Messier 71: an unusual globular cluster: This spectacular NASA/ESA Hubble Space Telescope image shows a bright scattering of stars in the small constellation of Sagitta (the Arrow). This is the centre of the globular cluster Messier 71, a great ball of ancient stars on the edge of our galaxy around 13 000 light-years from Earth. M71 is around 27 light-years across. Globular clusters are like galactic suburbs, pockets of stars that exist on the edge of major galaxies. These clusters are tightly bound together by their gravitational attraction, hence their spherical shape and their name: globulus means “little sphere” in Latin. Around 150 such globular clusters are known to exist around our Milky Way, each one of them containing several hundred thousand stars. Messier 71 has been known for a long time, having been first spotted in the mid eighteenth century by Swiss astronomer Jean-Philippe de Cheseaux. Cheseaux discovered a number of nebulae in his career, and also spent much time studying religion: one posthumously published work attempted to derive the exact date of Christ’s crucifixion from astronomical events noted in the Bible. Despite being a familiar object, Messier 71’s precise nature was disputed until recently. Was it simply an open cluster, a loosely bound group of stars? This was for many years the dominant view. But in the 1970s, astronomers came to the view that it is in fact a relatively sparse globular cluster. The stars in Messier 71, as is usual in such clusters, are relatively old, at around 9 to 10 billion years, and consequently are low in elements other than hydrogen and helium. This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys on Hubble. It is a combination of images taken through yellow (F606W — coloured blue) and near-infrared (F814W — coloured red) filters. The exposure times were 304 s and 324 s respectively. The field of view is about 3.4 arcminutes across.

Name: Messier 71
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Star Clusters
Constellation: Sagitta
Distance: 13000 light years
Release date: 23 August 2010, 10:00
Credit: ESA/Hubble and NASA

Young stars biting the cloud that feeds them: A billowing cloud of hydrogen in the Triangulum galaxy (Messier 33), about 2.7 million light-years away from Earth, glows with the energy released by hundreds of young, bright stars. This NASA/ESA Hubble Spare Telescope image provides the sharpest view of NGC 604 so far obtained. Some 1500 light-years across, this is one of the largest, brightest concentrations of ionised hydrogen (H II) in our local group of galaxies, and is a major centre of star formation. The gas in NGC 604, around nine tenths of it hydrogen, is gradually collapsing under the force of gravity to create new stars. Once these stars have formed, the vigorous ultraviolet radiation they emit excites the remaining gas in the cloud, making it glow a distinct shade of red. This colour is typical not only of NGC 604 but of other H II regions too. Although it is part of Messier 33 this object is so bright and prominent that it was given its own NGC number. The fierce ultraviolet radiation released by the stars that give these hydrogen clouds their distinctive glow is also the cause of their uneven appearance and eventual disappearance. The radiation and winds blowing from the surface of these stars gradually erode the cloud they formed from, causing the gases to slowly disperse. The complex structure of NGC 604, with irregular bubbles and wispy filament-like structures alongside denser, redder areas is due to the same forces that will eventually make the cloud disappear. The blister-like cavities show areas of stronger erosion of the cloud. While these areas appear dark in this photograph, they shine brightly at X-ray wavelengths. This image was created from images taken using the High Resolution Channel of Hubble's Advanced Camera for Surveys. It is a composite of images taken through a total of seven different filters spanning a huge range of wavelengths — from 220 nm in the ultraviolet all the way up to the near infrared at one micron. The field of view is about 31 by 22 arcseconds.

Name: Messier 33, NGC 604
Type: Local Universe : Nebula : Type : Star FormationLocal Universe : Galaxy : Type : Spiral
Category: Nebulae
Constellation: Triangulum
Distance: 3 million light years
Release date: 30 August 2010, 10:00
Credit: ESA/Hubble and NASA

An extraordinary celestial spiral: This remarkable picture from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope shows one of the most perfect geometrical forms created in space. It captures the formation of an unusual pre-planetary nebula, known as IRAS 23166+1655, around the star LL Pegasi (also known as AFGL 3068) in the constellation of Pegasus (the Winged Horse). The striking picture shows what appears to be a thin spiral pattern of astonishingly regularity winding around the star, which is itself hidden behind thick dust. The spiral pattern suggests a regular periodic origin for the nebula’s shape. The material forming the spiral is moving outwards a speed of about 50 000 km/hour and, by combining this speed with the distance between layers, astronomers calculate that the shells are each separated by about 800 years.The spiral is thought to arise because LL Pegasi is a binary system, with the star that is losing material and a companion star orbiting each other. The spacing between layers in the spiral is expected to directly reflect the orbital period of the binary, which is indeed estimated to be also about 800 years.The creation and shaping of planetary nebulae is an exciting area of stellar evolution. Stars with masses from about half that of the Sun up to about eight times that of the Sun do not explode as supernovae at the ends of their lives. Instead a more regal end awaits them as their outer layers of gas are shed and drift into space, creating striking and intricate structures that to Earth-bound observers often look like dramatic watercolour paintings. IRAS 23166+1655 is just starting this process and the central star has yet to emerge from the cocoon of enveloping dust.This picture was created from images from the Wide Field Channel of the Advanced Camera for Surveys on Hubble. Images through a yellow filter (F606W, coloured blue) were combined with images through a near-infra red filter (F804W, coloured red). The exposure times were 11 minutes and 22 minutes respectively and the field of view spans about 80 arcseconds.Links

Name: IRAS 23166+1655, LL Pegasi
Type: Milky Way : Star : Grouping : BinaryMilky Way : Nebula : Type : Planetary
Category: Nebulae
Constellation: Pegasus
Release date: 6 September 2010, 10:00
Credit: ESA/NASA & R. Sahai