Image of the Day: Globular Cluster Messier 69

January 4, 2014

Messier 69, a globular cluster in Sagittarius

NGC 6637

Image Credit: ESA/Hubble & NASA

Messier 69 (also known as NGC 6637) is a globular cluster of roughly 85 light-years across, with an estimated mass of 300,000 solar masses and an age of 13.06 billion years. It is located some 29,700 light-years away from Earth in the constellation of Sagittarius (the Archer), and only about 6,200 light-years away from the galactic center. It is a close neighbor of globular cluster Messier 70, with 1,800 light-years separating the two clusters.

The visually bright compact core of Messier 69 is less than half its diameter, only about 26 light-years, while its half-mass radius is about 7.2 light-years. As such, It is one of the smaller and fainter globular clusters in Messier’s catalog.

Globular clusters are collections of hundreds of thousands very old stars that orbit a galactic core as a satellite. They are tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The stellar concentration of Messier 69 is about average for a globular cluster. However, it is very poor in variable stars: only 8 have been discovered.

Remarkable is that, despite its age, Messier 69 is one of the most metal-rich globular clusters known, meaning that its stars show a relatively high abundance of elements heavier than the two most common elements in our Universe, hydrogen and helium. Nevertheless, this value is still significantly lower than that for the younger (Population I) stars like our Sun, indicating that even this globular cluster was formed at early cosmic times when the Universe contained less heavier elements, as these elements still had to be formed in successive generations of stars.

Studying the makeup of stars in globular clusters like this one has helped astronomers trace back the evolution of the cosmos.

Messier 69 forms a beautiful couple with the globular cluster Messier 70. Both can be seen at low magnification within the same image. The cluster is relatively bright, and can be seen through a binocular, although a telescope and a higher magnification is required in order to resolve stars.

This image is a combination of exposures taken in visible and near-infrared light by the Advanced Camera for Surveys on the Hubble Space Telescope. In this picture, foreground stars look big and golden when set against the backdrop of the thousands of white, silvery stars that make up Messier 69.

Anne’s Image of the Day: The Lagoon Nebula

October 21, 2013

The Lagoon Nebula, an emission nebula in Sagittarius

Messier 8, NGC 6523, Sh2-25

Image Credit: R. Barba, N. Morrell et al. (UNLP), CTIO, NOAO, NSF

The Lagoon Nebula (also known as Messier 8, NGC 6523 and Sh2-25) is an emission nebula that measures about 110 by 50 light-years, located between 4,000 – 6,000 light-years away from Earth in the constellation of Sagittarius (the Archer), and hence, toward the center of our Milky Way galaxy.

The massive stars hiding within the heart of this star-forming (H II region) give off enormous amounts of ultraviolet radiation, ionizing the gas and causing it to shine, as well as sculpting the surrounding nebula into strange shapes.

This giant cloud of glowing interstellar gas and dust is currently undergoing vivid star formation, and has already formed a considerable open cluster in the Eastern half of the Lagoon Nebula. Cataloged as NGC 6530, this cluster is estimated to be about 2 million years old and contains 50–100 young stars that scatter in a moderate range of brightness.

One of the remarkable features of the Lagoon Nebula is the presence of dark nebulae known as ‘Bok globules’ which are collapsing protostellar clouds with diameters of about 10,000 AU (1 AU = 1 Astronomical Unit = the distance between the Earth and the Sun = 149,597,871 kilometers or 92,955,807 miles). Some of the more conspicuous Bok globules have been cataloged in E.E. Barnard’s catalog of dark nebulae: the comet-shaped Barnard 88 (B88), the small Barnard 89 (B89), and the long, narrow black Barnard 296 (B296).

At its center, the Lagoon Nebula contains a structure known as the Hourglass Nebula, which should not be confused with the better known Hourglass Nebula (MyCn 18) in the constellation of Musca. The Lagoon Nebula’s Hourglass Nebula is situated in a region where vivid star formation is taking place. The bright emission is caused by heavy excitation of very hot, young stars; the illuminator of the Hourglass Nebula is the hot star Herschel 36 (mag 9.5, spectral class O7).

Closely by this Hourglass Nebula is the apparently brightest of the stars associated with the Lagoon Nebula, 9 Sagittarii (mag 5.97, spectral class O5), which surely contributes a lot of the high energy radiation which excites the nebula to shine.

In 2006 the first four Herbig-Haro objects were detected within the Hourglass Nebula, also including HH 870. Herbig-Haro objects are narrow jets of gas and matter, ejected by newly born stars at speeds of several hundred kilometers per second – named after astronomers George Herbig and Guillermo Haro who studied the outflows in the 1950s. They collide with nearby gas and dust in the interstellar medium, producing bright shock fronts that glow as the gas is heated by friction. This provides the first direct evidence of active star formation by accretion within the Hourglass.

The Lagoon Nebula is one of only two stellar nurseries which, on a clear night, is faintly visible to the naked eye from mid-northern latitudes. Like many nebulas, it appears pink in time-exposure color photos but is gray to the eye peering through binoculars or a telescope, human vision having poor color sensitivity at low light levels.

