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| Fig 4.1 The Milky Way |
General introduction and appearance
The Milky Way is part of the Local Group of galaxies. It appears as a hazy band of white light in the night sky arching across the celestial sphere. The plane of the Milky Way is inclined by about 60° to the ecliptic. The Galactic North Pole is situated at right ascension 12h 49m, declinations +27.4° near beta Comae Berenices. The Galactic South Pole is near alpha Sculptoris. The center of the galaxy is in the direction of Sagittarius.
The Milky Way looks brightest toward the galactic center, in the direction of Sagittarius. The Milky Way has a relatively low surface brightness due to the gases and dust that fills the galactic disk, preventing us from seeing the bright galactic center. It is difficult to see from bright urban or suburban areas because of the light pollution.
The Milky Way Galaxy is approximately 100,000 light years in diameter, and is widely believed to be 1,000 light years thick. The galaxy is believed to contain between 200 billion and 400 billion stars. The exact figure depends on the number of very low-mass stars, which is highly uncertain.
Who discovered the Milky Way Galaxy?
In 1755, Immanuel Kant proposed that the Milky Way was a large collection of stars held together by mutual gravity. This collection of stars must be rotating and flattened as a disk, with the Solar System embedded within the disk.
Uranus discoverer, William Herschel attempted to actually map out the shape of the Milky Way in 1785, but he did not realize that large portions of the galaxy are obscured by gas and dust, which hide its true shape.
It wasn’t until the 1920s, when Edwin Hubble provided conclusive evidence that the spiral nebulae in the sky were actually whole other galaxies. This helped astronomers to understand the true nature and shape of the Milky Way, and also discover the true size and scale of the Universe around us.
Formation of the Milky Way Galaxy
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| Fig 4.2 The Milky Way forming |
However, not all of the stars in the Milky Way date back to the primordial Universe. The 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. 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. It is known as 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.
Older stars in the Milky Way are to be found distributed spherically in the galactic halo, meaning that the galaxy had a spherical shape to start out. Younger stars in the galaxy are located in the disk, and as it started to get heavier, the mutual orbit of material started the galaxy spinning, which resulted in the spiral one sees in representations of the Milky Way.Age of stars in the Milky Way Galaxy
The age of the oldest star discovered so far in the Milky Way, HE 1523-0901, is estimated to be about 13.2 billion years, which is about the same age as the galaxy. It was determined that the elapsed time between the rise of the first generation of stars in the Milky Way and the first generation of stars in the cluster was between 200 and 300 million years. By including the estimated age of the stars in the globular cluster, 13.4 billion years, the estimated age of the oldest stars in the Milky Way is 13.6 billion years.
Components of the Milky Way Galaxy
The Milky Way Galaxy has three major components.
Firstly, it has a thin disk consisting of young and intermediate age stars. This disk contains gas and is actively forming new stars. Dust in the disk makes it appear orange usually. As it absorbs blue light more than red light, the stars are made to appear reddish. The spiral arms in the disk are regions of active star formation.
Secondly, it consists of a halo which is rather spherical in shape and contains little gas, dust, or star formation. The clusters found in the halo are globular clusters (approximately 100 of them) and contains very old stars. Dating of globular clusters by their turnoff points indicates that they may be as old as 15 billion years and are the oldest components of the galaxy.
Thirdly, it contains a nucleus or central bulge. The nuclear bulge or core contains the highest density of stars in the galaxy. Although at visible wavelengths the core of the galaxy is obscured by dust, gas, and stars, there is some evidence that violent processes may be taking place there.
