Sagittarius Star Cloud

The Sagittarius Star Cloud (also known as Delle Caustiche, Messier 24, IC 4715) is a star cloud in the constellation of Sagittarius, approximately 600 light years wide, which was discovered by Charles Messier in 1764.

The stars, clusters and other objects comprising M24 are part of the Sagittarius or Sagittarius-Carina arms of the Milky Way galaxy. Messier described M24 as a "large nebulosity containing many stars" and gave its dimensions as being some 1.5° across, a description that fits the star cloud rather well. Some sources, improperly, identify M24 as the faint cluster NGC 6603.

M24 fills a space of significant volume to a depth of 10,000 to 16,000 light-years. This is the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view.

Amazing Milky Way

The Milky Way band

Image extracted from the amazing ALMA planetarium show, produced by ESO and the Association of French Language Planetariums. The emphasis of the show is the incomparable scientific adventure of the ALMA project. A young female astronomer guides the audience through the story that includes unique animations and footage, leading the viewer from the first observations by Galileo, 400 years ago, to the world of modern astronomy, moving from the visible wavelength domain to explore the millimetre-wave view of the Universe, and leaving light-polluted cities for unique settings in some of the highest and driest places on Earth.

Image from Cosmic Origins


Access Road to the ALMA Site

Acces road to the ALMA site in Chajnantor. The image was taken on December 2004.

Dark matter halo

The dark matter halo is the hypothetical gravitational core of a galaxy, consisting of dark matter.

The nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but there are two popular theories: either the halo is composed of weakly-interacting elementary particles known as WIMPs, or it is home to large numbers of small, dark bodies known as MACHOs. It seems unlikely that the halo is composed of large quantities of gas and dust, because both ought to be detectable through observations. Searches for gravitational microlensing events in the halo of the Milky Way show that the number of MACHOs is likely not sufficient to account for the required mass.

The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity. The dark matter halo is the location of nearly all of the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy. The luminous matter makes up approximately 90,000,000,000 solar masses. The dark matter halo is likely to include around 600,000,000,000 to 3,000,000,000,000 solar masses of dark matter.