The Milky Way appears like a strip of non-homogeneous white light and almost separating the sky into two pieces. It can be seen during clear night skies, just a few miles outside the city-region due to the light pollution. You have a better chance seeing the Milky Way during low temperature night skies.
Photo of the Milky Way taken in Aeschi bei Spiez, Switzerland.
© 2018 - @Rubencress - Licensed for Steemit use only.
This white stripe represents a projection on the celestial sphere of the densest region in one gigantic star system. This system is a home to our Sun, many different stars, star clusters, nebulae and interstellar matter. It is called the Milky Way - or simply put, our Galaxy.
In the Greek language gala is a word for milk. Of course, many civilizations were inspired and astonished with this beautiful white stripe and here are some of the other names:
- Birds' Path - Used in several Uralic languages and in the Baltic languages.
- Silver River - The Chinese name is used throughout East Asia
- River of Heaven - The Japanese name as well as an alternative name in Chinese
- Winter's Alley - Scandinavian people have called the galaxy Winter's Alley (Vintergatan), as the galaxy is most clearly visible during the winter at the Northern hemisphere.
- Straw Way - In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for straw. This term is used in my country Serbia as The Godfather's Straw.
Shape and structure
Over the centuries it was believed that our star system is infinite and contains all of the objects we see on the sky. It was infinite homogeneous heliocentric model. However, there were other ideas (Immanuel Kant, XVIII century) of existance many other star systems similar to ours. English astronomer William Herschel accepted this idea, and started searching for other systems and counting stars in different directions with his telescope in 1875.
First accurate model was proposed by Harlow Shapley in his works in period of 1917 to 1919, based on his study about distribution of globular star clusters. You can read about globular star clusters in one of my previous posts, they are spherically symmetrically distributed around the Galaxy. Shapley concluded that the center of it lay in the direction of the Sagittarius constellation.
This artist's concept depicts the most up-to-date information about the shape of our own Milky Way galaxy. [2]
The diameter of the Milky Way is about 30 kpc and the distance from the center to the Sun is approximately 8.5 kpc, this value may vary depending on the calculation method. With the constant development of technology and telescopes we were finally able to observe and study other galaxies, which expended our knowledge and helped in understanding of our own Galaxy
Edwin Hubble (HST is named after this guy) classified all galaxies by their shape, and created the famous tuning fork diagram. Milky Way is classified as a SBb galaxy type.
The Hubble Classification scheme for galaxies, often referred to as the “tuning fork” diagram.[3]
Milky Way Morphology
In a spiral galaxy such as Milky Way, the stars, gas, and dust are organized into a subsystems called bulge, a disk which contains spiral arms, and a halo. Elliptical galaxies have a bulge-like central region and a halo.
Galactic Disk
The galactic disk is the flattened component of a spiral galaxy. It contains many ordinary objects in their middle age (about 1 - 5 billion years): The Sun and other middle age stars, some older open clusters, and the youngest globular clusters.
On the galactic plane that goes through the center of the disk we have a non-homogeneous spiral structure with a thickness of about 200-300 kpc, also known as thin disk. These structures are spiral arms. Matter density inside the spiral arms and thin disk is 30 times higher than in the thick disk, containing the youngest stars(the hottest ones), huge amount of interstellar matter and star associations and youngest open clusters. All the objects inside the thin disk are younger than 1 billion year.
Halo
As we could see on the schetamtic view above, the galactic disk is immersed into the spheroidal halo. Halo contains old stars and globular star clusters. Stars in these clusters have the lowest metallicity, and it is estimated that they are between 10 to 13 billion years old, representing the first stellar generation.
There are also - galactic corona (made of hot ionized gas) and dark matter halo, which are few times larger than the halo we can actually observe and study. However, even if we cannot observe these two halos, we have proof of their existence in galaxy rotation curve and gravitational effects.
Bulge
Bulge is an ellipsoidal central region whitin a 4 kpc radius on a galactic plane, it contains highest concentration of stars, molecular clouds and globular clusters.
Center of the Milky Way, Sagittarius A (Sgt A*) is also the strongest radio and gamma ray source in the Galaxy. Estimated mass in the center of the galxy is between 3 × 106M☉
Massive core with small and compact size with non-thermal radiation and chaotic movement of mass with inner-flow inside the Sagittarius A, it indicates that in the center of the Milky Way we have supermassive black hole.
An illustration of the edge-on view of the Milky Way with all the components. [5]
Stellar Population
During 1944 Walter Baade proposed a classification of stars and other objects in the galaxy into two main categories depending on their age and chemical composition (metallicity). This classification was slightely changed in 1978 adding the third group of objects.
Young, hot and bright stars which are found in spiral arms and have the high metallicity are categorized as population I. Older, less bright stars, concentrated in the galactic halo and bulge with low metallicity are categorized as population II. Population I or metal-rich stars, are symmetrically distributed in relation to the galactic plane, while population II or metal-poor stars are spherically symmetrically distributed in relation to galactic center. The Sun like stars are between population I and II.
Population III stars represents a hypothetical first generation stars, extremely massive and hot with no metals(metals in the astrophysics are considered all chemical elements heavier than helium). This population of stars has not been detected yet, but it is used in cosmological simulations of the early Universe.
Once again, I want to express my gratitude to 
Your feedback is always welcome! If you have any comments, questions or advice I would gladly read and answer them.
References:
- Carroll and Ostlie, Introduction to Modern Astrophysics
- NASA
- Swinburne University of Technology
- Wikipedia
- Beyond Earthly Skies
