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stars we see in our night sky are contained within our own
Milky Way. A galaxy is a gravitationally bound system containing billions of stars along with interstellar gases and dust.
Because we are located within the Milky Way galaxy and have never been able to view it from the outside, it can be difficult to determine the exact appearance of our galaxy. By studying other galaxies and by observations of the material within our own galaxy, astronomers recognize the Milky Way as a spiral galaxy, a flattened disk with a central galactic bulge and spiral arms curving out from the centre. The entire galaxy rotates about the centre, with our Sun traveling at about 230 kilometres per second and completing one orbit every 200 million years. Surrounding the flattened disk is the galactic halo, a faint and roughly spherical region of old stars and star clusters. From the Earth, the Milky Way appears as a hazy, luminous band of light which stretches across the night sky. The hazy cloud we see from a dark site is our view looking out through the galactic disk. It is a collection of millions of stars along with glowing gases which are so far away and condensed that they appear as a luminous haze instead of individual points of light.
Our galactic centre cannot be seen in optical light due to heavy obscuration from foreground stars and interstellar matter; it has been estimated that we can only see a tenth of the distance to the centre. A large amount of energy can be detected at the centre of our galaxy, however, and this energy is believed to be generated by a super-massive black hole. A black hole is an object which has an incredible mass packed into a small volume, causing an extremely high density and immense gravitational pull that will not allow even light to escape. Black holes are bizarre objects which are very complex and difficult to understand. Initially thought to be far-fetched, black holes are now known to be an important part of the universe. They generate a strong gravitational field which warps space and time around them and bends rays of light.
Because they are dark, black holes are impossible to see visually, but can be detected by other methods. Gas particles in orbit around a black hole will accelerate to incredible speeds as they near it, causing the gases to heat to tremendous temperatures and emit X-rays. These X-rays help astronomers classify virtually invisible stars as black hole candidates. The material situated near galactic centres is observed to be moving at incredible velocities; in order to accelerate the material to such high speeds and then have enough gravity to prevent it from flying off into space, there must be a massive object in the centre of these galaxies. The volume occupied by this object is known to be relatively small, and with such a high mass, its density is such that the object must be a huge black hole. Because of this, it is believed that many, if not all, spiral galaxies like our own contain a super-massive black hole at their centre.
The Milky Way galaxy is an immense collection of material, and the size of our galaxy is difficult to imagine. One light year is the distance light travels in one year, travelling at a speed of over one billion kilometres per hour (light would travel around the Earths equator 7.5 times in one second). A light year equals nearly 10 trillion kilometres, an unimaginable distance on its own. The circular disk of our galaxy is even more incomprehensible, with a diameter of approximately 100,000 light years. The Sun is located about two thirds of the way out from the centre, and the thickness of the disk at our location is about one thousand light years. The furthest stars visible with the naked eye are only a couple of thousand light years away; the Milky Way galaxy is truly a huge collection of material.
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Our solar system and the stars visible in our sky comprise an extremely small portion of our galaxy. In a dark sky nearly 3000 stars are visible to the naked eye, but the Milky Way galaxy contains billions of stars. These stars often form in clusters, when a large interstellar cloud collapses and fragments into several smaller protostars. It is believed that many stars have planets in orbit around them, creating solar systems. Because stars are so far away, it is very difficult to detect their orbitting planets, although astronomers have identified a few hundred such stars. This is done by measuring slight perturbations, or variations in the stars apparent location as it is influenced by the planets gravity. Astronomers are able to detect such variations, due to the sheer size of the planet, and as a result, are able to infer the existence of other solar systems in our galaxy. Many of the planets discovered thus far are significantly larger than our largest planet, Jupiter.
In addition to the countless number of stars in the Milky Way, the interstellar medium in our galaxy contains gas and dust particles which can form beautiful nebulae. A nebula is a cloud of interstellar gas which can be lit by nearby stars to form a glowing gas. Through a telescope, nebulae are rather dim and usually appear green, but a camera with sensitive film and a long exposure time reveals much more detail and colour. There are many different types of nebulae existing in various shapes and sizes. Planetary nebulae are roughly circular in shape and are caused by the death of a low mass star which ejects its outer layers of gas. An emission nebula is a glowing cloud of hot interstellar matter which is energized by young, hot stars imbedded within the gas. A reflection nebula simply reflects the light of nearby stars and is more faint and less common than emission nebulae. Not all nebulae glow; dark nebulae absorb the light of background stars and create a dark patch seemingly void of stars. Dark nebulae are composed of the same type of gas as emission and reflection nebulae, but they lack the imbedded stars necessary to illuminate the gas and instead block the light from stars located behind it. While planetary nebulae are generally circular in appearance, other nebulae can be virtually any shape depending on the arrangement of the gases which are the source of the nebula.
