The author's aim is to popularise the science of astronomy in a series of relaxed, easy to read, and easy to undestand articles
Credit to NASA
Unless otherwise stated, photos on this page are credited to NASA.
My thanks to NASA - without these images, these webpages would not have been possible.
N.B: Please note, all my articles are best read on desktops and laptops
In this fourth page of a series of guides to the night sky, I look at some more of the most prominent objects which can be seen. And there is a lot more than might at first be apparent. Look up at the night sky and what do you see? You may see the Moon, and you will see little points of light, the vast majority of which are stars. But I have covered the Moon and the stars in previous pages.
So what else is there up there waiting to be seen by the naked eye or maybe through a pair of ordinary binoculars?
Well there are planets of course, but also other moons which orbit those planets, and there's shooting stars and comets and star clusters and nebulae and even galaxies - all of which can be discovered by anyone who has a keen eye and a little patience.
And you will, to be honest, require just a little more patience and effort to try to find some of these objects, because some are less predictable in their location in the night sky, and the clarity of others is quite hazy - but it is well worthwhile trying, because the objects which I will be talking about on this page include some of the most spectacularly bizarre objects which you could ever hope to see in your lifetime.
Objects are described in sequence, more or less according to their distance from us.
These Four Pages
The four pages in this series are as follows:
- A Beginner's Guide to the Night Sky - the identification of the types of objects you can see in the sky at night.
- A Beginner's Guide to the Moon - what are the surface features on the Moon which we can see from Earth?
- A Beginner's Guide to the Stars - stars are not all the same. Some are like our Sun, but others that we can see with the naked eye have a true vastness or a brightness which is almost unimaginable.
- A Beginner's Guide to Naked Eye and Binocular Objects in the Night Sky - other outstanding heavenly bodies to look out for in the sky at night.
Contents of Page Four
- Artificial Satellites
- Shooting Stars and Meteor Showers
- The Five Visible Planets
- The Moons of Jupiter
- Open Star Clusters - Pleiades and Hyades
- The Orion Nebula
- Globular Clusters
- Galaxies - Our Galaxy
- The Milky Way
- The Magellanic Clouds
- The Andromeda Galaxy
First we will discuss the objects which can be seen closest of all to the Earth, and these are not natural heavenly bodies at all, though they certainly have everything to do with space. These are the artificial satellites which man has put into orbit around the Earth. I mentioned on Page 1, that a slow moving non-flashing light in the sky may well be one of these satellites. The reason is that several are large enough to be visible to the naked eye if sunlight is shining on them. They can even be quite bright.
Brightest of all is the International Space Station (ISS), which is orbiting the Earth at an altitude of about 350 kilometres (217 miles) and a speed of more than 27,000 kph (17,000 mph). At this altitude and speed, the Space Station takes several minutes to cross the sky. At 90 m (300 ft) in length, and made of shiny metal and highly reflective solar panels, it can indeed shine more brightly in the sky than any star or planet. (Many other satellites, as well as the Hubble Space Telescope, may also become visible as they orbit the Earth).
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The fact that satellites shine by reflected sunlight has two important consequences:
1) The best time to see satellites is for an hour or two before dawn or just after dusk. This is because at these times, the Sun is only just below the horizon at ground level, and can still shine on objects high up in the sky. But in the middle of the night, the satellite is usually too much in the shadow of the Earth to catch the rays of the Sun.
2) Sometimes a satellite which is dim or invisible to the naked eye will suddenly become visible for several seconds before fading once more. This is simply because the satellite happens to orientate in a way which reflects the Sun's light towards us, and then as it moves across the sky, its orientation changes and the reflection is lost - a bit like using a mirror or piece of glass (or metal) to flash a signal.
- You can find out when a satellite is due to pass over your locality by using the interactive map at this Nasa page. It's a little tricky to use, but give it a shot.
Seven Selected Perseid Meteors Caught on Video
Video uploaded by 06solareclipse
Shooting Stars and Meteor Showers
Anyone who spends time looking at the sky at night will occasionally see a bright, yet all too brief, streak of light rapidly flashing past before vanishing into darkness. This is a shooting star. Nothing at all to do with stars, but rather a small piece of rocky space material - usually no bigger than a grain of sand - which is drawn into our atmosphere by the Earth's gravity. The friction of colliding with the atmosphere at great speed causes the object, known as a meteoroid, to heat up intensely and burst into flames. This is the shooting star, and because of its tiny size, it usually flashes only very briefly - a second or two - before burning up harmlessly. (Occasionally much larger meteoroids are drawn in, and these create a more intense and long lasting fireball. And if they are large enough, they may even survive entry into the atmosphere and strike the ground as a meteorite.
