Starship Asterisk*

[img3="Credit: ESO, VLT, FORS"]http://cdn.eso.org/images/screen/potw1531a.jpg[/img3][hr][/hr]

The bright star in the centre of this image is not the star of this show. At the bottom centre is a rather unremarkable smudge of red which is in fact a rare and valuable object.

First discovered by amateur Japanese astronomer, Yukio Sakurai, in 1996, and noted as a nova-like object, Sakurai’s discovery turned out to be far more interesting than the supernova he initially supposed it to be.

The object is actually a small white dwarf star undergoing a helium flash — one of only a handful of examples of such an event ever witnessed by astronomers.

Normally, the white dwarf stage is the last in the life cycle of a low-mass star. In some cases, however, the star reignites in a helium flash and expands to return to a red giant state, ejecting huge amounts of gas and dust in the process, before once again shrinking to become a white dwarf.

It is a dramatic and short-lived series of events, and Sakurai’s Object has allowed astronomers a very rare opportunity to study the events in real time. The white dwarf emits sufficient ultraviolet radiation to illuminate the gas it has expelled, which can just be seen in this image as the ring of red material.

This image was taken using the FORS instrument, mounted on ESO’s Very Large Telescope.

[img3="Credit: ESA/Hubble and NASA
Acknowledgement: Judy Schmidt (geckzilla.com)"]http://cdn.spacetelescope.org/archives/ ... w1531a.jpg[/img3][hr][/hr]

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. It is located in the constellation of Sagittarius (The Archer), roughly 6000 light-years away from us. The rich glow of the cloud is just over half a light-year across — humongous compared to its tiny central star — but still a little gem on a cosmic scale.

When stars like the Sun enter retirement, they shed their outer layers into space to create glowing clouds of gas called planetary nebulae. This ejection of mass is uneven, and planetary nebulae can have very complex shapes. NGC 6818 shows knotty filament-like structures and distinct layers of material, with a bright and enclosed central bubble surrounded by a larger, more diffuse cloud.

Scientists believe that the stellar wind from the central star propels the outflowing material, sculpting the elongated shape of NGC 6818. As this fast wind smashes through the slower-moving cloud it creates particularly bright blowouts at the bubble’s outer layers.

Hubble previously imaged this nebula back in 1997 with its Wide Field Planetary Camera 2, using a mix of filters that highlighted emission from ionised oxygen and hydrogen (opo9811h). This image, while from the same camera, uses different filters to reveal a different view of the nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.

bystander wrote:White Dwarf Resurrection
ESO Picture of the Week | 2015 Aug 03

[img3="Credit: ESO, VLT, FORS"]http://cdn.eso.org/images/screen/potw1531a.jpg[/img3][hr][/hr]

The bright star in the centre of this image is not the star of this show. At the bottom centre is a rather unremarkable smudge of red which is in fact a rare and valuable object.

First discovered by amateur Japanese astronomer, Yukio Sakurai, in 1996, and noted as a nova-like object, Sakurai’s discovery turned out to be far more interesting than the supernova he initially supposed it to be.

The object is actually a small white dwarf star undergoing a helium flash — one of only a handful of examples of such an event ever witnessed by astronomers.

Normally, the white dwarf stage is the last in the life cycle of a low-mass star. In some cases, however, the star reignites in a helium flash and expands to return to a red giant state, ejecting huge amounts of gas and dust in the process, before once again shrinking to become a white dwarf.

It is a dramatic and short-lived series of events, and Sakurai’s Object has allowed astronomers a very rare opportunity to study the events in real time. The white dwarf emits sufficient ultraviolet radiation to illuminate the gas it has expelled, which can just be seen in this image as the ring of red material.

This image was taken using the FORS instrument, mounted on ESO’s Very Large Telescope.

This extraordinary bubble, glowing like the ghost of a star in the haunting darkness of space, may appear supernatural and mysterious, but it is a familiar astronomical object: a planetary nebula, the remnants of a dying star. This is the best view of the little-known object ESO 378-1 yet obtained and was captured by ESO's Very Large Telescope in northern Chile.

[img3="Credit: ESO/VLT"]http://cdn.eso.org/images/screen/eso1532a.jpg[/img3][hr][/hr]

Nicknamed the Southern Owl Nebula, this shimmering orb is a planetary nebula with a diameter of almost four light-years. Its informal name relates to its visual cousin in the northern hemisphere, the Owl Nebula. ESO 378-1 [1], which is also catalogued as PN K1-22 and PN G283.6+25.3, is located in the constellation of Hydra (The Female Water Snake).

