Lecture 1: The Stars

Ancient people knew a great deal about the sky since knowing the seasons was literally an issue of life and death. To know when to plant and when to harvest is crucial if you are going to starve if the crops fail! And since it was a matter of life and death, then astronomy and the calender got linked into the religion of that culture, so the astronomical alignments of sites like Stonehenge are only part of its function in the ancient culture which built it. Some nice links on archeoastronomy, and a lovely site from the british museum on Babylonian astronomy.

But as well as the cycle of the seasons, there is the more immediate cycle of day and night. While we are all aware of the suns motion, where it rises in the East(ish - depending on the season), has its maximum height above the horizon when its due south, and then sets in the West(ish - depending on the season), that of the stars is not so obvious. These would be more familiar to ancient people than to us, firstly because we live a lot more of our lives indoors in a culture which is not obviously linked to agriculture, and secondly because we mostly live in places where there are many street lamps, and their light drowns out all but the brightest stars. So what do we see in the night sky ?

Stars - and more stars, the Moon, Planets, and a milky band of light called the Milky Way. This lecture concentrates on stars, and we'll deal with the rest later.

The stars do not appear to move relative to each other - we now know that this is because they are so far away that their motions bring no appreciable change in their positions on human timescales (on 100,000 years then they do change a little - see the second part of the plough animation below). So the same stars in the same relative positions have been seen by all human cultures. And all cultures seemed to have played 'join the dots' and come up with pictures (termed constellations), and stories to weave around them. The way we 'join the dots' is mainly based on the Greeks. More on constellations.

When you look at the stars they have different apparent brightnesses. Starmaps generally use the size of the dot as a measure of stellar brightness. You already know from pervious sections of this module that stars have different intrinsic luminosities AND they are at different distances from us. The Greeks are responsible for a vile way to talk about the apparent brightness of an object - magnitudes, which iyou've already done. The Greeks classified stars into first class, second class, third class all the way down to sixth class (ones which are only just visible to the naked eye on a clear dark night), with the first class ones being the brightest. Each successive magnitude is about 2.5x fainter than the previous one. Sadly this system is still in use! Its now defined so that two stars which differ by a factor of 100 in brightness have magnitudes which are different by 5 units (with the fainter star having the bigger magnitude!) i.e. we relate magnitude to flux via
m₂-m₁=2.5 log f₁/f₂
On these definitions then Rigel (blue supergiant bottom right star in Orion) has a magnitude of about zero. Sirius (the brightest star) is about 4x brighter so has a magnitude of -1.5. The Sun has a magnitude of -26.85, while faintest galaxies which can be seen with big telescopes have magnitudes of ~30, i.e. 10 billion times fainter than the faintest stars we can see with our eyes. With just binoculars or a small telesope you can see objects that are 100x fainter (or 5 magnitudes bigger)

Now for some nasty stuff where you have to think in 3 dimensions! Firstly, we think of directions N-S-E-W looking DOWN on the Earths surface. Then west is to the left when we face north. But now we want to use the same dirctions to map the sky not the earth, so east and west reverse on skymaps. (Take a piece of paper, draw the normal points of the compass on it. Then lift it above your head!)

To work out how the stars move during the night and during the year you need to know that the earth goes around the sun once a year and that the earth rotates on its axis once every 24 hours, and that the axis of the earths rotation is tilted with respect to the axis of the earths orbit by 23.5 degrees. Always draw diagrams so that you are looking down on the north pole (i.e. you are looking south). Then the earth rotates anticlockwise on its axis, and anticlockwise around the sun. (All the planets also go anticlockwise around the sun and the moon goes anticlockwise around the earth).

The stars appear to move during the night and during the year so its a bit complicated to give the position in terms of altitude above the horizon in a given direction as these keep changing. Funnily enough, a better way to characterise postions on the sky and to describe what we see from earth is the ancient concept of stars fixed onto a sphere. If the earth spins anticlockwise, then this imaginary sphere rotates clockwise. We can define coordinates on the sphere in many way, but the obvius one is to use latitude and longitude. Extrapolate the Earths rotation axis onto this imaginary sphere, and call these the north and south celestial poles. Extrapolate the Earths equator onto the sphere and get the celestial equator. In the Northern hemisphere there is a brightish star which happens to be rather close to the north celestial pole - this is Polaris, or the pole star. There isn't such a convienient star marking the south celestial pole. The key to what you see is then the angle which the point directly above your head (the zenith) makes with the celestial pole. The meridian is the imaginary circle running through the point over your head (the zenith) and through the north and south celestial poles. This is all much easier to see in a diagram of the Celestial sphere.

Lets deal with dec first as its very easy. North celestial pole has dec of +90, celestial equator has dec of 0, south celestial pole has dec of -90 - its like latitude. circles of constant dec are parallel to the celestial equator. If we view the celestial sphere from the outside then the daily paths of the stars are very simple. Every star makes a daily circle around the Earth, and each stars circle is determined only by its declination. But there are two other considerations which determine what we SEE - we live on the earth so the planet blocks half the sky, and we can only see stars at night. We'll just deal with the planet for a start, and ignore day/night.

If you are at the Earths north pole then your zenith is the same as the north celestial pole, so you see all the stars moving in concentric circles centered on the north celestial pole directly above you. If you were looking south then the celestial sphere would rotate clockwise (as the earth goes anticlockwise) but since you are looking north then the celestial sphere goes anticlockwise around the pole star. The celestial equator runs along the horizon so all stars with positive dec are circumpolar and never set, while all stars with negative dec are never visible. All directions are south, so there is no meridian

Now sit on the equator. the zenith makes an angle of 90 degrees with the north celestial pole so the NCP is on the horizon. the zenith is perpendicular to the earths rotation axis. Stars with dec of 0 rise due east, and set due west. stars with positive dec rise north of E, and set N of W. Stars with negative dec rise south of E, and set S of W. Always see half of these stars circle so all stars are visible for 12 hours.

As you move away from the north pole to, say, Durham at latitude 55, then the NCP has an altitude which is equal to the latitude. Looking north you see some stars moving in anticlockwise circles round the north celestial pole. stars which have declination within 'latitude' degrees of the ncp are circumpolar ie those with dec bigger than 35 degrees. These never set so are visible for 24 hours, doing anticlockwise circles round the NCP. Stars closer to 'latitude' degrees of the SCP are never visible ie stars with dec less than -35 degrees never rise in durham. Maximum altitude is 90-lat+dec, minimum is 90-lat-dec. Stars on the celestial equator have maximum altitude of 90-latitude=35 degrees. But since they are on the celestial equator they rise due east and set due west and are visible for 12 hours. Ones with dec bigger than zero are visible for more than 12 hours, ones with dec smaller than zero are visible for less than 12 hours.