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Hubble Chases a Small Stellar Galaxy in the Hunting Dog

by NASA’s Marshall Space Flight Center
 
On a clear evening in April of 1789, the renowned astronomer William Herschel continued his unrelenting survey of the night sky, hunting for new cosmic objects — and found cause to celebrate! He spotted this bright spiral galaxy, named NGC 4707, lurking in the constellation of Canes Venatici or The Hunting Dog. NGC 4707 lies roughly 22 million light-years from Earth. NGC stands for “New General Catalogue of Nebulae and Clusters of Stars.” Over two centuries later, the NASA/ESA Hubble Space Telescope is able to “chase down” and view the same galaxy in far greater detail than Herschel could, allowing us to appreciate the intricacies and characteristics of NGC 4707 as never before. This striking image comprises observations from Hubble’s Advanced Camera for Surveys (ACS), one of a handful of high-resolution instruments currently aboard the space telescope. Herschel himself reportedly described NGC 4707 as a “small, stellar” galaxy; while it is classified as a spiral (type Sm), its overall shape, center, and spiral arms are very loose and undefined, and its central bulge is either very small or non-existent. It instead appears as a rough sprinkling of stars and bright flashes of blue on a dark canvas. The blue smudges seen across the frame highlight regions of recent or ongoing star formation, with newborn stars glowing in bright, intense shades of cyan and turquoise. 

About two-thirds of spiral galaxies show a characteristic bar shape like NGC 613. Our own galaxy appears to have one of these bars through its midline as well.

NGC 613 lies 65 million light-years away in the constellation of The Sculptor. It was first noted by the English astronomer William Herschel in 1798 and later by John Louis Emil Dreyer, a Danish-Irish astronomer, who recorded the object in his 1888 New General Catalogue of Nebulae and Clusters of Stars, hence the letters “NGC.”

NGC 613’s core looks bright and uniformly white in this image as a result of the combined light shining from the high concentration of stars packed into the core, but lurking at the center of this brilliance lies a dark secret.

As with nearly all spiral galaxies, a monstrous black hole resides at the heart of NGC 613. Its mass is estimated at about 10 times that of the Milky Way’s supermassive black hole and it is consuming stars, gas and dust. As this matter descends into the black hole’s maw it radiates away energy and spews out radio waves. However, when looking at the galaxy in the optical and infrared wavelengths used to take this image, there is no trace of the dark heart.

Object Names: NGC 613

Image credit: ESA/Hubble andamp; NASA and S. Smartt (Queen’s University Belfast); Acknowledgement: Robert Gendler

Text credit: European Space Agency

Hubble Space Telescope

Time And Space

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Stellar Lab in Sagittarius

The small smattering of bright blue stars in the upper left of this vast new 615 megapixel ESO image is the perfect cosmic laboratory in which to study the life and death of stars. Known as Messier 18 this star cluster contains stars that formed together from the same massive cloud of gas and dust. This image, which also features red clouds of glowing hydrogen and dark filaments of dust, was captured by the VLT Survey Telescope (VST) located at ESO’s Paranal Observatory in Chile.

Messier 18 was discovered and catalogued in 1764 by Charles Messier — for whom the Messier Objects are named — during his search for comet-like objects [1]. It lies within the Milky Way, approximately 4600 light-years away in the constellation of Sagittarius, and consists of many sibling stars loosely bound together in what is known as an open cluster.

There are over 1000 known open star clusters within the Milky Way, with a wide range of properties, such as size and age, that provide astronomers with clues to how stars form, evolve and die. The main appeal of these clusters is that all of their stars are born together out of the same material.

In Messier 18 the blue and white colours of the stellar population indicate that the cluster’s stars are very young, probably only around 30 million years old. Being siblings means that any differences between the stars will only be due to their masses, and not their distance from Earth or the composition of the material they formed from. This makes clusters very useful in refining theories of star formation and evolution.

Astronomers now know that most stars do form in groups, forged from the same cloud of gas that collapsed in on itself due to the attractive force of gravity. The cloud of leftover gas and dust — or molecular cloud — that envelops the new stars is often blown away by their strong stellar winds, weakening the gravitational shackles that bind them. Over time, loosely bound stellar siblings like those pictured here will often go their separate ways as interactions with other neighbouring stars or massive gas clouds nudge, or pull, the stars apart. Our own star, the Sun, was most likely once part of a cluster very much like Messier 18 until its companions were gradually distributed across the Milky Way.

