An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. A group often forms a recognisable pattern, The Pleiades and Praesepe are great examples. Open clusters reside in our Milky Way Galaxy.
Globular Clusters are a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centres. They look like fuzzy grey blobs tm because they contain tens of thousands of stars held together by their mutual gravity. All of the globulars that can be seen in the sky are part of our Milky Way Galaxy, and there are about 200 of them that surround our galaxy like a halo. M13 in HERCULES is a northern favourite.
A Planetary Nebula is an old term that has nothing to do with the planets. Instead, it is a round or symmetrical nebula that is formed by the expanding shell of gas and dust around an ageing star. At its centre is a white dwarf star. When our Sun dies, it will create a planetary nebula. These objects have diameters of a few light years and are located in our galaxy. The Ring Nebula, M57, in LYRA is a favourite.
A Nebula is a giant hydrogen gas cloud that is located in our galaxy. Within these clouds, concentrations of gas can occur and gravitationally condense to form stars and accompanying planets. A set of stars created by a nebula is known as an Open Cluster. The Orion Nebula, M42 is a favourite. The nebulae we can see are inside our galaxy.
Galaxies contain billions of stars. All galaxies are beyond our Milky Way Galaxy, where our Sun resides. When you are observing a galaxy, you are looking through our galaxy into the true depths of the universe. The Andromeda Galaxy, M31 is the most distant object that can be seen with the naked eye.
A Double Star is a star that looks like one star but when magnified sufficiently (from 6x to 200x), it separates into two or more stars. Some are very pretty because of contrasting colours. Castor in GEMINI is a favourite and Albireo in CYGNUS is well liked for its blue & gold colours.
Starting from New Moon, the Moon cycles through phases every 29 days, 12 hours, 44 minutes, 3 seconds. It is 2,160 miles in diameter and averages 239,000 miles from Earth. A New Moon is not visible in the sky because the Moon is positioned very close to the Sun. Solar eclipses occur at New Moon. The best time to observe the Moon is during a phase because the craters appear their sharpest near the terminator, the line that separates the illuminated side (day side) from the dark side (night side).
The planets are best observed with a telescope using magnifications from 50x to 200x.
The five “naked-eye” planets are Mercury, Venus, Mars, Jupiter and Saturn.
Due to its very close proximity to the Sun, Mercury can be very difficult to observe. It’s usually visible just before sunrise and just after sunset.
Venus is extremely bright and hugs close to the Sun, so you see it for a short time in the west after sunset or in the east before sunrise.
Jupiter can be out all night and always outshines any star. Everyone enjoys its 4 Galilean moons and cloud bands, easily visible at 50x. It is possible to see the moons with well-focused binoculars.
Saturn is popular because of its beautiful rings.
Mars gets close to Earth about every 2 years at which time it is very bright. This is the best time to observe it but you need higher magnifications around 150x to see the surface coloration.
Light Year (ly) & Nearest Stars
A Light Year (ly) is a unit of length, it is equal to the distance light travels in one year. Since light moves at the rate of 186,282 miles a second, one light year is nearly 6 trillion miles long.
The closest night-time star visible to the naked eye is Alpha (α) Centauri in the constellation CENTAURUS. Alpha Centauri shines brightly at magnitude –0.01 and is just 4.4 light years away. The very closest star is Proxima in CENTARUS at just 4.22 ly away. It is too faint to see with the eyes because it shines at magnitude +11. The second closest star visible to the naked eye is Sirius at 8.6 ly followed by Epsilon (ε) Eridani at 10.5 ly and Procyon at 11.4 ly.