<p><br></p><div class="text-justify">

<center>![solar-system-11111_1280.jpg](https://cdn.steemitimages.com/DQmW2Pbcjsrx5NZafzDVfnKYusv1C23xysEGhACMRLVrdNH/solar-system-11111_1280.jpg)</center>
<center><sup>
[Pixabay, WikiImages; public domain](https://pixabay.com/illustrations/solar-system-planet-planetary-system-11111/)</sup></center>
According to a popular early-19th-century lullaby written by Jane Taylor which goes thus; 

&gt;“Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky.<sup>[source]( https://en.m.wikipedia.org/wiki/Twinkle,_Twinkle,_Little_Star)</sup> 

<p>The word, <i>“Twinkle, twinkle, little star”</i>, is where the trouble starts. Stars begin to twinkle in the atmosphere when they are disturbed and those disturbances, in turn, lead to distortion of the refractive index of air pockets. So, viewing the stars with our naked eyes make the image formed on the retina to roam a little bit around. So far so much that using a very big telescope won’t lessen the issue on the image formed on the CCD sensor. The image of the star will blur over a small region after some time of continuous viewing rather than the normal small point-like image a star would produce in ideal conditions. The word “seeing” of the stars according to the astronomers, differs with weather conditions and the state of the atmosphere.</p>

<p>Almost all the modern earth-based observatories are situated on towering mountains, where the atmosphere above the telescopes is fine and thin, and it’s preferably in terrains where the cloud is sparse or too low to influence the viewing. There’s a popular joke that says that if you truly like sunshine in tropical climates, then become an astronomer!</p>

<center>![milky-way-916523_1280.jpg](https://cdn.steemitimages.com/DQmVq4WneSJGJ3XNTLwdq383Cea58ZeWJNaFBHcg33BuEqV/milky-way-916523_1280.jpg)</center>
<center><sup>
[Pixabay, Skeeze; public domain](https://pixabay.com/photos/milky-way-rocks-night-landscape-916523/)</sup></center>

<p>Apart from mounting telescopes on high mountains, another panacea is to place the telescope very well above the atmosphere in earth’s orbit just like the Hubble Space Telescope and some other temporary telescopes such as the Hipparcos satellite (High Precision Parallax Collecting Satellite. The Hubble telescope has a small aperture of about 2.4 m as compared with ground-based telescopes an example of which is the Very Large Telescope system at the La Silla Pinara Mountain Observatory in Chile with four 8.2 m telescopes whose outputs can be combined. The bitter truth is that all these large telescopes would still have “seeing” problems except with the support of an artificial star which is called the <b>Laser Guide Star (LGS)</b>.</p>

<center>![The_most_powerful_laser_guide_star_system_in_the_world_sees_first_light_at_the_Paranal_Observatory.jpg](https://cdn.steemitimages.com/DQmck1LjSQ28sckq7nH5XwtP5jPtxTpzPAyzrT8J4WLXZPC/The_most_powerful_laser_guide_star_system_in_the_world_sees_first_light_at_the_Paranal_Observatory.jpg)</center>
<center><sup>Powerful laser guide star system at the Paranal Observatory
[Wikimedia, ESO/G. Hüdepohl; CC BY-SA 4.0](https://commons.m.wikimedia.org/wiki/File:The_most_powerful_laser_guide_star_system_in_the_world_sees_first_light_at_the_Paranal_Observatory.jpg#mw-jump-to-license)</sup></center>

<p>At a distance of 90 km above the Earth, the atmosphere is composed of layers of sodium atoms. An adjusted laser with clearly described wavelength is launched in the direction of any object the astronomers want to look into. Sodium atoms are then converted to ions over a small space and shine brightly enough for a small ancillary telescope to observe. The expected signal is well known and any deviations from the norm caused by atmospheric conditions are measured. Rapid, automatic computer work enables signals to be sent to a series of pistons underneath the mirror of the large telescope being used. This flexes to return the light from the star or galaxy, received after an atmospheric disturbance as a distorted wavefront, to be sent to the detector in its perfect shape in a process known as <b>adaptive optics</b>. Adaptive optics is also used at the world's largest single telescope, <b>The Gran Telescopio Canaris</b> in the Canary Islands of Spain. </p>

