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A few galaxies are bright enough to see something of them with the naked eye. For example, in the northern hemisphere we can see the centre of the Andromeda Galaxy (about #2.5# million light years away) as a fuzzy patch of light.

In order to capture a decent image with a camera attached to a telescope, the telescope is kept pointing at the same part of the sky using a right ascension drive. This motorised mechanism cancels out the effect of the rotation of the Earth. If the sky is clear enough and there is sufficiently little light pollution, a long exposure can be used to capture faint details of a distant galaxy.

Locations in the sky are referenced using one of a small number of coordinate systems. Once you have a coordinate system, you can systematically point your telescope at different patches of the sky to catalogue what you can see. For example, the Mount Palomar #200# inch telescope was used to produce a star atlas in the 1950's consisting of #935# pairs of #14# inch photographic plates, recording stars and galaxies down to magnitude #20#.

Nowadays we know where many galaxies can be found, so can immediately point telescopes in the right direction to observe them.

Concerning "zooming in", it is interesting to note that nature sometimes gives us a helping hand to see some of the most distant galaxies. Sometimes clusters of galaxies can form a gravitational lens, magnifying stars and galaxies beyond them.


Black holes are "seen" through their interactions with other objects. The various interactions show up in X-ray emissions, the motion of surrounding stars, and most recently gravitational waves.


X-ray emissions

Most likely black holes are detected through X-ray emissions. If a massive star that has collapsed is part of a bibary system, the dense core can pull gas from the remaining "live" star. The gas spirals inwards, gains energy from the gravitational field, and it gets so hot that its emissions are mostly X-rays. A compact object that emits primarily X-rays, next to a light emitting star, is lilely a black hole if it appears to have enough mass. Cygnus X-1 is the best known such candidate.

A larger-scale version of this process is seen in the center of some galaxies, producing emissions so bright that that they outshine all the surrounding stars (https://en.wikipedia.org/wiki/Quasar).

Motion of Surrounding Stars

Massive black holes, containing millions of solar masses or more, can exert strong gravitational forces on surrounding stars. We detect this effect through the rapid, sharply curved motion of the stars. The central black hole in our galaxy is detected in this way (http://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html).

Gravitational Waves

The most certain way to detect black holes is to detect the gravitational waves produced when they collide with other black holes or other objects. The gravitational waves are made of the same "stuff" as the gravity of the black holes and so offer the clearest indication of such black holes. Recently the Laser Interferometer Gravitational-wave Observatory (LIGO) detected such gravitational waves from a black-hole collision (http://news.mit.edu/2016/second-time-ligo-detects-gravitational-waves-0615).


Actually the Earth doesn't orbit the Sun.


In reality no two bodies orbit each other. They actually orbit the centre of mass of the system which is called the barycentre.

The Earth and Moon both orbit about their centre of mass called the Earth-Moon barycentre.

In the case of the solar system, the Sun and all of the planets and other bodies always orbit around the Solar System Barycentre (SSB).

So, the focus of the Earth's orbit is the SSB which is in constant motion.

The position of the SSB is constantly changing. It can be anywhere between the centre of the Sun and two solar radii from the centre of the Sun. This depends on the relative positions of the planets and other bodies.

The diagram shows the position of the SSB over a period of decades.

enter image source here


The strong force holds atomic nuclei together and the weak force causes radioactive decay.


The strong nuclear force is responsible for binding protons and neutrons together in an atomic nucleus. It is strong and short ranged and has to overcome the electromagnetic force which is pushing positively charged protons apart.

A good example of the strong force is the fusion process which happens in smaller stars such as our sun. Positively charge protons repel each other. At the extreme temperatures and pressures in the sun's core, two protons can get close enough together for the strong nuclear force to bind them into a bi-proton or Helium-2 nucleus.

A bi-proton is very unstable and most of them fly apart. For the fusion process to continue to produce Deuterium the weak nuclear force is required.

The weak nuclear force is responsible for radioactive decay by being able to convert a proton into a neutron of vice versa. To be more precise it converts an up quark to a down quark or vice versa by means of the W boson. In the case of fusion a proton is converted into a neutron, a positron and an electron neutrino.

#W^+ ->e^++nu_e#

In fact the strong nuclear force doesn't really exist. Early theories described the strong nuclear force as binding protons and neutrons using the pion as the force transmitting boson. We now now that protons, neutrons and pions are composite particles consisting of quarks bound by the colour force transmitted by gluons. So, the strong force is actually a residual effect of the colour force extending beyond the inside of protons and neutrons to bind them together.


Fundamental forces are independent, non fundamental forces can be explained in terms of fundamental forces.


It is better to use the term interaction rather than force as two of the four fundamental interactions are not really forces.

Electromagnetism the the fundamental interaction which causes attraction, repulsion and motion of charged particles. The photon is the boson which mediates the interaction.

The colour force is a fundamental interaction which binds quarks into mesons and baryons. Gluons and the bosons which mediate the interaction.

The weak force is a fundamental force which causes beta radioactivity. It can convert a proton into a neutron, a positron and an electron neutrino. The W and Z bosons mediate the interaction.

Gravity is the fundamental interaction which causes masses to attract each other. It is a consequence of the curvature of spacetime.

The strong nuclear force used to be considered a fundamental interaction, but it is now known to be a residual effect of the colour force.

Forces such as friction are actually the result of electromagnetic interaction between electrons.


The Doppler effect which changes the spectra of of stars and galaxies toward the red or longer wave lengths tells scientists that the universe is expanding at an every increasing rate.


The red shift first observed by Edwin Hubble changed the way scientists viewed the universe. The red shift is part of the Doppler affect waves coming toward the observer move toward the blue side of the spectrum as the waves get closer together, and waves move toward the red side of the spectrum as the waves get further apart. Before Hubble's observations of the change in the spectra of stars scientists believed that the universe was static. The belief was that the universe had always existed in its "present" observational state. This view was consistent with the philosophy of material realism that matter and energy are all that has ever existed or ever will exist.

After the observations of the red shift doppler effect were widely accepted the static steady state view of the universe was abandoned. The Big Bang theory began to be developed in response to the empirical evidence of an expanding universe.

Recently ( in 1998) observations of the rate of expansion of the universe is increasing. The observations were based on the spectra of super novas, made by the Lawrence Berkley National Laboratory.
The hypothesis was that the rate of the expansion should be slowing down. The

The hypothesis was based on the theory that the universe was eternal and would recycle alternating between Big Bangs and Big Crushes. The Hypothesis was proven wrong by the observations of the changes in spectra of the galaxies.

The changes of the spectra of the galaxies tell scientists that our present universe had a beginning, will have an ending and that matter and energy are not eternal or self existent.

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