Picture of the Week 2008

20 March 2008

Recoating Yepun's mirror

The 8.2-m primary mirror of Yepun, Unit Telescope 4 of ESO's Very Large Telescope, after its recoating in early March

11 March 2008

The Orion Nebula Spied by Hawk-I

The central region of the Orion Nebula (M42, NGC 1976) as seen in the near-infrared by the High Acuity Wide field K-band Imager (HAWK-I) instrument at ESO's Very Large Telescope at Paranal.

25 February 2008

ALMA Transporters Arrive in Chile

Arrival of the ALMA Antenna Transporters at the Operations Support Facility (OSF) in Chile as the convoy passed through the Valle de Luna.

11 February 2008

Light Echoes (Artist's impression)

Taking advantage of the presence of light echoes, a team of astronomers have used an ESO telescope to measure, at the 1% precision level, the distance of a Cepheid — a class of variable stars that constitutes one of the first steps in the cosmic distance ladder. 

The determination of the distance to RS Pup, following the method of the American astronomer Robert Havlen, is based on the measurement of the phase difference between the variation of the star and the variation of isolated nebular features. Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star. The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope. As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time. This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well.

By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300,000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup.

This artist's illustration is not to scale.

29 January 2008

The Growing-up of a Star: the disc around MWC 147

Artist's impression of the disc of matter surrounding the young stellar object MWC 147 as inferred from observations made with ESO's Very Large Telescope Interferometer. Thanks to these observations, astronomers have probed the inner parts of the disc of material surrounding the MWC 147, witnessing how it gains its mass before becoming an adult.

A slice has been cut to show the inner structure better. The disc extends out to 100 times the distance between the Earth and the Sun (100 Astronomical Units — 100 AU). It is inclined by about 50 degrees as seen from Earth. The dust in the outer disc emits mainly at mid-infrared wavelengths, while close to the star there is also strong near-infrared emission from very hot gas. This gas is transported towards the forming star, increasing its mass at a rate of 7 millionths of the mass of the Sun — or about 2 times the mass of the Earth — per year.

21 January 2008

Cosmic Interactions

Colour-composite image of the triplet of galaxies, catalogued as NGC 7173 (top), 7174 (bottom right) and 7176 (bottom left), and located 106 million light-years away towards the constellation of Piscis Austrinus (the 'Southern Fish'). This triplet of galaxies makes up part of the Hickson Compact Group HCG 90. 

NGC 7173 and 7176 are elliptical galaxies, while NGC 7174 is a spiral galaxy with quite disturbed dust lanes and a long, twisted tail. This seems to indicate that the two lower galaxies - whose combined shape bears some resemblance to that of a sleeping baby - are currently interacting. Astronomers have suggested that the three galaxies will finally merge. 

The size of the image is about 5.3 arcminutes. The image is based on data obtained with the VLT FORS1 instrument on ESO's Very Large Telescope through three different filters, B, V, and R. The data were extracted from the ESO Science Archive and fully processed by Henri Boffin (ESO). 

14 January 2008

Robert's Quartet

Robert's Quartet is a family of four very different galaxies, located at a distance of about 160 million light-years, close to the centre of the southern constellation of the Phoenix. Its members are NGC 87, NGC 88, NGC 89 and NGC 92, discovered by John Herschel in the 1830s.

NGC 87 (upper right) is an irregular galaxy similar to the satellites of our Milky Way, the Magellanic Clouds. NGC 88 (centre) is a spiral galaxy with an external diffuse envelope, most probably composed of gas. NGC 89 (lower middle) is another spiral galaxy with two large spiral arms. The largest member of the system, NGC 92 (left), is a spiral Sa galaxy with an unusual appearance. One of its arms, about 100,000 light-years long, has been distorted by interactions and contains a large quantity of dust.

7 January 2008

The White Penitents*

These bizarre snow and ice formations, called "penitentes", form in high-altitude regions such as the Chajnantor plain, close to where the ALMA array will be located.

These are ice blades produced by the competition between sublimation and melting of the snow. At Chajnantor at the summer solstice, the Sun is close to the zenith at noon, and penitents are vertical. This image was taken in December 2005.

This image is available as a mounted image in the ESOshop

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