George's Random Astronomical Object

Object 27: WR 40

Podcast release date: 10 August 2020

Right ascension: 11:06:17.2

Declination: -65:30:35

Epoch: J2000

Constellation: Carina

Corresponding Earth location: 200 km northeast of the Balleny Islands near Antarctica

WR 40 is located at a distance of about 14400 light years (about 4400 pc) [1,2] in the constellation Carina. Interestingly, if you do a web search for WR 40, you will either turn up the Polish multiple rocket launch system called the WR-40 Langusta, a radio model called the WR 40, and information on how fast wide receivers in the National Football League Scouting Combine are able to run the 40 yard dash.

The star WR 40 seems to also be commonly called HD 96548 and RCW 58. The name WR 40 may be the most descriptive in a way, as the letters stand for the last names of Charles Wolf and George Rayet, the two astronomers who first discovered the class of stars now known as Wolf-Rayet stars. These stars are all known to be very massive, very hot, and very blue stars, and based on analyses of the stars' spectra in the visible part of the electromagnetic spectrum, they appear to be blowing out huge amounts of gas in the form of stellar winds.

A Wolf-Rayet star is thought to form out of a star with masses between 10 and 25 times the mass of the Sun [3] that initially would have been powered by the fusion of hydrogen into helium in its core. Over time, the core would run out of hydrogen and fill up with helium. While, in a star like the Sun, this process will take 10 billion years, in a star that is 10 time larger than the Sun, this could happen after a just few million years. When the core of one of these very massive star fills up with helium, the gravitational forces are strong enough to trigger the fusion of helium into carbon. The fusion of hydrogen into helium continues in a shell around the core. When the helium is used up in the core, the fusion of carbon into heavier elements begins, and the fusion of helium into carbon shifts into a new shell-like layer in the star. This continues for a while until the core fills with iron, which cannot be fused to produce energy. When this happens, the star will collapse, and then a reverse shockwave will cause the star to explode as a supernova. However, before the star becomes a supernova, it may become so hot that it blows away its outer layer of hydrogen gas, exposing the shell where helium is being fused into carbon. The hydrogen gas ends up being blown away at very high speeds, and the star appears abnormally bright. This point in time before the star becomes a supernova is when the star is classified as a Wolf-Rayet star [3].

So, WR 40 is one of these Wolf-Rayet stars. It's also one of the brighter Wolf-Rayet stars in the sky as seen from Earth. It has an apparent magnitude of around 7.7 [4]. For comparison, the brightest stars in Earth's nighttime sky have magnitudes between -1.5 and 0.5, and the faintest stars that can be seen without a telescope have apparent magnitudes between 5 and 6. WR 40 isn't quite visible to the naked eye, but it would be easy to see with even a small telescope, although it would be very difficult to identify which exact star in the constellation Carina is WR 40, and I'm not going to attempt to describe how you can find it.

What is really interesting about WR 40 is that it is capable of firing 40 rockets in 20 seconds [5]. No, wait, those are the web search results about the Polish multiple rocket launch system again. What is really interesting about WR 40 is that it's brightness looks very variable. This variability was first found during observations in the mid-1970s when astronomers were just beginning to use electronic devices instead of photographic plates for measuring the brightnesses of stars. The first people to make these types of measurements of WR 40 thought that they saw the brightness of the star varying with a period of about 4 days and 19 hours, and they interpreted this as evidence that the Wolf-Rayet star was in an eclipsing binary star system with a neutron star and that the brightness changed when one star passed behind the other [6]. However, subsequent observations turned up periods of variability that ranged from as short as 1 day to as long as 7 days [7,8,9,10]. At one point in the mid-1990s, a science paper was published stating that all of old measurements actually gave misleading results and that the actual variability was a combination of a 12.3 day period and a 17.5 day period that could mimic all of the shorter periodic variations that people thought they were seeing [11]. One of these periods was supposed to be related to the Wolf-Rayet star eclipsing the neutron star and the other was supposed to be related to sunspots on one of the two stars. However, by 1998, which was more than 20 years after WR 40 had been found to be variable, astronomers concluded that it and other variable Wolf-Rayet stars like it did not vary because they were in eclipsing binary star systems but because their cores cause rather complex oscillations that affect the stellar winds from the stars [12,13]. In the case of WR 40, these oscillations cause variations in brightness that are slightly more likely to be around 4.5 days, although a very broad range of oscillations are possible [13].

What I find so fascinating about this is that some people have been working on trying to understand this specific star for a very long time. One of the first people to identify WR 40 as variable was listed as one of the authors on a science paper about the star published just last year [13]. Moreover, while the initial hypothesis about WR 40 being in a binary star system was incorrect, some of the same astronomers that developed that inital hypothesis were willing to disprove it and replace it with a better one. I just think that's cool.

References

[1] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[2] Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1

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

[4] Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog

[5] Army Technology, Langusta WR-40 Rocket Launcher, 2020

[6] Moffat, A. F. J. and Isserstedt, J., The variable, single-line Wolf-Rayet star HD 96548 with a low-mass companion., 1980, Astronomy & Astrophysics, 91, 147

[7] Moffat, A. F. J., Wolf-Rayet stars with compact companions, 1983, in Wolf-Rayet stars: Progenitors of supernovae?, 13

[8] Smith, L. J. et al., UV and optical observations of variability in the WR+compact candidate HD 96548., 1985, Astronomy & Astrophysics, 146, 307

[9] van Genderen, A. M. et al., A high precision photometric investigation of the micro-variations of Wolf-Rayet stars., 1987, Astronomy & Astrophysics, 185, 131

[10] Gosset, E. et al., Analysis of the photometric variability of WR 40., 1989, Monthly Notices of the Royal Astronomical Society, 238, 97

[11] Matthews, J. M. and Moffat, A. F. J., WR 40 : coherence or chaos ?, 1994, Astronomy & Astrophysics, 283, 493

[12] Marchenko, S. V. et al., A comprehensive variability study of the enigmatic WN8 stars: final results., 1998, Monthly Notices of the Royal Astronomical Society, 294, 642

[13] Ramiaramanantsoa, Tahina et al., The chaotic wind of WR 40 as probed by BRITE, 2019, Monthly Notices of the Royal Astronomical Society, 490, 5921

Podcast and Website: George J. Bendo

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

Last update: 9 August 2020