Cosmic distances. How do we know how far away objects are in the night-time sky?
For nearby bodies – ie the planets and the moon – we can use radar. But when it comes to stars another method is required. Parallax uses trigonometry and was the way the first reliable distances to the stars were calculated. The principle is simple and a good practical demonstration is to raise a finger at arm’s length then alternately shut one eye then the other. The finger will jump position as seen against the more distant background. This is because it is being viewed from two slightly different positions based on the gap between your eyes. Measure this separation and add the angle of displacement and you have what you need to find the distance to your finger. Scaling this up to cosmic level was so difficult that it took hundreds of years to accomplish the feat. Instruments of unprecedented accuracy were required that could, by way of analogy, measure the diameter of a 5p coin at a distance of 2.5 miles. The distorting effects of starlight passing through the earth’s atmosphere and the expansion and contraction of a telescope with temperature changes also needed to be considered. The baseline used was the diameter of the earth’s orbit around the sun – 186 million miles. It fell to German Friedrich Bessel to achieve the breakthrough in 1838 when he discovered the star 61 Cygni to be 10.4 light years away (the modern figure is 11.4). These days earth based telescopes still use the technique and can measure objects up to 300 light years distant, but satellites like Gaia above the atmosphere can gauge distances up to 16,000 light years. When distances get greater other methods are needed, such as standard candles. If we can discover the true luminosity of an object then we can work out how far away it is (light falls off at a known rate over distance). Such objects include variable stars and supernovae. We can also get a sense of distance by taking the spectra of galaxies – the faster they are moving away the more they are shifted to the red. The speediest galaxies are the furthest away and the relationship between distance and speed is called the Hubble Constant.
Magnitude scale. How do we measure the brightness of objects? How dim can the human eye see?
A healthy human eye can see stars down to about magnitude 6 in dark skies. The magnitude scale is a little confusing and was invented by Greek astronomer, Hipparcus. In its modern usage each division denotes that an object varies 2.56 times in brightness. The lower the number the brighter the object. Negatives are allowed and the sub-zero readings are reserved for very bright bodies, ie Jupiter, Mars, the star Sirius. Under town skies you can typically see down to between magnitude 3 and 4 on a good night.
Stars. What are they, how are they different from each other, what can you deduce about them simply by looking at colour? Are they always alone, or do they have companions?
Stars are the same class of object as our day-time sun, but so far away we see them as points of light. They are powered by nuclear fusion – typically converting hydrogen into helium. This is caused by the immense pressure and high temperature at their core (more than 15 million centigrade). But stars have differences. Some are big, some small, some live a long time (10-20 billion years), others are short lived (ie a few million years). Like humans they are born, have a middle age and die. More massive stars have the shortest lives as they consume their fuel supply rapidly. The colour of a star indicates its temperature – hotter ones are blue and white, cooler ones red and yellow. Our sun is a yellow star. Many stars are also in multiple systems with other nearby suns, orbiting a common centre of gravity. Such objects can look beautiful in telescopes. An example is Almach – a lovely yellow and blue binary in Andromeda. It takes light about 7 days to travel between them.
Planets. What are they and what are exo-planets?
The term planet is derived from the Greek word for wanderer. Even ancient civilisations realised they moved differently to the background stars. They can be rocky – Mercury, Venus, Mars or Earth- or gaseous (and much bigger) like Jupiter, Saturn, Uranus and Neptune. They all orbit the sun and are only visible because they reflect the sun’s light. The smallest planet is Mercury (3,100 miles diameter) and the largest Jupiter (87,000 miles). Pluto was once designated as a planet, but in 2006 it was demoted to become a dwarf planet following the discovery of similarly size objects in the frigid depths of the solar system. To become a fully fledged planet an object must be massive enough to be spherical (ie have enough gravity to pull itself into a globe) and also dominate its orbit (something that Pluto does not do as it crosses the path of Neptune). We suspect most of the stars we see in our sky also have planetary systems – so called exo-planets. There are number of ways we can deduce this – for example detecting the subtle dimming of stars when a planet passes in front of them. The number of exo-planets is growing every week and is now in the low thousands. A good example is Pollux, chief star in Gemini.
Constellations. Are they random patterns in the night sky invented by humans or do they have scientific significance? How did non-Greek civilisations view the same night sky?
