Our Sun is the only star we have been able to study in any detail as it so much nearer than all the others. It is part of a galaxy known as the Milky Way, which contains about 100 million stars. The Sun revolves around the centre of the galaxy at a speed of 220 km/s, taking 225 million years to make a complete circuit. The Sun is a yellow star with a surface temperature of about 5500 ºC. There are cooler, dimmer (~4500 ºC) regions known as sunspots. In these regions the convection that usually takes place in the Sun's outer layers is restrained by strong local magentic fields (~4000 guass). These fields are also responsble for prominences, events where solar material is thrown up into a loop. Such an event, from June 1992, is shown on the right. The Sun foillows an 11 year cycle where it's level of magnetic activity rises and declines. It's last maximum was in 2001. Magnetic activity also gives rise to the corona, a region of tenuous gas around the Sun that is heated to 2 miilion °C. But because the gas is so tenuous it gives off little light compared to the disc. The Sun also fires out high energy particles into space, the solar wind. At solar maximum a coronal mass ejection may occur where a large quantity of particles are thrown out into space. The Earth's magnetic field protects us from these particles, though it funnels these into the magnetic poles to create the Aurora Borealis (Northern Lights) and the Aurora Australis (Southern Lights).

The Moon frequently passes between the Sun and Earth, causing an eclipse. Here's an image of a partial eclipse from November 1966.

The Sun has a diameter of 1.4 million km. This is 400 times larger than that of the moon and, coincidentally it is 400 times further away than the moon. This means that the moon can cover completely cover up the Sun from our view, a total eclipse. In this special circumstance, we can see the corona. The corona appears brightest at solar maximum, but if solar minimum it should be possible to see a bipolar shape following the field lines. The eclipse shown here toook place in 1983, close to a solar minimum.

Here's a photo of M31, the Andromeda galaxy, and it's companion M32. It lies 2 million light years from us but is slowly moving nearer. It's a spiral galaxy, but viewed edge-on.

It is easier to see the spiral structure of this galaxy, the Whirlpool galaxy, M51, in Canes Venatici.

Here's a giant elliptical galaxy NGC 205. Ellipctical gtalxy are very different to sprials. The stars oscillate inwards and outwards through the centre. HST images of old elliptical galaxies indicate that they change little in appearance with age, whereas the the older spiral ga.laxies have arms that are less well defined.

Spiral galaxies contain a wide variety of differnt stars. Elliptical galaxies however contain only old stars.

Some galaxies are shaped like a sombrero hat (M104 in the constellatoin on Virgo). It's basically a tightly wound spiral galaxy with a large core.

Some galaxies are irregular is shape, like M82 here in Ursa Major.

Some galaxies, just look weird, like the cartwheel galaxy here.

What we has happened, is that two galaxies have collided head-on. In such a collision the stars in the galaxies concerned move straight through unimpeded but the gas collides. The effective doubling of the gas density leads to an intense wave of starbirth.

The coolision took place about 500 milion years ago, and new stars are being born in the outer ring.

Here is a pair of interacting galxies, NGC 6872 and IC 1470. Gravitational interactions have effectively extened the spiral arms of the larger galaxy (NGC 9872).

    Here's another part of the same gas cloud in Orion. The cloud actually covers more sky that the whole constellation, but only parts of it are illuminated. It includes the famous horsehead nebula. The horsehead shape is caused by cold, unionised gas, passing in front of the ionised gas behind.

    Here's the Trifid nebula. It lies in the opposite side of the sky, in Sagittarius.

   Here's the Lagoon nebula, which is also in Sagittarius..

    Here's an HST image of the eagle nebula. I can't see an eagle shape anywhere, it's a bit like the roche moutonée (mystery number 33).

   Here's a photograph of the Pleiades, or seven sisters. It's a good object to look at with binoculars and can found by following the line of Orion's belt to the east. It is an open cluster, a young cluster of about 400 stars, all of which were formed at rounghly the same time.

This open cluster is best viewed through a small telescope, and it is absolutely beautiful. It's M11, a.k.a. The Wikd Duck Cluster in the conctellation of Scutum. It's vaguely fan-shaped.

    Here's an example of a globular cluster, M13, in the constellation of Pegasus. Globular clusters are dense aggregation of old, Population II stars. Although there are many of these associated with our galaxy, a lot of them are displaced somewhat from the galactic disk. They're actually similar to the giant elliptical galaxies, but much smaller.

    I could show you move photos of globular clusters, but when you've seen one, you've seen them all. Having said that, they can often very beautiful objects to look at through a small telescope.

    Here's photograph of part of the central region of M15, taken with the Faint Object Camera on the HST. This image covers an area of about 2 light years and it filled with hot blue stars. The nearest star to Earth apart from the Sun is Centuri, is 4.5 light years away and the nearest giants are several hundred light years away. If the Earth was placed in the centre of this cluster, the sky would be ablaze. The stars are close to each other that have become stripped of their outer layers and we can see exposed cores.

