Stellar Evolution: Life Cycles of Stars

The lives of the stars

Stars are not fixed and unchanging. Like living things, they are born, they shine for millions or billions of years, and eventually they die. What happens at the end depends almost entirely on one thing: the star's mass. This module follows the life cycles of a Sun-like star and a much more massive star.

Words for a star's life

Four ideas run through this topic:

A balancing act

A star stays stable because two forces are in balance. Gravity pulls its material inward, while the radiation pressure from nuclear fusion in the core pushes outward. While these balance, the star sits on the main sequence. When the core fuel runs low, fusion slows, gravity wins for a time, and the star begins to change.

What holds a star up?

On the main sequence, what balances the inward pull of gravity?

• The outward radiation pressure from nuclear fusion
• The star spinning quickly on its axis
• The gravitational pull of nearby planets
• The magnetic field of the galaxy

Two very different endings

A star's mass decides its fate. A Sun-like star ends quietly; a much more massive star ends in a violent explosion.

Life of a Sun-like star

Put the stages in the life cycle of a Sun-mass star into order.

• Nebula, a cloud of gas and dust
• Main sequence, fusing hydrogen steadily
• Red giant, swollen and cooler
• Planetary nebula, outer layers drift away
• White dwarf, a hot dense core
• Black dwarf, cooled and dark

A massive star's life

Select the TWO stages that belong to a MUCH MORE MASSIVE star's life cycle.

• Red supergiant
• Supernova
• Planetary nebula
• White dwarf

Match the remnant

Degeneracy pressure and a limit

When fusion stops, what holds a stellar remnant up? In a white dwarf it is electron pressure; in a neutron star it is neutron pressure. But electron pressure has a limit. The Chandrasekhar Limit, about 1.4 solar masses, is the greatest mass a white dwarf can have. Above it, electron pressure fails and the core collapses further, into a neutron star or a black hole.

The Chandrasekhar Limit

What is the Chandrasekhar Limit, the maximum mass of a white dwarf, in solar masses? Give your answer to one decimal place.

The life of a star in words

A star is born in a _____ of gas and dust and spends most of its life on the _____ sequence, fusing hydrogen. A Sun-like star then swells into a _____ giant. A much more massive star ends its life by exploding as a _____. The dense remnant left by a Sun-like star is a white _____.

Naming the stars

In the Bayer naming system, what does the name Alpha Centauri tell you?

• It is a bright star in the constellation Centaurus
• It is the tenth object listed in a catalogue
• It is a planet orbiting the Sun
• It is the distance to the star in light years

Evidence for black holes

Tap the TWO statements that are TRUE about black holes.

• A black hole cannot be seen directly because not even light escapes it.
• Black holes are found by the X-rays given off as matter falls into them.
• Black holes glow brightly and are easy to see with the naked eye.
• Astronomers detect black holes by the sound they make.
• A black hole has no effect on the stars around it.

Follow a star's fate

Follow two stars to the ends of their lives. Make three good choices.

• A star like our Sun runs low on fuel. What does it swell into?
• That Sun-like star sheds its outer layers. What remnant does it leave behind?
• A star far more massive than the Sun explodes as a supernova. If its core is massive enough, what can it become?

Compare two lives

Compare the life cycle of a Sun-mass star with that of a much more massive star. Write in clear, full sentences.

• Describe the stages a Sun-mass star passes through to its final remnant
• Describe the stages a much more massive star passes through to its final remnant
• Explain the part played by radiation pressure and gravity
• Explain how the star's mass decides its fate, including the Chandrasekhar Limit