Comet

Hale-Bob (Comet) captured with SpaceEngine.

A Comet is an object that it revolves around the Sun like planets and asteroids, but it has a very eccentric orbit (It can be elliptical, parabolic or hyperbolic), it consists of ice, dust and rock.

Comets are object with irregular shaped objects such as satellites, but when they approach the Sun, they heat up and the ice sublimes creating an atmosphere termed "The Coma" and a Tail.
The Coma surround the nucleus and the Tail is a linear sections consisting of dusty or gas dragged out of the nucleus by the solar wind and radiation.

Some comets are "periodic comets" they have an elliptical orbits, as approaches to the Sun, they sublimate their mass (Forming a Coma and Tail), but these disappear when consumed all their mass.
Other comets approach the Sun only once, they have a parabolic or hyperbolic orbit, therefore, after to approach the sun they head into interstellar space.

Star cluster

Globular Cluster, catch taken with SpaceEngine.

Star Clusters are astronomical objects, they are composed of many stars.

There are two types of clusters: Globular Clusters and Open Clusters.

Globular Clusters are groups of old stars, they have between  100,000 and 1 million stars and are denser than open clusters.

In the Milky Way globular clusters are located in the galactic Halo.

Open Clusters are groups of young stars, they have a few thousand stars, these stars were formed from the same giant molecular cloud, and are united by their mutual gravity.

In the Milky Way open clusters are located in the galactic disk.

Ceres

Ceres, catch taken with realist brightness
since SpaceEngine. 

Ceres is the fifth nearest planet to the Sun (the nearest dwarf planet), and it is the smallest planet in the solar system, it is also the only dwarf planet that isn't a TNO (Trans-Neptunian object) Its name comes from the roman deity Ceres.

Ceres is a Dwarf Planet (Cool Selena), located between Mars and Jupiter, in the main Asteroid Belt, it has a mass of 9.43 x 10^20 Kg and a diameter of 952.4 Km, therefore, its density is 2.07 g/cm³ (Approximately).

Although it is less massive planet, Ceres is the largest and most massive object in the Asteroid Belt (its mass represents a third of the total mass of the Asteroid Belt), this planet in the past was considered a asteroid, but it has a lot of mass to be an asteroid and it has very little mass to be a planet, so since 2006 the International Astronomical Union considers that Ceres is a Dwarf  Planet.

Its rotation period is 9.07 hours and its orbital period is 1682 days.  This planet doesn't have natural satellites, because it has little gravity (0,28 m/s² or 0,029 g on its surface).