George's Random Astronomical Object

Object 21: NGC 281

Podcast release date: 18 May 2020

Right ascension: 00:52:25.1

Declination: +56:33:54

Epoch: J2000

Constellation: Cassiopeia

Corresponding Earth location: Farmland just outside of Veinge, Sweden

The NGC 281 nebula is also known as the Pacman Nebula. I think the name Pacman nebula alone is sufficient to give you a good idea of what the object looks like. It has angular size on sky is around 30 arcminutes or half a degree (approximately the same as our Moon) [1,2]. If we scaled the real Pacman up to the size of NGC 281 then each pixel would be 2.31 arcminutes across.

The Pacman Nebula is an emission nebula created by the hot young massive stars forming within it. These stars are very hot meaning they emit enough ultraviolet light to ionise gas surrounding them. The young cluster in larger NGC 281 complex is known as IC 1590. We know that these stars must be young, as the more massive a star the shorter its lifetime, so to see a region of ionisation mean there must have been a recent period of star formation in the vicinity.

It is the on going star formation in the NGC 281 region which makes it particularly interesting to astronomers. As the star formation in the region is fairly dispersed, compared to the concentrated star formation seen to take place in Giant molecular clouds, NGC 281 is as a great testbed for studying different modes of star formation. In particular NGC 281 provides potential examples of triggered star formation [3].

The most typical mode of star formation occurs spontaneously within molecular clouds when clumps of gas become sufficiently dense to begin to collapse under gravity. Triggered star formation on the other hand occurs in the presence of some additional influence, primarily the impact of a previous generation of stars through either a supernova explosion or the outward radiation pressure from existing stars. Both these factors generate an outward force which gathers up material creating the densities required for star formation to take place.

NGC 281 seems to possess two regions of triggered star formation both supernovae triggered, from a previous generation of stars, and triggered by the interaction of a molecular cloud with the edges of the Pacman Nebula itself [3]. As such, comparison between two different modes of triggered star formation can also potentially be made. However, star formation is generally a messy process so study on this topic is still ongoing.

NGC 281 also contains several examples of my favourite types of astronomical object, the maser. Maser is an acronym which stands for Microwave Amplification by Stimulated Emission of Radiation, so the same as laser but with an 'm'. In the case of those observed in NGC 281 we are talking about water masers emitting primarily at a frequency of 22 GHz, (or a wavelength of 1.36cm) which is a little more toward the radio regime than microwaves, but nobody uses the term raser.

Masers are interesting objects when observed in astrophysical sources for a couple of reasons. Firstly, to create a maser with a certain molecule requires a specific set of environmental conditions, for example the correct abundance of that molecule, at the right temperature, in the presence of a particular amount of radiation at the right frequency. So if you see a maser in a given source you know that around that those environmental requirements have been met. Better yet you might find two different species of maser around the same astronomical source, like methanol and water. As masers of methanol and water require different conditions to trigger them, you can use their relative position in a region to give you clues as to what is happening around your target of interest [4]. The second reason masers are interesting a useful is that they are small, but exceptionally bright. Masers, particularly water masers, are amongst the brightest sources seen at radio frequencies. As a comparison the some water masers are 10 billion times brighter than the Sun at these frequencies.

Their brightness and compact nature makes them hard to miss in observations and allows exceptionally accurate positions to be measured by using very long baseline interferometry. Once you have a couple of high accuracy positions for a source measured at different times you can use techniques of parallax to find a reliable distance to the maser and the astronomical object your are interested in [3]. In the case of NGC 281 this technique has been applied by astronomers to provide a distance to the target of 2.9 kpc [3,5,6] or around 90 million trillion metres. Given this the Pacman Nebula is approximately 25 parsecs across (or the distance to Proxima Centuari and back 9.7 times) so each hypothetical Pacman Nebula pixel would be a little under 2 parsecs across or a little under 1 and a half times the distance to Proxima Centuari.


[1] Roth, J., Portrait of NGC 281, 2018, Astronomy Picture of the Day

[2] Goldman, Don, PacMan Nebula in Narrowband, 2010, Astrodon Imaging

[3] Sharma, Saurabh et al., Multiwavelength Study of the NGC 281 Region, 2012, Publications of the Astronomical Society of Japan, 64, 107

[4] Fish, Vincent L., Masers and star formation, 2007, IAU Symposium, 242, 71

[5] Reid, M. J. et al., Trigonometric Parallaxes of High Mass Star Forming Regions: The Structure and Kinematics of the Milky Way, 2014, Astrophysical Journal, 783, 130

[6] Shull, J. Michael and Danforth, Charles W., Distances to Galactic OB Stars: Photometry versus Parallax, 2019, Astrophysical Journal, 882, 180

Podcast and Website: George J. Bendo

Special Guest Contribution: Adam Avison

Additional Audio Editing: Adam Avison

Music: Immersion by Sascha Ende, which is distributed by under a Creative Commons 4.0 Attribution License

Sound Effects: craigsmith, Dalibor, ivolipa, jameswrowles, metrostock99, Samulis, straget, and Xulie at The Freesound Project

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


© George Bendo 2020. See the acknowledgments page for additional information.

Last update: 18 May 2020