The main source of energy for the stars is the thermonuclear fusion of hydrogen in its core. The core which mainly consists of hydrogen, helium and traces of heavier elements gets compressed under its own gravity until it reaches a particular temperature and sufficient internal pressure builts up to trigger the nuclear reaction. Once the reaction starts, the star is all set to shine and radiate heat for the next few million years. The star’s internal pressure prevents it from collapsing under its own gravity. Once all the hydrogen in its core gets exhausted, another set of nuclear reaction starts in which helium fuses to form heavier elements.
Stargazing Again : Observe a few bright stars closely with the naked eye for a little while. You can easily determine their color. Yes!!! Stars vary in their color too…Depending on the temperature, the color of stars may be blue, white, yellow, orange or red. The hottest stars are blue and the coolest ones are red. According to the surface temperature, stars are classified into 7 classes :
Class O, B, A, F, G, K, M. Class O stars are the hottest and Class M stars are the coolest. The Sun is a yellow, Class G star.
All this time, in all my previous posts I have been mentioning about stars, its groups and stuff like that. So what are these stars made up of? What makes them so bright and huge? Where did they come from? Stars are the biggest thermonuclear furnaces you can ever find. They are bodies of hot, glowing gas which vary enormously in size, mass and temperature. Relative to the sun, which is also a star
· Some are about 450 times smaller and some are 1000 times bigger.
· Masses range from about a twentieth to over 50 solar masses.
· Surface temperatures vary from about 3000 to 50,000 degrees.
The Sun has a surface temperature of 5,500 degrees which is quite close to the lower extreme. So the Sun is SUPPOSED to be one of the cooler stars. Coming from a place close to the Equator, I wouldn’t say so. Every year during the months of May and June, people in here experience the wrath of the Sun. Just imagine if it had a temperature close to the other extreme. Mann…we will be burned to death even if the Earth was ten times farther to the sun than it is now.
Assuming that we somehow manage to make it to the Central Bulge of the Milky Way galaxy, this is what we will find. The galactic disc will start bulging as we move towards the centre and its diameter varies between 70,000 to 100,000 light years. The central bulge can be divided into two parts- the Nucleus and the Halo. The halo is a less dense region surrounding the nucleus, which contains some of the oldest stars. At a distance of 7500 light years from the centre, we can actually find the oldest star yet discovered in our galaxy – HE 1523-0901. A star in its Red Giant phase, its age is estimated to be about 13.2 billion years which is nearly the same as the estimated age of the universe.
As we move closer to the exact centre of the galaxy, we can experience a gravitational force that we have never felt before. A Supermassive Black Hole welcomes us!!! Black Holes are objects whose gravitational pull is so strong that not even light can escape from it. Named Sagittarius A*, it is a source of intense radio waves and is four million times more massive than the sun. Astronomers have long suspected the presence of a supermassive black hole at the center, by observing the behavior of the stars around it.
So before it can suck us in for eternity, let’s tighten our seat belts and head back to the peaceful Earth…..
Aakash Ganga is the other name given to our galaxy – The Milky Way. I have always wondered why the ancient astronomers have given it such a funny name. I found the reason recently while I was reading an encyclopedia about space. On a clear night, you can see a dense band of stars that stretches across the sky. Our ancient astronomers thought this resembled a trail of spilled milk. So that’s how it got this funny name…
Now that we know how Milky Way got its name, let’s take a tour of this spectacular galaxy. We, the Earthlings lie about 25,000 light years from the centre of the galaxy which means it will take us a whopping 25,000 years to reach the centre even if we travel at the speed of light (With the present technology, we can’t even imagine to travel that far). The Galaxy stretches 100,000 light years across. As we move towards the centre of the galaxy, we can find that it becomes denser than the spiral arms. Unlike at the spiral arms, where you find mainly hot, young, blue stars, towards the galactic center we can find a number of older red and yellow stars. Also, you can clearly see that the inner stars are traveling much faster than the outer ones.
Now let’s see how galaxies are classified and the characteristics of each of these types:
1) Spiral Galaxies: A spiral galaxy appears like a huge disk (bright spiral arms) emerging out from a central bulge. The Milky Way is a giant spiral galaxy, 100,000 light years in diameter and a mass of approximately a trillion times that of the sun. Spiral Galaxies rotate slowly. The Milky Way takes about 250 million years to make a complete rotation. In the spiral galaxies new stars are constantly born from dust and gas.
2) Elliptical Galaxies: It has been found that elliptical galaxies are formed when all the gas is made into stars before it has time to form a disk. They are oval shaped and are composed of older stars, since few new stars form in them. They rotate very slowly or sometimes do not rotate at all. They appear to be very bright at the center and grow faint at the outer edges.
3) Irregular Galaxies: They have no particular shape and are mostly chaotic in appearance. They make up about a quarter of all galaxies in the universe and consist of a number of bright young stars. Some of these galaxies might once have been spiral or elliptical in shape but were deformed due to gravitational action.
The first thing that comes to our mind when we talk about galaxies is the word ‘Milky Way’. Yes…It is the most famous galaxy in the universe. The Solar System, in which we live in, is only a tiny part of the Milky Way galaxy and is at a distance of 25,000 light years from its centre. So, what is a galaxy..??
A galaxy is a group of stars, dust and gas bound together by Gravity. Scientists estimate that there are more than 100 billion galaxies in the visible universe. The smallest galaxies contain less than a billion stars while the largest ones contain upto 3000 billion stars. Only three galaxies can be seen outside the Milky Way with the naked eye – The Andromeda (2 million light years away), The Large Magellanic Cloud (160,000 light years away) and the Small Magellanic Cloud (180,000 light years away). A Light Year is the distance that light travels in vacuum in a year. Like I said, we are dealing with enormous distances. These galaxies differ in size and shape. The diameter of the galaxies varies from a few thousand to a half - million light years.
For enthusiastic Star Gazers: Try spotting the Andromeda Galaxy – For this you need a nice pair of Binoculars. First locate the Great Square of Pegasus in the northern sky, spot the star Mirach which is to the left of the bright star Alpheratz. Then locate a fairly bright star slightly above Mirach and continue to move your binoculars in that direction until you find the ‘Little Cloud’ – The Great Andromeda.