Like all planetary nebulae, ESO 378-1 is a relatively short-lived phenomenon, lasting only a few tens of thousands of years, compared to a typical stellar lifetime of several billion years [2].

Planetary nebulae are created by the ejected and expanding gas of dying stars. Although they are brilliant and intriguing objects in the initial stages of formation, these bubbles fade away as their constituent gas moves away and the central stars grow dimmer.

For a planetary nebula to form, the aging star must have a mass less than about eight times that of the Sun. Stars that are heavier than this limit will end their lives in dramatic fashion as supernova explosions.

As these less massive stars grow old they start to lose their outer layers of gas to stellar winds. After most of these outer layers have dissipated, the remaining hot stellar core starts to emit ultraviolet radiation which then ionises the surrounding gas. This ionisation causes the expanding shell of ghostly gas to begin to glow in bright colours. ...

[img3="Credit: ESA/Hubble, NASA, and S. Smartt (Queen's University Belfast)
Acknowledgements: Nick Rose and Flickr user penninecloud"]http://cdn.spacetelescope.org/archives/ ... w1532a.jpg[/img3][hr][/hr]

Bursts of pink and red, dark lanes of mottled cosmic dust, and a bright scattering of stars — this NASA/ESA Hubble Space Telescope image shows part of a messy barred spiral galaxy known as NGC 428. It lies approximately 48 million light-years away from Earth in the constellation of Cetus (The Sea Monster).

Although a spiral shape is still just about visible in this close-up shot, overall NGC 428’s spiral structure appears to be quite distorted and warped, thought to be a result of a collision between two galaxies. There also appears to be a substantial amount of star formation occurring within NGC 428 — another telltale sign of a merger. When galaxies collide their clouds of gas can merge, creating intense shocks and hot pockets of gas and often triggering new waves of star formation.

NGC 428 was discovered by William Herschel in December 1786. More recently a type Ia supernova designated SN2013ct was discovered within the galaxy by Stuart Parker of the BOSS (Backyard Observatory Supernova Search) project in Australia and New Zealand, although it is unfortunately not visible in this image.

This image was captured by Hubble’s Advanced Camera for Surveys (ACS) and Wide Field and Planetary Camera 2 (WFPC2). ...

Ann wrote:...
According to this page, a 450 nm filter, corresponding to about this color, was mapped both as blue and as green. But a 606 nm filter, corresponding to about this color, was also mapped as green.

So there is a lot of mapped green color in that picture, but little blue. ...

[img3="Credit: ESO/Babak Tafreshi (TWAN)"]http://cdn.eso.org/images/screen/potw1533a.jpg[/img3][hr][/hr]

In this panoramic image taken by Babak Tafreshi — a member of the ESO Ultra HD Expedition team — the ALMA Observatory’s antennas appear to take in the sight of the Milky Way, arching like a galactic rainbow of dust and stars over the Chajnantor Plateau in the Chilean Andes.

Located 5000 metres above sea level, the Chajnantor Plateau serves as an ideal hunting ground for ALMA. The array uses observations at the millimetre and submillimetre wavelengths to explore the coldest parts of the Universe. The superb aridness of the plateau means that far less submillimetre radiation is absorbed and attenuated by water vapour than would be the case closer to sea level.

As ALMA seeks to capture the secrets of the Universe, Babak Tafreshi and his fellow team members seek to capture the beauty and grandeur of ESO’s observatories and their unusual surroundings. Comprising four celebrated astrophotographers and ESO Photo Ambassadors, the ESO Ultra HD expedition aimed to raise public awareness of the ongoing work in Chile through astonishing Ultra HD photos and videos.

[img3="Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt"]http://cdn.spacetelescope.org/archives/ ... w1533a.jpg[/img3][hr][/hr]

Here we see the spectacular cosmic pairing of the star Hen 2-427 — more commonly known as WR 124 — and the nebula M1-67 which surrounds it. Both objects, captured here by the NASA/ESA Hubble Space Telescope are found in the constellation of Sagittarius and lie 15 000 light-years away.

The star Hen 2-427 shines brightly at the very centre of this explosive image and around the hot clumps of gas are ejected into space at over 150 000 kilometres per hour.

Hen 2-427 is a Wolf–Rayet star, named after the astronomers Charles Wolf and Georges Rayet. Wolf–Rayet are super-hot stars characterised by a fierce ejection of mass.

The nebula M1-67 is estimated to be no more than 10 000 years old — just a baby in astronomical terms — but what a beautiful and magnificent sight it makes.

A version of this image was released in 1998, but has now been re-reduced with the latest software.