Although shooting stars can be seen sporadically at any time of night or any time of year, there are occasions when a much higher than normal frequency of these events takes place, and with much greater predictability. These are the 'meteor showers', and they occur because of comets (see below) - dirty snowballs of rock and ice which orbit the Sun and which tend to leave a trail of particles in their wake. Each year the Earth in its orbit round the Sun will pass through this particulate debris and draw some of it into our atmosphere. When that happens shooting stars may be seen at a rate of as many as one per minute (occasionally, the Earth may pass through especially dense regions creating very intense meteor storms with dozens of shooting stars every minute). Each specific meteor shower appears to radiate out from the same small area of space each year and is named after the constellation in which this area is found. (Thus for example, the Leonid Meteor Shower appears to be generated in the Constellation Leo).
- There follows a table listing some of the very best meteor showers. A good page for detailing peak viewing times throughout the year plus information about viewing conditions, can be found at the Earthsky website.
Major Meteor Showers - Time of Year for Viewing
1ST - 5TH JAN
3RD - 4TH JAN
15TH - 28TH APR
21ST - 22ND APR
19TH APR - 28TH MAY
5TH - 6TH MAY
17TH JUL - 24TH AUG
12TH - 13TH AUG
2ND OCT - 7TH NOV
20TH - 21ST OCT
14TH - 21ST NOV
16TH - 17TH NOV
12TH - 16TH DEC
13TH - 14TH DEC
Apart from shooting stars (which are really an atmospheric phenomenon, even though they originate in outer space) planets and moons are the only objects in our own Solar System which are easily and predictably visible in the night sky. What is a planet? Well a planet is an astronomical body which orbits a star like the Sun. To be a planet, a body must be big enough for its own mass-induced gravity to pull it into a roughly spherical ball-like shape, but nonetheless, all planets are much much smaller than the Sun or any star you can see in the night sky. Some are rocky, and some are gaseous. Earth of course, is a planet, and five of the other planets (Mercury, Venus, Mars, Jupiter and Saturn) are quite easily visible in the night sky (a sixth - Uranus - can be seen with binoculars, but only if you know exactly where to look. You won't see Neptune).
How do you tell a planet, which shines by the reflected light of the Sun, from the much more distant stars which shine by their own light? Various methods can be employed and these are outlined on Page 1, but undoubtedly the best method is by getting to know the star constellations. Unlike the stars which remain fixed in the patterns which form constellations, planets wander from one constellation to the next (not quickly, but over a period of months). The Greeks recognised this, so without knowing exactly what they were, they christened these strange points of light 'planetae' which means 'wanderers'.
However, although the planets move relative to the stars, they will only ever be found in certain constellations. This is because the other planets orbit the Sun in roughly the same plane as our Earth, and so they can only be seen against the backdrop of those constellations which happen to lie in the same equatorial plane - these are the well known constellations of the zodiac.
Therefore in the northern hemisphere, the planets will NEVER appear in the part of the sky above the North Pole (ie: close to the Pole Star), and likewise in the southern hemisphere, the planets will NEVER appear very far south. They will always be in the region of sky which lies towards the equator.
The Five Visible Planets
Mercury can actually quite a bright object in the sky, certainly comparable to some brighter stars, and yet many people will never have seen it knowingly or unknowingly. The reason is because it is the closest planet to the Sun, and therefore never far from the Sun in the sky. Lost in the daylight glare, it is only visible shortly after sundown or shortly before sunrise, and rarely during hours of total darkness. Although planets don't generally twinkle in quite the same way as stars do (the full explanation is on Page 1), Mercury does tend to flicker, firstly because it has such a small diameter, and secondly because we see it very low on the horizon where the light from the planet has to pass through much more dust. Mercury can shine more brightly than Sirius (the brightest star), but often will appear rather like a dim star.
Venus, like Mercury, orbits closer to the Sun than we do, so again it never moves very far from the Sun. However it is visible for longer in the late evening or early morning sky, and may be present for about three hours after sunset or before sunrise. Once you know Venus, it is unmistakable. During the time it is in the night sky, highly reflective carbon dioxide clouds make it the very brightest object after the Moon. It shines 6 to 15 times more brightly than Sirius, and can sometimes be visible in the daytime. (Planets can vary in brightness much more than stars because their revolution round the Sun sometimes takes them much further away from the Earth. In the case of the innermost planets Mercury and Venus, they also experience phases like the Moon, when more or less of the disc is illuminated by the Sun, as seen from the Earth).
