Red and Brown Dwarf Stars

A dwarf star is a star that is relatively small and dim. Astronomers describe a stars’ luminosity in comparison to the brightness of the sun. Those much more luminous are giants, those much dimmer are dwarves. Additionally, astronomers graph stars based on their luminosity and temperature. This is on a Hertzsprung-Russell diagram, which shows us where red and brown dwarf stars place compared to other stars.
Graph of stars

You can see the relative luminosity and temperature of red and brown dwarf stars compared to other stars on the Hertzsprung-Russell diagram. Credit: Image created by the European Southern Observatory.

Red Dwarf Stars

Red dwarf stars are small stars as they are smaller than half the size of our Sun. They are cooler than most stars and also fainter. This means they are found on the bottom of the Hertzsprung-Russell diagram, near both axis. Red dwarf stars are extremely common and are found all around the Milky Way Galaxy.
Red Dwarf vs Planet.

Although red dwarf stars are small compared to the sun, they tower over this small planet.

Formation and Characteristics

Red dwarf stars form like other main-sequence stars. In simple terms, a clump of dust and gas are exposed to heat and fusion, and a star is formed. The low temperature of these stars means that they burn through hydrogen slower than a massive star like the Sun. Their lifetimes may extend to trillions of years. This is far longer than the lifetime of Sun-sized stars.
Photo of Starburst by Hubble.

Red dwarf stars form just the same as any other main-sequence stars.

Why are red dwarf stars red?

Because red dwarf stars only burn a little bit of fuel at a time, they are not very hot compared to other stars. Think of a fire. The coolest part of the fire is at the top of the flame where it glows red. The hotter part in the middle glows yellow, and the hottest part near the fuel glows blue. Stars work the same way. Their temperature determines what color they are. Thus, we can determine how hot a star is by its color.
Red Dwarf vs Sun and others.

Red dwarf stars are extremely small compared to the size of the Sun. Credit: This image is from the European Southern Observatory.

Brown Dwarf Stars

A brown dwarf has the same characteristics as a star but does not have enough mass for nuclear fusion. Without nuclearfusion, the star cannot glow. Brown dwarfs are not regular stars because they do not glow. However, they are not regular giant planets either. 
Brown Dwarf star.

These stars are dimmer than the Sun and red dwarf stars.

Brown Dwarfs vs. Planets

When you look up at the night sky, it is easy to mistake a star for a planet, or vice versa. However, planets shine by reflected light and stars shine by producing their own light. Stars go through a process known as nuclear fusion. This process releases a gigantic amount of energy which causes the star to shine. Planets form from leftover particles of dust. They don’t have enough energy to produce their own light. Brown dwarf stars, although small compared to other stars, are large relative to planets
Brown Dwarf Comparison

Brown dwarf stars are extremely small compared to the Sun and other dwarf stars. Credit: This image is taken from a book titled 50 Years of Brown Dwarfs – From Prediction to Discovery to Forefront of Research by Viki Joergens.

Other Great Resources:

The Difference between Big Planets and Brown Dwarfs: https://www.universetoday.com/138338/wheres-line-massive-planet-brown-dwarf-star/
 
 
Written by: Varsha Rammohan.