There are 88 constellations in the western view of the night sky and 48 of these were known to the Greeks and reflect their belief systems. Whether they ‘invented’ them is a different matter as they were influenced by the Sumerians and Babylonians. Each civilisation saw star patterns in very different ways. The Chinese view of the heavens looks nothing at all like the Greek one, highlighting that constellations are human constructs and have no scientific basis. They are random patterns of stars, often at very different distances. But they retain their utility because they are an excellent way of navigating the night sky – putting order into what otherwise would be chaos.
What are deep sky objects? Which are the best for viewing with basic instruments? When can you see them?
The view of a dark sky with the naked eye is breathtaking and we can do lots of enjoyable astronomy simply by gazing upwards. But telescopes and binoculars open up a hidden sky full of beauty. Bodies outside our solar system are often referred to as deep sky objects. They include open clusters, which are families of stars born around the same time and bound by gravity. Some look quite scattered, but others are more compact. Nearby ones include the magnificent Double Cluster in Perseus – 7,500 light years away. It is also visible to the naked eye as a hazy patch. Globular clusters are more densely packed, forming a spherical swarm of as many as one million suns. Famous examples include M13 in Hercules and M15 in Pegasus. Under dark skies they are spectacular. Nebula is the Greek word for cloud and we see a few of these objects too in small telescopes. The easiest is the Great Orion Nebula (M42) – visible in the winter – a huge cloud of mainly hydrogen gas where stars are being formed. There are also so-called planetary nebulae – although they have no connection with the aforementioned gas clouds. Instead they are dying stars that cast off their outer atmosphere to form amazing shapes. Showpiece objects for small telescopes include the Ring Nebula (M57), the Dumbbell (M27) and Blue Snowball. All these objects reside in our galaxy – the Milky Way.
What else can we see in dark skies?
Shooting stars can be seen all year round, but occur in greater numbers during meteor showers. The best ones are the Perseids, Orionids, Leonids and, king of them all, Geminids. They may look dramatic, leaving behind glowing trails, but they are caused by bits of space debris no larger than a peppercorn moving at high speed (7 – 45 miles per second) and burning up in the earth’s atmosphere. They are usually associated with comets, which leave a plume of debris behind when they travel into the inner solar system. Every year in its orbit the earth intercepts this field of ‘cosmic litter’. You will see more shooting stars in dark skies and the best policy is to look high in the east and sit back. Don’t bother with optical aids – your eyes are the best thing for this job.
The Northern Lights are magical and they can be seen from North Yorkshire. The further north you are and the darker your sky the better they look. But displays are quite rare and depend on increased solar activity. Aurora apps will give you a few hours notice. If you are lucky enough to be out under a clear sky during a display look to the north. Like a rain shower, it can build up slowly over a few hours and initially look like light pollution. But eventually you may see subtle tints of green and red and soaring light pillars.
The Milky Way and the realm of the galaxies. What is a galaxy, are they all the same size and shape? How far away are they and which are the brighter ones that we can easily see from earth?
The river of light we can see overhead in the Autumn is the Milky Way – it is one arm of our own spiral galaxy, which confusingly we also call the Milky Way. The darker the sky the better it looks. Our galaxy, which has 200 billions suns, including our own, is arrayed in a disc that side on looks like two fried eggs stuck back to back. Face on it looks like a whirlpool. The whole complex rotates about the core once in 200 million years. But there are other galaxies – each separated from each other by vast tracts of empty space. The furthest thing you can see with your naked eye under a dark sky is the Andromeda Galaxy (M31). Looking like the Milky Way, it lies at 2.5 million light years away. You can see it with the naked eye, but binoculars are better, revealing a bright core and faint arms. It was only the 1920s that we understood the nature of this object. Using photography stars were identified in its arms which we could use as a standard candle. Once the distance was computed it was clear Andromeda was far beyond the bounds of our own galaxy and was a huge star city in its own right. Other galaxies within the view of binoculars under dark skies include M51 (The Whirlpool), M81 and M82 (Bodes and the Cigar Galaxies) and in spring M66 and 65 in Leo. They look better in telescopes and the bigger the better. Galaxies are classified by their shape and as well as the handsome spirals, there are also round and elliptical galaxies and others called irregulars.
Dark Skies UK 