1. Luminosity: How bright a star is, which is related to its size and temperature. It is measured in solar luminosities (L☉), where 1 L☉ is the luminosity of our Sun.
2. Spectral Type: The star's surface temperature, determined by the color of light it emits. Spectral types are denoted by letters, from hottest to coolest:
* O: Blue (25,000-50,000 K)
* B: Blue-white (10,000-25,000 K)
* A: White (7,500-10,000 K)
* F: Yellow-white (6,000-7,500 K)
* G: Yellow (5,200-6,000 K)
* K: Orange (3,700-5,200 K)
* M: Red (2,400-3,700 K)
How Size and Temperature Relate on the H-R Diagram:
* Main Sequence: Most stars, including our Sun, fall on the main sequence, a diagonal band on the H-R diagram. Stars on the main sequence are fusing hydrogen into helium in their cores. Larger, hotter stars are more luminous and located in the top left corner of the main sequence, while smaller, cooler stars are less luminous and located in the bottom right corner.
* Giants and Supergiants: Stars that have evolved off the main sequence and have become larger and more luminous than main sequence stars. They are located above the main sequence.
* White Dwarfs: The dense, hot remnants of stars that have exhausted their nuclear fuel. They are located in the bottom left corner of the diagram.
Example:
* Our Sun is a G2V star, meaning it is a yellow, main sequence star.
* Sirius, the brightest star in the night sky, is an A1V star, meaning it is a white, main sequence star.
* Betelgeuse, a red supergiant star, is an M2Iab star, meaning it is red and has a much larger size and luminosity than the Sun.
Understanding the H-R diagram helps astronomers classify and study stars, revealing their evolution, age, and properties.
