Hold your arm at full length, then close one eye & use the hand shapes shown above to measure the angular distance between the stars.

(Ain't anatomy wonderful!)

The apparent magnitude of a star is a measure of how bright it appears from Earth. The scale was introduced over 2,000 years ago by the Greek astronomer Hipparchus, who grouped stars into six categories. The brightest 20 or so were deemed to be 'first magnitude', slightly dimmer stars 'second magnitude', and so on until the barely visible stars were classed as 'sixth magnitude'.

Later it was recognised that our eyesight, once it has been given time to get used to darkness, has a logarithmic response. i.e. a Mag. 1 star is actually 2.512 times brighter than a Mag. 2 star, or 6.310 times brighter than a Mag. 3 star (2.512 x 2.512 = 6.310).

The six Magnitudes thus corresponds to a 2.512⁶ difference in brightness or 100x.

Today the scale has now been extended, so that brighter objects can have an apparent magnitude of 0 or even negative. The brightest star Sirius, for example, has an apparent magnitude of -1.44 and the Sun is a whopping -26.74, due to it's close proximity to Earth.

Diffuse Nebula is a term given to clouds of interstellar gas & dust which do not have well defined boundaries. If the clouds are large and massive enough they are frequently places of new star formation, with associations or clusters of stars. If the stars formed are large enough, and emit sufficient radiation to ionise the surrounding gas & cause it to shine, then the nebula is called an emission nebula. These are usually dominated by a red hue, from the recombination of Hydrogen II. On the other hand if the stars are not hot enough to create ionisation, their light is seen as scattering off the dust & gas creating a reflection nebula with a white or blue-ish hue.

Sometimes regions of both emission and reflection can be seen in the same nebula, for example in the Triffid Nebula shown below. (This image was taken by a LAS member using the free Open University remote telescope 'COAST' in Teneriffe.)

M20 - Triffid Nebula (OU COAST scope)

The clouds of gas and dust will after a few million years, have all been used up in forming new stars and planets, or blown away by the stellar wind from the newly formed stars. Once that has happened, only the open cluster formed by those stars will be visible.

Open Clusters are groups of up to a few thousand stars which are loosely gravitationally bound, and which formed within molecular clouds at roughly the same time. More than 1,000 such clusters have been found within the Milky Way.

Because open clusters are situated in the galactic disc and have fewer stars than their globular counterparts, they are more prone to disruption and typically last about a few hundred million years.

Younger open clusters which still are surrounded by molecular clouds, can form emission and reflection nebula, depending on the size & luminosity of the stars. If the stars are hot enough, ultraviolet radiation causes hydrogen to ionise, emitting reddish light in an emission nebula. If the stars are slightly cooler and their light isn't sufficent to ionise the gas, it can be seen to scatter off dust particles in a reflection nebula.

Over time, the stellar winds from the stars cause the molecular cloud to dissipate, leaving behind an open cluster.

NGC869/884 - Double Cluster

For more information on open clusters see the Wikipedia entry.