Object 78: IC 1613

Podcast release date: 25 July 2022

Right ascension: 01:04:54.2


Epoch: ICRS

Constellation: Cetus

Corresponding Earth location: Tropical forests in the Republic of Congo

Once again, the random number generator has selected a location in the constellation Cetus, which is one of the dumbest constellations in the night sky because it's supposed to look like a whale but it actually looks like a bunch of randomly-placed stars. Despite this, Cetus actually contains many interesting objects, including this episode's object, which is the dwarf irregular galaxy IC 1613.

This galaxy looks sort of like a sphere of stars about half the width of the Moon with a rather large, amorphous-looking nebula located to its upper left (assuming that you are looking at it while facing south) and a few other nebulae scattered throughout it. It only contains about 100 million stars [1], which is about 1000 times smaller than the number of stars in the Milky Way. However, the galaxy is relatively close to Earth, which makes it easier to see. It's located at a distance of 2.36 million light years (724 kpc) from Earth [2] on the outskirts of the Local Group, which is the gravitationally bound group of galaxies that also contains our galaxy, the Andromeda Galaxy, Messier 33, and a bunch of other dwarf galaxies. It's actually possible to see this galaxy with medium- or large-sized amateur telescopes, and it even appears in the Caldwell Catalog, a list of amateur astronomy targets created as a complement to the Messier Catalogue. I'll discuss finding IC 1613 in the night sky later, but first, let's talk about a few things that make this object so interesting scientifically.

First of all, that really large amorphous nebula on the upper left side of the galaxy has attracted a lot of attention from astronomers over the years. That nebula is a site where lots of stars have formed or are in the process of forming, which makes it rather exotic looking in multiwavelength data. When stars initially form, they usually vary substantially in mass from 0.08 to 60 times the mass of the Sun. The most massive stars are also the brightest, hottest, and bluest. These massive stars also have very short lifespans in astronomical terms; after only a few million years, they first transform into red supergiants and then explode as supernovae. Because of this, these massive, hot blue stars are only found in locations where stars have recently formed. That large nebula in IC 1613 contains a huge cluster of these hot, massive blue stars [3, 4, 5]. The stellar winds from those hot blue stars are so strong that they have formed what astronomers call a superbubble, which is exactly what is sounds like: a bubble of really hot gas in space. For astronomers who want to understand the details of what happens when a bunch of stars form all at once in one place in a dwarf galaxy, IC 1613 is a great place to look. This region additionally contains a supernova remnant that, in terms of its X-ray emission, is one of the brightest supernova remnants in the Local Group [6, 7], and any X-ray astronomers interested in understanding the details of how supernovae work will probably want to take a look at this supernova remnant in IC 1613.

The second really interesting thing about IC 1613 is that it is relatively free of interstellar dust. Both our galaxy and many other spiral galaxies contain a lot of very small dust grains floating between the stars that cause a lot of problems when trying to measure the properties of stars within our galaxy and others. First, interstellar dust tends to absorb and scatter starlight, which makes stars look fainter. Interstellar dust will also tend to absorb or scatter blue light before red light, which makes stars look redder. However, IC 1613 contains hardly any interstellar dust, so the starlight that emerges from the galaxy looks the same as the light originally emitted by the stars. Additionally, the galaxy happens to be located in a place where, as seen from Earth, the interstellar dust in our own galaxy is relatively thin. This lack of dust obscuration along with the galaxy's relatively close proximity to Earth makes it an exciting place for astronomers to examine the individual stars in the galaxy.

One of the most interesting studies taking advantage of the dust-free characteristics of IC 1613 was an analysis comparing different ways to measure distances to galaxies. For very nearby galaxies, astronomers rely on being able to identify specific types of stars to do this. Astronomers would first accurately measure the distances to similar nearby stars within our own galaxy and also measure the total amount of light from these stars as seen from Earth, which then allows astronomers to compute the total amount of energy emitted by these stars. When astronomers then see these stars in other galaxies, they know how much light should be produced by these stars, and they can compare this to how many photons actually reach the Earth to calculate the distances to the galaxies containing these stars.

So, a group of astronomers led by Dylan Hatt accumulated a large pile of data on three specific classes of stars in IC 1613 [2]. The first class of stars are the brightest red giants that they can find in the galaxy. The second class of stars are Cepheids, which are variable stars that pulse regularly and which have average brightnesses that are directly related to the stars' pulsation periods. The third class of stars are RR Lyrae variable stars, which are named after the star RR Lyrae and which are similar to but smaller than Cepheids. The astronomers actually demonstrated that all three of these classes of stars yielded very similar but very accurate distances to IC 1613, and what's really interesting is that they showed that this was true when looking at either older or younger versions of these stars within the dwarf galaxy [2]. This analysis is quite important because it helps astronomers to understand how accurately they can measure distances to other galaxies using different methods.

