IC10 galaxy: a hypernova remnant?, 2007, Monthly Notices of the Royal Astronomical Society, 381, L26

[13] Heesen, V. et al., The non-thermal superbubble in IC 10: the generation of cosmic ray electrons caught in the act., 2015, Monthly Notices of the Royal Astronomical Society, 447, L1

[14] Steiner, James F. et al., On the Spin of the Black Hole in IC 10 X-1, 2016, Astrophysical Journal, 817, 154

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 27 June 2021