What is Pulsar?

A pulsar is a highly magnetized, high speed rotating neutron star that emits a beam of electromagnetic radiation. Pulsar is a short word for pulsating radio star.  Pulsars radiate two narrow beams of light in opposite directions but appear to flicker because they spin ( its similar the way a lighthouse is seen on the ocean by a observer). 

From earth pulsar may look like its blinking but in reality its not. When pulsar rotates, the beam of light also rotates and during rotation the beam moves away from earth view and when it comes back again and point towards earth and makes it like its blinking. As mentioned above that its similar the way a lighthouse is seen on the ocean by a observer. 

Pulsars pulses range roughly from milliseconds to seconds for an single pulsar. Most of the pulsars rotates at once per second and they are called as slow pulsars where as those pulsars rotates hundreds of times per second are known as milliseconds pulsars. 

How a pulsar is formed?

The formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star.  it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. The beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field. This rotation slows down over time as electromagnetic power is emitted. When a pulsar's spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called "death line"). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars born in the 13.6 billion year age of the universe, around 99% no longer pulsate.

Nemesis

The Nemesis Star - Our Sun's Companion

Scientists have been puzzled by an apparent cycle of mass extinctions that happen roughly every 26 million years (though some argue that it is more likely a 63 million year cycle). If the evidence holds true, and there are cycles of mass extinction on Earth, what could be the cause? Well some have suggested that there is a heavenly explanation. Specifically, our Sun might be in a binary system. And our Sun's companion -- deemed the Nemesis star -- is responsible for destroying life on Earth.

History of the Nemesis Star Theory

Analysis of the fossil record indicates that there are periods of time in history where a massive number of species, including all types of living creatures, become instinct. In 1984 researchers David Raup and Jack Sepkoski identified 12 such mass extinctions with each occurring roughly 26 million years after the previous event.
It should be noted that the accuracy of these claims has been widely challenged in the literature. At the very least, the 26 million year cycle length is often called into question, with periods closer to 63 million years often proposed. In either case, the last 25 years has seen a multitude of theories to try and explain this phenomenon.
In 1984 after the popularization of the mass extinction models two independent research groups (the first of Daniel Whitmire and Albert Jackson, and the second of Marc Davis, Piet Hut, and Richard Muller) devised theories for the cause of these mass extinctions that involved the presence of a companion star to our Sun. Essentially, each theory outlined how a star, yet to be detected, could disrupt the comets in the Oort Cloud and send them hurtling into the inner part of the solar system.
The logical question is, why is this theory any better than others that have been proposed? The main reason is that it provides a convenient explanation to observational data of the Oort cloud that scientists don't have an explanation for. Our Oort cloud has sharp, well-defined edges like those of Oort clouds around stars in other binary systems. Where as Oort clouds of isolated stars are far more diffuse. Additionally, researchers have noticed that most of the comet which make their way into the inner part of the solar system come from the same region of the Oort cloud. This indicates that there is some sort of gravitational disruption in that specific direction.
There was also recently discovered a dwarf planet, known as Sedna, that has a very unusual, and highly elliptical, orbit. At its farthest point it is nearly 1000 times further away from the Sun than the Earth is from the Sun. Scientists are baffled by its very existence, claiming that it could not possibly remain in its current orbit unless it is being influenced by another, massive object. So it would seem that even though no companion has yet been confirmed, it would seem likely given the observational data we have thus far.

What is the Nemesis Star and How Do We Find It?

