Anne’s Picture of the Day: Reflection/Emission Nebula NGC 6729

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February 17, 2013

NGC 6729, a reflection/emission nebula in Corona Australis

Image Credit: ESO/Sergey Stepanenko

NGC 6729 is a reflection and emission nebula (a star-forming region), located some 424 light-years away in the constellation of Corona Australis (The Southern Crown). It is part of one of the closest stellar nurseries to the Earth.

This image gives a close-up view of the dramatic effects that new-born stars have on the dust and gas from which they were born. Stars form deep within molecular clouds, and the earliest stages of their development cannot be seen in visible-light telescopes because of obscuration by dust. In this image there are very young, hot stars at the upper left of the picture. Although they cannot be seen directly, the havoc that they have wreaked on their surroundings dominates the picture.

High-speed jets of material that travel away from the baby stars at velocities as high as one million kilometres per hour are slamming into the surrounding gas and creating shock waves. These shocks cause the gas to shine and create the strangely coloured glowing arcs and blobs known as Herbig–Haro objects.

Herbig–Haro objects (HH) – after astronomers George Herbig and Guillermo Haro – are narrow jets of gas and matter ejected by young stars at speeds of 100 to 1000 kilometers per second that collide with the surrounding nebula, producing bright shock fronts that glow as the gas is heated by friction while the surrounding gas is excited by the high-energy radiation of nearby hot stars. They are ubiquitous in star-forming regions, and several are often seen around a single star, aligned along its rotational axis. The stellar jets seem to form as the swirling cloud of dust and gas surrounding a new star escapes.

These objects are transient phenomena, lasting not more than a few thousand years. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space.

In this view the Herbig–Haro objects form two lines marking out the probable directions of ejected material. One stretches from the upper left to the lower centre, ending in the bright, circular group of glowing blobs and arcs at the lower centre. The other starts near the left upper edge of the picture and extends towards the centre right. The peculiar scimitar-shaped bright feature at the upper left is probably mostly due to starlight being reflected from dust and is not a Herbig–Haro object.

This enhanced-colour picture was created from images taken using the FORS1 instrument on ESO’s Very Large Telescope. Images were taken through two different filters that isolate the light coming from glowing hydrogen (shown as orange) and glowing ionised sulphur (shown as blue). The different colours in different parts of this violent star formation region reflect different conditions, for example where ionised sulphur is glowing brightly (blue features) the velocities of the colliding material are relatively low. The data were selected from the ESO archive by Sergey Stepanenko as part of the Hidden Treasures competition. Sergey’s picture of NGC 6729 was ranked third in the competition.

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