Stars: Brightness, Colour and Life Cycles

Remember from Grade 6 and Lesson 1

In Grade 6 you learned that stars emit their own light and that the next nearest star is about 4.2 light-years away. In Lesson 1 you learned that the Sun is a medium-sized star powered by fusion. Now you will compare the Sun with other stars and follow a star from birth to death.

Look up: Orion

On a clear winter evening in Canada, look south for Orion, with its three-star belt. Its top-left star, Betelgeuse, looks orange-red. Its bottom-right star, Rigel, looks blue-white. Why are they different colours? And is Rigel brighter because it is closer, or because it really gives off more energy? Let's find out.

Characteristic 1: Brightness

Two different ideas get mixed up:

• Apparent brightness is how bright a star looks from Earth.
• Luminosity is how much energy a star really gives off.

A star can look bright because it is very luminous, or because it is close. Sirius, the brightest star in our night sky, is only about 8.6 light-years away. Many stars that look dimmer are actually far more luminous, but much farther away.

Astronomers measure brightness with magnitudes. Odd but true: smaller numbers mean brighter. Sirius has a magnitude of about −1.5; the faintest stars you can see in a dark sky are about magnitude 6.

Characteristic 2: Colour and temperature

A star's colour tells its surface temperature, just like a stove element glows red when warm and orange or white when hotter.

• Red stars are the coolest (about 3000 °C).
• Yellow stars like the Sun are medium (about 5500 °C).
• Blue-white stars are the hottest (10 000 °C to over 30 000 °C).

So Betelgeuse is a cool red star and Rigel is a hot blue-white star.

Characteristic 3: Size and mass

Stars range from red dwarfs much smaller than the Sun to supergiants like Betelgeuse, which is so large that, if it replaced the Sun, its surface would extend beyond the orbit of Mars. A star's mass is its most important property: it controls the star's temperature, luminosity, and how long it lives.

The Hertzsprung–Russell (H–R) diagram

In the early 1900s, astronomers Ejnar Hertzsprung and Henry Norris Russell plotted stars on a graph of temperature against luminosity. Stars did not land randomly: they formed groups.

• Most stars, including the Sun, lie on a diagonal band called the main sequence: hot stars are luminous, cool stars are dim.
• Red giants and supergiants are in the top right: cool but very luminous, because they are huge.
• White dwarfs are in the bottom left: hot but dim, because they are tiny.

Note that temperature on an H–R diagram traditionally decreases from left to right.

The life cycle of a star

1. Nebula: stars are born in cold clouds of gas and dust. The Orion Nebula, visible as a fuzzy patch in Orion's 'sword', is a star nursery.
2. Protostar: gravity pulls a clump of gas together. It heats up as it shrinks.
3. Main sequence: when the core reaches millions of degrees, hydrogen fusion begins. The star becomes stable, with gravity pulling in and fusion energy pushing out. Stars spend about 90% of their lives here.

What happens next depends on mass:

Sun-like stars (low to medium mass):

1. When core hydrogen runs low, the star swells into a red giant.
2. It gently pushes its outer layers off into space, making a glowing shell called a planetary nebula (nothing to do with planets; the name is historical).
3. The leftover core is a white dwarf, about the size of Earth, which slowly cools.

The Sun will become a red giant in about 5 billion years.

Massive stars (more than about 8 times the Sun's mass):

1. They burn through their fuel quickly (millions of years, not billions) and swell into supergiants.
2. Fusion builds heavier and heavier elements in the core, up to iron. Then the core collapses and the star explodes as a supernova, briefly outshining a whole galaxy.
3. What remains is a neutron star (about 20 km across but more massive than the Sun) or, for the most massive stars, a black hole.

More massive stars live shorter lives. Even though they have more fuel, they use it far faster.