There are two competing theories as to what the Nemesis star actually is. The theory put forth by Whitmire and Jackson stipulates that it is not a star at all, but rather a brown dwarf -- a protostar that never accumulates enough mass to ignite nuclear burning and become a star -- orbiting out in the Oort cloud. The typical mass of a brown dwarf is somewhere between that of a planetary gas giant (like Jupiter) and the lowest mass stars (about 8% the mass of our Sun).
Brown dwarfs are very difficult to detect since they are non-luminous -- since they do not readily radiate energy due to a lack of nuclear fusion or other exothermic reaction. Though, energy can be radiated due to their slow gravitational contraction heating the gas molecules within the object. For this reason, brown dwarfs should radiate light in the infrared.
Luckily a new infrared observatory, WISE, has been launched that should have the capability to detect such an object if it does exist. After WISE performs two complete scans of the sky researchers will be able to compile the data and map the positions and motions of all the nearby infrared objects, including Nemesis if it exists. However, it will take until at least 2013 to complete the scans and compile all the data.
But Richard Muller doesn't think that the Nemesis star is a brown dwarf at all. He and his colleagues have proposed that it is rather a small, cool star known as a red dwarf. These objects are typically much smaller than our Sun (though can be as much as half the solar mass). Muller argues that such a red dwarf is currently orbiting our Sun about 1.5 light years away (the Oort cloud extends roughly 1 light year from the Sun). But critics argue that there are problems with this theory.
If the Nemesis is a red dwarf, it would be much closer than the next closest star, Proxima Centauri (4.2 light years away). So first of all, we should have seen it already. And secondly, at 1.5 light years away it could not possibly be in a stable orbit so it could not explain the extinction cycle.
The first concern is completely valid and completely correct. We should have seen it. And actually, we may already have. There are many stars that we have observed and catalogued that we are still learning about. And given its proximity and slow proper motion, it would be difficult to determine its distance. So it may turn out that we detected the source long ago, but have only yet to realize it.
And Muller is quick to answer his critics on the second point and retort that a non-stable orbit is not a problem. In fact Nemesis has been spiraling out from our Sun for billions of years and will continue to do so. In fact, he predicts that within the next billion years or so it will no longer be bound by the Sun's gravity.

What Does This Mean For Us?

This is a question of great debate as well. There have been those that speculate that gravitational disruptions of the Oort cloud could cause comets to reign down on the inner solar system. While others believe that the Nemesis is innocuous. But we won't know for sure until we either find it or eliminate it as a possibility.

Formation of a Star

Stars begin as vast clouds of dust mainly hydrogen and helium left over from the supernova and formed Nebula. A good example of such as a dust cloud is the Orion Nebula.
Far from active stars, these nebulae remain cold and monotonous for ages.

These vast clouds can be hundreds of light years across. When some disturbance overcomes this balance and causes the cloud to begin collapsing.

These disturbance may come from a shockwave from a nearby supernova explosion or a distant supernova explosion, collision with another gas cloud, or the pressure wave of a galaxy’s spiral arms passing through the region.

These force moves though the cloud, particles collide and begin to form clumps. Individually, a clump attains more mass and therefore a stronger gravitational pull, attracting even more particles from the surrounding cloud and gets compressed.

As more and more matter falls into the clump, its center grows denser and hotter. Over the course of a million years, the clump grows into a small, dense body called a protostar. It continues to draw in even more gas and grows even hotter.

When the protostar becomes hot enough 6999727ºC. Its hydrogen atoms begin to fuse, producing helium and an outflow of energy in the process.
Material continues to flow into the protostar, providing increased mass and heat.

Now, what type of star it will become depends on amount of matter present in it.

Some object does not have enough mass for stellar ignition and become brown Dwarfs.

If a star has enough material, it can generate enough pressure and temperature at its core to begin fusion "a heavier isotope of hydrogen".
If enough mass collapses into the protostar, a bipolar flow occurs. Two massive gas jets erupt from the protostar as hydrogen fusion begins and blast the remaining gas and dust clear away from its fiery surface.

At this stage newly born star stabilizes. It is the point where its output exceeds its intake. The outward pressure from hydrogen fusion now counteracts gravity's inward pull. It is now a new born star and will remain until it burns through all its fuel.

If a protostar contains the mass of our Sun, or less, it undergoes a proton-proton chain reaction to convert hydrogen to helium. But if the star has about 1.3 times the mass of the Sun, it undergoes a carbon-nitrogen-oxygen cycle to convert hydrogen to helium.

Q. What is the lifespan of newly born stars?

A. It all depends on mass and how quickly it consumes its fule  i.e. Hydrogen.

Small red dwarf stars can last hundreds of billions of years, while large supergiants can consume their hydrogen within a few million years and die as supernova.

