THE UNIVERSE AND ITS STARS13.4 Stars: Old Age, Death, and New Life

Hertzsprung-Russell: Life History for the Stars

• We can keep track of the life cycle of star by looking at its mass. A star’s brightness, colour, size, and how long it will live depends on its mass.

• (see next slide) A system for the evolution of stars was created by Elnar Hertzsprung and Henry Norris Russell. Most stars fit on the “main sequence” line, which depends on its mass.

• All stars are compared to our Sun (our sun = 1 solar mass)• Stars do not go through the whole sequence as they age. It

stays in one stage until all the fuel is used up. Change in star’s energy production moves it to the upper-right to become a red giant or a supergiant.

Hertzsprung-Russell

Lower-right : Cooler, reddish stars. Small and dim.Upper-left: Very bright, hot bluish starsUpper-right: Cooler red giants and supergiants Lower-left: Hot and one third of solar mass.

Red Giant to White Dwarf

• After 10 billion years as a main sequence star:• Fusion will have changed most of Sun’s hydrogen to helium • Helium will form core inside shell of remaining hydrogen • Less hydrogen means the outward flow of energy slows down• Core begins to contract• Contraction heats the core• Outer layer expands and cools• This cooling and expansion creates a red giant • Sun will continue to expand for millions of years, completely

covering Venus, Mercury, and possibly Earth. • It will be a thousand times its original brightness • Majority of star’s mass converted to energy

• Eg. Star with initial mass of 10 solar masses becomes white dwarf of 1.4 solar masses in a diameter of about Earth’s size.

Red Giant to White Dwarf

• Heavies elements that can be created in a red giant are oxygen and carbon

• In the creation of a white dwarf, pieces of this dwarf will collide with pieces that were given off during the last stages of the red giant.

• The energy from these collisions illuminate these clouds of gas and dust and create a nebula (see next slide)

• A white dwarf will eventually fade and no longer be visible, thus becoming a black dwarf

Red Giant to White Dwarf

Supernovas

• Created from a star with initial mass of more than 10 solar masses – have the power to create the fusion of carbon nuclei as the star contracts

• Energy created can cause fusion of other (heavier) elements• With continuous temperature and pressure changes, these

stars collapse catastrophically• Collapsing of the stars with these heavy elements causes

them to scatter

Supernovas

The Crab Nebula

Neutron Stars

• If star’s starting mass is between 10 and 50 solar masses, the solar mass will produce a neutron star

• Crazy stuff!! The core of a neutron star is so dense that 150mL of the core would weigh millions of kilograms

Pulsar

• A type of neutron star that sends out light and high-energy radio waves

• If a pulsar is very small, it will release all its energy in the form of light and fade away

Black Holes

• Stars with a starting mass over 50 solar masses will become a supernova and produce heavy elements.

• If the mass of the material left is more than 4 solar masses, the core will collapse in on itself

• This great mass also causes great gravitational force. Not even light can escape. This results in a black hole.

• The observations around a black hold lead to indirect conclusions of its existence

Black Holes

CYU p436 #7, 19, 20