Ring nebula M57 (NGC 6720)

 

Profile :

 

Distance : ~2.300 ly

Right Ascension 18 : 53.6 (h:m)

Declination +33 : 02 (deg:m)

Visual Brightness 8.8 (mag)

Apparent Dimension 1.4x1.0 (arc min)

Diameter : 2.4 ly

History

Despite their name, planetary nebulae have nothing to do with planets. They were named originally because their discoverers observed them visually and they did not appear as stellar point sources, but rather as small diffuse objects that resembled the outer planets in our solar system such as Uranus and Neptune when seen in a telescope.

M57 was the second planetary nebula to be discovered (in January 1779), 15 years after the first one, M27.
Antoine Darquier de Pellepoix of Toulouse, France, who discovered the Ring Nebula. He first saw the Ring in January 1779 by using a telescope of about 3-inch aperture and he reported: "Nebula between gamma and beta Lyrae; it is very dull, but perfectly outlined; it is as large as Jupiter and resembles a planet which is fading".

A short time later, Charles Messier also saw it and added it to his catalogue of comet asqueraders listing it as M57. But like de Pellepoix, Messier’s telescope was too crude to give a true picture of what he was looking at.
"A cluster of light between Gamma and Beta Lyrae, discovered when looking for the Comet of 1779, which has passed it very close: it seems that this patch of light, which is round, must be composed of very small stars: with the best telescopes it is impossible to distinguish them; there stays only a suspicion that they are there." Messier reported this patch of light on the Chart of the Comet of 1779.

Not until six years later, in 1785, de Pellepoix comparison to a planet may have influenced William Herschel , who found the objects of this type resembling the planet newly discovered by him, Uranus, and introduced the name "Planetary Nebulae". Herschel described M57 as "a perforated nebula, or ring of stars;" this was the first mention of the ring shape. Oddly he did not count this most prominent representative in this object class, but described it as a "curiosity of the heavens", a peculiar object. Herschel also identified some of the superimposed stars, and correctly assumed that "none [of them] seems to belong to it."

About the discoverers

Charles Messier (June 26, 1730 - April 12, 1817)

Charles Messier, was a French Astronomer whose work on the discovery of comets led to the compilation of a catalog of deep sky objects known today as the Messier Catalogue of nebulae and star clusters.
Messier was born in Lorraine, France on June 26, 1730. His father died when he was 11, and since he was the tenth of twelve children he had very little opportunity for education. He developed an interest in astronomy as a boy after he saw the brilliant six-tailed comet of 1744.
He was eventually hired as a draftsman by Joseph-Nicholas de l’Isle, Astronomer to the French Navy. During this time, he learned to use astronomical instruments. He became a skilled observer, and was later promoted to clerk at the Marine Observatory at the Hotel de Cluny in Paris.

 

  Antoine Darquier de Pellepoix (Nov 23, 1718 - Jan 18, 1802)
  Darquier was born on November 23, 1718 in Toulouse (Rue Darquier 8), France. He worked at his hometown. Using a 2.5 inch achromatic refractor with 42 inch focal length (f/16.8) he observed sunspots and other astronomical phenomena. The observations made between 1748 and 1773 are collected in his book Observations Astronomiques faites Toulouse (Avignon 1777).
While searching for the comet of 1779 he discoverd the Ring nebula M 57 in January 1779 (just a few days before Messier) describing it as "a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet". Friedrich von Hahn found the central star in 1800.
William Herschel created the term "Planetary Nebula" in 1785 for those objects looking like a planet's disk - maybe adopting Darquier's description and still impressed by his finding of Uranus in 1781. Darquier observed the new planet in the same year and calculated its orbit. Between 1791 and 1798 he made an extensive star catalogue, later used by Lalande.
Darquier died on January 18, 1802 in Toulouse at the age of 83.

Formation of the nebula

 

 

 

 

 

 

 

 

When a star comes to age, having longly burned away all the hydrogene to helium in its core in its main sequence phase, and also (in the consequent red giant stadium), the helium to carbon and oxygene, its nuclear reactions come to an end in its core, while helium burning goes on in a shell.
This process makes the star expanding, and causes its outer layers to pulsate as a long-periodic Mira-type variable, which becomes more and more unstable, and loses mass in strong stellar winds.
The instability finally causes the ejection of a significant part of the star's mass in an expanding shell. The stellar core remains as an exremely hot, small central star, which emits high energetic radiation.

 
  The star at the center of the Ring nebula has a surface temperature of 216,000 degrees Farenheit or 120,000 degrees Celsius. Our own star, the Sun, is expected to undergo the same process in a couple of billion years. Planetary nebulae do not last long at all in cosmic terms, the shell of gas expands and diffuses, becoming invisible, and the star turns into a white dwarf.

The Ring Nebula is the most famous and among the brightest examples of what astronomers refer to as planetary nebulae.
For a long time the explanation for the Ring Nebula’s appearance was that the hazy disk was so much brighter around its edges that it looked like a ring; that we are looking through the rim of the gaseous shell lengthwise.
Therefore, there is much more gas in our line of sight and the refraction of the light from the central star makes it more luminous, because each particle acts as a prism or mirror, and reflects the rays back to us.
More recent research, however, has confirmed that it indeed is likely a ring, or torus of bright material surrounding its central star.

