Introduction to Astronomy

All of the stars visible in the night sky are in our part of the Milky Way galaxy, in which we live.  Our sun is partway along one of the spiral arms that constitute the outer regions of our galaxy. With a telescope, it is possible to see stars in neighbouring spiral arms of our galaxy.

Stars may be ‘organised’ a) in associations ie they seem to form a group in the sky, but are at different distances away from us, or  b) in star clusters, where the stars are actually similar distances from us, forming a local group of stars.

Interstellar distances are vast. Incredibly vast, as in mind-bogglingly mind-bogglingly vast. If our sun is represented as a model the size of a pea, the nearest star in our model would be 150 miles (240 Km) away. In fact our sun's diameter is about 115 times that of our own planet. The scale model above is about 300,000 BILLION times smaller than actual size! Suppose our Sun was the size of a punctuation full stop, then the nearest star would be 10 miles (16Km) away and the nearest galaxy to our own would be 5,250,000 miles (8,400,000Km) [21 x the Moon's distance] away. That's with the Sun represented by one very small full stop! The Sun is 1,500,000 BILLION times bigger than this!

If the solar system were the size of a table, the Andromeda Galaxy would lie at 10 times the distance to the moon and the most distant galaxies would lie at 60 times the distance to the Sun.

We use two measures of astronomical distance.  AU’s are astronomical units represent the distance between the Earth and the Sun ie about 150 million Km or 93 million miles. These tend to be used within solar systems, as with these units the nearest star to us is about 272,000 AU away. That’s over a quarter of a million times further away from us than our Sun. To the nearest star!!!!

For longer distances we use light years. Light travels about 300,000 Km (186,000 miles) per second. It takes light about 8 minutes to reach the Earth from the Sun ie to travel 1 AU.

In a year, light travels over 63,000 AU. This is a staggering 9,500,000,000,000 Km per year. The closest star to us is 4.3 light years away, 40,000,000,000,000 Km from us.

The nearest galaxy to ours is 2.5 million light years away from us, or in our Km system 21,000,000,000,000,000,000 km away.  That's 525,000 further than the nearest star! And most galaxies are much much much much further away, some at the very edge of the visible universe as viewed from the Hubble Space Telescope. In terms of the width of our galaxy, the next galaxy is about 17 times further, but our own galaxy is vast in itself! Our own galaxy is about 150,000 light years across and has maybe 400 thousand million stars, ie about 40 stars per person on our planet.

“... the total number of stars in the universe is greater than all the grains of sand on all the beaches on planet Earth.”

It is reckoned that there maybe 130 billion galaxies in the universe, with 50,000 billion billion stars, which works out at about 25 stars in the universe for each grain of sand around all of the world’s beaches.                                            (billion = 1,000,000,000)

Our galaxy is about average in size for a spiral galaxy.  But averages don’t say much about range of sizes. Some galaxies are much smaller, some much larger than ours. Galaxies may be spiral, ovoid or irregular in shape.

Our Sun is very much an average size star. It is 1.39 million Km in diameter, about 115 times the Earth’s diameter, but could contain the Earth 1.5 million times over! It is however a much brighter than average star brightness. Only 5% of stars are brighter than our Sun. More to follow  about stars and the Sun shortly!

The Sun has eight major or minor planets, several dwarf planets and numerous large and small asteroids in orbit. The major and minor planets lie in approximately the same plane, known as the Ecliptic.

Our Earth is the third planet out from the Sun and is the only planet with significant quantities of water and oxygen, necessary for Life. It is inclined at 23 degrees and rotates about it’s axis in 24 hours. This inclination to the orbital path gives Earth its Seasons. The Earth’s orbit is slightly elliptical, but the change in distance from the Sun is insignificant in comparison to the effect of the tilt in the Earth’s axis season-wise. The Earth’s diameter is 12,742 Km or 7964 miles.

The Earth has one moon, about a quarter of the size of the Earth, giving us larger tides in the Oceans. The Moon’s volume is about 1/49th of the Earth’s. Its Mass is about 1/80 ths of Earth’s. It is about 250,000 miles (384,000 Km) on average away from us, but this varies by a factor of about 1/11 as the Moon’s orbit is slightly elliptical.

As a coincidence, the Sun is 400 times the Moon’s diameter and is 400 times as far away. This gives Solar Eclipses with a total eclipse of the Sun by our Moon, but these happen very rarely for a given part of the Earth’s surface. Eclipses of the moon are much more commonplace, as the Earth is much bigger than the moon. Also, as the Moon travels into the Earth’s shadow, the eclipse is visible from the entire hemisphere. The Moon is visible when eclipsed by the Earth, by reflected light from the Earth.

