What are Nebulae?

Wednesday 20 September 2017 - 00:03:52 Posted by  Bobby

Nebulae / Nebula

  Nebulae and stars are inextricably intertwined. Stars are born from collapsing molecular clouds. Then, in their old age as red giants, stars enrich their surroundings with carbon that they have created from helium fusion. Heavier elements are produced by supergiant stars when they explode as supernovae, spewing those elements into space, perhaps to form planets, with elements such as iron, silicon, and gold, when another molecular cloud collapses.

Dark Nebulae

  Many dark nebulae are telescopic objects, but there are some fine ones for the unaided eye. The most obvious is the Coalsack, beside the Southern Cross. The Coalsack forms the head of a much larger object, the dark Emu of the Australian Aborigines. The southern end of the Great Rift, from Alpha Centauri through to Scorpius, forms the bird's body. The funnel cloud nebula, Le Gentil 2, the great Dark Nebula descending from Cepheus into Cygnus along Right Ascension 21 hours, is the second-best after the Coalsack. The funnel cloud nebula is still a naked-eye object with a 10-day-old gibbous moon visible in the sky.
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The Cone Nebula- NGC 2264, a dark nebula in the constellation Monoceros

  This nebula is a small dark nebula within NGC 2264, it is an innocuous pillar of gas and dust--a giant pillar residing in a turbulent star-forming region. The Cone Nebula resides 2,500 light-years away in the constellation Monoceros.

Reflection Nebulae

  Sometimes dust is made visible by reflecting starlight. If stars are embedded in dust, the resulting reflection nebula is usually just an amorphous glow around a star or stars, like M78. One exception that shows lots of detail is the 15 arc-seconds long, bright orange, Homunculus Nebula, a double lobed reflection nebula.
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Eta Carinae- a double lobed reflection nebula in the constellation Carina

  A huge, billowing pair of gas and dust clouds are captured in this stunning NASA image of the supermassive star. Eta Carinae was observed by Hubble in September 1995 with the Wide Field and Planetary Camera 2 (WFPC2). Images taken through red and near-ultraviolet filters were subsequently combined to produce the color image shown.

Emission Nebulae

  When star formation has begun in a molecular cloud, ultraviolet radiation from the most massive young spectral type O stars will ionize the surrounding hydrogen gas, exciting it to fluorescence, and destroying nearby volatile dust. We see this as an emission nebula, also called an HII region. The massive young stars' energetic stellar winds and ultraviolet radiation form cavities in the molecular cloud. If a cavity is on our side of the cloud, we see a showpiece object like the Orion Nebula, in which the highly luminous trapizium stars cause the walls of the cavity to shine. Many of the filaments of nebulosity in images of the Tarantula (NGC 2070), Orion (M42), Swan (M17), and Eta Carineae (NGC 3372) nebula are also visible at the eyepiece. But these nebula have too low a surface brightness for an observer to see the bright colors that are on images, with one partial exception, the Orion Nebula.
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The Eagle Nebula- NGC 6611, an emission nebula in the constellation Serpens

  Like a Winged fairy-tale creature on a pedestal, this is actually a billowing tower of cold gas and dust rising from a stellar nursery called the Eagle Nebula, NGC 6611. It is 9.5 light-years high, about twice the distance from our Sun to the next nearest star.

Planetary Nebulae

  Planetary nebulae were named by William Herschel for the resemblance that some of them have to the disc of a planet, especially a finger disc like that of his discovery, Uranus. One planetary, NGC 3242 in Hydra, is even called the Ghost of Jupiter. Planetaries are shells of gas that have been ejected from the outer layers of red giant stars at the end of their lives. The very hot exposed core of the star radiate strongly in the ultraviolet, and excites this shell of gas to fluorescence for a relatively brief period - tens of thousands to know more than a million years - before it completely dissipates into space.
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"Ghost of Jupiter"- NGC 3242, planetary nebula in the constellation Hydra

  This planetary nebula is most frequently called the Ghost of Jupiter, or Jupiter's Ghost due to its similar size to the planet, but it is also sometimes referred to as the Eye Nebula. The nebula measures around two light years long from end to end, and contains a central white dwarf with an apparent magnitude of eleven. The inner layers of the nebula were formed some 1,500 years ago. The two ends of the nebula are marked by FLIERs, lobes of fasting moving gas often tinted red in false-color pictures. NGC 3242 can easily be observed with amateur telescopes, and appears bluish-green to most observers. Larger telescopes can distinguish the outer halo as well.

Supernova Remnants

  Supernova explosions drive much of the star out into space. The original expansion velocity is typically around 3,100 - 7,500 miles per second. The tremendous shock wave compresses the interstellar medium, causing the xrays from plasma heated to a million degrees Kelvin, radio waves, and sometimes a visible light emission nebula. The Veil Nebula in Cygnus, a supernova remnant whose main arcs are NGC 6960 and NGC 6992, has a very complex from the filamentary appearance. A telescope equipped with an 0III filter reveals much of this marvelous detail, making the Veil a showpiece, under the constellation Cygnus. Although the beautiful Vela Supernova Remnant looks similar to the Veil Nebula on images, Vela's offering is a much more challenging object visually.

Observing Synchrotron Radiation

  The Crab Nebula, M1 in Taurus, is the remnant from the brilliant supernova observed in 1054 CE. The broad S-shape seen visually is from synchrotron radiation, produced by electrons moving at relativistic speeds in the incredible magnetic field produced by the pulsar spinning at 30 times per second. The Pulsar is the core of the star that went supernova. This is the only place in the sky that an amateur can see light produced by this mechanism, since in other supernova remnants synchrotron radiation is mainly confined to radio wave links. The reddest Red of the star that are so dramatic on photographs, and who's rate of explosion can be measured on images taken decades apart, or unfortunately not visible at the eyepiece with normal amateur equipment.
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The Crab Nebula- NGC 1952, Supernova remnant in the constellation Taurus

  Detail of the entire Crab Nebula, a 6-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this event nearly 1000 years ago, as did Native Americans. The blue area on the image is the synchrotron radiation.

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