Obser-vation

 

 

 

 

 

 

 

 

 

 

Located about 2,000 light years away, it has an apparent diameter of 1 arc minute and a real diameter of about one light year. At magnitude 9.0 its high surface brightness makes it an easy object, even from the city and even with small telescopes.

The constellation of Lyra was supposed to represent Apollo’s harp. Six fainter stars form a little geometric pattern of a parallelogram attached at its northern corner to an equal-sided triangle.

Vega gleams at the western part of the triangle. The two lowest stars in the parallelogram are Beta and Gamma Lyrae. Beta is sometimes also known as Sheliak. Between these two stars, but a trifle nearer to Gamma is where you will find the Ring Nebula.

 

 

  M57 is very easy to locate as it is situated between Beta and Gamma Lyrae, at about one-third the distance from Beta to Gamma (see picture).
It can be seen with binoculars as an almost stellar object, difficult to identify just because of its small apparent diameter. In smaller amateur telescopes, the ring becomes apparent at about 100 magnification, with a darker middle; a 12th-mag star is east of the planetary nebula, about 1' of the center. If ever color is notable, the Ring Nebula appears slightly greenish, not unexpected because most of its light is emitted in few green spectral lines.
Even in small scopes, a slight ellipticity can be noted, with major axis in a position angle of about 60 deg. With increasing aperture and under good condition, more and more detail becomes visible, but even in large instruments, the central star will be apparent only under exceptionally good conditions, or with the help of filters. In large instruments, several very faint foreground or background stars can be glimpsed within the nebula's extension under very good conditions.

Character

 

 

 

 

 

 

 

 

 

 

 

 

 

 

As most planetary nebulae, the Ring is much brighter visually at magnitude 8.8 than photographically at only 9.7 mag; a consequence of the fact that most light is emitted in very few particular spectral lines. Assuming a distance of 2,300 lightyears, this corresponds to an absolute magnitude of -0.3 visually (+0.5 photographically), or an intrinsic brightness of about 50 to 100 times that of our Sun. Even the 14.7-mag central star, of the size of a terrestrial planet, is only little fainter than our Sun with an absolute magnitude of about +5 or 6. Its apparent dimension of 1.4 arc minutes corresponds to a linear diameter of 0.9 lightyears (5.5 trillion miles or 8.8 trillion km, or 60,000 Astronomical Units), the halo extending out to a diameter of 2.4 lightyears.

Shown here is the star field close to the Ring Nebula. Plotted on it are the magnitude of field stars nearby as determined by Arne Henden (USNO-Flagstaff) who observed the field with a CCD on the USNO 1 meter telescope. Decimal places are omitted in the magnitudes so the periods are not confused with stars.
In a large telescope the central portion of the doughnut of M57 is bright with nebulosity, making the detection of the central star much more difficult than if it were isolated against a black sky background.
In addition to a large aperture, high magnification is also necessary to see this star, along with good transparency and great seeing, usually arc-second or better.

The mass of the nebular matter has been estimated at about 0.2 solar masses, the density at about 10,000 ions per ccm (cm^3). Its chemical composition has been determined as follows: On each Fluor (Fl) atom, the Ring Nebula contains 4.25 million atoms of Hydrogene (H), 337,500 Helium (He), 2,500 Oxygene (O), 1,250 Nitrogene (N), 375 Neon (Ne), 225 Sulfur (S), 30 Argon (Ar) and 9 Chlorine (Cl) atoms. It is expanding at 20 to 30 km/s, and approaching us at 21 km/s.

 

Similar nebula This nebula may look like a butterfly, but it’s bigger that our Solar System. NGC 2346 is, like the Ring nebula, a planetary nebula made of gas and dust. NGC 2346 is remarkable because its central star is known to be actually a very close pair of stars, orbiting each other every 16 days. It is believed that the binary star was originally more widely separated. when one component of the binary evolved, expanded in size, and became a red-giant star, it literally swallowed its companion star.
The companion star then spiralled downwards inside the red giant, and in the process spewed out gas into a ring around the binary system. Later on, when the hot core of the red giant was exposed, it developed a faster stellar wind, which emerged perpendicularly to the ring and inflated two huge "bubbles." This two-stage process is believed to have resulted in the butterfly-like shape of the nebula. NGC 2346 lies about 2,000 light-years away from us, and is about one-third of a light-year in size.

     
Exercise Finding Lyra / M57

Try to find the constellation of lyra or maybe even the ringnebula m57 in the night sky !
In order to find the lyra a star chart may be very useful. The easiest way to locate it is to look for Vega, lyra's star with the greatest magnitude. If you' ve found Vega then its easy to find the other stars belonging to Lyra.
M57 is located between beta and gamma lyra as mentioned above.

  Simple star chart

Sources

 

 

 

Books:

Kaler, James B. : Sterne und ihre Spektren: astronomische Signale aus Licht; aus dem Amerikan. von Ulrich Bastian; 1994; Ulm

Murdin, Paul: Flammendes Finale: spektakuläre Ergebnisse der Supernovaforschung; aus dem Engl. von Hilmar W. Duerbeck; 1991, Germany

 

Internet:

http://antwrp.gsfc.nasa.gov/apod/ap010729.html
http://antwrp.gsfc.nasa.gov/apod/ap950727.html
http://deepsky.astronomie.info/Lyr/m57/
http://www.allthesky.com/nebulae/m57-d.html
http://www.astro.wisc.edu/~dolan/constellations/constellations/Lyra.html

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written by Franziska Klingberg and Björn Beyersdorff