It is estimated to contain at least 100 billion stars, and possibly up to 400 billion stars. The figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 light years from our sun. Therefore, present estimates of the total number are uncertain. This can be compared to the one trillion (10) stars of the neighbouring Andromeda Galaxy.
The stellar disc of the Milky Way does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the Galaxy. Beyond a radius of about 40,000 light years, the number of stars drops much faster, for reasons that are not understood.
Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12000 light years–twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter (about an inch or 25 mm in diameter), the Milky Way would have a diameter of 2,000 kilometers. At 220 kilometers per second it takes the Solar System about 240 million years to complete one orbit of the Galaxy (a galactic year).
The Galactic halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach is at about 180,000 light years. At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.
The galactic disc, which bulges outward at the galactic center, has a diameter of 170–200,000 light years.
The exact distance from the Sun to the galactic center is debated. The latest estimates give distances to the Galactic center of 25–28,000 light years.
Movement of material around the galactic center shows that it has a compact object of very large mass. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is now confirmed to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.
The nature of the galaxy's bar is also actively debated, with estimates for its half-length and orientation spanning from 3,300–16,000 light years (short or a long bar) and 10–50 degrees. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.
In Greek mythology, Zeus places his son (the baby Heracles) whose mother was a mortal woman on Hera's breast while she is sleeping so that the baby will drink her divine milk and become immortal. However, Hera wakes up while she is breastfeeding the baby and realizes she is nursing a baby she does not know. According to Greek mythology, she then pushes the baby away and a stream of her milk sprays the night sky, making a faint band of light known as the Milky Way.
Movement
Movement, or motion, is the state of changing something's position—that is, changing where something is. A flying bird or a walking person are moving, because they change where they are from one place to another. There are many kinds of science and math related to movement.
Thanks to the work of scientists including Galileo Galilei and Issac Newton, we know that "position" and motion are relative. This means that an object's position depends on where it exists in relation to other objects. For example, a ball can be 5 feet away from a box, 3 feet away from a chair, and a foot away from a table. Here, the box, chair and the table helped me to decide the position of the ball. In other words, they acted as the reference points for my observation. By telling you how far the ball was from other objects, I told you its relative position.
Similarly, an object's "motion" is also relative. It depends on how its position changes in relation to other objects. Let us understand this with an example:
Suppose you are sitting inside a train. The train has not started moving yet. Let us call this Train A. You look out from the window and see another train moving in the opposite direction. Let us call this Train B. Now, when you look at Train B, it appears as if Train A is moving in the forward direction. But when you look at a nearby electric pole, you notice that train A was actually at rest and Train B was moving backwards.
From this, we conclude that we cannot decide whether a body is moving or at rest, unless we choose a frame of reference. In the above example, when we chose Train B as our frame of reference, Train A appeared to be moving forward. On the other hand, when we chose the electric pole as frame of reference, Train A appeared to be at rest and Train B appeared to move backward.
The study of motion, without considering its cause is called kinematics. In kinematics, we come across terms like speed, velocity, and acceleration. Dynamics is the branch of physics concerned with the study of causes and effects of motion. In dynamics we come across terms like force, inertia, work, energy and momentum.
Metaphor
Metaphor is a term for a figure of speech. It does not use a word in its basic literal sense. Instead, it uses a word in a kind of comparison. We run, and we also say rivers run. We may run into trouble, especially if we run up a bill at the bar.
So a metaphor uses words to make a picture in our mind. It takes a word from its original context, and uses it in another.
Metaphors are an essential part of language: it is not possible to speak or write without them. A simple example is the word "run". This has a basic meaning of "moving quickly" or "go with quick steps on alternate feet, never having both feet on the ground at the same time". The "Concise Oxford Dictionary" then gives
34 other uses as a verb; 21 uses as a noun; about 50 uses in short phrases. All of these are metaphors, although we do not usually notice this.
We use metaphors to make "indirect" comparisons, but without using 'like' or 'as' – because that would be a simile. A simile is a "direct" comparison: "Jane is like a child".
A metaphor very often uses the verb 'to be': "love "is" war", for example, not "love "is like" war" (that is a simile).
Poetry includes much metaphor, usually more than prose.
Spam is an example that any email user knows about – this word was originally a metaphor, from 'Spam', a type of canned meat. Servers putting unwanted email into somebody's inbox was similar to waiters putting unwanted Spam into food. This was originally suggested by a Monty Python scene. When we use a metaphor very often and we forget the old meaning, or forget that the two meanings are connected, this is a 'dead metaphor'.
Originally "metaphor" was a Greek word for 'transfer'. It came from "meta" ('beyond') and "pherein" ('carry'). So the word "metaphor" in English was a metaphor, too. Today in Greek, "metaphor" is a trolley (a thing that is pushed for carrying shopping or bags).
A simple metaphor has a single link between the subject and the metaphoric vehicle. The vehicle thus has a single meaning which is transferred directly to the subject.
In the simple metaphor, the effort to understand what the author or speaker intends is relatively low, and hence it may easily be used with a wider and less sophisticated audience.
A complex metaphor happens where a simple metaphor is based on a secondary metaphoric element. For example, using a metaphor of 'light' for 'understanding' may be complexified by saying 'throwing light' rather than 'shining light'. 'Throwing' is an extra metaphor for how light arrives.
A compound metaphor is one where there are multiple parts in the metaphor that are used to snag the listener. These parts may be enhancement words such as adverbs, adjectives, etc.
Each part in the compound metaphor may be used to signify an additional item of meaning.