Viewing the Earth and Moon from Space, the Earth is 40 times brighter than the Moon. The Moon is the only extra terrestrial world that man has been able to actually visit, though space probes have landed or flown past most nearer planets in the Solar System. Space probes have actually landed on Mars and Venus, as well of course, manned and unmanned on our Moon. Space probes have flown past Jupiter and Saturn and beyond!

Mercury, the closest planet to the Sun is about 1 ½ times the Moon’s size. Its relative smallness and closeness to the Sun make it difficult to observe, and then only near the horizon for a few minutes before/ after sunset, the times of the year when it is away from the Sun viewed from Earth.

Venus the next planet, is comparable in size to Earth, but is surrounded by clouds of C0₂ which retain the Sun’s heat, providing very inhospitable surface conditions. Venus is the brightest object in the night sky, and can only really be confused with Jupiter. Venus appears relatively close to the Sun, but is visible for up to about an hour before or after sunset, again when it is away from the Sun relative to the Earth. The crescent phases of Venus are easily visible in Binoculars.

Next out from the Earth is Mars, the Red Planet, considerably smaller than the Earth. It’s red colour is due to the presence of large amounts of Iron Oxide in its surface dust. Mars has a very tenuous atmosphere and small Polar Caps. The Polar Caps on Mars are water ice covered by a layer of frozen CO₂. There are also surface markings on Mars. But Mars has an eccentric orbit and is often much further away from the Earth. It is only occasionally, every few years, seen at closest approach to the Earth, making observing it usually more difficult. Mars can appear as a bright or lot less bright red ‘star’ in the night sky.

There is then a band of asteroids, possibly a collapsed planet, or more likely a planet which didn’t form because of the effect of Jupiter’s gravity. Next out is Jupiter itself.

Jupiter is a Gaseous Giant, and the largest planet orbiting the Sun. Much of its outer layers are frozen gas, above a rock core. The surface consists of liquefied gas in which intense storms rage, eg the Great Red Spot, sometimes lasting hundreds of years or more. Jupiter is 318 times as massive as the Earth, and is twice as massive as all the other planets combined.  Its diameter is about 12 times the Earth’s. Jupiter is the next brightest object in the night sky after Venus. Though much smaller, four of Jupiter’s moons are clearly visible in larger binoculars, and lie in the same plane to the planet. Telescopes show the banding around Jupiter’s surface clearly.

Next is Saturn, another gas giant. Saturn is distinctive in having a large ring system which is clearly visible in small telescopes. In binoculars, this gives an oval appearance. Saturn itself is about 2/3 the size of Jupiter ie about 8 times Earth's diameter. Saturn though varies in brightness depending on its orbital position to us, and looks to the naked eye like a bright or very bright yellow ‘star’. One of Saturn’s moons can be seen with a small telescope.

Uranus is a ‘small’ gas giant about 4 times Earth's diameter, but is too far away to see with the naked eye, as is Neptune (about 3.9 times Earth's diameter) which is more distant yet again from the Sun. The giant planets are all bigger than 48,000 Km in diameter.

Mercury, Venus, Mars, Jupiter and Saturn are all known from prehistoric times as they are all visible to the unaided eye and of course move over a time relative to the stars. Uranus and later Neptune were only discovered once telescopes had been invented, in 1781 and 1846 respectively.

Pluto, discovered in 1930, is smaller than Mercury and is now classified as a dwarf planet, along with Eris (actually bigger than Pluto), Ceres (orbiting between Mars and Jupiter) and Rapanui (Makemake).  There are also larger trans-Neptunian objects (TNOs) such as Sedna and others, though these are smaller than the main dwarf planets. Pluto is very feint and only visible in very large amateur telescopes.

Most of the dwarf planets and TNOs have eccentric or very eccentric orbits outside of the Ecliptic and their orbits may come inside the orbits of some of the outer planets.

A few interesting gravitational facts are:- the Moon has about a fifth of Earth's gravity, Jupiter 2.5 times and the Sun 28 times.

Orbitally, Mercury is just over a third of our orbit out, Venus just over 2/3rds of our orbit out, Mars is 1 ½ x our orbit out, Jupiter 5x our orbit out, Saturn nearly 10x our orbit out, Uranus nearly 20x our orbit out and Neptune 30x our orbit out, these being of course in AU, or Earth Sun comparative distances.

Courtesy of Bill Arnett is this description of part of our solar system:-  “One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon (pea size) would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away.” Thus the Solar System consists mainly of empty space! The stars we see in our night sky are far more distant objects!

