Third Stage: Once the core hits about 10,000,000 degrees Celsius, hydrogen atoms fuse together in the core... this forms*1 pointPlanetary NebulaMolecular CloudProtostarAccretion DiscSupernovaStar

The answer is Star.

Here's the step by step process:

1. The third stage in the life cycle of a star begins when the core of a nebula (a cloud of dust and gas in space) reaches a temperature of about 10,000,000 degrees Celsius.

2. At this temperature, hydrogen atoms in the core of the nebula begin to fuse together. This process is known as nuclear fusion.

3. The fusion of hydrogen atoms releases a huge amount of energy, causing the core to shine brightly. This is the birth of a new star.

4. Therefore, when hydrogen atoms fuse together at such high temperatures in the core of a nebula, a Star is formed.

The answer is Star.

Here's the step by step process:

1. The third stage in the life cycle of a star begins when the core of a nebula (a cloud of dust and gas in space) reaches a temperature of about 10,000,000 degrees Celsius.

2. At this temperature, hydrogen atoms in the core of the nebula start to fuse together. This process is known as nuclear fusion.

3. During nuclear fusion, four hydrogen nuclei combine to form one helium nucleus. This process releases a lot of energy in the form of light and heat, causing the nebula to glow.

4. The glowing nebula is now called a protostar. As the protostar continues to contract and heat up, it eventually becomes hot and dense enough to trigger more nuclear fusion reactions.

5. Once the protostar starts to generate its own heat and light through nuclear fusion, it officially becomes a star.