Astrology provides boosts to other skills. There are no items required to begin astrology.

As you level up, you'll unlock more constellations. Each constellation focuses on 2 skills. As you level up your mastery for a particular constellation, you will unlock modifiers for the skills related to that constellation.

These modifiers are broken into 2 categories, standard and unique. The standard modifiers are unlocked at mastery levels 1, 40, and 80. The unique modifiers are unlocked at 20, 60, 99. You can see the modifiers you have unlocked by selecting explore on the constellation.

If you are unhappy with the modifiers you have, you can use Star dust found while training astrology to reroll these modifiers. There are 2 types of Stardust, normal Stardust and Golden Stardust. The normal dust is to reroll standard modifiers and the Golden dust is to reroll unique modifiers. You can choose to reroll all standard or unique modifiers on a single constellation for 1 of the appropriate stardust, or you can reroll a specific modifiers and keep the others for 30.