Date: November 10th, 2012
Title: Encore: Wolf-Rayet Stars
Organization: RapidEye Observatory – a private observatory in rural Lee County, NC http://www.rapideye.us/astro/RapidEye-ClearSky.html
This podcast originally aired on February 24th, 2009
Description: A discussion about Wolf-Rayet Stars and how to observe NGC 2359 “Thor’s Helmet Nebula”, a Wolf-Rayet star powered Nebula.
Bio: I’ve been captivated by astronomy ever since I was a kid, living in NW Colorado where the Milky Way was bright enough to read by. I can be found most clear nights in my pasture with either my 4.5″ Dob, 10″ Dob, or my binoculars.
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Stellar Blowhards: Wolf-Rayet Stars
First identified by French Astronomers Charles Wolf and Georges Rayet in 1867, Wolf-Rayet, or W-R stars are exceptionally large stars that are quite rare. Current surveys of our own Milky Way galaxy estimate that there are 100 billion to 400 billion stars in the galaxy, however, we have found fewer than 300 Wolf-Rayet stars.
So what makes a Wolf-Rayet star so different from the average middle-aged yellow star like our sun. For starters they are very massive. They typically have a mass of at least 20 times that of our sun. Since they are big, as in heavy, they also burn very hot. Typically they have a surface temperature of 45,000 degrees F, which is 4 to 5 times hotter than our sun. Also, since they burn so very hot, they will burn through their fuel supply faster than our sun. Most Wolf-Rayet stars have life spans of only several million years as opposed to the 10 billion year life span of our sun. Finally, because they are burning so hot and fast, they have a tremendously strong stellar wind.
Wait, wind in space you ask? OK, let me explain.
The nuclear fusion process that fuels a star, releases tremendous amounts of outward pressure on the gasses that make up the star. Most of this pressure is overcome by the star’s gravity; however, some of the star’s outer atmosphere is propelled in to the solar system resulting in a constant outflow of charged particles we call the Solar Wind. Our sun’s Solar Wind is responsible for the lovely tails that comets will sometimes display, but for a star that is 20 times heavier and 5 times hotter than ours, its solar wind is a raging hurricane by comparison! In almost explosive bursts, this stronger solar wind not only hurls simple ions into space but also heavier elements such as Carbon, Oxygen, and Nitrogen. According to NASA, in a year a typical Wolf-Rayet star can expel as much mass as the Earth weighs.
So the typical Wolf-Rayet star has a large cloud of molecular gas surrounding it, which in and of itself is interesting but not the end of the story. All of the radiation the star emits slams into the gas resulting in several phenomena we can easily observe with typical amateur telescopes.
First, the gas becomes ionized and emits strongly along several emission lines that we can easily separate from other wavelengths of light being radiated by the star. Most Wolf-Rayet star nebulas are strong Oxygen III, or OIII emitters. That is the specific wavelength of light emitted when the ionized oxygen returns to its lower energy state. Most amateur astronomers keep an OIII filter in their eyepiece case to help separate the glow of wispy nebulas from background stars or more frequently, nearby light pollution.
Secondly, the radiation exerts a physical pressure on the gas and pushes it away from the star. This results in a nebula that resembles an expanding bubble, which is essentially what it is: an expanding bubble of gas in space.
But wait, there is more. Wolf-Rayet stars don’t live quietly, so it shouldn’t come as a surprise that they don’t die quietly either. When a Wolf-Rayet star dies, it often undergoes a supernova detonation. Some of these supernovae are sufficiently energetic to be classified as hypernova. And some of these hypernovae are accompanied by a gamma-ray burst as the core undergoes a gravitational collapse into the most awesome object in the Universe, a black hole: an object whose gravitational pull is so powerful nothing, not even light, can escape.
Fortunately, we don’t know of any Wolf-Rayet stars close enough to us to cause concern; however, there are some that are close enough to see with a typical backyard telescope.
The northern hemisphere’s premier Wolf-Rayet powered nebula can be found in the constellation Cygnus the Swan. Commonly known as the Crescent nebula, NGC 6888 is a very pretty nebula that is best positioned for observation between August and November, not now.
Thankfully, right now there is an even nicer (in my opinion anyways) Wolf-Rayet star powered nebula visible to most of the Northern Hemisphere and all of the Southern Hemisphere down in Canis Major. I’m of course referring to NGC 2359, also known as Thor’s Helmet or the Duck Nebula. Its about 15,000 light years away and to some observers resembles a duck. To others, including myself, it looks more like the classical depiction of the Norse God of Thunder, Thor’s winged helmet. It can be a little bit tricky to find by star hopping, but it is certainly worth the effort. In the show notes I’ve included links to several finder charts that will help you find Canis Major and the stars I will use as guideposts.
Lets start at Alpha Canis Majoris, better known as Sirius or the Dog Star. You can’t miss it. Just look to the south after dark and the brightest star you can see is Sirius. Next, find Gamma Canis Majoris. At magnitude 4, it should be visible from most suburban locations. It typically forms the top of the dog’s head in the classical Canis Major – stick dog figure. Its about 4.5 degrees east-northeast of Sirius and they should both be easily visible in the same field of view of most finder scopes. Draw an imaginary line in the sky between alpha and gamma and extend it further east-northeast about the same distance. Looking through the finder scope there should be a pair of 5th magnitude stars separated by a little more than one degree. These form an triangle with NGC 2359, with the nebula being northwest of the two stars by about 2 degrees. If you put a wide angle, wide field of view, finder eyepiece in your scope you should be able to find a small, feint smudge even without a filter.
Don’t worry, the view gets better. You need to increase the power and add a filter for the best view. Using my 10″ F/5 Dobsonian reflector, a widefield eyepiece that gives 125X, and an OIII or a typical narrowband Ultrablock filter, the nebula’s winged helmet namesake is easily visible. Study the image for a few minutes and relax. It takes a while for your eyes to dark adapt and see the subtle greenish/blue light that an OIII filter transmits. In the shownotes I’ve provided a link to an APOD photo that depicts the elements of the nebula that are visible using a reasonably sized telescope. Note – the vivid colors in the picture won’t be visible, but the bubble and wings should be discernible in most 8″ and larger telescopes.
This podcast is dedicated to my star party camping buddy, my 9 year old daughter. This week is her birthday and if the weather cooperates, I’ll go show her the Norse God of Thunder’s helmet after dinner tonight.
All Sky Map – Chart to find Canis Major:
Canis Major Detail:
APOD Picture of Thor’s Helmet:
More Details on Wolf-Rayet Stars:
End of podcast:
365 Days of Astronomy
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