Mars is one of two planets which could easily be mistaken for a star. At its brightest, it may be brighter than Sirius, but usually it resembles a slightly more modest star. Of course, Mars is well known for its orangey colour, and this will help to identify it, though be aware that a few of the brightest stars are also orangey in tint.
Jupiter, for many, will be the most prominent of all planets. Though not as brilliant as Venus, it is still brighter than any star, and it remains higher in the night sky for much longer than Venus. It shines with a steady light, and during the year appears to move more slowly through the constellations (because it is so much further away than Venus). Jupiter is worth looking at through binoculars to see its moons.
Saturn, like Mars, can be mistaken for a star, and must be located using Internet or newspaper night sky maps. Because of its distance, Saturn is often the dimmest of the five planets, but still looks like one of the brighter stars in the sky. (With naked eye and binocular observation this is all one can see of Saturn, but I will add that with a small, quality telescope, the rings of Saturn become clearly visible).
- (The following EarthSky page is an excellent guide to the planets which can be seen in the northern hemisphere tonight).
The Moons of Jupiter
A moon is a heavenly body whose primary orbit is not around the Sun, but around a planet. We of course have one very prominent Moon, and for most people reading this page, that will be the only moon they have ever seen. But most other planets in the Solar System have moons, and four of these which orbit Jupiter are visible with binoculars.
Try to find Jupiter in the night sky. If it is there, it will probably be quite easy to find (see above). Now look at this planet through a good, steadily held pair of binoculars. You may well see two, three or even four tiny pinpricks of light in a single plane on either side (all may be on one side, or they may be split, some on each side). These tiny pinpricks are neither planets or stars - they are moons - the only moons, other than our own, visible with binoculars. Called the Galilean moons, as they were first discovered by Galileo Galilei with his primitive telescope back in 1610, they are called Io, Europa, Ganymede and Callisto, and they are all among the largest moons in the Solar System. Indeed, Ganymede is the largest of all - bigger than the planet Mercury. Another - Io - is the most volcanically active body in the Solar System. And Europa is believed to hold deep oceans of liquid water under a thick icy crust, and scientists think there could possibly be life in these oceans. Think about that as you look at these little pinpricks of light!
Comets are accumulations of rock and ice which - like the planets - orbit around the Sun. However unlike the planets which have roughly circular revolutions, the orbits of the comets are extremely eccentric. Many which come within the orbit of Mercury at their closest approach actually go so far away beyond the realm of the planets that they do not return again to the environment of the Sun for many tens of thousands of years.
Some, however, have less extreme orbits, and the most famous of these is Halley's Comet, which 'only' goes as far away as Neptune before returning towards the Sun when it becomes visible to us. As a result its period of revolution is not measured in thousands of years, but is a mere 76 years. Incidentally Halley's Comet is named after Edmund Halley, who after studying the long history of cometary sightings and their orbital characteristics, predicted in 1705, that many of these cometary sightings were of one and the same object, and that its next return would in 1758. Although Halley died before this date, sure enough the comet did return, and from then on was named for the man who had worked out its revolutionary cycle. (Note: it is NOT Haley's Comet! Bill Haley was a pop star who called his 1950s pop group, The Comets. )
The brightness of a comet will depend on its closeness to the Earth, and its closeness to the Sun (because the comet reflects sunlight). Comets may appear star-like, or as fuzzy smudges, but when they are close to the Sun, solar radiation may create a stream of reflective particles which forms a tail stretching away from the comet, as in the illustration above. Bright comets may remain visible for several weeks before they head away and fade into the distance.
That's all to say about comets, because at the time of writing there are no clearly visible comets in the night sky, and Halley's Comet will not be returning until the year 2062. The next time a bright comet ventures into our neighbourhood, it will be reported in the news, so take note of the location, and see if you can spot it.
Open Star Clusters - Pleiades and Hyades
It's now time to leave the solar system and the realm of objects which can be seen within a few light hours of our Sun. Now we are venturing many light years distant into the realm of the stars. The first of the objects we can find here (apart from the individual stars) are the open star clusters - groups of stars in close proximity. The reason these stars are in close proximity is because they are relatively young stars which formed together out of a gas cloud. All stars move at different rates in space, but these stars simply haven't had time to move far apart from each other. In binoculars, they are perhaps the most distinctive of night sky objects, unmistakable in their form.