<center>![Adaptive_optics_system_full.svg.png](https://cdn.steemitimages.com/DQmbyMTx14DgUQVt27XQCJ1dVpihd6LGByD3gk17YbimUTi/Adaptive_optics_system_full.svg.png)</center>
<center><sup>Schematic illustration of an adaptive optics (AO) system.
[Wikimedia, 2pem; CC BY-SA 3.0](https://commons.m.wikimedia.org/wiki/File:Adaptive_optics_system_full.svg#mw-jump-to-license)</sup></center>

<h3>The ideas in this post</h3>

In this post, I will first discuss the major characteristics of the Solar System i.e the Sun and its planets, asteroids, and comets and how they were composed. I will then review the Sun as a distinctive star, the only star close enough to critique in detail. Our knowledge and understanding of the nearby universe are based solely on the information of different forms of electromagnetic radiation. Although the closest stars are mostly much larger and brighter than the Sun, they are so far away that they appear as tiny pinpoints of light, even in a large telescope. Yet, we are able to gain a surprising amount of information from the tiny amount of electromagnetic radiation reaching us. Astronomers have fitted the different kinds of a star into a pattern that links their sizes, brightness, and ages. This pattern is the <b>Hertzsprung-Russell diagram</b>.

<center>![454px-Hertzsprung-Russel_StarData.png](https://cdn.steemitimages.com/DQmeXxtH5sT9i7ZjtfG4qxgC4FzgZYLrUfTAyEqFjR15741/454px-Hertzsprung-Russel_StarData.png)</center>
<center><sup>Hertzsprung-Russell Diagram identifying many well known stars in the Milky Way galaxy
[Wikimedia, ESO; CC BY-SA 4.0](https://commons.m.wikimedia.org/wiki/File:Hertzsprung-Russel_StarData.png#mw-jump-to-license)</sup></center>

<p>The brightest stars we see are relatively close and lie in our own branch of our own galaxy which is called the <b>Milky Way Galaxy</b> and contain 100 billion stars. The physics of stars is mostly to do with their sources of internal energy, and how they reach equilibrium by matching energy emission with the rate of energy production from nuclear and gravitational processes.</p>

<p>Finally, I’ll consider what happens when a star's supply of energy is running out, and its steady-state turn into the dramatic processes that produce red giants, white dwarfs, supernovas, neutron stars, and black holes.</p>

<h3>THE SOLAR SYSTEM</h3>

The planets move around the Sun in elliptical orbits, held in their paths by the gravitational force between them and the Sun. The Solar System also includes many smaller rocky objects called asteroids which form a belt between the orbits of Mars and Jupiter. The total mass of the Solar System is about 2.0 × 10<sup>30</sup> kg. Some 99.9 percent of this is concentrated in the Sun. The largest planet, Jupiter for example, has a mass of 1.899 × 10<sup>27</sup> kg- less than 0.1 percent of the Solar System's mass.<p></p>

<h3>THE NUMBERS ARE ASTRONOMICAL</h3>

Astronomy requires very large numbers when distances are measured in standard SI units like metres and kilometres, so astronomers also use two other units of distance - the astronomical unit and the parsec. Popular astronomy also uses the light year.

<h4><center>1 Astronomical Unit, AU (i.e Earth-Sun) = 150 × 10<sup>9</sup> m</center></h4>

<h4><center>1 Light year, ly = 6.3 × 10<sup>4</sup> = 9.45 × 10<sup>15</sup> m</center></h4>

<h4><center>1 parsec, 1 pc = 3.26 ly = 3.1 × 10<sup>16</sup> m</center></h4>

<b>The astronomical unit</b>, AU, is the mean distance of the Earth from the Sun, 150 × 10<sup>9</sup> m. On this scale, Jupiter is just over 5 AU from the Sun and Pluto about 40 AU. The AU is a convenient unit for astronomers as it is the basis of a method for measuring astronomical distances using parallax. The AU gives rise to the second unit, the <b>parsec</b>, pc. Parsec is short for “parallax second” and is the distance of a star which appears to shift its position in the sky by 1 second of arc as the Earth moves from the end of one diameter of its orbit to the other. It has a value of 2.06 × 10<sup>5</sup> AU (3.1 × 10<sup>16</sup> m)

<center>![New_shot_of_Proxima_Centauri,_our_nearest_neighbour.jpg](https://cdn.steemitimages.com/DQmWsoCdZ2F7gf4EBZjy37Zcf1kMFXwMdamyD1V4noLyxyg/New_shot_of_Proxima_Centauri,_our_nearest_neighbour.jpg)</center>
<center><sup>New shot of Proxima Centauri, our nearest neighbour
[ESA/Hubble &amp; NASA, CC BY-SA 4.0](https://commons.m.wikimedia.org/wiki/File:New_shot_of_Proxima_Centauri,_our_nearest_neighbour.jpg#mw-jump-to-license)</sup></center>

<p>The light year, ly, is the distance travelled by light in 1 standard year, 9.5 × 10<sup>15</sup> m. 1 pc is about 3.3 ly. To get an idea of the scales measured by these units: the nearest star to Earth is Proxima Centauri, a very faint companion star of the brightest star in the constellation of the Centaur. It is at a distance of 4 × 10<sup>16</sup> m, which is the same as 4.2 ly or 1.3 pc.</p>

<h3> THE MAIN CHARACTERISTICS OF THE SOLAR SYSTEM</h3>

Generally, the planets are divided into two  groups, <b>the inner planets</b> and the <b>outer planets</b>. It is not easy to show them all to the same scale on a diagram, but with space probes and satellites, we can be provided with remarkable pictures of the planets and information about their composition and appearance, revealing hitherto unsuspected rings and small satellites.

<h4>THE INNER PLANETS</h4>

The four inner planets are Mercury, Venus, Earth, and Mars, which all lie within 1.5 AU of the Sun. Then there is a belt of 'minor planets' and asteroids at an average distance of 2.8 AU, containing an estimated 50 000 objects. The inner planets and the asteroids are rocky bodies made of materials similar to the material on and in the Earth. Their cores are iron and nickel and they have lighter elements (mainly in compounds) such as oxygen, silicon, aluminium, magnesium with smaller quantities of potassium, sodium, and calcium.

<h4>THE OUTER PLANETS</h4>

The outer planets, called the Jupiter group, are Jupiter, Saturn. Uranus and Neptune, plus an oddity, the ‘double planet’ Pluto-Charon. These planets are much further away from the Sun than the inner planets and are much larger both in size and in mass. They are also much less dense, suggesting that their composition is different from the inner planets’ composition. Their bulk must be mostly the light elements hydrogen and helium, which are gases at the planets’ surfaces, but liquid or even solid at depth.

<center>![Pluto-Charon_system-new.gif](https://cdn.steemitimages.com/DQmPTqHWngKDCPTpaCqsFAwZGQiQdTkD61wyxiEBmcCGD9Y/Pluto-Charon_system-new.gif)</center>
<center><sup>Pluto-Charon system
[Wikimedia, Tomruen, Own work; CC BY-SA 4.0](https://commons.m.wikimedia.org/wiki/File:Pluto-Charon_system-new.gif#mw-jump-to-license)</sup></center>

<p>Pluto was discovered only in 1930: it is very small and is likely to be a rocky planet. Pluto is no longer classified as a planet. The International Astronomical Union decided in 2006 to restrict the use of the word planet to bodies orbiting a star which are massive enough for their own gravity to form them into a spherical shape. Pluto is now a <b>dwarf planet</b>. There are also <b>periodic comets</b> such as Halley’s comet.</p>

<h4>PERIODIC COMETS</h4>

Periodic comets move in very large orbits which extend far beyond the orbit of Pluto, the furthest planet. Comets are much smaller than planets and are made of dust and the ‘ice’ of water and other gaseous elements. Comets may have their origin in a region filled with a large number of objects (estimated to be 10<sup>12</sup>) called the Oort Cloud. The Oort Cloud is more than a thousand times more distant than Pluto, at the outer limit of the Solar System.

<center>![640px-Comet_P1_McNaught02_-_23-01-07-edited.jpg](https://cdn.steemitimages.com/DQmYARTnXpUtW5UzpkDDNM73g7s6oLCPdxRkCcYaASSfhdT/640px-Comet_P1_McNaught02_-_23-01-07-edited.jpg)</center>
<center><sup>Comet P1
[Wikimedia, Soerfm • CC BY-SA 3.0](https://commons.m.wikimedia.org/wiki/File:Comet_P1_McNaught02_-_23-01-07-edited.jpg#mw-jump-to-license)</sup></center>

<h3>PLANETARY MOTION</h3>

All the planets are in orbit round the Sun and move in the same direction, which is anticlockwise when viewed from above our North Pole. Their paths all lie close to the same plane. The orbits are elliptical, which means that sometimes they are further from the Sun than at other times. Books of data tend to give the mean distance of the planet from the Sun. For most planets, this distance is usually quite close to the actual distance at any time, but Mercury and Pluto have more elliptical orbits. In fact, Pluto sometimes gets closer to the Sun than the next inner planet, Neptune.

<p>The Sun is at one focus of the planet's ellipse. So, accurate prediction of a planet's motion requires very careful mathematics and not only is the mathematics of elliptical motion more complicated than that of circular motion, but each planet is affected by the gravitational forces between itself and other planets. However, the orbits of most planets are so nearly circular that we can assume circular motion and get good enough results for our purposes.</p>

<center>![640px-PIA17046_-_Voyager_1_Goes_Interstellar.jpg](https://cdn.steemitimages.com/DQmSvYGcWd1PjGJf5ggEYb2cCCM57PH6reTSw6uG4SRhKZR/640px-PIA17046_-_Voyager_1_Goes_Interstellar.jpg)</center>
<center><sup>Oort Cloud
[NASA / JPL-Caltech • Public domain](https://commons.m.wikimedia.org/wiki/File:PIA17046_-_Voyager_1_Goes_Interstellar.jpg#mw-jump-to-license)</sup></center>

<h3>THE LAWS OF PLANETARY MOTION</h3>

The gravitational force acting on the planet produces an inward acceleration v<sup>2</sup>/r where r is the orbital speed of the planet at a distance r from the Sun. [Newton's law of gravitation](https://en.m.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation) tells us the size of the force:

<h4><center>GMm/ r<sup>2</sup></center></h4>

where G is the universal gravitational constant, M is the mass of the Sun and m is the mass of the planet. So, by applying Newton's second law of motion,

<b>force = mass x acceleration</b>, we have:

<h4><center>GMm/ r<sup>2</sup> = mv<sup>2</sup>/r</center></h4>

Making speed v the subject of the equation:

<h4><center>v = (GM/r)<sup>1/2</sup></center></h4>

which tells us that the speed of a planet in its orbit does not depend on its mass but only on its distance from the Sun. This is a very powerful result. For example, it allows us to calculate the mass of the Sun from the orbital speed of any planet. We also have to know the value of the gravitational constant G, a value which was not known in Newton’s day and remains one of the most difficult of fundamental physical constants to measure accurately.

<p>In my next post, I’ll be considering what happens when a star's supply of energy is running out and some other educative parts of astrophysics. Till then, I remain my humble self, @emperorhassy.</p>

<h2>Thanks for reading.</h2>

![](https://steemitimages.com/DQmUsDx6pen1XAB66xGNezuiZ9RYoXEv9U98MqHzTxFV1dE/15.png)    


<h5>REFERENCES</h5>

<sup>[Laser Guide Star (LGS)](https://en.m.wikipedia.org/wiki/Laser_guide_star)</sup>
<sup>[Hubble telescope](https://en.m.wikipedia.org/wiki/Hubble_Space_Telescope)</sup>
<sup>[Large Telescope system at the La Silla Pinara Mountain Observatory in Chile](https://en.m.wikipedia.org/wiki/La_Silla_Observatory)</sup>
<sup>[adaptive optics](https://en.m.wikipedia.org/wiki/Adaptive_optics)</sup>
<sup>[The world's largest single telescope, The Gran Telescopio Canaris in the Canary Islands of Spain.](https://m.esa.int/spaceinimages/Images/2019/03/Gran_Telescopio_Canarias_telescope)</sup>
<sup>[Hertzsprung-Russell diagram.](https://en.m.wikipedia.org/wiki/Hertzsprung–Russell_diagram)</sup>
<sup>[Milky Way Galaxy](https://en.m.wikipedia.org/wiki/Milky_Way)</sup>
<sup>[Charged Coupled Device (CCD)](https://en.m.wikipedia.org/wiki/Charge-coupled_device)</sup>
<sup>[Astronomy and space exploration](https://www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/astronomy-general/astrophysics)</sup>
<sup>[What is Astrophysics](https://www.google.com/amp/s/www.space.com/amp/26218-astrophysics.html)</sup>
<sup>[Astrophysics](https://en.m.wikipedia.org/wiki/Astrophysics)</sup>


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