We all have surely heard about the general theory of relativity, which was proposed by Albert Einstein. His theory states that gravity is a geometric property of space and time (known as spacetime), which is related to the subject's mass, energy, and momentum. So when a star of huge mass, energy, and momentum, is compressed into a compact small space, an infinite spacetime and a massive gravitational pull is created. The spacetime curvature caused by this mammoth gravity is so huge that even light gets sucked into this space. When stars of the solar system meet such a fate, they are termed as black holes.
Formation in Space
In the life cycle of a star, there is constant nuclear fusion at its core, which generates photons through electromagnetic radiations. This radiation is responsible for the outward pressure that perfectly balances the pull of gravity from the mass of the star. However, at the fag end of the star's life, when the nuclear fuel of the star starts diminishing, the outward pressure starts to reduce. Consequently, the inward gravitational pull increases manifold, thereby shrinking the star inward. Eventually, the core of the star collapses and becomes a black hole.
- One of the most basic facts about black holes, based on Einstein's theory of general relativity, is that the objects falling into it never reappear. Its gravity is so intense and powerful, that a gravitational time dilation happens, which causes time to stop.
- It has two sections: singularity and event horizon. The event horizon is its surface, whereas the singularity signifies its core. Anything to get sucked into one needs to be in the vicinity of the event horizon, else the object will not be sucked in. Event horizon is also known as - "point of no return". Singularity is the sole point of suction where the volume is zero and density is maximum. This is the point of infinite spacetime curvature.
- Its mass and size (radius of the event horizon) are directly proportional to each other. For instance, if one is 10 times heavier than another, its radius also ought to be 10 times larger than the other. In fact, if there is one with a mass equivalent to the Sun, it will have a radius of 3 kilometers only! The radius of this event horizon is also known as the Schwarzschild radius, which can be calculated by the formula: r=2GM/c2 (where r: Schwarzschild radius, G: gravitational force, M: mass, and c: speed of light or escape velocity.)
- With time, they shrink in size, as they constantly emit x-ray radiations. After a while, they nearly evaporate.
- Only stars 10-15 times as massive as the Sun can transform into black holes, as they can get compressed to the Schwarzschild radius. Smaller stars become white dwarfs or neutron stars.
- Last but not the least, even light gets sucked into the black hole once it crosses the event horizon. To escape from it, one needs an escape velocity greater than that of light, which is not possible!
- Currently, there are 14 known black holes in the universe.
- The one nearest to earth is known as V 4641. It is 1600 light years (16 quadrillion km) away.
- The next closest is Cygnus X-1, which is about 8000 light years (80 quadrillion km) away.
- There are 2 types: Stellar and super massive. A stellar is a small one, which is formed whenever a star collapses. On the other hand, a super massive one is an aggregation of many black holes. A super massive might contain a million to trillion Suns!
- At the center of the Milky Way galaxy, there is a massive one, which weighs about 4 million solar masses. Fortunately it is 30,000 light years away from earth and hence no need to get worried about being sucked in!
NASA is exploring more into the mysteries with the help of its space missions. XMM-Newton is a space telescope that was launched into the Earth's orbit in 1999 by NASA and the European Space Agency. This telescope has helped the scientists a great deal in unraveling many facts. This is because the telescope can observe the universe in a light containing high energy x-rays, which are mostly emissions of black holes.
The other space mission to be launched by NASA, is LISA (Laser Interferometer Space Antenna). LISA has 3 spacecraft which will orbit the Sun, all of them connected by laser beams. This space mission will aid in understanding the concept of gravitational waves of black holes. LISA will get launched in 2015. Not to miss an interesting fact, NASA's Galaxy Evolution Explorer, in 2006, had caught an interesting snapshot of a giant black hole gobbling up a star! This was the first of all times when the astronomers saw this entire process.
So, you must be wondering what if our Sun becomes a black hole? Relax! Our Sun is not massive enough to become one. It might become a white dwarf or a neutron star after several billion years, which is definitely not anytime in near future!