One open star cluster in the northern hemisphere is especially conspicuous, a little to the north west of Orion (featured on Page 3) in the constellation of Taurus. This is the Pleiades which is popularly known as the Seven Sisters, and with half-decent clarity of sky this is not really missable. Look at the sky with the naked eye and you should see a fuzzy cloud in the region indicated. If you look slightly to one side, so that the area is just visible in your peripheral vision, the cloud becomes more obvious (visual acuity in low light is actually greater at the periphery of our vision than at the centre - presumably a throw-back to our ancient ancestry when it was advantageous to be able to spot the movement of potential predators out of the corner of our eyes, even when not really paying attention). To see this star cluster at its very best, use binoculars, because the Pleiades - in binoculars - is perhaps the most attractive sight of all, next to the Moon. All the stars can be viewed in a single field, and lie at a distance of about 440 light years.
Even closer to Orion is the bright orangey star Aldebaran, also in Taurus. Look at Aldebaran in binoculars, and a large number of other dimmer stars will immediately appear in the field of view. This is the Hyades, a more dispersed open star cluster than the Pleiades. But why is it more dispersed? The Hyades is three times closer to us than the Pleiades, at a distance of about 150 light years. And the Hyades is believed to be at least 600 million years old, whilst the Pleiades is relatively young, at just over 100 million years old. These two factors explain the difference in appearance. The greater distance of the Pleiades means that they appear three times closer together than they would if they were as near to us as the Hyades, and the relative youth of the Pleiades means that the stars simply have not had time to move significantly apart from each other.
Many other open star clusters are worth looking for through binoculars, including Praesepe, known as 'The Beehive' in the constellation Cancer, which can be found between the bright star Regulus and the constellation Gemini. Two other star clusters are visible in the same binocular field of view as each other in the constellation of Perseus. And in the southern hemsiphere in the middle months of the year, two star clusters in the constellation Scorpius are easy to find close together in binoculars. These are the Butterfly Cluster and Ptolemy's Cluster.
The next class of objects are nebulae - clouds of interstellar dust and gas. Strictly speaking, nebulae are not one class of object, but several. Planetary nebulae (nothing to do with planets) are unstable dying stars which pulse off shells of ionised gas which radiates visible light. If the dying star is massive enough it may end its days in a violent explosion or supernova, which may leave behind it a chaotically nebulous swirl, the most famous example of which is the Crab Nebula. Sadly all of these types of nebula are too faint to be seen with the naked eye, or even to clearly see with binoculars, unless you know exactly where to look.
Perhaps of more interest to absolute beginners are reflection and emission nebulae. Reflection nebulae are dust and gas clouds which scatter the light of nearby stars. Often such nebulae appear bluish, because blue light is scattered or reflected more easily than other colours (the same principle which makes our sky appear blue). A bluish haze around the stars of the Pleiades Cluster may be apparent in binoculars - this is a reflection nebula. Emission nebulae are clouds which are highly energised by stars within the nebula; indeed stars may actually be forming as a result of the coalescence of gases under gravity. Depending on the gases present, these nebulae may vary in colour, but red is a common hue of such emission nebulae, as in the Orion Nebula below.
Some nebulae have no stars nearby, but can be seen simply because they obscure the sky behind them. They appear as a dark area, or dark nebula if they partially block the light of a bright nebula (such as the Horsehead Nebula in Orion) or if they block out the light of background stars (such as the Coalsack in front of the southern Milky Way).
The Orion Nebula
Most famous and easiest to locate of all nebulae is the Orion Nebula. Below the three-starred belt of Orion (see on Page 3) is a streaky and cloudy line commonly visualised as the sword of Orion (in the southern hemisphere it will be above the belt). Look at this sword through binoculars and you will see a few stars and one small area of light which remains just a cloudy patch. This patch is the Orion Nebula - a vast stellar nursery in which dust and gas contracting under its own gravitational forces coalesces into balls of super hot matter - the genesis of stars. The Orion Nebula is believed to be about 1500 light years distant, and 20 light years in diameter, and contains about 700-1000 stars in varying stages of formation.
We have already looked at open star clusters in which young stars formed out of a nebula remain in very close association in the night sky. But there are also globular clusters - a quite different type of star grouping. Globular clusters are vast, relatively densely packed balls, the largest of which contain many hundreds of thousands of stars. Unlike open clusters, the globular clusters tend to contain very old stars, and the exact nature of their formation remains a matter of conjecture. They also have a very peculiar distribution in space, mostly being found around the centre of our Galaxy (see the next section). One of the consequences of this is that all globular clusters are at great distance - almost all are further away than 15,000 light years.
Sadly, the result of this is that globular clusters are all faint, difficult objects to view without a telescope. Although some are theoretically visible to the naked eye, I would suggest that none in the northern hemisphere can be seen without the aid of binoculars unless you have extremely clear skies and good vision. What's more, none are to be found in the very distinctive constellations I described on Page 3. I therefore won't locate them on this page, but certainly after you have found all the other objects described, globular clusters will be next on the list.
