Star Circle

Star Circles are rings of tiny stars created by Star Posts or different objectives. In Sonic the Hedgehog games, Star Circles are rotating portals, which lead into Specials or Bonus Stages after activating the Star Post with required amount of Rings.

Sonic the Hedgehog 2
As first appearing in 16-bit version of Sonic the Hedgehog 2, the red and white Star Circle is created by each Star Post when the player has collected least fifty Rings. The player is then transported to the Special Stage by jumping on it. The Star Circle keeps rotating around the Star Post until it shrinks and disappears after couple of seconds. If the player does not manage to gain one of seven Chaos Emeralds from Special Stage, the player has to collect another set of fifty Rings and activate another Star Post. Star Circles cannot be activated after getting all seven Chaos Emeralds or in two player mode of the game at all.

Sonic Spinball
In 16-bit version of Sonic the Hedgehog Spinball, the player has to collect every Ring in one of the game's four pinball stages to gain access in the fourth Bonus Stage titled "Clucker's Defense". The Star Circle can be found in one of sections in each pinball stages, as entering in it will transport the player into the fourth Bonus Stage.

Sonic the Hedgehog 3 & Knuckles
In standalone Sonic the Hedgehog 3, the circle of tiny silver stars appear the same way as they did in Sonic the Hedgehog 2 after activating the Star Post with fifty Rings on the player's counter. The player can enter the Bonus Stage of the game, which is the Gumball Machine variation.

The Gumball Machine variation is not presented in Sonic & Knuckles, as there are two other types. By having 20-49 Rings creates the yellow Star Circle that leads to Slot Machine type of Bonus Stage and having more than 50 Rings creates red Star Circles leading into Glowing Spheres type of Bonus Stage. In locked-on Sonic 3 & Knuckles all three Bonus Stages are featured and they are accessed by different types of Star Circles with different amounts of Rings:

It can be noticed that after over fifty Rings, the pattern restarts with 70-84 Rings leading to Slot Machine-type, 85-99 Rings to Glowing Spheres-type and least 100 Rings to Gumball Machine-type.

Sonic Triple Trouble
In Sonic the Hedgehog Triple Trouble, there are Chaos Emerald Monitors found hidden in each Act of the game. When the player finds one with least 50 Rings on-hand, breaking it open creates the Star Circle to access the Special Stage.

In other media
In other media appearances of Sonic the Comic and Archie Comics, where Star Posts are featured, Star Circles are featured as dimensional portals created by post's technology. Although in none of appearances they are referred to as Star Circles but rather just portals or energy fields. In Sonic the Comic, Sonic has to run around the Star Post for charging it with Sonic Speed Energy and create the energy field, while in Archie Comics, dimensional portals are activated and deactivated by remote control.

Trivia

 * Knuckles the Echidna in Sonic the Hedgehog 2 gives an change for the player to have all previously collected Rings in the HUD counter after exiting the Special Stage as giving easy time to activate another Star Circle straight ahead. In the original game, the player loses all collected Rings after exiting the Special Stage.