NEBULA

                                     NEBULA

Around 150 AD, Claudius Ptolemaeus (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/Muslim 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.

On 26 November 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 wouldn't be performed until 1659 by Christiaan Huygens, who also believed him to be the first person to discover this nebulosity.

In 1715, Edmund 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 1751–53, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, with most of them being 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, wasting time.

The number of nebulae was then greatly expanded 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 starlight.

In about 1922, 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 diffuse nebulae, finding 29 that showed emission spectra and 33 had the continuous spectra of star light. In 1922, Hubble announced that nearly all nebulae 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 B1 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.

Most nebulae can be described as diffuse nebulae, which mean that they are extended and contain no well-defined boundaries. In visible light these nebulae may be divided into emission and reflection nebulae. Emission nebulae emit spectral radiation from ionized gas (mostly ionized hydrogen) they are often called HII regions (the term "HII" is used in professional astronomy to refer to ionized hydrogen).

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called "dark nebulae"

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.

TYPES OF NEBULAE

·         Ant Nebula

·         Barnard's Loop

·         Boomerang Nebula

·         Cat's Eye Nebula

·         Crab Nebula

·         Eagle Nebula

·         Eskimo Nebula

·         Eta Carinae Nebula

·         Fox Fur Nebula

·         Helix Nebula

·         Hourglass Nebula

·         Horsehead Nebula

·         Lagoon Nebula

·         Orion Nebula

·         Pelican Nebula

·         Red Square Nebula

·         Ring Nebula

·         Rosette Nebula

·         Tarantula Nebula

SOLAR SYSTEM

                                  SOLAR SYSTEM

Our solar system is made up of the sun and everything that travels around it. This includes eight planets and their natural satellites such as Earth's moon; dwarf planets such as Pluto and Ceres; asteroids; comets and meteoroids.

1. The sun is the center of our solar system.
2.  It contains almost all of the mass in our solar system and exerts a tremendous gravitational pull on planets and other bodies.
3. Our solar system formed about 4.6 billion years ago.
4. The four planets closest to the sun -- Mercury, Venus, Earth, and Mars -- are called the terrestrial planets because they have solid, rocky surfaces.
5. Two of the outer planets beyond the orbit of Mars -- Jupiter and Saturn -- are known as gas giants; the more distant Uranus and Neptune are called ice giants.
6. Most of the known dwarf planets exist in an icy zone beyond Neptune called the Kuiper Belt, which is also the point of origin for many comets.
7. Many objects in our solar system have atmospheres, including planets, some dwarf planets and even a couple moons.
8. Our solar system is located in the Orion Arm of the Milky Way Galaxy. There are most likely billions of other solar systems in our galaxy. And there are billions of galaxies in the Universe.
9. We measure distances in our solar system by Astronomical Units (AU). One AU is equal to the distance between the sun and the Earth, which is about 150 million km (93 million miles).

HISTORY

People believed Earth to be stationary at the center of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus had speculated on a heliocentric reordering of the cosmos, Nicolas Copernicus was the first to develop a mathematically predictive heliocentric system. In the 17th-century, Galileo Galileo, Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth. The invention of the telescope led to the discovery of further planets and moons. Improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena, such as mountains, craters, seasonal meteorological phenomena, such as clouds, dust storms and ice caps on the other planets.

Formation

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars. As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula, collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre. The planets formed by accretion from this disc, in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies.

MILKY WAY GALAXY

MILKY WAY GALAXY

The Milky Way Galaxy is an immense and very interesting place. Not only does it measure some 100,000–120,000 light-years in diameter, it is home to planet Earth, the birthplace of humanity. Our Solar System resides roughly 27,000 light-years away from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust particles called the Orion Arm.

But within these facts about the Milky Way lie some additional tidbits of information, all of which are sure to impress and inspire. Here are ten such facts, listed in no particular order.

FIVE FACTS

·         The Milky Way is a disk about 120,000 light years across with a central bulge that has a diameter of 12,000 light years (see the Guide to Space article for more information). The disk is far from perfectly flat though, as can be seen in the picture below. In fact, it is warped in shape, a fact which astronomers attribute to the galaxy’s two neighbours -the Large and Small Magellanic clouds.

·         Scientists believe that 90% of our galaxy’s mass consists of dark matter, which gives it a mysterious halo. That means that all of the “luminous matter” – i.e. that which we can see with the naked eye or a telescopes – makes up less than 10% of the mass of the Milky Way. Its halo is not the conventional glowing sort we tend to think of when picturing angels or observing comets.

·         As galaxies go, the Milky Way is a middleweight. The largest galaxy we know of, which is designated IC 1101, has over 100 trillion stars, and other large galaxies can have as many as a trillion. Dwarf galaxies such as the aforementioned Large Magellanic Cloud have about 10 billion stars. The Milky Way has between 100-400 billion stars; but when you look up into the night sky, the most you can see from any one point on the globe is about 2,500. This number is not fixed, however, because the Milky Way is constantly losing stars through supernovae, and producing new ones all the time (about seven per year).

·         Though it may not look like it to the casual observer, the Milky Way is full of dust and gas. This matter makes up a whopping 10-15% of the luminous/visible matter in our galaxy, with the remainder being the stars. Our galaxy is roughly 100,000 light years across, and we can only see about 6,000 light years into the disk in the visible spectrum. Still, when light pollution is not significant, the dusty ring of the Milky Way can be discerned in the night sky.

·         The thickness of the dust deflects visible light (as is explained here) but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space Telescope extremely valuable tools in mapping and studying the galaxy. Spitzer can peer through the dust to give us extraordinarily clear views of what is going on at the heart of the galaxy and in star-forming regions.

·         The Milky Way wasn’t always as it is today – a beautiful, warped spiral. It became its current size and shape by eating up other galaxies, and is still doing so today. In fact, the Cans Major Dwarf Galaxy is the closest galaxy to the Milky Way because its stars are currently being added to the Milky Way’s disk. And our galaxy has consumed others in its long history, such as the Sagittarius Dwarf Galaxy.

·         Currently, we can’t take a picture of the Milky Way from above. This is due to the fact that we are inside the galactic disk, about 26,000 light years from the galactic centre. It would be like trying to take a picture of your own house from the inside. This means that any of the beautiful pictures you’ve ever seen of a spiral galaxy that is supposedly the Milky Way is either a picture of another spiral galaxy, or the rendering of a talented artist.

·         Imaging the Milky Way from above is a long, long way off. However, this doesn’t mean that we can’t take breathtaking images of the Milky Way from our vantage point!

·         Most larger galaxies have a persuasiveness black hole (SMBH) at the centre, and the Milky Way is no exception. The centre of our galaxy is called Sagittarius A*, a massive source of radio waves that is believed to be a black hole that measures 22,500 kilo meters (14 million miles) across – about the size of Mercury’s orbit. But this is just the black hole itself.

·          

·         All of the mass trying to get into the black hole – called the accretion disk – forms a disk that has 4.6 million times the mass of our Sun and would fit inside the orbit of the Earth. Though like other black holes, Sgr A* tries to consume anything that happens to be nearby, star formation has been detected near this behemoth astronomical phenomenon.

·         The most recent estimates place the age of the Universe at about 13.7 billion years. Our Milky Way has been around for about 13.6 billion of those years, give or take another 800 million. The oldest stars in our the Milky Way are found in globular clusters, and the age of our galaxy is determined by measuring the age of these stars, and then extrapolating the age of what preceded them.

·         Though some of the constituents of the Milky Way have been around for a long time, the disk and bulge thems elves didn’t form until about 10-12 billion years ago. And that bulge may have formed earlier than the rest of the galaxy.