The Home Galaxy

The Home Galaxy

Debi Prasad Choudhary

Los Angeles, 11/2/2014

The river Ganges that flows from Himalayas to Bay of Bengal, actually originate in heaven. After washing the feet of the God, she descends on earth, using the head of another powerful god Siva as launching pad. The band of light that is spectacularly visible in rural sky is the heavenly portion of this sacred river called “Akas Ganga” “Ganges in the sky”. Without asking too many questions as to how is it sustained or will it fall down etc., I looked at this band of light in my childhood with great curiosity. Much civilization from ancient time looked at this band of light with utter amazement and constructed stories extending their local culture. The Greeks called it “milky circle” and Romans “milky road” or “milky way”.  It is interesting to note that in India, the heavenly portion of the imaginary river contained milk!!

From a rural dark sky the Milky Way looks like a long stretch of light with a dark band in the middle. It was disappointing to see through a small telescope that this part of the sky is just crowded with stars, which shattered my imagination of heavenly river. Later in my astronomy class I learnt that the Milky Way is one of the arm of our galaxy, which has a spiral shape. The dark lane in the middle is due to the presence of huge dark clouds with dust that obscures the view.

In early 19^th century, Sir William Herschel and his sister counted stars in 683 directions to figure out the structure of this object, but only after about one hundred years, Dutch astronomer Jacobus Kapteyn could determine its size and thickness. In early 20^th century, Harlow Shapley using the telescope in Mt. Wilson observatory in Los Angeles worked for his PhD thesis to figure out the size of the galaxy and locate its center. He used the star clusters in our galaxy to locate the center. There are two types of star clusters in our galaxy. In one of them stars are loosely scattered, called open clusters, which are mostly found along the Milky Way. The others are more compact, looks like “meat ball” in the galaxy, located mostly in the direction of Sagittarius constellation. The open clusters contain younger stars and some gas, while globular clusters are old and devoid of gas. The globular clusters are as old as the galaxy, some of which are also located above and below the galactic plane.

The distance to the globular clusters are measured by using variable stars, which become dim periodically depending on their original brightness. These are Cepheid variable stars, which expand and contract due to build up of heat by electrons from ionized helium. The heating results the outer envelop of the star expands leading to cooling and contraction. As the star expands the bigger star becomes more bright compared to the smaller star. The period of maximum to minimum brightness depend precisely on the original or initial brightness of the star. So, by detecting the variable period, which is easier, it is possible to measure the absolute brightness accurately. By comparing the absolute or real brightness and apparent brightness, the distance to the star is determined. This is so because the star becomes dimmer by the square of the distance. If a candle is placed at twice the distance, it looks dimmer by four times. This technique of determining the distance was found to be correct when it was applied to the stars located at known nearby distance that could be measured by simple parallax method.

Now, we know the size and structure of this galaxy in which we reside. The Milkyway Galaxy is  a “dinner plate” like flat disk with a central budge and a halo that surrounds the structure. The disk is so big that the light takes 80,000 years to travel from one side to the other. The sun, and the planets around it, is located at about two third of the radius of the galaxy from its enter and revolves around it once in about 250 million years. The galaxy does not rotate like a single rigid body. The stars at different distance from the center go around with different speeds, such that the stars nearer move faster than the ones situated at greater distance. While going around the center of the galaxy, the sun also swings up and down of the plane of the disk with a period of about 60 million years. As the sun passes the disk, the reservoir of ice cloud (a comet cloud known as Oort’s cloud) gets disturbed. This results a shower comets on earth leading to massive loss of life called “mass extinction”. The dinosaurs are believed to have extinct due to such an event about 65 million years ago!

The stars are continuously formed from huge clouds of dust and gas, which are constantly disturbed by the pattern of rotation. The stars evolve and die and produce new material. New stars are born from ashes of old stars. This is going on for past 10 to 13 billion years and perhaps will go on forever, until the existence of the universe.

The motion of stars around the center of our galaxy gives vital information about the distribution of matter. As the gravitational force governs the motion of stars around the galaxy, they should follow the pattern similar as the planets move around the sun. This pattern of motion is called Keplerian motion that follows the Kepler’s Laws. The measurements show that the stars very close to the center move as if they are attached rigidly and the ones far away move faster. This type of motion is possible only if the center harbors a very massive object and the galaxy is surrounded by enormous amount of matter. But, neither in the center nor around the galaxy such matter is observed. We do not see the matter in the galactic center, because it is confined in a very small space resulting a super massive black hole. The invisibility of the matter around the galaxy is even a bigger puzzle. Astronomers think that this invisible matter is indeed dark matter, which interact with visible matter through gravitational force. Almost 85% of the matter in our galaxy is dark matter, the properties of which are unknown at this time.

How exactly our galaxy formed? Did the stars got together to form the galaxy or they were born in the galaxy? What will happen in the end? These and may other similar questions students of astrophysics constantly ask. They make ever-bigger telescopes in space and on the ground to search the clues and test their ideas. One thing is clear. Most of these questions can be answered using modern physics. All our knowledge about our home galaxy is gathered only in past 50 to 60 years. If we do not know some thing today, we will know them soon.