. A newly formed protostar and a red giant are both located in the same region on the H-R diagram. Explain how you could distinguish between these two.

The first important thing to tackle is the Hertzsprung-Russell diagram itself. The H-R diagram is a scatter graph of stars that displays the stars' luminosity or absolute magnitude against their temperature or color. The diagram was created independently by Ejnar Hertzsprung and Henry Norris Russell in the early 20th century, and provides a very useful chart of stellar evolution. On the vertical axis, the diagram measures brightness, while on the horizontal axis it measures temperature (generally in reverse order with cooler stars on the right and hotter stars on the left). The stars on this diagram are normally distributed along a band known as the 'main sequence', with other clusters of stars at certain locations depending on their evolutionary stage.

Protostars are stars in the process of formation. They are located on the right side of the H-R diagram, where stars are cooler and more luminous. This is because they are still contracting and thus increasing their temperature and luminosity. Protostars are not always plotted on the H-R diagram as they are often obscured by gas and dust and not visible in optical light. When they burn through these obscurations, they move left onto the main sequence.

Red giants are much older stars that have exhausted the hydrogen fuel in their cores and have moved off the main sequence. They are located on the upper right of the H-R diagram. This is because they have expanded and cooled at their surfaces, resulting in a decrease in surface temperature - hence, they're 'red' - but a huge increase in size, which leads to a large luminosity.

To determine the difference between a newly formed protostar and a red giant star, check their temperatures and luminosity on the H-R diagram. A newly formed protostar is located on the right side meaning it is cooler and brighter relative to its size. A red giant is located at the top right of the diagram; it is even cooler (red) but much more luminous due to its larger size.