On a clear night, away from light polluted skies, about 5,000 stars can be seen without optical aid. This decreases to less than 2,000 in city skies. The Milky Way, our own galaxy is visible in darker skies. Most of the stars visible to the naked eye are within about 1,000 light years distance of us.

From ancient times, Constellation names have been used to describe apparent patterns made by brighter stars in the night sky. A common example is Ursa Major, also known as the big dipper or the plough or the great bear. The two stars at the outer edge of the plough are known as the pointers as they indicate the direction of the North star.

Over 10,000 years, the stars move noticeably relative to each other, hence the constellation shapes aren’t constant over thousands of years. Also, because the Earth wobbles a bit on its axis, a phenomenon known as precession occurs, and in say a thousand years time, the North Pole relative to the stars will have moved away from the ‘current’ North Star. In times gone by eg the Ancient Greeks, a different star was used to navigate by!

Our own Sun rotates about the centre of the galaxy once in about 250 million Earth years, as the spiral arm in which we live rotates, having made about 25 rotations since having formed! Stars are catalogued according to type. Our Sun is a ‘Yellow Dwarf’ star. The classifications depend on the size of the star and its emission spectra. For our purposes though, it is enough to describe a star by the colour it appears in the sky, and its real and relative brightness. ‘Red Giant’ stars are stars similar to our own that have expanded greatly later in the stars’ life. ‘White Dwarf’ stars are stars smaller than our own that are expending the last of their energy and have collapsed into a very dense but bright object. ‘Brown Dwarf’ stars have never collected enough material to become proper stars and glow rather less brightly than stars. ‘SuperGiant’ stars are much more massive and considerably brighter than our own Sun. The very brightest of these SuperGiants is 10 million times or so brighter than the Sun. SuperGiants eventually become SuperNova, ie they explode with the most extreme violence! ‘Red Dwarf’ stars at the opposite extreme are stars smaller and cooler than our Sun. They are the commonest type of star in the Cosmos. There are also in between stars, ‘Giants’ that may not eventually explode but which are quite a lot bigger than our Sun. ‘Dwarfs’ is just a term for normal size stars to distinguish them from Giants and SuperGiants (which are mostly blue in colour).  There are also stars intermediate in colour and size between our Sun and ‘Red Dwarfs’ ie orange or orange red ‘Dwarfs’.

But a small star say 10 light years away may be brighter in our night sky than a much larger and ‘relatively’ much brighter star say 150 light years away. But even the biggest stars discovered are still only pinpoints of light even in the biggest of our telescopes. No telescope built so far can show a star as a disk because of the immense distance even the closest is to us.

Taking a ‘snapshot’ of our galaxy, we are part of a local group of about 30 galaxies. Our own galaxy has two small nearby galaxies ‘attached’ to the Milky Way. These are the small and large Magellanic Clouds visible only in the Southern hemisphere. Around our galaxy are numerous ‘Globular’ star clusters, relatively dense clusters of tens to hundreds of thousands of stars or more. Within the spiral arms are some relatively less dense ‘Open’ star clusters with tens to hundreds of stars. Some stars, in fact, the majority of stars are binary systems or ‘double’ stars where two or more stars rotate about each other in orbit. Sometimes the components of binary systems are different types of stars. Some stars vary in brightness over a period of time and are called variable stars. There are also different types of Nebula. Planetary nebulae are the remains of stars which went supernova a long time ago. Some nebulae are reflection nebula, gaining their light from other stars. Other nebulae are emission nebula made up from glowing gases. Nebulae are made up from gases or interstellar dust. Some nebulae are star forming regions!

We know less about some of our own galaxy than we do of neighbouring galaxies! We cannot see the centre of our own galaxy. The clouds of dust are too dense in the direction of the centre for us to see through.  There are also other ‘star’ types that are invisible to the eye or even photographic plates or astronomical CCDs. These are of interest only to astronomers with infrared, or radio telescopes, or x-ray detectors.

But some of these other star types, as well as supernovas have been observed in distant galaxies. They often provide a means of calculating how far away these other galaxies are from us!

The galaxy closest to ours is the Andromeda galaxy, just visible to the naked eye as a fuzzy bit of sky in the darkest skies. It is clearly visible, (but without much detail) in binoculars and small telescopes. Light has taken from well before there were humans to reach us from there! The Andromeda galaxy is about 1 ½ times as big as ours and has two smaller subsidiary galaxies in orbit.

With a medium sized amateur telescope, galaxies as far away as 65 million light years or more can be seen. Light from these set off when there were Dinosaurs on Earth! With the Hubble space telescope, galaxies have been detected up to 14 billion light years away, nearly at the calculated/ guessed edge of space and time, back to shortly after the ‘Big Bang’ creation of the Universe.