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OzSky Star Safari - Southern Skies Showcase:
The Eta Carinae Nebula complex

Eta Carinae Nebula (NGC 3372, Caldwell 92) Eta Carinae Nebula - Credit: Dylan O'Donnell
Image Credit: Dylan O'Donnell

R.A: 10h 44' 29"
Dec: -59° 57' 26"
Mag: +1.0
Const: Carina (Car)
Best at: OzSky "Classic" Star Safari in April

The Eta Carinae Nebula is a large, complex area of bright and dark nebulosity in the southern constellation of Carina (Car).

Shining at an apparent magnitude of +1.0, the nebula lies at an estimated distance of 7,600 light-years (2,330 pc) from Earth in the Carina–Sagittarius Arm of the Milky Way. Eta Carinae is another one of the first targets of many observers at The OzSky Star Safaris, transiting the April meridian around 9pm at an impressive altitude of around 62° above the horizon.

The nebula has within its boundaries the large Carina OB1 association and several related open clusters, including numerous O-type stars and several Wolf–Rayet stars. Carina OB1 encompasses the star clusters Trumpler 14 and Trumpler 16.

Trumpler 14 is one of the youngest known star clusters at half a million years old. Trumpler 16 is the home of WR 25, currently the most luminous star known in our Milky Way galaxy, together with the less luminous but more massive and famous Eta Carinae star system and the O2 supergiant HD 93129A. Trumpler 15, Collinder 228, Collinder 232, NGC 3324, and NGC 3293 are also considered members of the association. NGC 3293 is the oldest and furthest from Trumpler 14, indicating sequential and ongoing star formation.

The nebula is one of the largest diffuse nebulae in our skies. Although it is some four times as large and even brighter than the famous Orion Nebula, the Eta Carina Nebula is much less well known to notrthern hemisphere observers due to its location in the glorious southern skies.

The Eta Carinae star itself is a highly luminous hypergiant star. Estimates of its mass range from 100 to 150 Solar masses, and its luminosity is about four million times that of the Sun.

This object is currently the most massive star that can be studied in great detail, because of its location and size. Several other known stars may be more luminous and more massive, but data on them is far less robust. (Caveat: Since examples such as the Pistol Star have been demoted by improved data, one should be skeptical of most available lists of "most massive stars". In 2006, Eta Carinae still had the highest confirmed luminosity, based on data across a broad range of wavelengths.) Stars with more than 80 times the mass of the Sun produce more than a million times as much light as the Sun.

They are quite rare - only a few dozen in a galaxy as big as ours - and they flirt with disaster near the Eddington limit, i.e., the outward pressure of their radiation is almost strong enough to counteract gravity. Stars that are more than 120 solar masses exceed the theoretical Eddington limit, and their gravity is barely strong enough to hold in its radiation and gas, resulting in a possible supernova or hypernova in the near future.

Eta Carinae's effects on the surrounding nebula can be seen directly. Dark globules and some other less visible objects have tails pointing directly away from the massive star. The entire nebula would have looked very different before the Great Eruption in the 1840s surrounded Eta Carinae with dust, drastically reducing the amount of ultraviolet light it put into the nebula.

The Homunculus Homunculus Nebula - Credit: Jon Morse, University of Colorado / NASA
Image Credit: Jon Morse, University of Colorado / NASA

Within the large bright nebula is a much smaller feature, immediately surrounding Eta Carinae itself, known as the Homunculus Nebula (from Latin meaning Little Man).

It is believed to have been ejected in an enormous outburst in 1841 which briefly made Eta Carinae the second-brightest star in the sky.

The Homunculus Nebula is a small HII region, with gas shocked into ionised and excited states.

It also absorbs much of the light from the extremely luminous central stellar system and re-radiates it as infrared (IR).

It is the brightest object in the sky at mid-IR wavelengths.

The Homunculus is always a favourite target for observers at The OzSky Star Safari, and on a good night the detail visible through some of the 3RF telescopes defies belief.

The Keyhole Nebula Keyhole Nebula - Credit: Wolfgang Promper
Image Credit: Wolfgang Promper

The Keyhole Nebula is a small dark cloud of cold molecules and dust within the Eta Carinae Nebula, containing bright filaments of hot, fluorescing gas, silhouetted against the much brighter background nebula.

The diameter of the Keyhole structure is approximately seven light-years (2.1 pc).

Its appearance has changed significantly since it was first observed, possibly due to changes in the ionising radiation from Eta Carinae.

The Keyhole Nebula does not have its own NGC designation. It is sometimes erroneously called NGC 3324, but that catalogue designation refers to a reflection and emission nebula just northwest of the Eta Carina Nebula (or to its embedded star cluster).

Mystic Mountain Mystic Mountain - Credit: NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team (STScI)
Image Credit: NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team (STScI)

Mystic Mountain is the term for a dust–gas pillar in the Carina Nebula, a photo of which was taken by Hubble Space Telescope on its 20th anniversary.

The area was observed by Hubble's Wide Field Camera 3 on 1–2 February 2010.

The pillar measures three light-years (0.92 pc) in height; nascent stars inside the pillar fire off gas jets, that stream from towering peaks.

There are at least eight known open clusters in the Carina Nebula:

  • Bochum 10 (Bo 10)
  • Bochum 11 (Bo 11)
  • Collinder 228 (Cr 228)
  • Collinder 232 (Cr 232)
  • Collinder 234 (Cr 234)
  • Trumpler 14 (Tr 14, Cr 230)
  • Trumpler 15 (Tr 15, Cr 231)
  • Trumpler 16 (Tr 16, Cr 233)

    Trumpler 14 is an open cluster with a diameter of six light-years (1.8 pc), located within the inner regions of the Eta Carinae Nebula, approximately 8,000 light-years (2,500 pc) from Earth. It is one of the main clusters of the Carina OB1 stellar association, which is the largest association in the Eta Carinae Nebula. About 2,000 stars have been identified in Trumpler 14 and the total mass of the cluster is estimated to be 4,300 Solar masses.

    Trumpler 16 is one of the main clusters of the Carina OB1 stellar association, which is the largest association in the Carina Nebula, and it is bigger and more massive than Trumpler 14.

    References: SkySafari 5 Pro (iPhone app); Wikipedia

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