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20.Telescope (a little bit of history)

  In 1608 Hans Lippershey, an eyeglass maker, submitted the first patent of telescope. But who made the first telescope, it is not clear. Later on, Galileo improved the design and started applying in Astronomy. This telescope had convex objective lens and concave eyepiece lens. In 1611 Kepler developed the design using convex lens in both objective and eyepiece and this design was more useful than the previous one developed by Galileo. Diagram of Galilean and Keplerian Telescope In 1655 Huygen was making powerful and enhanced Keplarian telescopes using compound eyepiece but those telescopes were proved inaccurate. In 1688 Isaac Newton built the first reflector telescope which consisted of a tiny flat diagonal mirror to reflect the incoming light on an eyepiece. Using achromatic lens, in 1733, Chester Moore Hall built a telescope. First Reflector Telescope built by Newton Diagram of Achromatic Lens When light of white color passes through a medium it splits into several colors. For exam
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Neutron Star (A Brief Introduction)

 

A Massive Star

Formation:

A star too heavy, more than about 8 solar masses, to follow the evolutionary path that leads to white dwarf has a different fate. The large mass of such a star causes it to collapse abruptly when out of fuel and then to explode violently. The explosion flings into space most of the star's mass. An event of this kind called a super-nova is billions of times brighter than the original star ever was. What is left after a supernova explosion is the remnant of the star whose mass is greater than 1.4 time the mass of the sun. As this star contracts gravitationally, its electrons become more and more energetic. When the Fermi energy reaches about 1.1 Mev, the average electron energy is 0.8 Mev which is the minimum energy needed for an electron to react with a proton to produce a neutron. This point is reached when the star's density is about 20 times greater than the mass of a white dwarf. From then on neutrons are produced untill most of the electrons and protons are gone. The neutrons, which are fermions, end up as a degenerate gas and their pressure supports the star against gravitational shrinkage.

A visualization of the size of a Neutron Star

Size:

Neutron stars are thought to be 10 to 15 K.M. in radius with masses between 1.4 to 3 times the mass of the sun. If the earth were this dense, it would fit into a large apartment house.

Some properties:

Stars have magnetic fields and as a star contracts into a neutron star, its surface field increases enormously. The magnetic field is produced by motions of electrons that remain in its interior and since they cannot lose energy, the field should persist for a time long compared with the age of the univerese. The magnetic field of a neutron star traps tails of ionized gas that radiates light, radio waves and X-rays.


Neutron stars trap tails of ionized gas that radiates light

If the magnetic axis is not alinged with the rotational axis when neutron star rotates the beam may sweep across the Earth, an astronomer on earth will receive bursts of radiation as the neutron star spins. This kind of neutron star is called pulsar. Thus a pulsar is like a lighthouse whose flashes are due to a rotating beam of light. Over 1000 pulsars have been discovered, all with periods between 0.0016 to 4 seconds.The best pulsar which is at the centre of the Crab Nebula, has a period of 0.033 second and it is increasing at a rate of 10^-5 second per year as the pulsar loses angular momentum.

Possible ways of a Neutron Star's death:

Pulsars or rotating neutron stars may radiate particle like neutrinos and lose energy, so they gradually slow down. There are three hypothetical endings for neutron stars.
1. Collapsing with neighbouring star or neutron star (if it is a binary system) and turning into a black hole.
2. Through losing energy spin slows down which means decrease in angular momentum and so centrifugal force decreases. Then it won't be able to hold the gravity and will collapse into a black hole.
3. When it can no more hold back the gravity, it explodes and disappears.

Possible death of Neutron Star

But the ending story is still on progress, these are just hypothesis.

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