When you look at the stars they have different apparent brightnesses. From the Evolving Universe part of this course you now know its because the 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. They 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!) On these definitions then we can relate any brightness to a magnitude. Rigel (blue supergiant bottom right star in Orion) has a magnitude of about zero. Sirius (the brightest star) is about 4x brighter and 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) - and then you see all these fuzzy objects which are not single stars. They first got catalogued by people looking for comets (faint fuzzy things!), and many are still called after the number one of these comet hunters - Messier - gave them. There are open clusters and globular clusters (both of which are very important in giving us a sample of stars of different masses but more or less the same age, chemical composition and, most importantly, distance from us - we can use these to test our models of stellar evolution). Other fuzzy objects are star forming regions - diffuse clouds of gas and dust lit up by the light of the young stars forming deep within the cloud. Yet others are associated with the end points of stellar evolution planetary nebulae and supernovae remnants. And of course there are the external galaxies Browse the whole Messier fuzzy object catalogue here
P> Starmaps generally use the size of the dot as a measure of stellar brightness. And bright stars are much easier to see than faint ones. So often the shapes we pick out for stars are not the full constellation but just the bright stars - for example just picking out the Plough rather than all the stars in the constellation of Ursa Major (Great Bear). Groups of stars like this which are NOT constellations are called asterisms. Other examples are the 'summer triangle' made by the bright stars Vega (in Lyra) Deneb (in Cygnus) and Altair (in Aquilus).Now we have a way to classify brightness we can look and see what we can see. And the most obvious thing is that
stars twinkle - this is because of the Earth's atmosphere and has nothing to do with the star! But not all stars have constant brightness - some are intrinsically variable. There are dramatically explosively variable stars - supernovae. Type II supernovae mark the end of the life of a massive star, Type I mark the end of a white dwarf, where accretion of material from a companion star finally pushes the mass over the limit where electron degeneracy pressure can hold it agains gravity. Supernovae are rare - we expect only one every hundred years in a galaxy like ours. The last was in 1680 so we are overdue for one. Supernovae are incredibly bright - 10 billion times brighter than the Sun at the peak, so they can be seen to very great distances. Images of recent supernovae in other galaxies can be found here - these are the ones which are used to get distances to remote galaxies and so track the expansion of the Universe. These explosions completely rip apart the white dwarf, and so are very different from the much less catastrophic novae. Matter accreting from a companion onto a white dwarf builds up on the surface until the density is so large that nuclear fusion of the hydrogen occurs. The explosion causes the star to become 10,000x brighter (10 magnitudes) within a few days, then dimming over timescales of a few months. But the amount of matter blown away in the explosion is tiny so accretion onto the white dwarf soon resumes.
Then there are more restrained variable stars. Stars on the main sequence are generally stable because of hydrostatic equilibrium, but as they evolve then they can pulse. The large envelope that forms in a red giant or supergiant star is not well coupled to the energy producing core. Suppose that the envelope expands a bit, then it cools, then it contracts... These pulses can be irregular as in the Mira (or long period) variables, or regular as in the Cepheids and RR Lyra stars.
Some variables do not intrinsically change in brightness! Stars in binary systems can eclipse each other giving periodic changes in apparent brightness. These eclipsing binaries are the ones which are very useful to get a fairly direct measure of star masses and radii - the most famous one of these is Algol, in Perseus. A nice compilation of many sorts of variable stars is given here