Stars change color with surface temperature and are sorted on the H-R diagram into main sequence, red giants, white dwarfs, and more.
Blue stars are hot and red stars are cooler, and most stars, including the Sun, sit on the main sequence.
Stars are born in nebulae and end as red giants or supernovae depending on mass; distance is measured in light-years.
On this page, press the star-type buttons to explore the H-R diagram and stellar life cycles.
How Do We Classify Stars?
⭐ Intuition — Stars Have Types Too
①Star color depends on surface temperature
②Blue star = hot (30,000 K), Red star = cool (3,000 K)
Stars in the night sky have slightly different colors. Which has the higher surface temperature, a blue star or a red star, and why?
1
The color of a star depends on its surface temperature.
2
The higher the surface temperature, the bluer it looks (about 30000 K), and the lower, the redder (about 3000 K), so a blue star is hotter.
▸ A blue star is hotter (blue ≈ 30000 K > red ≈ 3000 K)
The H-R diagram classifies stars by their surface temperature (color) and brightness.
Example 2
Distances to stars are so large that the light-year is used. What does one light-year mean, and about how many light-years away is the nearest star?
1
One light-year is the distance light travels in one year (about 9.46 × 10¹² km).
2
The nearest star after the Sun (Proxima Centauri) is about 4.2 light-years away.
▸ 1 light-year = the distance light travels in 1 year (≈ 9.46 × 10¹² km) / nearest star ≈ 4.2 light-years
Note that a light-year is a unit of distance, not of time.
Unit Test
Which statement about stars and the universe is correct?
①A red star has a higher surface temperature than a blue star
②One light-year is the distance light travels in one year
③A light-year is a unit of time
④All stars have the same size and brightness
⑤The Sun is classified as a red giant
▸ ② One light-year is the distance light travels in one year
1
The higher the surface temperature the bluer it is, so a blue star is hotter (① wrong), and a light-year is a unit of distance (③ wrong).
2
Stars differ in size and brightness (④ wrong), and the Sun is classified as a main-sequence star (⑤ wrong); one light-year is the distance light travels in one year (② correct).