Human Journey

A Look At The Ring Nebula

One of the brightest nebulae of the summer night sky is the Ring Nebula.  It was discovered by Antoine Darquier de Pellepoix in 1779  and independently rediscovered just days later by Charles Messier.  The nebula is also known as M57 as it is the 57th object in Messier’s list of non-comets.

As shown in the chart below, the Ring Nebula is located in the Constellation Lyra about halfway between Vega, the brightest star in the northern summer sky, and Albireo, which is in the neighboring constellation Cygnus, the swan. (credit: Star chart from Sizing Up The Universe by J. Richard Gott and Robert J.  Vanderbei.  Click on image for full summer chart.)

The Ring Nebula is a “planetary nebula” meaning that it is an expanding shell of gas expelled from a dying old star.

In earlier times (a few billion years ago), this star was much like our own sun.  But, eventually, it ran out of hydrogen in its core.  (Our sun will meet the same fate in about five billion years.)

In general, when a stellar core runs out of hydrogen, the nuclear reaction that allows the star to “burn” changes, the outer layers of the star expand, and the star becomes a so-called “red giant”.  During the time of transition, temperature instabilities develop causing the outer atmosphere to be expelled by hot super-winds.  The resulting expanding gaseous shell is energized by ultraviolet light from the central star allowing us to see the nebula.

The red outermost shell corresponds to hydrogen atoms emitting light at a particular wavelength (i.e., color) of 656.3 nanometers (one nanometer is one billionth of a meter).  The blue-green inner portion is caused by doubly ionized oxygen atoms emitting light at 495.7 and 500.7 nanometers.

(credit:  Robert J. Vanderbei.  Click on image for larger version.)

The faint dot at the center of the nebula is the dying star itself.   Originally, the star was about as luminous as our sun, but when it became a red giant, it was about 200 times more luminous than our sun.  Since then it has faded.   Today, it is classified as a white dwarf.  To us, it appears very faint because it has low luminosity and, more importantly, it is far away.  Specifically, it is 2,300 light-years away and so the central star has apparent magnitude of 15.8.

Direct measurement of its expansion suggests that the star first started expelling gas to form the nebula about 7,000 years ago.

Unlike the planets of our solar system, most deep sky objects hardly change over the course of a human lifetime.  If the weather will be clear where you live and you own a small telescope, you can take a peek at M57 tonight—it currently rises in the east roughly when the sun sets and is available for viewing all night.   If tonight is not good for you, you can visit it another time—it will still be there looking just as exotic as it ever does.

Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.
  • Aaron

    “take a peek”, not “take a peak”.

  • Michael Gregg

    The discussion is a little bit inaccurate. The little dot at the center of the Ring Nebula is a white dwarf, the remnant compact core of the original star. The white dwarf is much less luminous than our sun, although with a much hotter surface temperature, as one might expect for what used to be the hot interior of a star.

    Some readers might be confused by the author’s use of the words brightness and faintness. In my book, brightness and faintness describe how stars appear to us here on Earth, whereas luminosity is a measure of how much energy is emitted by a star. So I think it is clearer to say that, originally, the star was about as luminous as our Sun; some people might misconstrue the description provided as meaning that the original star appeared to be equally bright in our sky as the Sun. Because the original star was a pretty ordinary one, and at fairly large distance from us, it has always been just one of many stars in the sky as seen from Earth, and currently is in fact too faint to see with the unaided eye. And while it is worth taking a look with even a small telescope, the nebula is quite tiny and will take some perseverance to find, especially for first time observers.

  • Aaron: Thanks for catching the typo. I updated the posting correcting the error.

    Michael: Thanks for pointing out my ambiguous use of the term “brightness”. I changed it to “luminosity” where that term is better. I also added a mention that the star is now a white dwarf.

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