A star about the size of our sun takes roughly 50 million years to reach main sequence and maintains that level for approximately 10 billion years

This lifespan of sun began roughly 4.6 billion years ago, and will continue for about another 4.5 – 5.5 billion years, when it will deplete its supply of hydrogen, helium the elements will "swell" up, swallow Earth, and eventually die off into a small white dwarf. 

What is Star ?

A star is a luminous ball of plasma held together by its own gravity. The nearest star to Earth is the Sun. However, when Sun is compared with other stars in the universe it looks small dots and nothing else. In simple words Sun is very-very small when compared to other stars in the universe and known as Super Giants.

Many other stars are visible to the naked eye from Earth during the night, appearing as a fixed luminous points in the sky due to their immense distance from Earth but our Sun is close to Earth and due to this it looks very-very bright and makes DAY here at Earth. At the same time we get heat also.

Top 4 Biggest Stars In The Universe

As we already know that Sun is not bigger as compared to other stars in the universe. Above image is clearly showing the comparison of stars.

So, lets see the top 4 biggest stars in the universe.

1. VY Canis Majoris

VY Canis Majoris is a red hypergiant star located in the constellation Canis Major. It is among the largest known stars in our galaxy and has a radius of approximately 1,420 solar radii (990,000,000 km; 6.6 au), and is located about 1.2 kiloparsecs (3,900 light-years) from Earth. VY Canis Majoris is a high-luminosity M star with an effective temperature of about 3,500 K. 

VY Canis Majoris is surrounded by an extensive nebula, which contains condensations that were once regarded as companion stars. The nebula has been extensively studied with the aid of the Hubble Space Telescope. 
It has a complex structure that includes filaments and arcs, which were caused by past eruptions.

2. VV Cephei

VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 5,000 light years from Earth. It is both a B star and shell star.  

VV Cephei is an eclipsing binary with the second longest known period. The supergiant primary, known as VV Cephei A, is currently recognised as one of the largest stars in the galaxy, with an estimated radius of 1,050 R☉. 

VV Cephei experiences both primary and secondary eclipses during a 20.3 year orbit. The primary eclipses totally obscure the hot secondary star and last for nearly 18 months. Secondary eclipses are so shallow that they have not been detected photometrically since the secondary obscures such a small proportion of the large cool primary star.

VV Cephei A is fairly clearly identified as an M2 supergiant, and as such, it is given a temperature around 3,800 K. 5-8 times the radius of the Sun, 15-18 times the mass of the Sun, and around 25,000K. 

3. Mu Cephei

Mu Cephei  also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as the M2 Ia standard by which other stars are classified. 

Mu Cephei is visually nearly 100,000 times brighter than the Sun, with an absolute visible magnitude of Mv = −7.6. Summing radiation at all wavelengths gives a luminosity of around 280,000 L☉.  making it one of the most luminous red supergiants in the Milky Way. It is also one of the largest stars known at 1,260 R☉ (or 877,000,000 km). 

The photosphere of Mu Cephei has an estimated temperature of 3,750 K. It may be surrounded by a shell extending out to a distance at least equal to 0.33 times the star's radius with a temperature of 2,055 ± 25 K. This outer shell appears to contain molecular gases such as CO, H2O, and SiO.

Mu Cephei is nearing death. It has begun to fuse helium into carbon, whereas a main sequence star fuses hydrogen into helium. When a supergiant star has converted elements in its core to iron, the core collapses to produce a supernova and the star is destroyed, leaving behind a vast gaseous cloud and a small, dense remnant. For a star as massive as Mu Cephei the remnant is likely to be a black hole. The most massive red supergiants will evolve back to blue supergiants or Wolf-Rayet stars before their cores collapse, and Mu Cephei appears to be massive enough for this to happen. A post-red supergiant will produce a type IIn or type II-b supernova, while a Wolf Rayet star will produce a type Ib or Ic supernova.

4. WOH G64

WOH G64 is a red hypergiant star in the Large Magellanic Cloud satellite galaxy in the southern constellation of Dorado. It is 168,000 light years away from Earth and is one of the largest known stars, with a radius of 1,540 solar radii, corresponding to a volume some 3.65 billion times bigger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Jupiter.

WOG G64 varies regularly in brightness by over a magnitude with a primary period of around 800 days.