• SAO Astronomy News

Semester 2, 2004

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21st September, 2004: The Universe Beautiful

There are times when the sheer beauty of our Universe is overwhelming. The Cat's Eye Nebula, aka NGC 6543 is a case in point. Astronomers studying planetary nebulae like the Cat's Eye are doubly lucky! They extract important information about the end products of stellar evolution and how matter is returned to the interstellar medium and also get images like this.

The Cat's Eye Nebula, aka NGC 6543. The image is 1.2 arcminutes (1.2 light-years or 0.35 parsecs) wide and is made from observations using the filters F502N [O III], FR505N [O III] and F658N (Halpha+[N II]). Image Credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA) Acknowledgment: R. Corradi (Isaac Newton Group of Telescopes, Spain) and Z. Tsvetanov (NASA)

Hubble's Advanced Camera for Surveys (ACS) reveals eleven or even more concentric rings, or shells, with each shell representing the ejection of mass from the central star in a series of pulses at 1,500-year intervals.

NGC 6543, Cat's Eye Nebula Imaged with Nordic Optical Telescope Image Credits: R. Corradi (Isaac Newton Group) and D. Goncalves (Inst. Astrofisica de Canarias)

Astronomer, Romano Corradi, also made deep (wider field of view) images (above) using the Nordic Optical Telescope in two narrow-band filters: nitrogen (red) and oxygen (green and blue shades). This shows an even earlier history of mass ejections and probably a more complex interaction with the surrounding interstellar medium.

For more information see: Hubble Heritage * Nordic Optical Telescope

5th September, 2004: Global Surveyor spots Mars

Spots on Mars? Mars Global Surveyor (MGS) has captured a remarkable view of Martian dunes near the northen pole.

What are the black spots though? In the northern hemisphere of Mars spring approaches and the ice that normally covers the dunes begins to thaw showing underlying dark dunes. The newly exposed dark sand then accelerates the thawing process until summer when the entire dune is revealed.

A Mars Orbiter Camera (MOC) image shows sand dunes in the north polar region. This image is located near 82.8 N, 219.6 W and covers an area about 3 km across. Image Credit: NASA/JPL/Malin Space Science Systems

The Mars Global Surveyor continues to acquire images of Mars after beginning image surveying in March 1999 and is expected to continue through until December 2006.

For more information see: Malin Space Science Systems * Astronomy Picture of the Day * Mars Global Surveyor

26th August, 2004: Chandra spies amazing Cassiopeia A detail

Chandra X-ray observatory has spent one million seconds imaging the supernovae remnant Cassiopeia A (Cas A). A bright outer ring (in green) is ten light years in diameter and this shows the shock wave generated by the supernova explosion. A large jet-like structure (in red) protrudes beyond the (green) shock wave and can be seen in the upper left of the image. Red indicates silicon ions, and a counter-jet can also be seen on the lower right.

A 1 million second exposure by Chandra of Cassiopeia A. Image Credit: NASA/CXC/GSFC/U.Hwang et al.

X-ray spectra show that the jet and counter-jet are rich in silicon and relatively poor in iron suggesting that the jets formed soon after the initial explosion of the star (which expels large amounts of iron). A bright source evident in the centre of Cas A is presumed to be a neutron star created during the supernova event. Unlike pulsars (rotating neutron stars) in the Crab Nebula and Vela supernova remnants, the Cas A neutron star is quiet and faint, and doesn't shows evidence for pulsed radiation.

For more information see: Chandra X-ray Observatory * Chandra Press Release * Chandra Animations * Astronomy Picture of the Day

5th August, 2004: Cassini-Huygens at Saturn

Welcome to SAO semester 2, 2004.

On the 30th of June spacecraft Cassini-Huygens executed a 96 minute burn that provided a change in speed of 626 m/s, and was captured by Saturn's gravity. The closest approach to Saturn throughout the entire Cassini mission occured shortly before burn completion, a mere 20,000 km above the cloud tops of the planet when Cassini passed through the plane of the rings between the F and G rings.

Saturns magnificent rings show some of their intricate structure in this image taken on May 11, 2004, by the narrow angle camera. The rings are composed of billions of individual particles. Satellites visible in this image: Janus (181 kilometres across) above the rings, and icy Enceladus (499 kilometres across) below the rings. The image was taken in visible light from a distance of 26.3 million kilometres from Saturn. Image Credit: NASA/JPL/Space Science Institute

Whilst the Huygens probe will begin it's journey to the moon Titan in December, some early images from Cassini of Titan have already been taken. Cassini's visible and infrared mapping spectrometer (VIMS) pierced the smog that enshrouds Titan. This instrument, capable of mapping mineral and chemical features of the moon, reveals an exotic surface bearing a variety of materials in the south and a circular feature that may be a crater in the north.

Titan: Left - a variety of surface features at a wavelength of 2.0 microns. The darker areas are possibly regions of relatively pure water ice, while the brighter regions likely have a much higher amount of non-ice materials such as simple hydrocarbons. Middle - a wavelength of 2.8 microns shows a very dark surface almost everywhere, as expected for a surface of water ice and simple hydrocarbons. Right - at 5.0 microns, indicates dark icy regions and brighter hydrocarbon-rich materials. A bright cloud of methane particles is apparent in all three images near the south pole. Image Credit: NASA/JPL/Space Science Institute

For more information see: Cassini-Huygens home page (JPL) * VIMS home page