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.

Dark Nebula or absorption nebula are interstellar clouds that are dense enough to obscure light in the visible part of the spectrum, so that objects such as stars which are behind the cloud, cannot be seen.

Light hitting the cloud is absorped by sub-micrometre interstellar grains of dust, which are coated with frozen carbon monoxide and nitrogen. This effectively blocks the passage of light at visible wavelengths. The clouds are however transparent to infra-red and radio waves & also contain other organic compounds such as ammonia and formaldehyde.

The Great Rift - dark nebula region in the Milky Way

Clusters and large complexes of Dark nebula are associated with Giant Molecular Clouds, but smaller isolated examples known as Bok Globules can also be seen in HII regions. These typically only have a mass of 2-50 × that of the Sun.