The colour of a star depends on its surface temperature: stars with very hot surfaces look blue, and cooler ones look red.
Plot stars with surface temperature across and brightness up, and you get the H-R diagram; most stars, the Sun among them, sit on a band called the main sequence running from the upper left to the lower right.
Stars are born in nebulae, shine for a long time on the main sequence, and end their lives, depending on their mass, as white dwarfs or in a supernova explosion.
Choose a type of star in the H-R diagram below to light up where those stars gather; the second picture lays out the life of a Sun-like star step by step.
How are stars sorted?
⭐ The colour gives away the temperature
①A star's colour depends on its surface temperature
②A star at about 30000 K looks blue, the Sun at about 6000 K looks yellow, and a star at about 3000 K looks red
③Draw stars by surface temperature and brightness in an H-R (Hertzsprung-Russell) diagram and they do not scatter at random but fall into a few groups
④The band where most stars lie is the main sequence; red giants are at the upper right and white dwarfs at the lower left
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H-R diagram with surface temperature across and brightness up, showing where main-sequence stars, red giants and white dwarfs gather
The life of a star
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A star of about the Sun's mass going from nebula to protostar, main-sequence star, red giant and white dwarf
💫 Stars are born and grow old
①A nebula of gas and dust pulls together under gravity into a protostar, and when hydrogen fusion starts in its core it becomes a main-sequence star
②A star spends most of its life on the main sequence; the Sun has been there for about 4.6 billion years
③When a Sun-like star runs low on hydrogen it swells into a red giant, blows its outer layers off as a planetary nebula, and leaves its core behind as a white dwarf
④A star much heavier than the Sun becomes a red supergiant, explodes as a supernova and leaves a neutron star or a black hole
⑤The heavier the star, the faster it burns its fuel and the shorter it lives
How big is the universe?
🌌 Measuring distance with light
①The distance light travels in a year is one light-year, about 9.5 trillion km
②Proxima Centauri, the nearest star after the Sun, is about 4.2 light-years away
③Our galaxy, the Milky Way, is about 100000 light-years across
④The observable universe is estimated to be about 93 billion light-years across
⑤So the light we see tonight from a distant star left it long ago
Summary
Where stars sit on the H-R diagram
Main sequence: band from upper left to lower right / Red giants: upper right / White dwarfs: lower left
Across: surface temperature (hottest on the left); up: brightness (brightest at the top)
A main-sequence star releases energy by turning hydrogen into helium in its core
Unit of distance
1 light-year ≈ 9.46 × 1012 km
The distance light travels in one year
🎯 Points that often come up
①On the H-R diagram most stars lie on the main sequence, and the Sun is a main-sequence star
②The hotter the surface, the bluer the star; the cooler, the redder
③A Sun-like star goes nebula → protostar → main-sequence star → red giant → white dwarf
④The more massive a star, the shorter its life
⑤A light-year is a unit of distance, not of time
Worked examples and unit test
Example 1
Stars in the night sky differ slightly in colour. Which has the higher surface temperature, a blue star or a red star, and how can you tell?
1
A star's colour depends on its surface temperature.
2
The hotter the surface, the bluer it looks (about 30000 K); the cooler, the redder (about 3000 K). So the blue star is hotter.
▸ The blue star is hotter (blue star about 30000 K, red star about 3000 K)
A piece of charcoal glows red at first and yellow-white as it gets hotter. Thinking of stars the same way makes the link between colour and temperature easy to remember.
Example 2
Stars are so far away that we use a unit called the light-year. What does one light-year mean, and roughly how many light-years away is the nearest star after the Sun?
1
One light-year is the distance light travels in one year, about 9.46 × 10¹² km.
2
Proxima Centauri, the nearest star after the Sun, is about 4.2 light-years away.
▸ One light-year is the distance light travels in a year (about 9.46 × 10¹² km), and the nearest star is about 4.2 light-years away
The light arriving from Proxima Centauri tonight left that star about 4.2 years ago.
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).