Galaxy is any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars. Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants.
Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed and subsequently evolved. Galaxies are extremely varied not only in structure but also in the amount of activity observed. Some are the sites of vigorous star formation, with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars.
The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. Our concern would be Milky Way galaxy in which our solar system exists. The Milky Way Galaxy is a barred spiral galaxy composed of at least 100 billion stars. It is approximately 100,000 light years across and about 1000 light years thick. It has a central bulge that is about 10,000 light years in diameter. The interesting and important facts about our galaxy are listed here.
10) Its Origin Relates to its Name
The Milky Way Galaxy is just one of hundreds of billions of galaxies that populate our universe. The name Milky Way refers to the milky patch of sky which rings the Earth. You may have seen it if you live outside of a big city, it looks almost like very tenuous clouds to the naked eye. It was named in prehistory before anyone really knew what it was, so just called the "Milky Way" for its appearance. An interesting fact related to this is that the word galaxy actually comes from the Greek word, galactos for milk! The Latin version of Milky Way is Via Lactea, with "Via" meaning "Way" or "Road" and "Lactea" meaning "Milk". If you go to a dark sky location, the plane of the Milky Way appears as a swath of light across the night sky. There is a Greek myth for the origin of the Milky Way. That is that the baby Heracles is brought by Zeus to Hera to drink her milk so he could become immortal and the Milky Way is nothing more than spilt milk.
9) It was Formed Shortly After the Formation of Universe
With estimates of the age of the universe centered around 14 or so billion years old, estimates for the age of the Milky Way galaxy range from 800 million to 13.5 billion years old. But how did the galaxy get here? First off, there weren’t always stars in the Universe, and the Milky Way hasn’t been around forever. After the big bang happened, and the Universe cooled for a bit, all there was, was gas uniformly spread through-out. Small irregularities allowed the gas to coalesce into larger and larger enough clumps, heating up and eventually starting the nuclear fusion that powers stars. The stars started to gravitationally attract each other into larger groups. The oldest of these groups of stars are called globular clusters, and some of these clusters in the Milky Way galaxy date back to the very, very early Universe.
Not all of the stars in the Milky Way date back to the primordial Universe, though. Milky Way produces more than 7 stars per year, but it acquired much of its mass in another fashion. The Milky Way is often referred to as a "cannibal" galaxy, because during formation it swallowed up smaller galaxies. Astronomers think that this is how many larger galaxies have come to be the size they are today. In fact, the Milky Way is currently gobbling up another galaxy (and a stellar cluster) at this very moment. Called the Canis Major Dwarf Galaxy, the remnant stars are 45,000 light years from the galactic center, and a mere 25,000 light years from our Sun.
8) It Wobbles Squishes and Moves Continuously
Astronomers have discovered that besides being permanently in motion, our Milky Way galaxy also makes small wobbling or squishing movements. Being a barred spiral galaxy, Milky Way rotates around the Galactic centre. In addition to the regular Galactic rotation, scientists found the Milky Way moving perpendicular to the Galactic plane.The galaxy makes small wobbling or squishing movements. It acts like a huge flag fluttering in the wind, north to south, from the Galactic plane with forces coming from multiple directions, creating a chaotic wave pattern. However, the source of the forces is still not understood. Possible causes include spiral arms stirring things up or ripples caused by the passage of a smaller galaxy through our own.
The spin of our galaxy has a twisting effect on our local space that is a million times stronger than that caused by the spin of the Earth.
The Milky Way is part of a group of galaxies known as the Local Group. All of these are moving relative to each other due to their gravitational interaction with speeds of around 100 km/s or less. Calculating the velocities of the galaxies in the Local Group is difficult because there are probably members that have not yet been discovered because they are too dim or are obscured by the plane of the Milky Way. The radial velocities relative to the Milky Way are found by measuring Doppler shifts in the spectra of stars in the galaxies.
7) The Inter-stellar Medium
The region between the stars in a galaxy like the Milky Way is far from empty. These regions have very low densities (they constitute a vacuum far better that which can be produced artificially on the surface of the Earth). But are filled with gas, dust, magnetic fields, and charged particles. This is commonly termed the interstellar medium. Approximately 99% of the mass of the interstellar medium is in the form of gas with the remainder primarily in dust. The total mass of the gas and dust in the interstellar medium is about 15% of the total mass of visible matter in the Milky Way.
Interstellar dust grains are typically a fraction of a micron, irregularly shaped, and composed of carbon and/or silicates. Absorption of light by dust causes large dark regions in our galaxy and in other galaxies. These dust clouds are visible if they absorb the light coming through them. We then refer to these clouds as dark nebulae. On the other hand, light can reflect from clouds of dust and gas, giving rise to sometimes beautiful reflection nebulae.
