Virgo ( left )
Birth Of Stars
When stars are born, they form from existing gas and dust. This is called interstellar matter. When a cloud of interstellar matter crosses the spiral arm of a galaxy, it begins to form clumps. The gravitational forces within the clumps cause them to contract, forming protostar. The center of a protostar may reach a temperature of several millions of degrees Celsius. At this high temperature, a fusion reaction begins. The energy released by this reaction prevents the protostar to contract. Thus, a star has been formed.
A star is a hot object that emits light.
Why Do Stars Shine?
The stars sparkle because they are hot. But, where does a star get the energy that makes it hot? Albert Einstein discovered that mass could be transformed into a large quantity of energy. The stars are make up mainly of Hydrogen, which is the simplest element in the universe. A Hydrogen atom consists of a nucleus of proton and electron. The gravitational forces within the star compress the matter at the center of the star. This causes the protons that form the nuclei of Hydrogen atom to collide. Thus, four Hydrogen nuclei come together to form a nucleus of helium. The helium nucleus is made up of two protons and neutrons. During fusion, two protons are transformed into neutrons .The mass of helium nucleus is less than the total mass of the four protons that made it. The lost mass has becomes the energy and thus, stars twinkle in our night sky.
Colours Of Stars
The light that the star put out varies in colours from one star to another. The colour differences tell us how hot the star is. The stars have a whole range of colours, from Red to Orange, Yellow, White, and Blue White. The material the star is made of is so hot that it takes the form of gas, mostly Hydrogen. Star are huge balls of gases and gases are hot because of their high temperatures it emits light. The colour of the light tells us that the temperatures of the star’s surface.
Why Do Stars Die?
Many stars’ core is made up of very hot and compressed helium. When the temperatures reach up to 200 million Degrees Celsius. The helium nuclei began to react. The new nuclear reactions form heavier elements such as a Carbon, Nitrogen and oxygen. They produce energy that stops the star contracting. The star’s wrapping becomes so swollen that the star begins to lose its outer layer. At this stage, the star is known as a planetary nebula. In the planetary nebula, it has a blue-white star. This is the old core of a compressed and hot star that becomes exposed and lost its outer layers. When Hydrogen is consumed, the star’s core can no longer handle the weight and contracts. At this point, the temperature of the core rises, causing nuclei reactions in the layer surrounding the core and expanding the layers. Such stars are called White Dwarfs and made up of ice, Carbon and oxygen. They will start to cool over a period of thousands of millions years and becoming darker. Thus, they end their lives.
MILKY WAY
The Milky Way is the galaxy which homes our Solar System together with at least 200 billion other stars and their planets, and thousands of clusters and nebulae including at least almost all objects of Messier's catalog which are not galaxies on their own. All the objects in the Milky Way Galaxy orbit their common center of mass, called the Galactic Center (see below).
As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Radio astronomial investigations of the distribution of hydrogen clouds have revealed that the Milky Way is a spiral galaxy of Hubble type . It is still not clear if it has a bar structure or not.
The Milky Way Galaxy belongs to the local group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The closest of all is above-mentioned SagDEG at about 80,000 light years from us and some 50,000 light years from the Galactic Center, followed by the more conspicuous Large and Small Magellanic Cloud at 179,000 and 210,000 light years, respectively.
Similar to other galaxies, there occur supernovae in the Milky Way, at irregular intervals of time. If they are not too heavily obscured by interstellar matter, they can be, and have been seen as spectacular events from Earth. Unfortunately, none has yet appeared since the invention of the telescope (the last well observed supernova was studied by Johannes Kepler in 1604).
As we are situated within the outer regions of this galaxy, only about 20 light years above the equatorial symmetry plane but about 28,000 light years from the Galactic Center, the Milky Way shows up as luminous band spanning all around the sky along this symmetry plane, which is also called the "Galactic Equator". Its center lies in the direction of the constellation Sagittarius but very close to the border of both neighbor constellations Scorpius and Ophiuchus The distance of 28,000 light years has recently been confirmed by the data of ESA's astrometric satellite Hipparcos.
Milky Way pictures are wide-field exposures. Besides being attractive and often colorful, they are often suited to view the Milky Way objects (including nebulae and star clusters) in their celestial surroundings of field stars.
The band of light that we call the Milky Way is actually the plane of the disk of our galaxy. The Sun is one, rather faint, example of approximately 200,000,000,000 stars that make up our galaxy. These stars are mostly grouped into a flattened disk which has a bulge at its centre. The Sun is in this disk about two thirds of the way from its centre to its edge. When we look at the night sky we see the Milky Way when we look along the plane of this disk whereas when we look in other directions, out of the plane, we see far fewer stars.
The galaxies come in a variety of shapes , with the distribution of shapes depending in a way not yet completely understood on the evolution of galaxies. Our galaxy has arms of younger stars and gas that appear to spiral out from the centre. In fact the objects in these spiral arms are in almost circular orbits about the centre of the Galaxy. The Sun takes about 200 million years to complete one orbit around the centre. About 30 percent of all galaxies have spiral arms. Some have arms that spiral directly from the nucleus while others have a linear feature, called a bar, from whose ends the arms originate. One of the exa,ples of the galaxy which has a spiral orbits is M100 in the Virgo cluster. Unlike visible light, which is strongly absorbed by the intervening gas and dust, the infrared light is transmitted well enough to give us a clear view of the central region. Thus, we see the side of the bulge surrounding the galactic core from our position in the disk of the galaxy about 30,000 light years from the center. The centre of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick.
The majority of galaxies show no spiral features, nor are they flattened disks; they take the form of ellipsoids. The adjacent image shows an example, the giant elliptical galaxy in the center of the Virgo Cluster ( see below )Elliptical galaxies show only small evidence for young stars, dust or gas. They are very different in size ranging from giant ellipticals with masses of about 1 million million times that of the Sun to dwarf ellipticals with masses closer to those of the globular clusters.
There are many clusters of galaxies. Members of some of the closest can be seen with a small telescope in the constellations Virgo and Coma Berenices.
Virgo ( left )
Giant elliptical galaxy in the center of the Virgo Cluster.
Galactic
centre
We can trace clusters of galaxies out to the furthest distances that we can reach. Some of these clusters contain thousands of galaxies. Near their centres giant ellipticals are often found and it is thought that these arise from the collision of several galaxies which have combined.
Gifs/darkmatter.rosat.gif (below)
( A nice spiral, M100)
