Star Properties and Stellar Evolution

What are stars composed of? Super-hot gases of Hydrogen and Helium. The sun is 70% Hydrogen and 30% Helium.

What is the mass of stars? A star’s mass is compared to the sun’s mass. This is called a solar mass. Mass varies form.1 solar masses to >10 solar masses.

What is the size of stars? Vary from the size of Earth to 2,000 times the size of the sun.

What determines the color of a star? Surface Temperature Hot stars are blue-white Cool stars are red

What is Luminosity? It is the actual brightness of a star. Depends on size and temperature.

What is Apparent Magnitude? How bright a star appears to be from the Earth. Depends on distance.

Measuring Magnitude The magnitudes of the brightest stars are negative. Each number of magnitude equals a 2.5 difference from the previous number.

Examples Sirius = Sun = Betelgeuse = Moon = -12.6

How can stars of different distances be compared? Use their brightness if they were all an equal distance from Earth = 32 ly. Absolute Magnitude

Why do some stars change in brightness? Variable Stars 1.Pulsating stars – expand and contract 2.Cepheid Variables – used to find distances to galaxies that contain them 3.Eclipsing Binaries – 2 stars revolve around each other

Stellar Evolution NEBULA Clouds of gas and dust where stars are born. Cloud contracts and heats up.

Stellar Evolution Protostar Large glowing cloud sections that will eventually become stars.

Stellar Evolution Main Sequence At 10 million degrees, fusion begins and a star in born. It is in equilibrium. The sun is an example of this type of star.

Stellar Evolution Red Giant H is depleted. He begins to fuse. Star expands and outer edges cool and become red. (large, red cool = old stars)

Stellar Evolution stars less than 4x size of sun White Dwarf H and He fuel gone. Collapses to size of Earth.. Glows faintly, dim, hot, and small.

Stellar Evolution stars less than 4x size of sun Nova White dwarf flares up and brightens for a few years, then fades.

Stellar Evolution stars larger than 4x size of sun Supergiant Red giants grow seven larger and becomes yellow/orange.

Stellar Evolution stars larger than 4x size of sun Supernova Star explodes violently! Large Magellanic Cloud in 1987 – 170,000 ly away.

Stellar Evolution stars larger than 4x size of sun Neutron Star Core of supernova. So dense that 1 tsp = billion tons.

Stellar Evolution stars larger than 4x size of sun Black Hole Objects so dense that gravity will not allow even light to escape. Look for x- rays, black area, gravity pulling matter in.

Nebula Protostar Main Sequence Red Giant

Less than 4 solar massesGreater than 4 solar masses White Dwarf NOVA Super giant Supernova Neutron Star Black Hole

Stars are classified using surface temperature and absolute magnitude.

practice questions Animation of HR Diagram