A third interesting aspect of IC 1613 is that it is located on the periphery of the Local Group. I previously discussed in episode 55 another dwarf galaxy named the Wolf-Lundmark-Melotte (WLM) Galaxy that is also at the periphery of the Local Group. Galaxies like IC 1613 and the WLM Galaxy are popular to study because they are sufficiently close to the Milky Way that it's possible to produce detailed images of them, yet they are sufficiently far away from the Milky Way and other Local Group galaxies that they have not been gravitationally disturbed by those galaxies.

In the case of IC 1613, it was targeted because of its isolation for a study of gas within its halo. If it had been located closer to any other galaxy, this halo of gas would have been stripped away, but because it's in a relatively remote location, it has retained its gas halo [8]. Observations of the halo show how heavy elements from supernova explosions have been ejected out of the galaxy's star forming regions [8]. These types of measurements have been made in many other galaxies, but what makes IC 1613 special is that it is one of smallest galaxies in which this has been done [8]. Ultimately, these specific observations of IC 1613's halo are important for understanding how heavy elements are produced and ejected from dwarf galaxies more generally.

So, those are just a few of the many reasons why professional astronomers have spent time looking at IC 1613, but it is something that you can take a look at yourself with an amateur astronomy telescope, although it is a very challenging galaxy to find. Unfortunately, the galaxy is located in a nondescript part of the sky between the brightest stars in the lower half of the constellation Cetus, which looks like an irregular polygon, and the constellation Pisces, which, despite being very famous as one of the constellations in the zodiac, consists of a lot of relatively faint stars that form a really large, tilted V shape. The best thing I could suggest would be to identify the star Epsilon Piscium, which is one of the stars in the lower part of the V, and then go about 5.75 degrees south and a little to the east [9]. Even though the galaxy is in the Caldwell Catalog, which was created explicitly for amateur astronomers, it has a relatively faint surface brightness for an amateur astronomy target, so it will be virtually impossible to see in binoculars or a small telescope, and it will barely be visible in a 15 to 20 cm (6 to 8 inch) telescope [10]. In a relatively large amateur telescope like a 38 cm (15 inch) telescope, it's just barely possible to see structure in the galaxy [11]. I wouldn't recommend this as an object for beginners, but it would be an interesting challenge for experienced amateur astronomers tired of looking at easy-to-find Local Group galaxies like the Andromeda Galaxy and M33.


[1] McConnachie, Alan W., The Observed Properties of Dwarf Galaxies in and around the Local Group, 2012, Astronomical Journal, 144, 4

[2] Hatt, Dylan et al., The Carnegie-Chicago Hubble Program. II. The Distance to IC 1613: The Tip of the Red Giant Branch and RR Lyrae Period-luminosity Relations, 2017, Astrophysical Journal, 845, 146

[3] Borissova, J. et al., A catalogue of OB associations in IC 1613, 2004, Astronomy & Astrophysics, 413, 889

[4] Garcia, M. et al., The young stellar population of IC 1613. I. A new catalogue of OB associations, 2009, Astronomy & Astrophysics, 502, 1015

[5] Schlegel, E. M. et al., The Nearby Dwarf Irregular Galaxy IC 1613 and Its Complex Bubble Region: Chandra and VLA Observations, 2022, Astronomical Journal, 163, 66

[6] Schlegel, E. M. et al., The X-Ray-resolved Supernova Remnant S8 in the Dwarf Irregular Galaxy IC 1613, 2019, Astronomical Journal, 158, 137

[7] Fesen, Robert A. and Weil, Kathryn E., The Nature of the Young Supernova Remnant S8 in the Dwarf Galaxy IC 1613, 2020, Astrophysical Journal, 902, 19

[8] Zheng, Yong et al., Characterizing the Circumgalactic Medium of the Lowest-mass Galaxies: A Case Study of IC 1613, 2020, Astrophysical Journal, 905, 133

[9] O'Meara, Stephen James, Deep-Sky Companions: The Caldwell Objects, 2016

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

[11] King, Bob, Meet the Shy Side of the Local Group, 2017, Sky & Telescope


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: art6nik, dronemachine, Gerent, ivolipa, jameswrowles, janusnovak, kentspublicdomain, LittleBigSounds, milo03, and visual at The Freesound Project

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