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These so-called Pillars of Creation are part of an active star-forming region within the nebula. In this image of the Carina Nebula, you can spot tiny yellow and white dots inside pink dust clouds. Those tiny dots are newly-formed stars! Nebulae are made of dust and gases—mostly hydrogen and helium. The dust and gases in a nebula are very spread out, but gravity can slowly begin to pull together clumps of dust and gas. As these clumps get bigger and bigger, their gravity gets stronger and stronger.
Eventually, the clump of dust and gas gets so big that it collapses from its own gravity. The collapse causes the material at the center of the cloud to heat up-and this hot core is the beginning of a star. Nebulae exist in the space between the stars—also known as interstellar space. The closest known nebula to Earth is called the Helix Nebula.
It is the remnant of a dying star—possibly one like the Sun. It is approximately light-years away from Earth. Ionized hydrogen regions are very widespread across the Milky Way, and can even be seen in some external galaxies such as NGC in the Pinwheel galaxy Messier However, not all of them are bright and obvious for the visual observer, and some respond better to photographic efforts than to mere viewing.
The subtle red colours of most nebulae are not seen visually as the eye does not Approaching Skyline - Outputmessage - Nebulae (CD) red easily in the dark, and most objects are fainter than expected.
Courtesy of Andreas Fink, commons. Once stars are fully formed, converting hydrogen to helium, and generally in equilibrium, we see them as clusters of stars. The bright blue light from such stars or clusters is enough to give sufficient scattering to make the dust visible, and as the light is of short wavelengths, the frequency spectrum of many reflection nebulae is similar to that of the illuminating stars.
Reflection nebulae are not particularly rare but their relatively low light profile in comparison to light-emitting nebulae, such as HII regions, make them a little harder to see, and the nebulae are generally well located close to the stars. This makes the starlight overwhelming on occasion and renders visual representation of such nebulae difficult. Examples such as the Pleiades, or many of the Messier numbers such as M36 — M38 in Auriga, show us how stars born in HII regions stay together for long periods before interaction with the galaxy thins them out into individuals.
It is when the stable relationship of hydrogen burning ends that stars begin the next process of nebulae formation—dying in a planetary nebulae or exploding as a supernova. This ending depends on the mass of the star. A star larger than 15 times the mass of our sun will become a supernova, whilst those under this limit will become planetary nebulae.
Most stars convert hydrogen to helium for millions or even billions of years, but once the hydrogen begins to run out, the star is doomed. Once there is sufficient helium build-up in the core to significantly interfere with the hydrogen reactions, the core shuts down and begins to contract.
However, there is a lot of latent energy in the overlying layers from radiation attempting to escape the outer envelope. As a consequence of this radiation pressure, the star will begin to expand, as gravity works primarily upon the greater mass of the core and only has a relatively weak effect on the outer layers.
At this point the core contracts, the outer layers expand, and the star begins to cool. As it does this, the star changes colour and cools to become an orange K type sub-giant. The star then utilizes the energy of hydrogen-helium conversion, which now takes place in a shell around the inert hydrogen core rather than throughout the core as in its previous incarnation. Over time the star will continue to expand and cool until it becomes an M type red giant.
It is now large, luminous, and has Approaching Skyline - Outputmessage - Nebulae (CD) extensive solar wind, which is driving the material of the outermost layers off the star. This expulsion of material is important in the development of a planetary nebula. Eventually, gravity compresses the helium core until sufficient pressures and temperatures build up inside to fuse helium to carbon.
Once a new source of energy has been established, the star has a short stay of execution. However, there is insufficient helium fuel to power the star, and as this fuel becomes exhausted, the outer layers expand again with latent energy from the radiation release and they are eventually pushed off the star, lost to space with an increase in the power of the stellar wind at this stage. Once the luminous outer envelope of the star is lost, the naked core is all that is left: a small, hot remnant with a fraction of the luminosity of the whole star, and the object dims appreciably and makes its way Approaching Skyline - Outputmessage - Nebulae (CD) ending its days as a white dwarf.
That is what planetary nebulae are—the thrusting away of the envelope and the exposing of the white dwarf remnant. Observing planetary nebulae is not particularly difficult, as there are several good examples for amateur study.
Courtesy of Mohamad Abbas, commons. Conversely, the death of a massive star is a relatively rare event. This is partly because such stars are rare in numbers within the galaxy. Nevertheless, there are enough of these rare but exciting objects to become worthy of study, and they generally give themselves away due to the expulsion of materials in shells or nebulous clouds. Massive stars which are ending their lives as red giants, such as Betelguese in Orion, are also appropriate candidates for spectacular explosions.
Massive stars continue the fusion process from hydrogen to helium, through carbon, oxygen, nitrogen neon, aluminium, silicon, and others right up to the iron stage. Once the silicon is turned to iron in the core, the last exothermic process that holds the star up against gravity is over.
To make iron fuse into the next generation of heavier elements it is necessary to inject energy in to the star, as the process is endothermic—it needs energy just to keep going. No energy is available at this stage, and so the core falters and is squeezed by the overlying layers, and the materials break down allowing a huge implosion of the core.
This implosion rebounds, and the outer layers falling in under gravity are met by an enormous shock wave which causes the formation of elements heavier than iron on the periodic table in a process known as explosive nucleosynthesis.
The resultant explosion of the star spreads its outer layers into space at a very rapid acceleration—up to 60,km a second—and the light from the explosion is so bright that it can outshine entire galaxies for Approaching Skyline - Outputmessage - Nebulae (CD) brief period. The expanding gasses may be lit by radiation for a few months and by the conversion of Ni 56 to Fe 56 but the light fades eventually to leave an expanding patch of gasses. The core at this stage either becomes a black hole, dependent on how much mass has been shed by the core over the last gasps of its lifetime, or a neutron star like the one in the Crab nebula.
The synchrotron radiation from the neutron star, which has now become a pulsar, is then responsible for the ionization of the expanding nebula. By their brief and ethereal nature, there are very few supernova remnants available for amateur study, Approaching Skyline - Outputmessage - Nebulae (CD). The best known examples are the Crab nebula in Taurus and the Veil nebula in Cygnus. This new Hubble image—one among the largest ever produced with the Earth-orbiting observatory—shows the most detailed view so far of the entire Crab Nebula ever made.
The Crab is arguably the single most interesting object, as well as one of the most studied, in all of astronomy. The Crab Nebula is one of the most intricately structured and highly dynamical objects ever observed. Nebulae come in all shapes and sizes and different brightness.
Some will be right at the edge of your observing ability whilst others will be well within reach. The application of photography by many amateur astronomers today render images which would challenge those from a major observatory only 50 years ago.
The life cycle from gas cloud to star back to gas cloud is a reminder that the universe is constantly recycling materials, and that the rubbish of yesteryear is actually the future of stars, planets, and possibly even life elsewhere.
Getting to know and appreciate their beauty and their stories gives us a fresh perspective on our place in the cosmos and an endless vista of wonderful objects to observe. Martin Griffiths is an enthusiastic science communicator, writer and professional astronomer.
He was also responsible for surveying the sky quality of the Brecon Beacons National Park in their successful bid to gain International Dark Sky Association Dark Sky Reserve status in and is a consultant to the Hay Tourism Board for their annual dark sky festivals.
Griffiths is the Director of the Brecon Beacons Observatory, a public and education resource, fitted with a 30cm telescope situated in the Dark Sky Reserve.
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