While the distances involved in describing the Milky Way seem unbelievable, our galaxy is only a small fraction of the contents of the universe. There are billions of other galaxies which populate the known universe, both like and unlike our own. The largest galaxies are much larger than the Milky Way and contain trillions of stars, while the smallest can contain less than a million. The Milky Way has a couple of small satellite galaxies, but the nearest major galaxy, the Andromeda, is about 2.2 million light years away. The Andromeda Galaxy appears as a disk with a bright central bulge surrounded by spiral arms, much like the Milky Way. Throughout the first decades of the 20th Century, Edwin Hubble, a prominent astronomer and galaxy expert, observed several hundred different galaxies in an attempt to establish a galactic classification system based on general appearance. His work culminated in the creation of the Hubble Tuning Fork, which remains the commonly accepted classification system in use today. This system groups the billions of galaxies in the universe into one of four categories: spiral, barred-spiral, elliptical and irregular.
This is the diagram Edwin Hubble established (1925) as a scheme to assist with the classification of galaxies.
Spiral galaxies, like the Milky Way, have a bright, central galactic bulge surrounded by curved spiral arms of stars, gas and dust. The galactic bulge appears reddish because it is populated by old red stars, while the arms appear bluish because its light is dominated by young blue stars.
Elliptical galaxies have no apparent structure and no spiral arms. They appear as large spheres of uniform luminosity, composed almost entirely of old stars with virtually no gas or dust to promote new star growth. They vary between perfectly spherical to rather elongated and oval.
Barred spirals are much like a regular spiral except for the presence of an elongated bar of interstellar material extending through the central bulge, the spiral arms originating at the ends of the bar instead of from the centre. The spiral arms of both spiral and barred spiral galaxies vary between being tightly wrapped around the nucleus to being loose and widely spread.
Galaxies are situated at various orientations and angles as seen from the Earth, and may be difficult to identify depending on their orientation. Most galaxies in the universe are located within a galaxy cluster, a group of galaxies held together by their mutual gravitational attraction. Through a high powered telescope, a galaxy cluster can be an amazing sight of hundreds of galaxies concentrated in a small area of the sky. Large clusters, known as super clusters, span millions of light years and can contain a few thousand galaxies, each with about 100 billion stars. The Milky Way is part of a small cluster called the Local Group, and includes about 20 galaxies, most of which are elliptical or irregular. The space between galaxies within a cluster, and between clusters, is vast, cold, and virtually empty of any matter.
The formation of the universe is difficult, if not impossible, to understand completely because humans simply do not know what the conditions of the early universe were like. Because of this fact, the science of cosmology developed as a means of studying the origins of our universe. Although it is unlikely that cosmologists will ever fully comprehend the exact process from which our universe was born, there are numerous theories as to how it may have happened. The most widely accepted theory in the field of astronomy today is the Big Bang theory, first proposed in the 1920s. By observing physical properties of the universe, cosmologists speculate that time began about 10 to 15 billion years ago when all of the matter within the universe exploded from a singularity, a dense point with an infinitely small volume. The Big Bang theory is based upon three main supporting observations.
The first of these observations is that the universe appears to be expanding. By observing light from distant galaxies, it was discovered in the 1920s that this light is shifted towards the red end of the spectrum, implying that galaxies are receding from the Earth. The Big Bang theory states that this recession is not due to the movement of the galaxies through space, but instead is an expansion of space itself. Assuming that the universe is expanding as a whole and that it has been since the beginning of time, cosmologists extrapolate back in time to when the universe was a small point.
The second observation relates to the relative abundance of chemical elements within the universe. The Big Bang model predicts that the universe should be composed of approximately 75% hydrogen, 25% helium and small amounts of heavier elements. Although these predictions depend on the initial conditions of the early universe, which are nearly impossible to know accurately, the observable universe is nonetheless composed of about three-quarters hydrogen and one-quarter helium, along with small amounts of heavier elements.
The third observation concerns cosmic radiation. In 1948, a Russian astronomer named George Gamow speculated that the initial fireball of the Big Bang explosion should have left behind a uniformly distributed radiation which would fill the universe and cool as the universe expanded, and be visible in every direction of the sky. The Cosmic Background Radiation (CBR), as this radiation is called, was first detected in 1965 in the form of radio waves, and has a temperature of 2.7K. The discovery of this radiation swayed many astronomers to deem the Big Bang credible.
Every theory involves assumptions, and the Big Bang is no exception; however, despite being proposed in the 1920s, the model has survived the scrutiny of 80 years of technological advancements and competing theories, contributing to its credibility.
The Sun is but one of billions of stars located in the Milky Way galaxy. The stars visible in our night sky all belong to our galaxy, and it wasnt until the early 1900s that galaxies outside of our own were discovered. The Milky Way contains billions of stars along with the possibility of numerous solar systems in orbit around those stars. There are vast regions of space between the stars which are sparsely filled by gas and dust particles. In areas of denser gas, a nebula may be visible either as an illuminated region or as a dark patch hiding background stars. It is thought that our galaxy, along with almost all similar galaxies, contains a super-massive black hole at its centre. The Milky Way is one of billions of galaxies in the universe, which can be grouped into four main categories by means of the Hubble Tuning Fork: spiral, barred spiral, elliptical and irregular. Galaxies are associated with galaxy clusters, collections of numerous galaxies held in proximity due to their gravity. Galaxies contain billions of stars and a large amount of interstellar matter, but the vast space between galaxies is cold, dark, and void of any particles.