Dust has two major effects on light passing through it:
1) The light is dimmed by the dust; this is called interstellar extinction.
2) The light that does pass through the dust is depleted in blue wavelengths because the size of the dust grains favours scattering blue light. This is called interstellar reddening, because the resultant transmitted light is redder than it would have been otherwise. This implies that transmitted light will be redder, but reflected light will be bluer. On Earth, the blueness of the sky is due to similar effects in scattering of light from molecules in the atmosphere.
6) It is Disk Shaped Spiral Galaxy
Edwin Hubble studied galaxies and classified them into various types of elliptical and spiral galaxies. The spiral galaxies were characterized by disk shapes with spiral arms. It stood to reason that because the Milky Way was disk-shaped and because spiral galaxies were disk-shaped, the Milky Way was probably a spiral galaxy.
If we look at the structure of the Milky Way as it would appear from the outside, we can see the the Galactic disk. This is where most of the Milky Way's stars are located. The disk is made of old and young stars, as well as vast amounts of gas and dust. Stars within the disk orbit the galactic center in roughly circular orbits. (Gravitational interactions between the stars cause the circular motions to have some up-and-down motion, like horses on a merry-go-round). The disk itself is broken up into these parts: Nucleus,The centre of the disk; Bulge,This is the area around the nucleus, including the immediate areas above and below the plane of the disk and Spiral arms, These areas extend outward from the centre. Our solar system is located in one of the spiral arms of the Milky Way.
The Milky Way's gravity acts on two smaller satellite galaxies called the Large and Small Magellanic Clouds. They orbit below the plane of the Milky Way and are visible in the Southern Hemisphere. The Large Magellanic Cloud is about 70,000 light years in diameter and 160,000 light years away from the Milky Way. Astronomers think that the Milky Way is actually siphoning off gas and dust from these satellite galaxies as they orbit.
5) Andromeda Galaxy is fast approaching us
Colliding galaxies are becoming more frequent in the Universe. The recent evidence from space points to the nature of some galaxies to expand at the cost of other galaxies, that is, the massive galaxies are exerting a gravitational force on the smaller ones and thus drawing them in making a larger, composite galaxy. Galaxies are not in a balanced state but are under strain from forces exerted by other galaxies.
As the galaxies formed were of varying sizes and the bigger ones started to exert force on the smaller ones, the bigger ones grew in size by eating the smaller ones. In the past, this was all speculation. But, advances research has made it possible to detect the merging of galaxies. Merging is a complex process which takes millions of years to complete. When a small galaxy starts to orbit around a larger one, the gravitational force of the latter will distort the former. They will orbit each other many times before the eventual collision and subsequent merging.
Currently, Andromeda is at a distance of 2.3 million light years from the Milky Way and it will take millions of years for this merger. Even though scientists have estimated the speed with which these galaxies are approaching as 120km/sec, they are unable to measure accurately the sideways speed of the Andromeda Galaxy. If the sideways speed is more than what they have calculated, then there is a chance that this collision will never happen.
4) It has Three Main Components
The three main components are globular clusters, open clusters and stellar associations. The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains more than 150 globular clusters (the exact number is uncertain because of obscuration by dust in the Milky Way band, which probably prevents some globular clusters from being seen). They are arranged in a nearly spherical halo around the Milky Way, with relatively few toward the galactic plane but a heavy concentration toward the centre. Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 Suns. The most luminous are 50 times brighter. The masses of globular clusters range from a few thousand to more than 1,000,000 solar masses. The clusters are very large, with diameters measuring from 10 to as much as 300 light-years.
Clusters smaller and less massive than the globular clusters are found in the plane of the Galaxy intermixed with the majority of the system’s stars, including the Sun. These objects are the open clusters, called so because they generally have a more open, loose appearance than typical globular clusters. The largest percentage of known open clusters has a brightness equivalent to 500 solar luminosities. Most open clusters have small masses on the order of 50 solar masses. Open clusters have diameters of only 2 or 3 to about 20 light-years, with the majority being less than 5 light-years across. The most important structural difference from the globular one’s is their small total mass and relative looseness, which result from their comparatively large core radii. These two features have disastrous consequences as far as their ultimate fate is concerned, because open clusters are not sufficiently gravitationally bound to be able to withstand the disruptive tidal effects in the Galaxy.