In the southern hemisphere, there is one significant exception. Omega Centauri lies at a distance of about 16-18,000 light years, and appears to the naked eye as a faint, and fairly unimpressive fuzzy star. But to be visible at all at such a distance bespeaks a stupendous intrinsic brightness. Omega Centauri is a class apart from other globular clusters, at least ten times more massive, encompassing 10 million stars in a sphere with a diameter of 150-230 light years. So large is it, in fact, that some have suggested it may not be a true globular cluster, but rather the nucleus of a dwarf galaxy (see later) which at some date in the past collided with our Galaxy.
In the northern hemisphere, Omega Centauri will only be visible from lower latitudes (about 40º North), and is best seen in the late spring, low down on the southern horizon.
Galaxies - Our Galaxy
I think it is now time to consider the heavenly body which encompasses all that we have so far discussed on this page and on the previous pages in this series. Planets and moons, comets, stars and star clusters and nebulae - everything is contained within a galaxy. So what is a galaxy? Basically it is a mass of star systems, dust and gas all held together by gravity in a gigantic rotating whole.
The size of galaxies can vary hugely from a mere 10 million stars (as we have mentioned previously, Omega Centauri may be such a dwarf galaxy) to great giant galaxies containing 100 trillion stars. Galaxies are also known to vary from spheroidal or elliptical to flattened disc-like forms known as spirals because concentrated swirls of stars and dust and gas give the galaxy a spiral appearance, as in the image above. Some dwarf galaxies have no clear definable shape and these are classified as irregular.
That is all I will say about galaxies in general, because this page is devoted to the observations which beginners can make with the naked eye or binoculars, and sadly the images shown here are quite impossible to resolve even in small telescopes. Big, high resolution telescopes are required to see any detail of this kind.
But it's important to understand the basics of our own galaxy, its shape, and our position within it. Observations and calculations suggest that our galaxy is of the flattened spiral kind and is believed to be similar in appearance to the two images shown above right. It probably contains at least 200 billion stars, and is shaped like a flat plate or disc with a central bulge or nucleus, with spiral arms of stars and dust swirling around this nucleus. The whole disc is about 100,000 light years in diameter, but away from the nucleus, the galaxy is no more than about 10,000 light years thick. Most of the mass is contained in the central nucleus, but our Sun is in one of the spiral arms about 30,000 light years from that centre, as shown in the images below. Incidentally the globular clusters which we mentioned earlier are distributed around the outside of the nucleus. and far away from the plane of the disc, which is why there are none close to our Solar System.
The form of our galaxy and our location within it, of course cannot be seen in the night sky, but it does have a bearing on the next object we look at, and indeed the whole appearance of the night sky.
The Milky Way
Everybody is familiar with the band of soft white liight we call the Milky Way, but sadly few in the developed world can easily see this extraordinary spectacle nowadays, as the hazy light is lost in the glow of street lamps and house lights. Even the full Moon is enough to wash away the Milky Way. But if one goes to a dark, unpolluted site, then the Milky Way has to be one of the most spectacular of all sights in the heavens. For a long time, its true nature was a mystery, although many, including some of the ancient Greeks, had speculated that it might possibly be composed of a myriad of stars too far distant to be seen as individual points of light. It was not until the invention of the telescope that this was, in fact, proved to be the case.
The Milky Way is a band of stars at great distance, but why does it appear thus? The answer lies in the description of our galaxy in the previous section and in the images shown above. Our galaxy is disc-like, and our Sun is in one of the outer arms of the galaxy. Now because the disc is relatively flat, if we look away from the plane of the disc, we only see sparsely distributed stars in the darkness of space, and absolutely nothing beyond a distance of a few thousand light years. But if we look in the plane of the disc, towards or directly away from the Galactic centre, we see many millions of stars, the vast majority of which are tens of thousands of light years distant, far too distant to be resolved into individual points of light; instead they merge into a fuzzy band of light - the Milky Way. When we look at the Milky Way we are therefore looking in a very real sense not just at stars, but at our galaxy.
Many people refer to our galaxy simply as 'The Galaxy' to distinguish it from all others, but today, the term 'Milky Way' is often used not just for the band of light, but also for our galaxy itself. We therefore live in 'The Milky Way Galaxy'. Everything we have described so far, and 99% of what we can see in the night sky, is within our Milky Way Galaxy. But there are just a few things easily visible to the beginner which are not within it ...