Black Holes

If you had read my article on ‘The Life of the Star’, you would have known that a Black Hole is created when a huge and massive star (more than 2.5 times the mass of the sun) runs out of hydrogen and collapses into a Supernova explosion. If the core of the star remains the explosion it becomes a black hole. But what exactly is a Black Hole? Why is it different from other dead stars?

This image shows how a Black Hole may look like in the Milky Way.

When such a massive star is crushed into a smaller volume, the gravitational attraction of the star increases and hence the escape velocity. The gravitational attraction of a Black Hole is very high that it pulls out anything that is closer to it. The escape velocity of a Black Hole is more than the velocity of light i.e. an object in a Black Hole should travel faster than the speed of Light to escape the gravity of Black Hole. And hence even light cannot escape its gravity. Eventually nothing can escape out of a Black Hole as nothing can travel faster than light. This is the same reason that the Black Hole is invisible. You can only see an object when it reflects light. But since Black Hole does not reflect light it is invisible.

NOTE: None of the images of the Black Holes are real. They are artistic works to show how a Black Hole may look like. A Black Hole can not be photographed. It can only be detected.

There are three types of Black Holes – Stellar Black Holes, Supermassive Black Holes and Miniature Black Holes.

A Stellar Black Hole is a Black Hole created by a single star as explained above. It can be seen in almost every other galaxy.

This is an image of a Stellar Black Hole.

A Supermassive Black Hole is a Black Hole whose mass is between 105 and 1010 times the mass of the sun. It is known that most of the galaxies, including the Milky Way contains a Supermassive Black Hole in the center. It could have formed when a Stellar Black Hole start growing by pulling stars of huge mass by means of gravity. Supermassive Black Holes are only found in the center of the galaxy.

Image of a Supermassive Black Hole.

A Miniature Black Hole has not been identified but some theories say that Miniature Black Holes might have formed a short while after the Big Bang.

Questions may arise such as ‘How a Black Hole was discovered if it is invisible?’ and ‘Who discovered the first Black Hole?’ Here are the answers.

Before answering them let me explain about Binary Star System. A binary star system consists of two stars very close to each other and also moves around each other. Most of the stars we see in the night sky are binary stars though it looks like one twinkling star. Sirius, the brightest star in the sky is a binary star. Sirius A is bigger and brighter than Sirius B which can be seen only through a telescope. Sun, of course is a single star. Distance between two binary stars may be about 20 to 50 times the distance between Sun and Earth. But the distance between two single stars will be in Light years.

Image of Sirius A (bright one) and Sirius B (smaller one above it).

Consider a binary system of stars where one of the stars is a black hole and the other a normal star. If the normal star's envelope gets close enough to the black hole, then the fierce gravity of the black hole can rip out gas from the normal star which is then swallowed by the black hole.

However, due to the conservation of angular momentum, the gas cannot plunge straight into the black hole, but must orbit it for some time before it gets sucked. Thus, a disc like structure is formed around the black hole from which gas is pulled slowly into the black hole. When the gas orbits the black hole in the disc, its temperature is raised to several millions of degrees which emits radiation in the X-ray part of the spectrum (by the first note that I explained above). Thus, when we detect X-ray sources in the sky, then we know that there is gas which has been heated to several million degrees, and one of the mechanisms to achieve that is the accretion disc around the black hole. Now about actual discovery: In the early 1970s, an intense X-ray source was found in the constellation Cygnus called Cygnus X-1. As the years passed, in the 1972, Cygnus X-1 was identified with a star known by its classification number HDE226868 (which is a radio source). Soon evidence was found that it is a binary star system with a period of about 5.6 days.

By the special Theory of Relativity, no information can travel faster than the speed of light. Hence, a celestial object cannot change its luminosity on a time scale shorter than the time taken for the light to reach from one side of it to the other. Analysis of Cygnus X-1 showed that its emission had luminosity variations on time scales as short as thousandths of a second, suggesting that the object was only a few kilometers wide. Thus evidence was found that one of the stars was a compact object. Finally, astronomers used the binary star system to determine the mass of the compact object and found that it was greater than the critical mass, so that it was most likely a black hole. That is about the discovery of the first black hole in our universe.

Look for Mars

This is the perfect time to watch Mars in the sky. Mars will come to its opposition on December 24th, 2007 in the constellation of Gemini. By opposition I mean that Sun, Earth and Mars will lie in a perfect straight line is such a way that Mars will be exactly opposite to the Sun from Earth. Mars will be seen over head at midnight on its opposition.

But six days earlier to it, that is today, December 18th, 2007, Mars will be at its closest to the Earth. Mars will be just 88.42 million km from Earth and from today for a week it will be the perfect time to look out for Mars in the sky. The magnitude of Mars will be -1.5. Magnitude is nothing but the brightness of an object in the sky. The object is brighter when it is negative. Sun, the brightest object in the sky is of magnitude -26.8. The brightest star, Sirius is of magnitude -1.46. So Mars will be bright enough to be noticed.

Look out at the east about an hour or two after sunset. If you don’t know which is east look at the direction opposite to the direction where the sun had set. Between North-East and East you should be able to see a bright star which is Mars. Be careful not to misjudge Sirius, the brightest star, with Mars as Sirius will also be present somewhere nearby. Mars will be a little reddish compared to Sirius.

