What kind of stars are found here?

A star in this region of the Hertzsprung-Russell diagram has a temperature of roughly 3,000 kelvin (3,000 K), a luminosity five thousand times fainter than that of the Sun (0.0002 × LSolar), and a radius one-twentieth that of the Sun (R = 0.05 × RSolar). This star lies along the Main Sequence, where most stars (including the Sun) are found. The low temperature indicates extremely red colours; these stars would be easiest to detect using an infrared camera.

Try to read the values of L, T, and R for yourself from the diagram. Do you estimate values for the luminosity, temperature, and size of the star similar to those listed above?

Hertzsprung-Russell Diagram. The x-axis is labeled 'Surface Temperature' (in units of kelvins) with high temperatures around 60,000 on the left and low temperatures around 3,000 on the right. The points representing stars which appear furthest to the left are drawn in blue, those in the middle temperature range are drawn in yellow and orange, and those furthest to the right are drawn in red. The y-axis is labeled Luminosity (in units of solar luminosity), with low luminosities around 0.0001 at the bottom and high luminosities around 200,000 at the top. A third parameter, Radius (in units of solar radii), is also labeled. Lines of constant radius extend from the upper-left to the lower-right, covering the whole space with a set of parallel lines. In the lower-left corner we find the line labeled 'R is equal to 0.001 solar radii'; successive lines are labeled 0.01, 0.1, 1, 10, 100, and 1,000 solar radii, with the line for 'R is equal to 1,000 solar radii' being located in the upper-right corner. A series of blue, yellow, and orange points scattered along the 'R is equal to 0.01 solar radii' line is made up of white dwarf stars. A large set of red (and a few yellow and orange) points made up of giant stars appears between the 1 and 1,000 solar radii lines. The 1 solar radii line extends from the upper-left corner to the lower-right corner. The Main Sequence (a curved sequence of blue, yellow, orange, and red points) mainly follows the 0.1 to 10 solar radii lines; at the high luminosity end it curves up to slightly higher luminosities and at the low luminosity end it curves down to slightly lower luminosities. In addition, there are three green lines on the figure which intersect around the point near to 3,000 kelvins and 0.0002 solar luminosities. One points down to the x-axis, one points left to the y-axis, and one is parallel to the black lines of constant radius (being drawn at a radius of around 0.05 solar radii).

How much fainter is the faintest star in this region than the brightest star on the Hertzsprung-Russell diagram with the same temperature?

We can trace the green horizontal line over to the y-axis, and determine that the faintest stars in this region (just below the green cross) have a luminosity of 0.0001 (one-ten-thousandth as bright as the Sun).

Now shift your eyes vertically, up to the top of the diagram. Identify the brightest stars (those nearest to the top) with the same temperature as our faint star (they will lie directly above our star). How bright are these giant stars? If you again trace your way over to the y-axis, you will find that they are 100,000 times brighter than the Sun.

The different between these stars is 100,000 ÷ 0.0001, or a factor of a billion! Clearly giant stars and faint dwarfs must be rather different from each other!