Wash and trim the rhubarb. Dice the rhubarb and transfer to a large bowl. Add the sugar and stir together. Cover and let sit for a minimum of 4 hours on the counter or up to overnight in the fridge, stirring occasionally to encourage the sugar to dissolve.
Preheat the oven to 250°F. Make sure the jars are clean and then place on a baking sheet in the oven for at least 15 minutes before canning. Add the canning lids to a small saucepan with water and place over low heat in barely simmering water. Keep clean tongs and a towel nearby. Gather a clean ladle, funnel, and damp cloth and have ready to go. Have a potholder ready for the hot pot.
When the canning equipment is all sterilized and ready to go, begin to cook the jam. It is imperative that the jars and jam are both hot when canning. Tie the star anise in a small piece of cheesecloth.
Transfer the rhubarb along with its juices to a large pot. Add the lemon juice, grated ginger, cinnamon stick, and star anise. Bring to a boil over medium high heat, stirring constantly. Optionally, add a teaspoon of butter to help prevent the jam from foaming. Maintain the boil, adjusting the temperature to prevent the jam from boiling over. Skim periodically to remove any excess foam.
Continue to boil the jam until it reaches a temperature of 218°F. This will take from 15 to 20 minutes.
When the jam reaches 218°F, immediately transfer the pot to the potholder. Use tongs to remove a jar from the oven. Use the funnel and ladle to fill the jar with the hot jam, leaving a half inch space from the top.
Use the damp cloth to wipe the rim of the jar to clean any spills. Use the tongs to remove one of the canning lids from the hot water and wipe dry with the clean towel. Place the lid on the jar and then add the screw top. Hold the jar with a towel and screw the lid on as tightly as possible. Set the jar aside being sure not to press down on the top of the lid - this could give the false impression that the jar has sealed when it hasn't. Repeat with the remaining jam.
Check the jam jars after an hour or two to see if they have sealed. A properly sealed jam jar will make a small popping sound as the lid seals and the top will go from looking rounded to indented. Any jam that hasn't sealed can be stored in the fridge and used within a few weeks. Sealed jam can be labelled and stored in the pantry for at least a year. When opening the jam, check that it is still sealed and for any signs of mold or spoiling, discarding if you notice any spoilage. Enjoy!