Check out this site. It is a digital planetarium which shows the complete night sky. I’ll try to help you to find Mars using this.

www.neave.com/planetarium/

Set the date and the time on the top left corner of the page. Set the latitude and the longitude of the city you are living at present. For latitude the positive value is north and the negative is south. For longitude positive is east and negative is west.

For example set to latitude to 13 and longitude to 80 (For Chennai, Tamil Nadu, India) and set the time to 18:00:00 and the date to 18th December, 2007. Rotate the sky such that you face east. Near the horizon on the east you should be able to see a tiny red star, which labels as Betelgeuse in the constellation of Orion. Mars should be seen somewhere around the left of the star. But you cannot spot it in this website as it only shows the stars. Try to spot those stars you can spot Mars easily.

In this picture you can see the full moon. The white streak of light is the movement of the space station and a tiny dot below it is the Mars.

Hope you find Mars. You can go to a planetarium nearby to view Mars through the telescope. If you are lucky you may be able to see its two satellites – Phobos and Deimos.

The Life of a Star

What is a Star? A star is a hot body of glowing gases that emits light and undergoes nuclear reaction. This is the only difference one can point out between a star and a planet. There are billions and billions of stars found in our galaxy, The Milky Way. There are thousands of galaxies in the Universe. It is said that there are more stars in the universe than the number of grains of sand found in all the beaches of the Earth. It is predicted that there are 100 billion stars in the Milky Way. Stars vary in their size, mass, temperature, density, etc. No two stars are alike just as no two humans are alike (left alone the identical twins). But yes, the stars do look alike as you watch them in the night but if you watch them carefully you can see that some stars are bright and some are comparatively less bright.

Stars, just like life, have birth and death of its own. Stars live for a period of time and then it dies. By saying stars I also include the Sun. At average stars live upto 10 billion years. Let’s see how a star is born.

Birth of a Star:

Usually a star is born in a region of high density Nebula. Nebula is nothing but a cloud of dust in the space in which stars and planets are born. Nebulae are visible to our naked eye as a tiny coloured patch of light. The Orion Nebula is one of the brightest nebula situated in the Orion Constellation.

The picture above shows the orion nebula at the bottom left and horse head nebula at the top right. This nebulae is found in the constellation of Orion.

When the nebula condenses and contract under its own gravity it creates a new star. The region of condensing matter will begin to heat up and it starts to glow. These glowing bodies are called as protostars.

The above picture shows a protostar in a nebula. The x-ray version does not show the protostar but you can clearly see them in an Infrared version of the image.

When a protostar contains enough matter the central temperature reaches 15 million degrees centigrade. At this temperature nuclear reactions starts where Hydrogen fuses to form Helium. The star then begins to release energy stopping it from contracting. Now it is called as Main Sequence Star. Sun is in Main Sequence Star level. A star is said to be in its Main Sequence Level for 10 billion years before it starts to die. Sun is said to be 5 billion years old and it is said to live for 5 billion more years.

Death of a Star:

A star is considered dead when all the hydrogen is burnt into helium. But what happens to a star after its death? There are two possibilities based on their mass.

Mass of the Star is under 1.5 times the mass of the Sun:

If the mass of the star is less than 1.5 times the mass of the Sun then as the hydrogen gets less the star begins to expand. The expanding star is called as a Red Giant.

The above picture shows the size comparison of a Red Giant with the Sun and the Earth.

The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula.

The picture above shows the Boomerang planetary nebula where a shell covers around a star.

The remaining core (that’s 80% of the original star) is now in its final stages. The core becomes a White Dwarf the star eventually cools and dims. Sirius, the brighest star in the sky is a White Dwarf. When it stops shining, the now dead star is called a Black Dwarf.

The picture below shows the white dwarfs which are circled.



Mass of the Star is greater than 1.5 times the mass of the Sun:

If the mass of the star is greater than 1.5 times the mass of the Sun then as the hydrogen gets less the star begins to expand just as the previous case. But here the star becomes massive in size and it is called as a Red SuperGiant.

The above picture is a size comparion of a Red SuperGiant Aldebaran with the Sun.

The SuperGiant then starts of with a helium core surrounded by a shell of cooling, expanding gas. In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. The actual supernova shines brighter than the entire galaxy for a short time. The bright object at the top left corner (arrowed) is a supernova explosion.

Sometimes the core of the star survives the explosion. If the surviving core is between 1.5 to 3 times the mass of the sun, it contracts to become a tiny, very dense Neutron Star. If the core is much greater than 3 times the mass of the sun, the core contracts to become a Black Hole.

Picture of the Month - December 2007

If you take a picture of the Sun at the same time each day for a year, would it remain in the same position? The answer is no.

The shape traced out by the Sun over the course of a year is called an Analemma. The Sun's apparent shift is caused by the Earth's motion around the Sun when combined with the tilt of the Earth's rotation axis. Earth does not move around the Sun in a perfect circle. The Earth's orbit is rather elliptical. So at some part of the year Sun is closer to the Earth and at some part it is farthest. Earth's axis is tilted at an angle of 23° which gives out seasons. During summer the Sun will appear at its highest point of the analemma and lowest during winter.

Analemmas created from different Earth latitudes would appear at least slightly different, as well as analemmas created at a different time each day. The analemma pictured on top was built up by Sun photographs taken from 1998 August through 1999 August from Ukraine. The foreground picture from the same location was taken during the early evening in 1999 July.