This image is taken with the Curtis-Schmidt Telescope, and shows the nebula’s emission in three exact colors specifically emitted by hydrogen, oxygen, and sulfur.

Anne’s Image of the Day: Open Cluster Messier 18

October 12, 2013

Messier 18, an open cluster in Sagittarius

NGC 6613

Image Credit: Two Micron All Sky Survey (2MASS) / IPAC

Messier 18 (also known as NGC 6613) is a relatively faint open star cluster of about 17 light-years across, located some 4,900 light-years away from Earth in the constellation of Sagittarius (the Archer), between the Omega Nebula (Messier 17) and the Sagittarius Star Cloud (Messier 24).

This cluster is quite young; its age is estimated at 32 million years. It contains mostly small stars, although there are also several bright blue as well as bright yellow or orange stars, together with a small amount of nebulosity – or dust.

Because of its distance and the lack of really high-quality observations of Messier 18, no one’s really sure as to how many stars are in there. As it is situated in the constellation of Sagittarius – the crowded center of our Milky Way galaxy – it is difficult to determine which stars are members of Messier 18 and which stars are not.

Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Anne’s Image of the Day: The Trifid Nebula

September 17, 2013

The Trifid Nebula, a star-forming region in Sagittarius

Messier 20, M20, NGC 6514

Image Credit & Copyright: R. Jay GaBany, Cosmotography 

The Trifid Nebula (designated Messier 20 and NGC 6514) is a star-forming (H II) region of some 40 light-years across, located about 5,200 light-years away from Earth in the constellation of Sagittarius (the Archer). It is estimated to be only 300,000 years old.

Stars, the Sun included, were born within clouds of dusty gas such as the Trifid Nebula. This young nebula contains enough material to make many thousands of suns. Within it a number of young hot stars have already formed.

The Trifid Nebula, meaning ‘divided into three lobes’, is an unusual combination of a red emission nebula with a young open star cluster near its center, surrounded by a blue reflection nebula which is particularly conspicuous to the northern end, and a dark absorption nebula (the apparent ‘gaps’ within the emission nebula that cause the trifid appearance; these are also designated Barnard 85).

The hydrogen molecules in the emission nebula at the Trifid’s core is heated by hundreds of brilliant young stars causing it to emit red light. The dense part of the nebula is a stellar nursery full of embryonic stars (NASA’s Spitzer Space Telescope discovered 30 embryonic stars and 120 newborn stars, not seen in visible light images). The star cluster, known as C 1759-230, may well be the youngest star cluster in our Milky Way galaxy.

The blue color of the reflection nebula comes from cosmic dust grains which preferentially reflect the blue component of starlight as it scatters light from bright new stars that were formed nearby. The largest and hottest of these stars shines most brightly in the hot, blue portion of the visible spectrum.

In some parts of the nebula there are so many dust grains that they hide the glowing gas, producing the dark absorption lanes, which were created in the atmospheres of cool giant stars and in the debris from supernovae explosions. Within these dark lanes, the remains of previous star births and deaths continue to collapse under gravity’s unrelenting attraction. The rising density, pressure and temperature inside these dark blobs will eventually trigger the formation of new stars.

Close-up images show a finger-like stalk in the emission nebula that points from the head of a very dense cloud directly toward the star that powers the Trifid nebula, which is actually a triple system of extremely hot stars. This stalk is a prominent example of evaporating gaseous globules, or ‘EGGs’. The stalk has survived because its tip is a knot of gas that is dense enough to resist being eaten away by the powerful radiation from the star.

The Trifid nebula is situated roughly 2 degrees northwest of the larger Lagoon Nebula (Messier 8). It is rather faint: a telescope is needed in order to see a star surrounded by nebulosity.

Anne’s Image of the Day: Planetary Nebula ESO 456-67

August 5, 2013

ESO 456-67, a planetary nebula in Sagittarius

ESO 456-67, a planetary nebula  in Sagittarius

Image Credit: Jean-Christophe Lambry, ESA/Hubble & NASA

ESO 456-67 is a planetary nebula located some 10,000 light-years away from Earth in the constellation of Sagittarius (the Archer), in the southern sky. It is approaching us at approximately 93 kilometers per second.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose in the 18th century because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through small optical telescopes. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all.

Planetary nebulae represent the final brief stage in the life of a medium-sized star like our Sun. While consuming the last of the fuel in its core, the dying star (collapsing from a red giant to a white dwarf) expels a large portion of its outer envelope. This material then becomes heated by the radiation from the stellar remnant and radiates, producing glowing clouds of gas that can show complex structures, as the ejection of mass from the star is uneven in both time and direction.

Over the next several thousand years, ESO 456-67 will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

In this image of ESO 456-67, it is possible to see the various layers of material expelled by the central star. Each appears in a different hue — red, orange, yellow, and green-tinted bands of gas are visible, with clear patches of space at the heart of the nebula.

It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions — to name but a few.

This image is taken with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope using three different color filters. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Jean-Christophe Lambry.