One of the most interesting and sophisticated creations of the nature around us is the galaxy, people gain a lot of knowledge about it, but it still seems to be unidentified and fascinating our minds.

We usually talk about the galaxy or Milky Way galaxy. Milky Way comes from the translation of the Latin word Via Lactea, which came in its turn from the Greek Galaksias. Galaxy is considered to be the home of the solar system, but there are billions of galaxies in the universe. The first person who in stated that the Milky Way consists of several distant stars was Democritus.


The word “milky” is associated with the white light coming across the celestial sphere, which one is able to see from the Earth. Relative the celestial equator the Milky Way is till the constellation of Cassiopeia in the north direction and till the constellation of Crux in the south direction. The Milky Way divides the sky at night into two equal hemispheres, this proves that the solar system is close to the galactic plane.

The diameter of the Milky Way galaxy is about 80-100 thousand light years, the thickness is about 3.000 light years and about 250-300 thousand of light years in circumference. It consists of 200 billion stars. If to reduce the galaxy to the size of 130 km in diameter, the solar system would be 2mm in width.

The age of the Galaxy is about 13.6 billion years – thus as almost as old as the universe.

Since the year 1980 the astronomers stated that the Milky Way is not a usual but a barred spiral. The galactic disk is of the diameter of 100.000 light years; the Sun is about 27.700 light years from the galactic center.

There is a large compact object, which is called a supermassive black hole, it is usually associated with the Sagittarius A radio source. The scientists think that most of the galaxies have supermassive black holes at their centers. Another typical feature for most galaxies is the fact that the orbital speed of most starts has little to do with the distance from the center. The typical stella speed is 210-240 km/s.

All spiral galaxies have their “arms”, each spiral arm is a logarithmic spiral with pith of 12 degrees. There are four spiral arms, all of them start at the center of galaxy. Their names are:

- 3 kpc and Pereus Arm

- Norma and Cygnus Arm

- Crux and Scutum Arm

- Carina and Sagittaius Arm.

The smaller arm, called Orion Arm contains the Sun and the solar system.

Stars are the parts of the galaxy which people are able to see every night from any place on the Earth. They are born in nebulae. The protostars are formed when the clouds of dust and gas collapse under gravitational forces. Later on they have to go through another collapse and then they form main sequences of stars. Some people were used to think that stars stay the same all the time, but this is not correct as time passes, they become older and the core out of hydrogen and then out of helium ruins and the outer layers become cool and thus the star is less bright.

Stella nurseries are in the centers of giant molecular clouds (GMC) – “large regions of space heavily populated with gas and dust and cool enough for molecular to form” (2). The size of GMC is from one to several hundreds of light years in diameter and they contain hundreds of thousands or even millions of solar masses of material.

The collapse of the GMC is a slow process. The cloud breaks first into lumps and they are more easily collapse than the other parts of the cloud. Out of these lumps were formed stars. During the stage of protostars they are heating up, and the collapse gets slower, but still doesn’t stop. Later comes the moment when the center or the core of the protostar is so hot that it is enough to start fusing hydrogen into helium, soon the star is able to enter the so-called main sequence. At this point the life of the star begins. The whole life actually is the process of fusing the atoms together and fighting against gravitational collapse, the length of the star life is different depending upon the mass – from several million to several billion years.

Our sun “was busy” with fusing hydrogen into helium within five billion years, and the scientists think that it will go on doing it for another five billion years.

As it was already mentioned the mass of the stars is different. The stars with low mass (much less than the mass of the Sun) are not able to get beyond hydrogen burning. As soon as the most of the hydrogen is used by the star it starts cooling and soon collapses into “brown dwarf”.

The stars that have the mass close to the mass of the Sun are considered to be intermediate mass stars. They usually go through the stage, when they are called red giants, the radiation blows away most of the outer layers, as soon as the star cools it will become a white dwarf.

The most beautiful is the end of the high mass stars, They go through the red supergiant phase. The temperature is high enough to fuse the core into iron, as iron can not stand the gravity the core collapses. The star becomes either a neutron star or a black hole.

Black holes are big mysteries in our science. “If the core of a massive star at the end of its life has more mass then three Suns, the core collapses into a singularity, called a black hole” (4).

Singularity in this case means the point of infinite density, when a finite quantity of mass is squished into zero volume. Infinities are strongly argued about among astrophysicists, thus there is no unique conception concerning the real universe.

It is next to impossible for the astronomers to watch the whole cycle of a star as it lives from 40.000 to 10 billion years. There are millions of stars, which the scientists can watch at different stages of their life cycle. It is logical to ask then what is the source of the information about starts for the scientists? It is easy – different stella populations are able to tell the astronomers about the history of the galaxy. The stars of different mass have different life length, if we need the information about recent formations in the galaxy we should study the big stars and if we need to know about longer formations – we will look at small stars.

Some information comes also from kinematics of the stars. In our galaxy the younger stars have thinner disks then the older ones. This happens so because of the collapse of the Milky Way.

Not only the masses of the stars are different, colors and temperatures differ as well. Astronomers detect the colors of the stars taking the spectra (like splitting the light with the help of the prism as Newton did). The color is also somehow connected with the temperature of the star – for example the blue stars are hotter than red ones. In general the temperature of stars varies from 3000 K to 50.000 K. The temperature brings another difference – cool stars have molecules like Titanium oxide on their surface, hot stars – have ionized atoms.

Each year about 3 solar masses are formed, these are approximately as big as the Sun, but the smaller stars are more numerous.

Sources:

1. The life cycle of a star, Dave Kornreich, 1999, pp.2-15
2. Are all stars all the same? , Jagadheep D. Pandian, 2001, pp.1-6
3. Stars are born and die each day, Amelie Saintonge, 2004, pp. 2-19
4. http://www.can-do.com/uci/ssi2003/starlife.html
5. http://en.wikipedia.org/wiki/Stellar_evolution
6. The life cycle of a Sun-like star, J. Mcgray, 1998, pp.5-8
7. Light in the sky, A.Grey, 1999, pp. 112-115
8. Solar physics and stars, overview of the book by Alice Szeto, 2004, pp.1-2
9. Life Cycle of Black Hole Emissions Seen for First Time , Robert Roy Britt, 2002, pp.1-10
10. Life cycle. Britannica Student Encyclopedia. from Encyclopædia Britannica Premium


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