Helix Nebula
Encyclopedia
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
(PN) located in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Aquarius
Aquarius (constellation)
Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-bearer" or "cup-bearer", and its symbol is , a representation of water....
. Discovered by Karl Ludwig Harding
Karl Ludwig Harding
Karl Ludwig Harding was a German astronomer notable for having discovered the asteroid 3 Juno.-Biography:...
, probably before 1824, this object is one of the closest to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
of all the bright planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e. The estimated distance is about 215 parsecs or 700 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s. It is similar in appearance to the Ring Nebula
Ring Nebula
The famously named "Ring Nebula" appears in the northern constellation of Lyra and is located in the Carina–Sagittarius Arm...
, whose size, age, and physical characteristics are similar to the Dumbbell Nebula
Dumbbell Nebula
The Dumbbell Nebula is a planetary nebula in the constellation Vulpecula, at a distance of about 1,360 light years....
, varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix has often been referred to as the Eye of God on the Internet, since about 2003.
General information
The Helix Nebula is an example of a planetary nebulaPlanetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
, or 'planetary' formed at the end of a star's evolution. Gases from the star in the surrounding space appear, from our vantage point
Perspective (visual)
Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...
, as if we are looking down a helix
Helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
structure. The remnant central stellar core, known as a planetary nebula nucleus or PNN, is destined to become a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
star. The observed glow of the central star is so energetic that it causes the previously expelled gases to brightly fluoresce.
The Helix Nebula in the constellation of Aquarius
Aquarius (constellation)
Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-bearer" or "cup-bearer", and its symbol is , a representation of water....
lies about 700 light-years away, spanning about 0.8 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
or 2.5 light-years. Recent images by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
of the Helix Nebula are a composite of newly released images from the ACS
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...
instrument and the wide-angle images from the Mosaic Camera on the WIYN
WIYN Consortium
WIYN Consortium consists of the University of Wisconsin–Madison , Indiana University , Yale University , and the National Optical Astronomy Observatories...
0.9-metre telescope at Kitt Peak National Observatory
Kitt Peak National Observatory
The Kitt Peak National Observatory is a United States astronomical observatory located on 2,096 m Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O'odham Nation, southwest of Tucson...
.
Currently, the age is estimated to be 10,600 years, based solely upon a measured expansion rate of 31 km·s−1.
Structure
The Helix Nebula is thought to be shaped like a prolate spheroidProlate spheroid
A prolate spheroid is a spheroid in which the polar axis is greater than the equatorial diameter. Prolate spheroids stand in contrast to oblate spheroids...
with strong density concentrations toward the filled disk along the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
ial plane, whose major axis is inclined about 21° to 37° from our vantage point. The size of the inner disk is 8×19 arcmin in diameter (0.52 pc); the outer torus is 12×22 arcmin in diameter (0.77 pc); and the outer-most ring is about 25 arcmin in diameter (1.76 pc). We see the outer-most ring as flattened on one side due to its colliding with the ambient interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
.
Expansion of the whole planetary nebula structure is estimated to have occurred in the last 6,560 years, and 12,100 years for the inner disk. Spectroscopically, the outer ring's expansion rate is 40 km·s−1, and about 32 km·s−1 for the inner disk.
Knots
The Helix Nebula was the first planetary nebula discovered to contain knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric (from the PNNCentral Star
Central Star consists of four residential skyscrapers under construction in Busan, South Korea. The tallest landmark tower measures tall and has 58 floors. Ground was broken by POSCO E&C in 2007 and is completing in 2011....
) and are described as "cometary", each containing bright cusps (local photoionization fronts) and tails. All extend away from the PNN in a radial direction. Excluding the tails, they are (very approximately) the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photon
Lyc photon
Lyc photon or Ly continuum photon or Lyman continuum photon are a kind of photon emitted from stars. Hydrogen is ionized by absorption of Lyc photons...
s from the (PNN). There are more than 20,000 cometary knots estimated to be in the Helix Nebula.
The excitation temperature
Excitation temperature
The Excitation Temperature is defined for a population of particles via the Boltzmann factor...
varies across the Helix nebula. The rotational-vibrational temperature ranges from 1800 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
in a cometary knot located in the inner region of the nebula are about 2.5′ (arcmin) from the central PNN, calculated at about 900 K in the outer region at the distance of 5.6′.