Stars

In Scrap Clicker 2
Stars are the primary measurement of progress in the game. Stars passively increase Scrap production similarly to Golden Scrap.

Purchasing Stars
The player's first 10 Stars are earned or "purchased" by resetting their Barrels, Scrap upgrades, and Golden Scrap. Each Star purchased requires the player to have accumulated a certain amount of Golden Scrap first and multiplies their Scrap production by 10 for each Star owned.

Unlocks
At 4 Stars, the player unlocks the Scrapyard, allowing them to spend Scrap to reduce the Golden Scrap required to purchase Stars. The Scrapyard becomes inaccessible once the player reaches 10 Stars. At 5 stars, the GS Multipliers are unlocked.

Upgrading Stars
Once the player has accumulated 10 Stars, the player can no longer purchase more; instead, at 4 million Golden Scrap (often referred to as "x10.5 Stars"), the player is afforded the ability to individually upgrade and increment the Scrap production multiplier for each Star.

In order to upgrade Stars, the player must spend Golden Scrap, Magnets, and Star Fragments. Because upgrades become more expensive the more a Star is upgraded, it is most cost-effective to upgrade them evenly. At the first few star upgrades, Golden Scrap and Fragments are the most expensive, but this changes until ~x30, then Magnets become the main problem of upgrading stars and at ~x90 they're the only one, if played right.

Milestones
Many of the game's features are unlocked at certain Star milestones:

Pre 10 stars milestones

 * 1st Star - Unlocks Teams.
 * 1st Star - Unlocks Leaderboards.
 * 4th Star - Unlocks Scrapyard.
 * 5th Star - Unlocks GS Multipliers.
 * 8th Star - Unlocks Team Leagues.

10 stars+ milestones

 * 10th Star - Removes the first Scrapyard.
 * - Unlocks Star upgrading.
 * x12 Stars - Unlocks Scrapyard v2.
 * x15 Stars - Unlocks Bricks and Brick Multipliers.
 * x20 Stars - Unlocks creating a team, plus Team Challenges and Change Tokens.
 * x30 Stars - Unlocks Global Challenge and Combine Tokens.
 * x40 Stars - Unlocks Skill Tree

Skill Tree milestones

 * x40 Stars - Unlocks Steel Beams
 * x75 Stars - Unlocks Scrapyard v3.
 * x100 Stars - Unlocks Wrenches.
 * x115 Stars - Unlocks Scrapyard v4.
 * x130 Stars - Unlocks Tires.
 * x250 Stars - Unlocks Scrapyard v5.
 * x400 Stars - Unlocks Barrel Mastery.

Star Bombs
Star bombs are a community-coined term for upgrading multiple stars in quick succession. There is no practical benefit on waiting to upgrade, but some may argue there is a certain style to it.

It actually has disbenefit, upgrading the weakest stars is cheaper and yields more scrap production.

Here are some terms that may be used to describe the size of a star bomb:
 * 1-2: No bomb
 * 3-5: Very small bomb, Micro bomb
 * 6-9: Small bomb
 * 10-24: Medium Bomb
 * 25+: Big Bomb

Star Wall
Another community term. The Star Wall used to be the point, where upgrading stars takes almost ages. It seems to not exist anymore thanks to Merge Boost and slow cost jumps.

Video
There's a video made by Schrottii, showing upgrading stars, mostly focusing on a star bomb.

In Scrap Collector
Stars were added to the game on Version 2.0. They used to give (additive) +100% Scrap per Star, but since Version 2.1 they give x5 until 10 stars, the point at which the boost is calculated with the following formula: $$50 * 1.3^{Stars - 10}$$. They cost $$10000 + 10000n$$(where n are the Stars) until 10 Stars, but from then it becomes difficult to extract a formula for the GS cost.

It's important to note that Stars won't take away from you any extra GS you've got, so you can save some more before buying a Star. Stars won't reset your Magnet Upgrades either. They will just take the GS they need when you buy them, making it possible to buy several Stars at once.

Star Info
Sterne Estrellas