    One might think that once has completely converted all of it's hydrogen into helium it would merely cool down and stop shining. This is not the case! When the core starts to run out of hydrogen it beocomes isothermal and slowly contracts. Hydrogen fusion continues to take place in shell around the core. Heat transfer in the outer layers becomes completely convective and it expands enormously. It becomes a red giant.Eventually the core temperature becomes hot enough for helium to fuse to form carbon and oxygen. The fusion rate initally is very fast, the helium flash, but it slows down quickly. As the fusion rate slows down the star's diameter shrinks to become a sort-of main sequence star again. Eventually all of the helium in the core will run out and core contracts again. The envelope of the star expands once again, more so this time, and so the star becomes a red supergiant or asymptotic giant. In a star like this, one half of the star's mass is concentrated within the central one millionth of it's volume. The Sun in this state, is expected to have a diameter of a diameter of 40 million km, 27 times it's current diameter. The luminousity of the star may become variable, where it periodically brightens and fades. These stars are known as Cepheid variables. There is a defined ratio bewteen the luminosity and it's preiod of fluctuations (brighter Cepheids have longer periods). These stars are very useful to astronomers as by observing Cepheids in other galaxies, we can determine how far away they are. The most famous red supergiant in the sky is Betelgeuse (which by some people is pronounced 'Beetle juice'). It's the 10th brightest star in the sky and the 'top-left' star in Orion. It looks slightly yellowish. It's diameter is larger than that of the Earth's orbit.

   
    As previously stated, stellar lifetimes vary enormously. A star like the Sun will probably spend about 10 billion years fuisng hydrogen in it's core, the remainder of it's life will take less than 1 billion years.

    The envelope of an asymptotic giant is only loosely bound to the core and radiation pressure from the core ultimately causes it to dissapate. Initailly the star dissappears from site as the expanding shell is too cool to radiate. But then as core continues to collapse it heats up and starts to emit a large amount of ultra-violet radiation. That causes the shell to reionise and to start shining again. The result is planetary nebula. This is the most famous example, the ring nebula (M57) in the constellation of Lyra. It is a fine object to look at through a small telescope.  The outer diameter of shell is about half a light year. The core of the star is visible at the centre. The star loses about 20% of its mass in this event.
    One might expect the expanding shell to be spherical, but seems rarely to be the case as most stars have at least one companion star. Stellar wind from the compananion star may cause the shell to form a more tubular structure. Indeed we think that shell around th Ring Nebula is tubular.

    Here's another planetary nebula, the dumbbell nebula (M27), which lies 1000 light years away in the constellation of Vulpecula. This was taken with the Very Large Telescope at Cerro Paranal in Chile. Through my telescope, it is visible as a fuzzy patch.

    Here's an HST image of another planetary nebula, the Catseye Nebula (NGC 6543). The expanding shell of gas has become very distorted.

    Planetary nebulae are called thus because when first observed it was thought that they looked like planets. This one, through a small telescope, looks a small blue diffuse disc. The surface central of the central star can be very hot, some more than 200 000°C. Planetary nebulae only 'exist' for a few thousand years. Before the expanding shell becomes too tenous and cold. The star loses about 20% of its mass in this phase. The remaining star cools and becomes a dim white dwarf.

    White dwarfs are strange objects. It consists, primarily, of a dense gaseous mixture of carbon and oxygen nuclei and electrons. So dense indeed, ~100 000 000 times the density of water, that the electrons are degenerate (in the lowest energy states that they can be in) and are chiefly responsible for supporting the star against further gravitational collapse*. If you had a chocolate cake weighing 500g, you would expect it to occupy twice the volume of one weighing 250g. The reverse applies to white dwarfs, a white dwarf that weighs 1 solar mass occupies roughly half the volume of a white dwarf weighing 0.5 solar masses. The smallest ones (in volume) are only slightly larger than the Earth. No white dwarf though can weigh more than 1.44 solar masses, the Chandrsekar limit.

    The first white to be discovered was discovered by Fredrich Wilhelm Bessel**. It is the companion of the brightest star in the sky, Sirius. Many white dwarfs in binary systems have been discovered. If the white dwarf is close to its companion it may draw material away from its companion. This gas heats up and, on occasions, may ignite in a nuclear explosion. This event is called a nova and star can be magnitudes brighter over a period of several years. The nova means 'new' in latin and the name is given because it looks as though a new star has appeared. On the right are two HST images of Nova Cygni 1992. The first one was taken in May 1993 (before the COSTAR repair mission) and the second in January 1994 (afterwards). There is a hot shell of gas moving outwards at 1700 km/s.

    It was indeed a powerful explosion. However, if a white dwarf accrestes mass onto itself, and in doing so attains a mass greater than the Chandrasekar limit, it contracts to a point where it is hot enough for carbon fusion to start. In less than a second the star explodes in what is called a type 1a Supernova. For a period of few weeks, a type 1a supernova can easily oushine an entire galaxy, with a peak luminosty greater than 10 billion solar luminosities.