The dark lanes that snake through this image are murky filaments of cosmic dust, blocking out the light from distant stars. The contrasting faint reddish clouds that seem to weave between the stars are composed of ionised hydrogen gas. The gas glows because young, extremely hot stars like these are emitting intense ultraviolet light which strips the surrounding gas of its electrons and causes it to emit the faint glow seen in this image. Given the right conditions, this material could one day collapse in on itself and provide the Milky Way with yet another brood of stars — a star formation process that may continue indefinitely (eso1535).

This mammoth 30 577 x 20 108 pixel image was captured using the OmegaCAM camera, which is attached to the VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile.

Notes


[1] Messier 18 is also listed in the New General Catalogue as NGC 6613.

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berlin: Sun, stars, and Mercury, photographed by SOHO, 15th June 2014.

1 photo per hour for 24 hours. Mercury is between Earth and the Sun.

“Alnath” is derived from the Arabic النطح (“the butting”, as of the bull, Taurus), also rendered Elnath or El Nath. Since it is the second brightest star in Taurus, it is also known as Beta Tauri (or β Tau). Before the modern borders of the constellations were defined, Alnath was also sometimes considered to be shared with the neighboring constellation of Auriga, earning it yet another designation; Gamma Aurigae (γ Aur).

NGC 1746 is the New General Catalogue number for the loose group of stars seen at far right. At the time of their designation in 1863, the stars were thought to be an open cluster - gravitationally bound and probably sharing an origin. More recent studies show that they are not related, and only appear as a group from Earth’s perspective.

Image credit: NASA/ESA/GSFC. Animation: AgeOfDestruction.

A cosmic trick of the eye

Stars of different masses end their lives in different ways. While truly massive stars go out in a blaze of glory, intermediate-mass stars — those between roughly one and eight times the mass of the Sun — are somewhat quieter, forming cosmic objects known as planetary nebulas.

Named because of their vague resemblance to planets when seen through early, low-resolution telescopes, planetary nebulas are created when a dying star flings off its outer layers of gas into space. This cloud forms an expanding shell around the central star, while the star itself slowly cools to become a white dwarf. This is what has happened in this NASA/ESA Hubble Space Telescope image, taken in 2007, which shows a planetary nebula known as NGC 2371.

NGC 2371 resides 4300 light-years away from us, in the constellation of Gemini. It is one of the largest planetary nebulas known, measuring roughly three light-years across. Its progenitor star can be seen here as a pinprick of orange–-red light, surrounded by a green, blue and aqua-tinged puff of gas. This shell appears to have a regular, elliptical shape that is sliced in half by a dark lane running through the nebula, which also encompasses the central star.

This dark feature misled astronomers when NGC 2371 was initially catalogued because the two lobes visually resembled two objects, not one. As a result of this confusion, the nebula has two names in William Herschel’s New General Catalogue: NGC 2371 and 2372 (often combined as NGC 2371/2 or NGC 2371-2).

Two prominent pink patches are also visible on either side of the central star. These features are thought to be knots of gas, most likely jets, thrown off by the star at some point in the past. Their pink colour indicates that they are cooler and denser than their surroundings.

The nebula’s central star was once similar to the Sun, but is now only a shadow of its former self. It is slowly cooling after energetically shedding most of its gas, but has a long way to go yet. It currently boasts a scorching surface temperature of over 130 000ºC – some 25 times hotter than the surface of the Sun – and glows with the luminosity of at least 700 Suns.

The hot ultraviolet radiation streaming outwards into the nebula energises the gas it touches, causing NGC 2371 to glow in the beautiful aquamarine colours seen in this image.

This picture was taken in November 2007 by Hubble’s Wide Field Planetary Camera 2. It is a false-colour image created with a combination of filters to detect light coming from sulphur and nitrogen (shown in red), hydrogen (green) and oxygen (blue). The observations were gathered as part of the Hubble Heritage project.

Copyright NASA/ESA/Hubble Heritage Team (STScI/AURA)