Spawns a star familiar that slowly travels toward the Boss Room, fading through doors as it travels towards it. Once it reaches the boss room, the familiar stands in the center of the boss room for the rest of the floor.
The familiar emits an aura that, while Isaac stands inside it, gives him a 1.8× damage multiplier, a 2.5× fire rate multiplier, and homing tears. It also gives Isaac a 50% chance to ignore damage similarly to Cone Head's effect except with a higher chance.
Notes
In The Void, the familiar will travel to the boss room that contains Delirium.
In Greed Mode, the star will travel directly to the position where Isaac spawns upon entering a new floor.
On the last floor, it will move to the upper part of Ultra Greed's room.
The star will travel through Red Rooms if they create an optimal path to the boss room.
The star can backtrack into rooms it already went into if a more optimal path is created by a Red Room.
During the Ascent, the star will linger in the boss room of each floor, and will not progress backwards through the stage.
The star will travel through the hole to the Mother fight and be present in the fight. It will be centered on the top half of the room, and will not move when the arena expands.
The star will be present against Dogma, but does not follow for The Beast.
If the door to Mega Satan is opened, the Star of Bethlehem will enter it instead of the boss room containing ??? or The Lamb.
The Star will move backwards towards Mega Satan's door if the door is opened later during the floor (such as by using Dad's Key).
For Tainted Forgotten, the bonuses from the star are only applied if Tainted Forgotten is within its aura, not Tainted Soul.
If either Tainted Lazarus or Dead Tainted Lazarus have picked up the star, the familiar will spawn in the room where the character holding it becomes active, which may not be the starting room. It will only move while the character holding it is active, effectively making it take twice as long to travel to the boss room unless Tainted Lazarus also has Birthright.
During Backasswards (challenge #31), the star acts as usual; originating in the starting room and gradually moving towards the boss room.
The star will be copied in the Mirrored World and will simultaneously travel in both worlds.
Entering and leaving the Knife Piece 2 area will reset the star and make it reappear in the room with the minecart.
The star exhibits a "rubber banding" effect in its movement. If it is in a room that is closer to the Boss Room than the one Isaac is in, it will move slower; if the opposite is true, it will move faster.
This item is a reference to the Star of Bethlehem, which appears in the Nativity story of the Gospel of Matthew, signifying the birth of Jesus.
Bugs
Bug!
If Isaac uses Pause, the light aura will be stuck in the last position before Pause was used even if the familiar continues without it. This is only a visual glitch, Isaac still has to be near the star to receive its benefits. If Pause’s effect ends the light instantly disappears and respawns back around the star again. The light will still fade when the star leaves the room.
Boss Room, fading through doors as it travels towards it. Once it reaches the boss room, the familiar stands in the center of the boss room for the rest of the floor.
Cone Head's effect except with a higher chance.
The Void, the familiar will travel to the boss room that contains 
Delirium.
Ultra Greed's room.
Blood Donation Machines and 
Devil Beggars for free, provided they are spawned via 
XIV - Temperance or 
XX - Judgement cards, respectively.
Secret Rooms and 
Curse Rooms on its path to the boss room.
Red Rooms if they create an optimal path to the boss room.
Dogma, but does not follow for 
The Beast.
Mega Satan is opened, the Star of Bethlehem will enter it instead of the boss room containing 
??? or 
The Lamb.
Dad's Key).
Tainted Forgotten, the bonuses from the star are only applied if Tainted Forgotten is within its aura, not 
Tainted Soul.
Tainted Lazarus or 
Dead Tainted Lazarus have picked up the star, the familiar will spawn in the room where the character holding it becomes active, which may not be the starting room. It will only move while the character holding it is active, effectively making it take twice as long to travel to the boss room unless Tainted Lazarus also has 
Birthright.
Knife Piece 2 area will reset the star and make it reappear in the room with the minecart.
Blood Rights/
IV Bag/
Kamikaze!/
Razor Blade: The aura has a chance to block the damage inflicted, but the effects will still occur.
Rock Bottom: Isaac will keep the stat upgrades even after leaving the aura. However, it won't keep the homing effect outside of the aura.
Sprinkler: The tears that the Sprinkler shoots will also be buffed, along with its rotation speed.
BFFS!: The star appears larger, but the aura's size does not increase and neither do the stat gains.
Adrenaline: Isaac's damage before Adrenaline's flat damage increase is increased by 1.2×, then damage after is increased by 1.5×.
Bloody Lust: Isaac's damage before Bloody Lust's flat damage increase is increased by 1.2×, then damage after is increased by 1.5×.
Hallowed Ground: The damage multiplier from the aura does not stack with white poop.
Immaculate Heart: The damage multiplier is reduced to 1.5× while in the aura.
Ipecac: Isaac's damage before Ipecac's flat damage increase is increased by 1.2×, then damage after is increased 1.5×.
Bug!
Pause, the light aura will be stuck in the last position before Pause was used even if the familiar continues without it. This is only a visual glitch, Isaac still has to be near the star to receive its benefits. If Pause’s effect ends the light instantly disappears and respawns back around the star again. The light will still fade when the star leaves the room.
Bethany Unlocks 
Delirium Unlocks 
The Binding of Isaac: Rebirth
