Monday, 29 June 2015

Nebula - Lagoon

The Lagoon Nebula (Messier 8 or M8, and as NGC 6523) is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula and as a H II region.

The Lagoon Nebula is estimated to be between 4,000-6,000 light years from the Earth. In the sky of Earth, it spans 90' by 40', translates to an actual dimension of 110 by 50 light years.

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Saturday, 27 June 2015

Massive Old Stars

Eta Carinea

Eta Carinae formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity over five million times that of the Sun, located around 7500 light-years (2300 parsecs) distant in the direction of the constellation Carina...

First recorded as a 4th magnitude star, it brightened considerably over the period 1837 to 1856 in an event known as the Great Eruption. Eta Carinae became the second brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility. It has brightened consistently since about 1940, peaking above magnitude 4.5 in 2014.

Eta Carinae is circumpolar south of latitude 30°S, so it is never visible north of latitude 30°N.

The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is a peculiar star similar to a luminous blue variable (LBV) that initially had around 150 solar masses and has since lost at least 30. Because of its mass and the stage of its life, it is expected to explode as a supernova or hypernova in the astronomically near future.

This is currently the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula. The weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.


Wednesday, 24 June 2015

Galactic Disk

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the galaxy's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.

While the disk contains dust which obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars. Open clusters are also located primarily in the disk.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought, the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the 2004 discovery of the Outer Arm extension of the Cygnus Arm. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Juri? and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.

Gaseous halo

In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the galaxy itself. The temperature of this halo gas is between 1 million and 2.5 million kelvin, a few hundred times hotter than the surface of the sun.

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way. If the finding that the mass of the halo is comparable to the mass of the galaxy is confirmed, it could be the identity of the missing baryons around the Milky Way.

Wednesday, 10 June 2015

Lascaux Caves

Astronomy History Caved in France?!

….the setting of a complex of caves in south-western France famous for

its Paleolithic cave paintings. The original caves are located near the village of Montignac, in the department of Dordogne. They contain some of the best-known Upper Paleolithic art. These paintings are estimated to be 17,300 years old. They primarily consist of images of large animals, most of which are known from fossil evidence to have lived in the area at the time. In 1979, Lascaux was added to the UNESCO World Heritage Sites list along with other prehistoric sites in the Vézère valley.