NEBULA

A nebula is a formation in space which is constituted mostly of helium, dust, and other gases in various concentrations. The shape and size of a nebula varies, and as such there can be various types of formations having different sizes. Very often, they are huge in size, and their diameters can be a number of light years across. It is derived from Latin, and as such means a cloud.It is not clear how exactly a nebula is formed, but it is believed that they are formed by the collapsing of interstellar medium, which then come together because of the gravitational pull of the particles. Nebula is an important object for observation by researchers, who derive significant information about the formation process of stars and planets. A nebula is capable of exerting a gravitational pull, and the force is derived from the particles which come together to form the nebula. With more particles coming together, not only does the nebula increase in proportion, but its gravitational pull also gathers more power and intensity. A nebula is also believed to be one of the primary stages in the formation of stars. Scientists are of the opinion that a nebula can be used to create a trajectory of stellar evolution. There are various nebulae that are in existence at present, though numerous others may exist in the far-flung corners of space that remain to be observed. Some of the most popular ones are Pelican, Crab, Eagle, and Ring Nebula, with Ring being among the most observed ones on the planet. Nebulae are regions where new stars are beginning to form.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 700 light-years away from Earth. That means even if you could travel at the speed of light, it would still take you 700 years to get there!

• How do we know what nebulae look like? 

Astronomers use very powerful telescopes to take pictures of faraway nebulae. Space telescopes such as NASA's Spitzer Space Telescope and the Hubble Space Telescope have captured many images of faraway nebulae

• By the way, what is a supernova explosion? 

A supernova is the biggest explosion that humans have ever seen. Each blast is the extremely bright, super-powerful explosion of a star.

One type of supernova is caused by the “last hurrah” of a dying massive star. This happens when a star at least five times the mass of our sun goes out with a fantastic bang!Massive stars burn huge amounts of nuclear fuel at their cores, or centers. This produces tons of energy, so the center gets very hot. Heat generates pressure, and the pressure created by a star’s nuclear burning also keeps that star from collapsing.A star is in balance between two opposite forces. The star’s gravity tries to squeeze the star into the smallest, tightest ball possible. But the nuclear fuel burning in the star’s core creates strong outward pressure. This outward push resists the inward squeeze of gravity.When a massive star runs out of fuel, it cools off. This causes the pressure to drop. Gravity wins out, and the star suddenly collapses. Imagine something one million times the mass of Earth collapsing in 15 seconds! The collapse happens so quickly that it creates enormous shock waves that cause the outer part of the star to explode!Usually a very dense core is left behind, along with an expanding cloud of hot gas called a nebula. A supernova of a star more than about 10 times the size of our sun may leave behind the densest objects in the universe—black holes.

A second type of supernova can happen in systems where two stars orbit one another and at least one of those stars is an Earth-sized white dwarf. A white dwarf is what's left after a star the size of our sun has run out of fuel. If one white dwarf collides with another or pulls too much matter from its nearby star, the white dwarf can explode. Kaboom!

Observational history:

Around 150 AD, Ptolemy recorded in books VII–VIII of his Almagest, five stars that appeared nebulous. He also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star. The first true nebula, as distinct from a star cluster, was mentioned by the Persian astronomer Abd al-Rahman al-Sufi, in his Book of Fixed Stars (964). He noted "a little cloud" where the Andromeda Galaxy is located. He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster. The supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054.

In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659, by Christiaan Huygens, who also believed he was the first person to discover this nebulosity.

In 1715, Edmond Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751 to 1753, Nicolas-Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 "nebulae" (now called Messier objects, which included what are now known to be galaxies) by 1781; his interest was detecting comets, and these were objects that might be mistaken for them.

The number of nebulae was then greatly increased by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786. A second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster.

Beginning in 1864, William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had the emission spectrum of a gas. The rest showed a continuous spectrum and thus were thought to consist of a mass of stars. A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster. Thus the nebula radiates by reflected star light.

About 1923, following the Great Debate, it had become clear that many "nebulae" were in fact galaxies far from our own.

Slipher and Edwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light. In 1932, Hubble announced that nearly all nebula are associated with stars, and their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter (including all O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars. Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner.

• What are the 4 types of nebulae?

Types of Nebulae

1. Galaxies.
2. Globular Clusters.
3. Open Clusters.
4. Emission Nebulae.
5. Reflection Nebulae.
6. Dark Nebulae.
7. Planetary Nebulae.
8. Supernova Remnants

• Some names of nebulae :

1. Notable named nebulae
2. Ant Nebula.
3. Barnard's Loop.
4. Boomerang Nebula.
5. Cat's Eye Nebula.
6. Crab Nebula.
7. Eagle Nebula.
8. Eskimo Nebula.
9. Carina Nebula.

• How many nebulae are there?

Right now, we know of about 3,000 planetary nebulae in our galaxy, the Milky Way, out of roughly 200 billion stars. Most of them are near the center of our galaxy. They come in many shapes, but most are spherical, elliptical, or bipolar.

• Some pictures of the nebulae: