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The HR diagram;
Luminosity is plotted with the brighter ones higher, and the spectral type is plotted hottest to coldest:left to right. The stars on the diagonal band, represents a main sequence; these have hydrogen cores. There are some stars within the wider, sparser verticalish band, from up and to the right towards the colder and brighter: this is the giant sequencce, which consists of red giant stars. The few stars at the top of the diagram are supergiants; blue on the left side and red on the right. (more or less.) Then there are the stars located below the diagonal band, at the bottom center, are white dwarfs.

Main sequence stars are plot according to brightness and tempreture but these dependon mass. Hence the diagonal shows a trend of high to low mass stars. (high as in higher mass than the sun.) YSO aren't usually plotted and neutron stars and black holes are too dim to plot with normal stars.

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Why is one star brighter than another?
Because Brightness is determined due to tempreture and surface area: the hotter something burns the brighter light it omits, and the more surface area, the more space there is to burn.

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White Dwarfs; which are near the bottom due to theur small size, hence they're not the brightest of stars, and due to the fact they only fade and not die, they slide down, and to the right, on the diagram as they grow fainter and colder. They aren't on the right side, because they grow too faint they fall below the range of the diagram.

Supergiants; which are at the top end of the diagram as the are huge in mass, up to around 1000 times larger than the sun, hence they are naturally bright also. The supergiants are generally the same height: indicating that the blue supergiants on the left, are smaller than the red one, on the right. Because the red supergiants, which are colder, must be larger to produce the same total amount of light as the blue which are brighter.

Main Sequence; which are on the diagonal band, from upper left to lower right, due to the fact they burn hydrogen in their cores. The position they are on the diagram depends on their size, as that is the main differential. Although the ones on the left are hotter and therefore brighter. The main sequence stars on the right are the dim, cold, red dwarfs.

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The colour of a star is subjective. The graph below was created by Hertzsprung and Russell. They plotted the spectral type on the horizontal axis, which is the parameter assigned to a star based on its spectrum; which is the way the light appears on a prism or a spectrograph.

The main spectral types are; O, B, A, F, G, K and M, from the hottest to the coolest, the following table contains the general properties of each spectral type.

These 'classes' are then split into another group, the 'spectral type', in more specific detail, such as a star may be a B class, but specifically it will be B2. This gives the temperature of a star, hence the lower the subdivision the hotter the star. The subdivison goes from 0-9, but then they are again split into roman numerals, the 'luminosity class', this refers to the size and its average density. Below is a table of the lumiosity class.

Sometimes a star description of Ia or b will be used, this is to indicate that it is a brighter supergiant, and a Ib is a fainter one.

A star that has a large mass will concentrate its core with a fierce nuclear fire, which produces more energy than a star of a lower mass. Hence a massive main sequence star is bigger, hotter and brighter. This is a fundamental point of stellar astrophysics; mass determines class.

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Young stellar objects;
Newborn stars technically, which are still surrounded by their birth cloud; this class includes T Tauri stars, the first of their type stars, and Herbig-Haro objects, they are actually just gas that are sent in opposite directions from a new star, they are usually hidden in the birth cloud. New stars are usually created in HII regions, which act as a nusery for stars, like the Orion Nebula, which is noted to be a birth place for stars over the last 1 or 2 million years or so.





Main sequence stars;
These are stars that have shed their birth clouds and shine through nuclear fusion. The smallest of these is the red dwarfs, which have little mass but are common amongst the stars, however popular they may be, you cannot see even the closest one (without telescopic aid), Proxima Centauri, the nearest known star, because even that is beyond the sun. As previously mentioned although the red dwarfs are dim, they will outlast the life cycle of our sun.






Next are the red giants, which are the opposite in the fact that they are much larger than our sun, around the size of the orbit of earth, 938,900,000 km. A red giant doesn't have hydrogen burning at it's core, it burns it in a particular region just outside it, aptly named the hydrogen-burning shell, it does this because it has already turned all it's core hydrogen into helium through nuclear fusion.

There are some stars that are alot bigger than a red giant, which are called red supergiants, which can typically be around 1000 or 2000 times bigger than our sun.

Then there is the End states of stellar evolution, which are central stars of planetary nebulae, which are little stars at the center of some specific larger nebula, visible in the planetary nebula of NGC 2392 in Gemini. (image)
The central stars of the planetary nebula turn into white dwarfs, as they are the remains of sun-like stars. The actual nebuala, composed of stars made from gas which a star expelled, fade and blow away. This leaves behind the stars which become white dwarfs.




White dwarfs; which while they are called 'white' dwarfs, they have a range of keeping to white, yellow or possibly red, as the colour of the stars depends on how hot they burn. White dwarfs are the remains of sun-like stars, which never really die, they just fade away.

The essence of a white dwarf is whilst it still gives off heat, it doesn't burn anymore. They are the second second most common stars, after red dwarfs, dispite the fact that even the closest one is still too far away to see without proper equipment. Although the white dwarfs may be small and compact, they have the same mass as the sun,1.989E30 kg , despite the fact that they only occupy the space of the earth.

Supernovas, are enormous explosions which can destroy stars. These are the main varieties.
The first is called Type 2, which is an explosion of a larger, brighter star (bigger than our sun), the star would have been a red supergiant. (possibly burning bright enough to be a blue one.) When it explodes it leaves behind a neutron star, it mayimplode leaving behind a black hole.



The second type is type 1a, which is brighter, hence it is used to measure the expansion of the universe. (Which is only growing faster.) This type of supernova also explodes, but in a reliable manner, they explode due to their 'binary systems', where gas from one star flows dowm to another, which builds up an outer hot layer; which then reaches critical and explodes. (without the build up reaching 'critical' there is no explosion, and no, there is no event when the build up passes critical.)

The 'binary systems', as far as we know, come from two stars; one a white dwarf and the other like the sun, with the dwarf taking gas from the larger. Although there is evidence that some Type 1a supernovas are eased by the combination of two white dwarfs in the system instead.

Then there are Neutron stars, which are stars, that whilst they outweight (via mass) the white dwarfs, they are tiny in comparison. Whilst they may be small, at around 1 or 2 dozen miles across but has a mass that could beat that of the sun.

There are some stars called 'pulsars', which are highly magnetized, spinning neutron star that produces radiation; these beams of radiation sometimes pass satalies which cause brief spurts, which are called 'pulsars', hence the name. The 'pulse' of radiation can be used as a tell of how fast the star spins (ranging a few hundred times per second or once every few seconds.)

And finally there are black holes which are dense, compact objects with matter cram packed into them, meaning the amount of gravity is so strong that it prevents anything from escaping, (including light) some physicists theorize that objects within have left our universe. Black holes are detected by the effects they have on surrounding objects, that is that the matter surrounding them becomes hot and moves quickly, although it never gets organised. It should also be noted that some matter is shot out at a fraction of the speed of light, (186,000 miles per second in a vacuum.) from the matter that swirls within a black hole. There is also the fact that stars orbit around the black hole due to the gravity it omits.

There are three basic kinds of black holes;
Stellar mass black holes, which have the mass of a star; from three times the size of the suns up to around a hundred times that, (although non of that mass have yet been discovered.) They are around the size of a neutron star. They form in supernova explosions, although other means are possible.

Supermassive black hole;
These have masses of hundreds of thousands to more than 20 billion times the mass of the sun. These supermassive black holes are located, generally, at the core of galaxies; either they 'grow' there or the galaxy forms around them. Astronimers believe that there is one in every galaxy; or at least in every full size galacy, as they are unsure about the dwarf galaxies.
The size of these, which the diameter of the event horizon that is given, that is the spherical surface around the black hole, which is where the velocity needed to escape the hole is equal to the velocity of light, 299,792,458 metres per second. When outside of the horizon, the velocity needed to escape is smaller, hence light and high-speed matter can leave.

Intermediate mass black hole;
These are black holes that aren't really very well known to us. Hence most imformation on them is guess work and theories; they have estimated masses of a few hundred to ten thousands the mass of the sun. As it is more massive than any star, hence it probably didn't form from the collapse of a single star. although they haven't been found in the central regions of galaxy, hence we don't really have any idea where they come from or how they are formed.

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I figured i might as well research into the qualities of a star, hence here we are.

This is the general life cycle of a star, "with about the same mass as the sun" (1.989E30 kg), according to 'astronomy for dummies'.

1 - Gas and dust in a cool nebula condense, forming a young stellar object (YSO)
2 - The YSO disperses its remaining birth cloud as it shrinks and the hydrogen ignites. (nuclear fusion is beginning)
3 - As the hydrogen burns the star begins the main sequence
4 - After using all the hydrogen in the core, the shell ignites.
5 - With the shell burning, the energy released due to the fire causes the star to expand and glow brighter. The expanding makes the surface; cooler, larger and redder, meaning the star has become a red giant.
6 - The stellar winds that blow off the star remove the outer layers over time,this forms a planetary nebula around the remaining core.
7 - the nebula then expands, dissipates, leaving a mere core.
8 - Now a white dwarf star, the core cools and fades.

Before i explain that, it should be noted that stars with a higher mass than the sun have a different life cycle; they explode as supernovas and leave neutron stars or black holes instead of producing nebulae and dying as white dwarfs. Also the life cycle of a star with a mass greater than the sun only lives a few million years being exploding, whereas the sun may last for around 10 billion years.

The stars with a mass lower than that of the sun can't really have what is called a life cycle, they begin the same, as YSO, before joining the main sequence as normal, however they never progress past red dwarfs. (When I say never, a red dwarf burns its hydrogen fuel so slowly it won't run out. Admittedly it could, if given long enough,but that length of time is greater than the age of the universe at present, hence every red dwarf that has ever existed still exists.) What happens is that because the smaller the mass, the less fierce the fire, the longer it lasts; although I may be oversimplifing stellar astrophysics. But that is the core principle, as the opposite is true also; the bigger the mass, the fiercer and faster the fire.

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This ones about gravity, admittedly a little random, but within the topic of astrology it explains a few things.

First the general idea of gravity as brought to us by Sir Isaac Newton, an english scientist (1642-1727). His idea was that gravity acts as a force between two objects, one based on mass and seperation. Basically it boiled down to; the greater the object, the greater the force, however the greater the distance, the less force.

Where Newton's idea is fine and dandy for the common gravity, Albert Einstein's theory of gravity explains why stars near the sun in an eclipse seem out of position, why gravitational lensing is found when we observe deep space, why, as earth turns, warped space and time are dragged with it, and why black holes exist. Einstein's theory works because he thought of gravity not as a force, but as the bending of time and space due to the presence of a object with a large mass, like a star.

Despite not being as correct as Einstein, Newton did get alot right, Einstein just carried on his idea and ended up a little ways down the road with some more information. Newton did manage to explain some things though, like; why the moon orbits the earth and why the earth orbits the sun and why the sun orbits the milky way, etc, why the planets are round, and why combinations of gas and dust forms a new star.

The point of this is that all of space is constantly moving, including us. The earth rotates on its axis, taking a day for one complete turn, as it orbits the sun, taking a year to complete, as it travels with the sun in orbit around the milky way, taking 226 million years to complete once (otherwise known as a galatic year), as it moves with the milky way around the center of the local group of galaxies, as it moves through the universe with the local group as part of the hubble flow.
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Why do stars near the sun in an eclipse seem out of position?
Because, as discovered by Arthur Stanley Eddington, astrophysicist, who tested Einstein's theory that space was distorted by gravity, hence light that would pass through it would not travel in a straight line, as it would follow the curved patten by the distorted space, so long as the mass of the object was great enough; hence Eddington tested it on the objects of the greatest mass he knew of. He tested this by observing the stars during a eclipse of the sun, revealing distant ones close to the sun, if Einstein was right, the gravity of the sun would shift the the stars to different positions, compared to when they are visible after the sun has shifted. This was viewed in the 1919 eclipse, with totality at around 6 minutes, at the sun in front of Hyades, a cluster of particularly bright stars.

Why gravitational lensing is found when we observe deep space?
Well, gravitational lensing is the distribution of matter between a distant source and an observer, it is capable of bending the light from the source as it travels towards the observer. They are created as a consequence of gravity and are basically a warped, closer image of an object that is farer away, hence it is also known as 'mother nature's telescope'.

Why as the earth turns warped space and time are dragged with it?
It works because as a ball in goo, when spinning, will cover itself in the goo, before coninuing and pulling more goo with it, earth works in the same way, although with space-time being pulled instead of goo. This occurs due to the gravity of earth, and the fact that the curvature of spacetime being connected to the energy and momentum of whichever object of matter is present. 

Why black holes exist?
Most black holes form from the remains of a star that dies in a supernova explosion, so long as the star is large enough, as the smaller ones become dense neutron stars. (that are a kind that cannot trap light due to their small stature.) A black hole occurs when the total mass of a star is large enough, around three times of the mass of the sun, it will, theorectically, collapse under the gravity. When the star collapses, it nears the 'surface' of the event horizon, and the time dimension of the star stands still, hence the star cannot collapse any more, where it becomes a frozen collapsing mass. Its a natural phenomenia.

Why the moon orbits the earth, etc?
It's mainly due to gravity, for example if you swing a weight in a circle around you, the weight moves because you move, the earth is you, the moon is the weight and gravity is the string.

Why the planets are round?
Because their gravitational field in the core of the planet, pulls everything towards it, and the only way to get everything towards the center as much as possible is to cut corners, hence a sphere.

Why combinations of gas and dust can form a new star?
A star is formed inside a dense interstellar cloud, made from gas and dust, at around 10 to 20K (just above absolute zero.) which allows the gases to bind together in high densities, once the denser areas of the coloud core collapse under their own gravity a star actually begins to form. The core has around 10 4 solar masses in the cloud, although the cores are denser than the outer cloud hence why the collapse at first. The cloud will then seperate into clups of around 0.1 parsecs in size and 10 - 20 solar masses in mass. The clumps then become protostars, with their own gravity, and loose gas, remaining from the original cloud, enters the center; which in turn release kinetic energy to form heat, building up the temperature and pressure until it becomes an IR (infared) source.

18

I'm taking this opportunity to return to the Moon, just because I found some awesome more scientific, information from this "astronomy for dummies" book i found.

The Moon is exactly 2,160 miles in diameter, the moon has nothing particular spectacular in its atmosphere; a bland combination of hydrogen, helium, neon, argon atoms, along with a few trace items. Its core is mostly rock, although some experts think it has a small molten iron core, and it has a mass of only 1/81 of the mass of earth, and it has a density of about 3.3 times that of earth.

It should be noted that aside from in a lunar eclipse, half the moon is always in sunlight and the other half always in night. A lunar eclipse occurs when a full moon is in line with the sun and the earth, where the moon is completely in the earth's shadow, 'the umbra'. Hence no light is reflected from the sun on the moon, because the earth in the way, simple. However when if you can recall the 'red moon eclipse' of December the 10th of 2011.

The way that works is that some of the light from the sun gets bent around the earth, red and orange mostly, still shines on the moon, which is reflects as usual, turning it red. The eclipse of the moon is as common as an eclipse of the sun; however the moon is more widely viewed when it does happen. This is because in an eclipse of the sun it is visible only along a band called 'the path of totality', whereas the earth's shadow falls across the moon, and all of that half that the moon has risen in can see it. (namely due to the small size of the moon in comparison to the sun.)

The moon has craters of every size across the whole of it, the largest of it is 'The South Pole Aitken Basin' that is around 1,600 miles across. These are caused by debris of the universe striking the surface, such as asteroids, meteorites and comets. Whereas the smaller craters that are have been found on the samples brought back from the moon exploration, are discovered to be caused by micrometeorites, the smaller rock particles that are found in space among the rest of the debris, essentially, the dust of the universe.

Although the moon has no volcanoes it has experienced some volcanism, this has occurred within the small volcanic domes or hills, which are connected with a variation of channels on top of the surface as lava tubes, called rilles. The moon has large lava pools that reside at the bottom of the larger impact basins, the craters, these are called maria, Latin for 'seas'. These were first mistaken for pools of water, in the early years, however if they were they would reflect light brighter, from the sun, just as water here is want to do.

The larger areas of brighter light from the moon are actually heavily cratered areas, called the lunar highlands. Whilst the maria have some craters, the highlands have far more per square mile, meaning that the maria are younger. This indicates that the large impacts that caused the maria removed the preexisting craters, with the lava flow removing any new craters that are formed. The craters that exist in the maria now are ones that have been formed after the lava froze.

The 'soil' that covers the moon, which in reality is just fine rock dust, comes from the numerous impacts of debris that have beaten against the moon over the ages. There have been traces of ice stuck to the dust particles in some cases (along with mercury and silver), these are commonly when the dust is pulled from the bottom of craters near the poles. Because the sun never reaches that height and thus the bottoms of said craters are shadowed, leaving these the coldest places on the moon, in one the south pole crater the temperature was below -400 degrees Fahrenheit.

The near side of the moon, will be visible when the sky is hazy or a little cloudy, and at most times it is visible even during the day, at which point you can see craters with even a small telescope, although with a larger more equipped one, there will be thousands of indentations on the moon; impact craters, maria, lunar highlands, rilles along with central peaks1, lunar mountains2, and rays3.

The 'dark side' of the moon, shows a dim glow, despite the sun not shining on it, the glow is called earthshine. It's like the red glow of the moon in an eclipse, earthshine is the sunlight that is absorbed into the earths atmosphere and projected back onto the moon, as the light is bent enough to land there. You will never see this effect on a full moon, although it is most commonly noted on a crescent moon. In fact in a full moon, you can barely see anything, due to the sun is high enough not cast many shadows, hence you can't really see any indentation on the surface of the moon.

To properly catalogue the moon, there are charts, just as there are for the stars, although due note they only show the one side of the moon, as you don't need one for the side you cannot see. The following site has a literal map of the moon that is interactive enough to see details, with alterations to when you are viewing available, Moon View, there are also a variety of books available if you are willing to look. There is also google moon, which is another interactive site, and I have also found the helpful guardian webpage wherein there is a report including some images of the original 1951 map of the moon created by H. Percy Wilkins (1896-1960), which can be see in detail at the National Museums Greenwich.
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1 - Central peaks surround some if not most craters, as they are the result of the displacement of the surface due to an impact.
2 - Lunar mountains are central peaks where the crater has been effected; either disguised or removed, and the peak has been left standing alone, hence mountain.
3 - Rays, are the term for the bright lines that blow outwards from a crater, these are made up of debris of the surface and the dust.

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The constellations: part two.
This is the last forty-four of the constellations.