Definition of Astrophysics: Astrophysics is mainly the branch of space science which applies the laws of physics and chemistry to describe the birth, life and death of planets, stars, galaxies, nebulae and other objects in the universe. This consists of two sibling sciences, astronomy and cosmology, and the lines connecting them smudge.

Introduction to Astrophysics:

The two major divisions in this field are theoretical and observational astrophysics. There is no such thing as experimental astrophysics as the scales and objects being noticed or viewed is far too large or far away to experiment on with modern technology. Since light takes time to travel to us on Earth, the most far-away regions of the universe are in reality windows into the ancient universe, when the universe was far denser and more energetic. As this field at times deals by the theories of early, compact universe, it can overlap strongly with particle physics that gives predictions of how matter would behave in the prehistoric universe.

Astrophysicists are acknowledged for studying such phenomena as galaxies, black holes, super-clusters, quasars, neutron stars, the Big Bang, cosmic strings, dark matter and energy, stellar evolution, the cosmic microwave background radiation and numerous others. The cosmos is a good arena for studying the pure physics because on such huge scales, the specific kind of element making up objects becomes less important, and more general variables, like mass and velocity, take primacy. At times, this field is termed as 'the study of the very large and very small'.

Most of the significant insights to mankind's understanding of the universe have been contributed through astrophysicists. They have predicted the probable age of the universe, the size of the observable universe, how long the Sun will last before it exhausts its nuclear fuel, the commonness of black holes and various other exotic celestial bodies, what the universe looked like billions of years ago, the average temperature of interstellar or intergalactic space, the shapes of galaxies, and the manner that matter is distributed across the visible universe. Astrophysics for all time continues to grow and generate new insights to the structure of the universe.

Milestones in astrophysics:

As the only way we interact with far-away objects is by observing the radiation they emit, much of astrophysics has to do by deducing theories which illustrate the methods that generate this radiation, and give ideas for how to extract nearly all information from it. The first ideas regarding the nature of stars emerged in the mid-19th century from the blossoming science of spectral analysis that means observing the specific frequencies of light that specific substances absorb and emit whenever heated. Spectral analysis remains necessary to the triumvirate of space sciences, both the guiding and testing new theories.

Early spectroscopy provided the very first proof that stars have substances as well present on Earth. Spectroscopy revealed that a few nebulae are purely gaseous, whereas some contain stars. This later helped cement the idea that a few nebulae were not nebulae at all - they were other galaxies!

In the year of early 1920s, Cecilia Payne introduced, by using spectroscopy, that stars are mainly hydrogen (that is, at least until their old age). The spectra of stars as well allowed astrophysicists to find out the speed at which they move toward or away from Earth. Just similar to the sound a vehicle emits is different moving toward us or away from us, as a result of the Doppler shift, the spectra of stars will change in the same manner. In year 1930, by joining the Doppler shift and Einstein's theory of general relativity, Edwin Hubble provided solid proof that the universe is expanding. This is as well predicted by Einstein's theory, and altogether forms the foundation of the Big Bang Theory.

As well in the mid-19th century, the physicists Lord Kelvin (or William Thomson) and Gustav Von Helmholtz considered that gravitational collapse could power the sun, however ultimately realized that energy generated in this manner would only last 100,000 years. Fifty years later, Einstein's famous E = mc² equation provide astrophysicists the first clue to what the true source of energy might be (however it turns out that gravitational collapse does play a significant role). As nuclear physics, quantum mechanics and particle physics rise in the first half of the 20th century, it became probable to formulate the theories for how nuclear fusion could power stars. Such theories illustrate how stars form, live and die, and successfully describe the observed distribution of kinds of stars, their spectra, luminosities, ages and other characteristics.

Astrophysics is basically the physics of stars and other far-away bodies in the universe, however it as well hits close to home. According to the Big Bang Theory, the first stars were nearly entirely hydrogen. The nuclear fusion method that energizes them smashes together hydrogen atoms to make the heavier element helium. In the year 1957, the husband-and-wife astronomer team of Geoffrey and Margaret Burbidge, all along with physicists William Alfred Fowler and Fred Hoyle, illustrated how, as stars age, they form heavier and heavier elements, which they pass on to later generations of stars in ever-greater quantities. It is only in the final phases of the lives of more recent stars that the elements making up the Earth, like iron (32.1 percent), oxygen (30.1 percent), silicon (15.1 percent) are generated. The other of such elements is carbon that together with oxygen, form the bulk of the mass of all living things comprising us. Therefore, astrophysics states us that, as we are not all stars, we are all stardust.

Copernicus gets Solar System geometry, but no scale:

Copernicus (early 1500's!) computed the relative size of the planetary orbits to ≈ 1%. His unit, that is, the astronomical unit (AU), was of unknown size. The method was geometry:

When one supposes circular orbits and constant velocities all along orbits (can ensure this observationally), then the dates of opposition/quadrature/greatest elongation/conjunction provide the relative geometry.

Parallax of Mars, transits of Venus determine scale:

Placing a scale on things needs that any one distance be known both in AU and in km.

1671: French expedition to the Cayenne (French Guiana, home of hot peppers) computed parallax of Mars at opposition:

They got the wrong answer (or right!) to ≈ 10%.

Halley, in the year 1716 (age 60) illustrated that the transit of Venus in the year 1761 and 1769 could be employed. He is familiar that he would be dead by then and told all young astronomers regarding it. (Such transits are quite infrequent: next ones were 1874, 1882, June 7, 2004 and June 5, 2012.)

This will be noted that there is a geometrical magnification factor which helps:

D/d = 0.72/0.28

In the year 1761 and 1769 different discrepant results ± 10% were obtained. Not till year1835 was an accurate value obtained by Encke from these similar measurements: Gauss had, for the meantime, introduced least squares for getting combining observations!

The modern value is:

1 AU = 149, 597, 870 ±1 km.

Newton's Law of Gravitation proposes mass of Sun:

Given the AU, we can make use of Newton's Law of Gravitation to obtain the mass of the Sun. This is most likely the first illustration of the true astrophysics:

M = 2.00 x 10³³ g

(The modern value is 1:989 x 10³³ g).

Stellar distances from Parallax across Earth's orbit:

The first exact stellar distances came from the parallax across the Earth's orbit:

As,

1 arcsec = 1 arcsec x (1^o/3600 arcsec) (π rad/180^o)

1 arcsec = (1/206265) rad

Therefore, 1 pc ≡ 206265 AU (= 3.26 ly).

By the help of modest astrometry telescopes (and lots of attempt) direct parallaxes are possible out to 10^-50 pc (0.1 to 0.02 arcsec). Beyond that, indirect processes should be employed.

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