Scale of the universe looking at the sky

[These blogs are created for my online astronomy class in California State University Northridge. Since they may interest wider audience, I publish them on Blogger. All comments are welcome.]

Scale of the universe looking at the sky

Debi Prasad Choudhary

Los Angeles, 1/30/2014

While growing up in a small remote village, I used to listen stories from my granny about the mythological characters who could change their size as they like and can move large distances in a matter of fraction of a second. They can become like mountains or like an ant in a moment. Once a Hindu God, known as Hanumanji in his childhood, woke up in the morning and was hungry.  His mother was attending to some other household activity. He saw the rising sun and thought of it as a ripened fruit. Immediately, he jumped and ate the sun. The whole world became dark. Other Gods were afraid and prayed Hunmanji to eject the sun, which he obliged and saved the creation. I was really fascinated with these stories and believed them that gave me a sense of scale of the earth, sun and all others I saw around me. Later in school, I read the story of Gulliver’s travels, where he encounters Lilliput and giants. Even though I did not believe them, I wished they were true.

Later, I went to far away places from my village for higher studies that took 24 to 36 hours of train journey. This really gave me a sense of the size of landscape in which I lived. Much later, on my first visit abroad to Germany, when I used to talk to my wife in India, some times I used to ask her go outside and see if it is daylight or darkness of night. Even though, I knew that earth is round and my wife’s place would be in nighttime when Germany would be in day, this fact fascinated me. Even now, the fact that east and west coast of United States has two different times separated by three hours fascinates me. Last summer, while attending a conference, one of my old professors asked me “do you know how much is a trillion dollar?” I was a bit confused, as I can never imagine what a trillion dollar means. We figured out that if you place trillion of one dollar bills one over the other, the height would be the distance to the moon!! You might like to calculate this fascinating fact!!

Compared to my home, my town and me the universe, of which I am a part, is big. There are, of course, much tiny things around. They are so tiny that when you shine light onto them to see, they move due to the push of light particles. So, it is impossible to precisely determine their position. There is a whole different subject of dealing with these tiny objects. But, astronomy is all about big things and large distances.

It is fascinating to know that there are things beyond what we know to exist like our home, our town or our earth. There are other planets moving around the sun like our earth. There are billions of suns in our galaxy and billions of galaxies are in this universe. In this scale of structure, the nearest distance to a star, which is our sun, is about 150,000,000 km called one astronomical unit. It is convenient to write this in scientific notation as 1.5×10⁸ km. The distance is so much that light takes about 8 minutes to come to us from the sun. The speed of light is about 300,000 km/s and nothing in the universe can travel faster then light. In order to express the size of a galaxy, we need a bigger unit of distance. One convenient unit is the distance travelled by light in one year. This is known as light year (Ly), a unit of distance NOT time!! The size of a galaxy is 80,000 Ly, that means light takes 80,000 years to come from one side of the galaxy to another side. If you go by a car it will take many many years, during which time the sun itself will die!!

This is the vast universe we live in. Knowing that make us smart and humbling. That is the single most benefit of learning astronomy. How do we know that universe is this big? In past 100 years, we have explored the universe with modern physics that has also given us airplanes, cars, iPods and cell phones. We can watch a movie sitting on the college lawn. The laws of physics that are used to make these devices are employed to explore the universe. Sir Isac Newton, a British scientist showed that the same laws of physics govern the motion of objects both on earth and in heavens. So, we are confident that what we know is consistent. Today we are equipped to ask big questions like: how the universe was created? How life was created? These are the questions all civilizations asked. Every civilization answered in their own way. We hope to give answer using robust science. We do not have answers to all questions at this time. But my granny did not have an iPhone either. Exploring the answer is the goal of modern science. We look at the sky and explore the property of the stars and all that we can see.

While in elementary school, we used to sleep outside in summer time and look at the sky. The village sky was much darker those days, filled with bright and faint stars. Many bright stars gave an appearance of patterns that are called constellations. These groups of stars just appear as one pattern at this time and do not have any physical association. Several thousand years ago, you might not see such patterns as the stars move in the sky. My granny narrated stories about two group of constellation that impressed me in my childhood. One of them was Orion constellation depicted as the great warier, general of Gods, known as Mahakal.  The other one was great dipper, called saptarashi (seven sages). One of the sages is the sixth star (located in the tail) named as Vasistha, one of the great-learned sages of Hindu mythology. This is a visual binary star that has a faint companion, named as the Arundhti, the faithful wife of Vasistha. Many years later, when my granny died, I wanted to name one of these stars after her!!! This companion star is the limit of visibility for human eye, which can often be used for eye test. If you can see this star with unaided eye, you are good!!

There are twelve such prominent patterns known as Zodiac. Since they happen to align with the plane in which the sun moves in the sky, they have special significance in ancient time. Depending on the position of the sun in the background of this zodiac constellation during the birth of an individual, our constellation signs are decided. For example, I am a Leo. When I was in 5^th grade, my father invited an astrologer and found out my major life events based the zodiac sign. He was worried, because I used be sick very often for prolonged period, primarily because of lack of medical facility in the village. Anyway, I took interest and waited for better times predicted by the astrologer. Later, while in college, I used to look at the astrological predictions printed in local newspapers. If the prediction was bad, I really used to get depressed and look at another newspaper that gave prediction for a good day. I used to convenience myself, may be the later newspaper is more correct. I gave up this exercise after a while, when I did not find any such correlation with what they say and what actually happens. Now, of course, I know that these predictions are baseless and as good as my granny telling me how good my day is going to be.

After my granny died, I started looking for the brightest star for her. I learned that a star that looks bright might not be really bright if it is located nearer to us compared to a bright star at a greater distance. The brightness that appears is known as apparent brightness. The astronomers have devised techniques to determine distances to the stars. So, they can tell their actual brightness, known as absolute brightness. These are measured in magnitude scale. In this apparent magnitude scale the sun is -26.74. The brightest star is -1.47. A human eye can see a star of 6^th magnitude. The more negative is the brighter.

The stars are huge balls of glowing mostly hydrogen located at far away distances. For us they are just point sources of light, radiating energy in all directions. The energy received on earth over a square meter per second is the energy flux (F) that actually decides the brightness of the star. If two stars A and B differ by one magnitude, the flux ratio would be F_A/F_B = (2.512)^(m_B^-m_A⁾. 

So, now while I know that Hanumanji could not have eaten the sun or the star patterns do not decide my day, I know that the laws of physics using which the TV set in my living room is made, govern the motion of stars and their lifestyle. In my later career, I have been using them to explore this amazing creation.

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You should use the material in the following links to explore the night sky in your locality with the help of Internet resources. Familiarizing the objects in the sky will not only give you intellectual pleasure but also help you in navigation and other useful activity.

(1) Install GoogleSky on your computer.

(2) Exercises to identify the constellations on local sky.