Which quantities are plotted on an H-R diagram? a. brightness on the horizontal axis; luminosity on the vertical axis b. brightness on the horizontal axis; temperature on the vertical axis c. luminosity on the vertical axis; temperature on the horizontal axis d. main sequence on the vertical axis; spectral type on the horizontal axis

Stellar luminosity is the total amount of energy that a star emits per second. Think of it like the wattage of a light bulb: the higher the wattage, the brighter the bulb. Similarly, a star with higher luminosity appears brighter.
Luminosity is denoted in units of solar luminosity \(L_\odot\), which refers to the luminosity of our Sun. This makes it easier to compare other stars to our Sun. The formula used to describe luminosity is:
\[ L = 4 \pi R^2 \sigma T^4 \] where:

• R is the star's radius.
• T is the star's surface temperature.
• \( \sigma \) is the Stefan-Boltzmann constant.
• \( 4 \pi \) is a geometric factor.

As you can see, both the size and the temperature of a star influence its luminosity.

Stellar temperature is a measure of how hot the surface of a star is. Astronomers use a unit called Kelvin (K) to describe these temperatures.
Here's a cool fact: the temperature of a star determines its color. Hot stars appear blue, while cooler stars appear red. Stars in the middle range of temperature appear yellow or white, just like our Sun.
The surface temperature of a star can be estimated using its spectrum, which shows the light emitted by the star at different wavelengths. Wien's displacement law helps in this calculation:
\[ \lambda_{max} = \frac{b}{T} \] where:

• \( \lambda_{max} \) is the wavelength at which the star emits most of its light.
• \( T \) is the temperature.
• \( b \) is Wien's constant.

The H-R diagram's horizontal axis represents temperature, decreasing from left (hotter stars) to right (cooler stars).

Astronomers classify stars into different types to understand their properties and life cycle. One method of classification is by spectral type, which groups stars based on their surface temperature and color.
The spectral types, in order of decreasing temperature, are:

• O (blue, very hot)
• B (blue-white)
• A (white)
• F (yellow-white)
• G (yellow, like our Sun)
• K (orange)
• M (red, cool)

Each spectral type is further divided into sub-categories using numbers from 0 to 9.
Another important classification shown in the H-R diagram is the evolutionary stage of stars. Main sequence stars, giants, and white dwarfs are all placed differently on the diagram.
Understanding where a star falls on the H-R diagram helps astronomers predict its future evolution and past history.