The star community

The star community

Debi Prasad Choudhary

Los Angeles 03/16/2013

My childhood fascination to know the stars was based on the fact that many of them govern our lives. But than, I was also suspicious that what the other stars are doing? How far they are? And if they are shaped like us and hanging in the space. What do they eat? I never dared to ask these questions to my elders and wise man, who predicted my future using the stars. But, in my first astronomy class it became clear that these objects are truly powerful even though they have nothing to do with my future.

Twinkling of stars of different brightness and color, mainly blue and red, is a priceless spectacular sight at a dark location under clear sky. Why some stars are brighter than others and what their color mean? Major part of the last century was spent in answering these questions.

Like our sun, the stars are hot balls of hydrogen, helium and other elements glowing at temperatures ranging from 2000 to 30,000 degree kelvin. Kelvin is a temperature scale in which 310.9 degree kelvin is -100 degree Fahrenheit. The red stars are cooler than the blue stars and our sun is a yellow star with temperature of about 5800 degree K. More accurate temperature of the star is determined by measuring the amount of light removed by some elements such as hydrogen, Calcium or Iron in selected wavelengths.

The brightness of the stars depends on their temperature and size. If a star is two times hotter than another star of same size, it would be 16 times brighter as predicted from the simple laws of physics. If the radius of a star is two times larger than another star of same temperature, it would be four times brighter because its surface area would increase by four times. In reality the stellar brightness is a combined effect of temperature and size.

If a star of same brightness to another star were located nearer to us, it would look brighter. A similar effect we might observe as the brightness of the headlight of an approaching car would increase as it becomes nearer to us. So, it is important to know the distance to the stars to determine its real brightness. Among many methods, the distance to the nearer stars is determined by using methods similar to stereopsis with which we gain depth perception. Our depth perception is because of looking at the same object by two eyes located on our face that are laterally separated by a small distance. We can observe this effect by placing our thump in front of the forehead by stretching the hand. When viewed by one eye while closing another the finger would appear to be in front of different background. As our eyes are separated by few inches, we can distinguish depth at horizontal difference of 30 seconds of arc.

Exactly same method is used to determine distances to relatively nearby stars. Photograph of the star field in the sky are taken at different times of the year when the earth is located on the opposite side of the sun. Since earth is at different locations while going around the sun, nearby stars change position with respect to far away stars similar to what we see in case of thump experiment. Since the distance of the earth between two positions is 2 Astronomical Unit we can determine the small angle p and distance to the star as 1/p. Remember one astronomical unit is the distance between sun and earth about 92 million miles We can measure distance to about 100,000 stars using this method. The distance to the stars are so large that we need to devise another unit, which the distance at which a star makes 1 second of arc. To travel this distance light takes about 3.26 years. As we know this distance can also be 3.26 light years.

When we see a source of light such as a lamp or a headlight of a car from a distance it looks dimmer than when we see this from nearby. If the distance is twice far, the lamp appears four times as dimmer. The source becomes dimmer as the square of the increased distance. So, by knowing the distance to the lamp, we can find the true or absolute brightness. Similar way, we can determine the absolute brightness in magnitude scale by knowing its distance. In order to compare the brightness of different stars we determine their absolute brightness at a standard distance of 32.6 light years.

There are two more properties, mass and diameter or size, that are commonly measured by using binary stars. Binary stars circle around a common center similar to the sun and the planets. In our galaxy about 85% of the stars are in binary system. So, we can get the mass and radius of majority of stars using these methods. The spectral lines from these two stars display Doppler shifts that can be used to determine the orbital velocity. When a faint star goes in front of a bright star the brightness of the system change periodically. The amount of time the brightness decreases we determine the diameter of the star using its distance from us. From the period of changes we can determine the mass of the system by using the Kepler’s law.  In practice, however, the planes of stellar orbits around the common center are oriented randomly so determining mass and diameter accurately is a very difficult job. There are several other methods for these measurements that are used to determine one property independently, which gives confidence in the results.

It is possible to study the long-term evolution of a group of human or tree in a forest from their common properties. For example, in a forest you can see some saplings, young, old and dying trees, which are at different stages of their evolution. Without waiting to observe the birth, growth and death of a single tree, properties of trees at different stage are used to study their evolution. Similarly, after gathering the absolute brightness and temperature of many stars, we examine their properties to understand their life story. According to their temperature, stars are classified into groups of O B A F G K M; O being hotter and M being cooler stars. (Astronomers remember this sequence with the rime Oh Be A Find Girl Kiss Me.)

In circa 1910, a Danish astronomer and chemist Ejnar Hurtzsprung and American astronomer Henry Norris Russell, found that most stars fall into a sequence, called Main Sequence, in which cool stars are fainter and hot stars are brighter. There are more faint, cool stars than bright hot stars, because hot bright stars are heavier, and rapidly consume their nuclear fuel that keeps them bright. So, hot bright stars die faster than cool faint stars. There are some stars, which are brighter than what is expected from their temperature if they were Main Sequence stars. These stars, known as giants and super giants, appear brighter because of their large size. Such situation in the life of a star happens when the nuclear fuel runs out just before their death. There also some stars that appear fainter than what would be expected due to their temperature if they were Main Sequence stars. These stars are known as white dwarfs, which are high-density compact hot core of dead stars that no longer produce light.

Stars are born out of huge dust and gas clouds, and spend most of their life producing light by converting one element to another. In the end they die and become dust and gas. From these material new stars are born. These stars also produce the elements in our body, so we may as well be called “star stuff”!! This grand cosmic evolution gives us a sense of belonging to vastness that surrounds us. It makes us generous. Today, I know that even if the stars do not control my life or my future, I have a real intimate relationship with these shining objects in the sky.