Starship Asterisk*

An Anarchic Region of Star Formation
European Southern Observatory | 2013 May 02

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The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud.

NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula (Messier 8, eso0936). Although it is usually overlooked in favour of its distinguished companion, NGC 6559 has the leading role in this new picture.

The gas in the clouds of NGC 6559, mainly hydrogen, is the raw material for star formation. When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The centre of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine.

These brilliant hot young stars born out of the cloud energise the hydrogen gas still present around them in the nebula ^[1]. The gas then re-emits this energy, producing the glowing threadlike red cloud seen near the centre of the image. This object is known as an emission nebula.

But NGC 6559 is not just made out of hydrogen gas. It also contains solid particles of dust, made of heavier elements, such as carbon, iron or silicon. The bluish patch next to the red emission nebula shows the light from the recently formed stars being scattered — reflected in many different directions — by the microscopic particles in the nebula. Known to astronomers as a reflection nebula, this type of object usually appears blue because the scattering is more efficient for these shorter wavelengths of light ^[2].

In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image. To look through the clouds at what lies behind, astronomers would need to observe the nebula using longer wavelengths that would not be absorbed.

The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559.

This eye-catching image of star formation was captured by the Danish Faint Object Spectrograph and Camera (DFOSC) on the 1.54-metre Danish Telescope at La Silla in Chile. This national telescope has been in use at La Silla since 1979 and was recently refurbished to turn it into a remote-controlled state-of-the-art telescope.

[list=1]Notes

[*] These young stars are usually of spectral type O and B, with temperatures between 10 000 and 60 000 K, which radiate huge amounts of high energy ultraviolet light that ionises the hydrogen atoms.

[*] Rayleigh scattering, named after the British physicist Lord Rayleigh, happens when light is scattered off particles of material that are much smaller than the wavelength of the light. It is much more effective for short wavelengths of light, that is, wavelengths corresponding to the blue end of the visible spectrum, so the result is a bluish diffuse light. This is the same mechanism that explains the blue colour of the daytime cloud-free sky.[/list]
Credit: ESO

http://asterisk.apod.com/viewtopic.php?t=31296#p198310

It's a great picture, but I'm a little disappointed in the caption.

http://www.eso.org/public/news/eso1320/ wrote:
Known to astronomers as a reflection nebula, this type of object usually appears blue because the scattering is more efficient for these shorter wavelengths of light [2].

Certainly, but the stars inside reflection nebulae are usually intrinsically blue, too. Any star that is hot enough to ionize a red emission nebula is itself intrinsically blue.

But it is unusual for emission and reflection nebulae to be so sharply separated, if they are powered by the same star(s). The Trifid Nebula is an example of a nebula where the emission and reflection components are well separated, but the large blue reflection nebula is either powered by another star than the one that ionizes the emission nebula, or else this is actually an example of where dust scatters short-wave blue starlight far from the star, but doesn't scatter more nearby long-wave red Ha light.

The star that ionizes the red part of the Trifid Nebula is a hot young O-type star. Because the star is so young, there is still some dust in its vicinity, and the innermost region of the Trifid is faintly bluish from dust that reflects the stars' blue light. But the bluish inner reflection nebula merges smoothly with the red emission nebula outside it. This is very different from the appearance of NGC 6559.

Here is my interpretation of it. The blue stars inside the blue reflection nebula next to NGC 6559 are not hot enough to ionize the kind of emission nebula that surrounds the central star of the Trifid Nebula. The stars' blue light is scattered by the dust cloud that surrounds them, but their ultraviolet light is not powerful enough to ionize the hydrogen that is undoubtedly there in the blue reflection nebula.

Instead, the red emission nebula is a ridge, a piled-up ridge of hydrogen that is pummeled by stellar winds from two directions. The blue stars inside the blue reflection nebula undoubtedly produce a stellar wind that pushes the gas outside it outwards. But the same gas is feeling the stellar wind of the B-type blue star that can be seen in the lower left corner of the ESO picture.
Another example of a ridge can be seen in the core of the Heart Nebula, IC 1805. However, there are several O-type stars in this nebula, and all of it is ionized by the hot stars. Nevertheless, there is a ridge in the center of it, where gas and dust is being pummeled by stellar winds from two directions.

That's why I think that NGC 6559 is a ridge of gas being "pushed" from two directions by B-type stars, which themselves are not hot enough to ionize an emission nebula around them.

Ann