Bring in the Clouds

The Tarantula Nebula

Tarantula Nebula, a thousand light years across. Turbulent filaments of gas and void form the legs of the spider. The multi-colors shown each represent a dominant chemical element being energetically excited, such as hydrogen (red) and oxygen (green and blue).
Credit: ESO

The Tarantula Nebula is one of the most impressive views in the Southern sky.

Visible to the unaided eye in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way that is located in the direction of the southern constellation Doradus at a distance of about 170,000 light-years, this huge nebula is the prototype of what astronomers refer to as a "Giant HII region". In this complex of glowing gas and very hot and luminous stars, the gas is mainly composed of protons and electrons, which are kept apart by energetic photons emitted by the stars in this area.

The images were produced by two ESO astronomers who are impressed by this sky region. Nausicaa Delmotte did the observations for her thesis and notes that: "many of the nebulae and clusters seen in these photos would stand out prominently if they were located elsewhere in the sky and not this close to the core of the spectacular Tarantula complex.".

The Tarantula Nebula (also designated 30 Doradus) owes its name to the arrangement of its brightest patches of nebulosity that somewhat resemble the legs of a spider. They extend from a central "body" where a cluster of hot stars (designated "R136") resides that illuminate the nebula. This name, of the biggest spiders on the Earth, is also very fitting in view of the gigantic proportions of the celestial nebula – it measures nearly 1,000 light years across!

While the central regions of 30 Doradus may be compared to a tarantula, the entangled filaments in the outskirts of this nebula – some of which are seen in images obtained with the WFI camera at the ESO/MPG 2.2-m telescope at the La Silla Observatory – could well be likened to a cobweb. They testify to an ongoing history of very vigorous activity and make this spectacular sky region a showcase of dramatic effects caused by the tremendous output of energy from the most massive stars known.

Intricate colors

The marvellous richness of the filament colors is due to the varying conditions in the interstellar gas in this region. The red in these images is caused by emission of excited hydrogen atoms, the green shades correspond to emission from oxygen atoms from which two electrons ("doubly-ionized oxygen") have been "knocked off" by the energetic radiation of hot stars in the R136 cluster. The intensity of this emission increases towards R136, explaining the yellowish color near the edge of the images. A blue colour is contributed by singly-ionized atoms of oxygen.

Other atoms like nitrogen and sulfur at different levels of ionization also add to the emission of the nebula at specific wavelengths. The observed colors thus probe the physical condition of the emitting gas and the temperature of the stars that excite it. The intricate appearance of the filaments is mostly a consequence of turbulence in the interstellar gas, of the magnetic fields, and of the energy input by the massive stars in the neighborhood.

Supernovae blow interstellar "bubbles"

Detailed investigations of a large ring-shaped nebula show that it represents an "interstellar bubble" which was "blown" by supernovae explosions, most probably happening millions of years ago, as massive stars near the center of this structure ended their comparatively short lives in glorious flashes.

Turbulent filaments in the Tarantula form a cobweb of dust and voids.
Credit: ESO

A closer inspection shows that another supernova exploded somewhat later near the rim, forming a bright and more compact nebula known as SNR 0543-689. Other supernovae in this general field exploded even more recently, such as the one that created the remnant B0544-6910 only a few tens of thousands of years ago, a blink of an eye by all astronomical standards.

Nebulae with built-in powerhouses

Not all the nebulae seen in this region are caused by supernovae, however. The glow of N 164, a bright, extended red-yellow nebula just below DEM L 299, is mostly due to its own "private" powerhouse, that consists of several massive stars deeply embedded in its interior.

The same holds for DEM L 297, the somewhat smaller and fainter nebula to the right of DEM L 299. It is divided into two half-circle formed segments by a dark lane of interstellar dust in front of it. Indeed, within the Tarantula complex many such dark and dusty clouds are seen in silhouette as they obscure bright nebulosity behind them.

Many stellar clusters

The outskirts of the Tarantula Nebula are also rich in stellar clusters. One of them, NGC 2093, has relatively few stars and is relatively young, just a few tens of millions of years. It appears that its stars have already excavated a sizeable cavity around them that is now relatively void of gas.

An older and much more compact cluster, NGC 2108, resembles the globular clusters in our own Galaxy, but it formed much more recently, about 600 million years ago. Still, NGC 2108 is much older than the Tarantula complex and it is quite possible that in its "youth" it was the core of another giant HII region that has since dissolved into interstellar space.

Nausicaa Delmotte’s description is echoed by her colleague, Fernando Comeron: "This amazing concentration of clusters, HII regions, supernova remnants, and extremely hot and luminous stars in a single region makes the Tarantula in the LMC a unique celestial object, unrivalled in our own Galaxy and other nearby galaxies!"

Timelines: Probitive of Stellar Evolution

- Hubble Space Telescope launches aboard Space Shuttle Discovery, as Earth Orbiting Observatory

- Hubble Space Telescope finds evidence of black hole in the center of M87
- Hubble Key Project begins studying Cepheid variable stars to better define Hubble Constant, and the size of the universe

The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
Credit: NASA

- Sidney van den Bergh and Gustav Tammann debate Hubble Constant and the scale of the universe

- Jim Peebles and Michael Turner debate nature of universe and whether cosmology is solved

- John Cowan confirms age estimates of globular clusters and universe by dating metal-poor stars
- Wendy Freedman and Allan Sandage debate Hubble Constant and the scale of universe

- Hubble Space Telescope detects an atmosphere around an extrasolar planet

- Chandra X-ray Observatory finds evidence for new matter in "quark stars", matter so dense it exceeds terrestrial nuclear material with 1.2 million degree temperatures

- Final mission in NASA Great Observatory series, the infrared observatory, or Spitzer Space Telescope, finds evidence for organic molecules in intergalactic regions
- Microwave measurements precisely date the Big Bang at 13.7 billion years ago, with a remarkable 1% error prediction

- French COROT mission will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission

- Kepler, Extrasolar Terrestrial Planet Detection Mission, designed to look for transiting or earth-size planets that eclipse their parent stars [survey 100,000 stars]. Scientists expect to find thousands of planets, and perhaps 50 Earth-like candidates.
- Likely de-orbit for Hubble Space Telescope? [date announced is highly fluid but assumes no planned shuttle visits from NASA]

- Planned launch for NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope.
- Planned launch for Space Interferometery Mission (SIM)

- Planned launch for TPF and Darwin missions

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