Even younger than open clusters, stellar associations are very loose groupings of young stars that share a common place and time of origin but that are not generally tied closely enough together gravitationally to form a stable cluster. Stellar associations are limited strictly to the plane of the Galaxy and appear only in regions of the system where star formation is occurring, notably in the spiral arms. The brightest are even brighter than the brightest globular clusters. They have absolute luminosities as bright as any star in the Galaxy—on the order of one million times the luminosity of the Sun. Such stars have very short lifetimes, only lasting a few million years. The sizes of stellar associations are large; the average diameter of those in the Galaxy is about 250 light-years. They are so large and loosely structured that their self-gravitation is insufficient to hold them together, and in a matter of a few million years the members disperse into surrounding space, becoming separate and unconnected stars in the galactic field.
3) Beyond the boundaries of Milky Way- The Galactic Halo
A galactic halo is nothing more than a spherical distribution of matter surrounding a galaxy. For a spiral galaxy such as the Milky Way, the matter of this region may include very old stars (Population II), globular clusters, ionized gas and dark matter, and it is clearly separated from the main galactic disc .The main disc of our galaxy rotates at a speed of 500,000 mph and it is surrounded by the inner halo rotating at 50,000 mph in the same direction. The outer halo rotates in the opposite direction at about 100,000 mph. These regions mainly consist of (a) Population II stars and globular clusters, (b) ionized gas and (c) dark matter.
a) Population II stars are older and with lower metallicity compared to the younger Population I stars that belong to the main galactic disc. Globular clusters, on the other hand, are spherical concentrations of very old stars found only in the halo of our galaxy. Since they contain no gas, it is not possible to create new stars within the globular clusters.
b) Another component of the Milky Way's halo is the hot ionized gas that originates from supernova remnants being expelled through galactic chimneys. These are pipes or vents of hot gas that transport this matter away from the galactic disc. After the gas is cooled, it is pulled back into the disc by gravitational forces.
c) The outer part occupies a larger region and mostly consists of dark matter. The presence of dark matter can only be detected through its gravitational effect on the motion of stars.
2) Scientists Discovered Black Hole at its Centre
What lies at the centre of the Milky Way? For a long time, astronomers have suspected that a black hole lurks at the heart of our Galaxy, but could not be sure. After 15 years of regular monitoring of the Galactic Centre scientists finally obtained conclusive evidence. Astronomers were able to watch individual stars using the telescopes, with unprecedented accuracy as they moved around the Galactic Centre. Their paths conclusively showed that they must be orbiting in the immense gravitational grip of a super-massive black hole, almost three million times more massive than our Sun. The observations also revealed flashes of infrared light emerging from the region at regular intervals. While the exact cause of this phenomenon remains unknown, observers have suggested the black hole may be spinning rapidly. Whatever is happening, the black hole's life is not all peace and quiet.
Once an object falls through the horizon of a black hole, it is lost forever. "It’s an exit door from our universe". "You walk through that door, you’re not coming back." Super-massive black holes are the most extreme objects predicted by Albert Einstein’s theory of gravity — where, "gravity completely goes haywire and crushes an enormous mass into an incredibly close space."
The spectacle of a gas cloud hurtling toward a black hole, right in our cosmic backyard is the most highly anticipated cinematic event for astronomers. At the centre of our galaxy lies a super-massive black hole by the name of Sagittarius A* (and yes, the * is actually part of its name). With an extraordinary mass of four million suns, this black hole wields a hefty gravitational pull, which it’s using to rip apart a gas cloud many times the size of our planet. The epic tug-of-war between black hole and matter will be the first astronomers have ever been able to observe. The prey in the black hole’s sights is a gas cloud known as G2, which was first identified in 2011. As the black hole’s gravity draws the gas cloud closer, it spins and stretches the gas cloud. The gases are speeding up and heating up as G2 gets closer. Recent analysis of observations from the VLT shows that the front end of the cloud has finally reached its closest approach to the black hole.
1) It includes Our Solar System
Did you ever wonder where in the galaxy we are? Our home lies within the disk of the Milky Way. It's the disk where most of the galaxy's gas and dust is located. As a result this is where most new stars are being born. Just as the Earth has an equator, so too does the galaxy and we are about 14 light years above what's called the equatorial symmetry plane. As to the thickness of the disk, most current estimates put it at around 1,000 light years thick. Obviously our solar system lies very close to the galaxy's equator. As to our distance from the centre of the galaxy, the best guess is that we are 26,000 to 28,000 light years from the centre.
The Sun is fundamental to our life on Earth. It is a huge ball of very hot gas. At its centre the temperature is 15 million degrees centigrade. A continuous reaction generates energy, some of which we receive as light and other forms of radiation. The Sun is part of a system of hundreds of billions of stars- the Milky Way. The Sun is currently 5–30 parsecs ( 16–98 light-year ) from the central plane of the Galactic disk.
The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Centre. The Sun's orbit about the Galaxy is expected to be roughly elliptical. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term.
