T5L3 Stars part 1

The nebula contains a lot of gas, which is the material that stars form from.

Each star's mass...

• Stars with more mass DO NOT last longer than stars with less mass.
• Stars with less mass (than our sun) use their fuel slowly can last for up to 200 billion years.
• A medium-mass star like the sun will last for about 10 billion years.
• The sun is about 4.6 billion years old, so it’s about halfway through its life span.
• Stars with more mass than the sun, may last only about 10 million years, that’s a 10th of 1% of the life span of our sun.

White Dwarfs

The Hubble Space Telescope captured this image of a white dwarf. The white do in the center is the dense remaining core of the star. the glowing cloud of gas surrounds the white dwarf and eventually blows off all its outer layers.

• When stars begin to run out of fuel the core shrinks and its outer portion expands.
• Depending on its mass, the star becomes either a red giant or a supergiant.
• Low-mass stars and medium-mass stars take billions of years to use up their fuel.
• As they start to run out of fuel the outer layers expand and they become red giants.
• The outer parts continue to grow larger and drift out into space, forming a glowing cloud of gas called a planetary nebula.
• The blue-white core that is left behind cools and becomes a white dwarf.
• White dwarfs are about the size of Earth but about one million times denser than the sun.
• They have no fuel, but glow faintly from leftover energy.
• After billions of years white dwarfs stop glowing and become a black dwarf.

Supernovas

• These stars quickly proceed into brilliant supergiants.
• When a supergiant runs out of fuel, it explodes suddenly.
• Within hours, the star blazes millions of times brighter.
• The explosion is called a supernova.
• After a supernova, some of the material from the star expands into space.
• This material may become part of the nebula
• A supernova provides enough energy to create the heaviest elements.

Neutron Stars, Pulsars, and Black Holes

• After a supergiant explodes, some material from the star is left behind.
• This material may form a neutron star.
• Neutron stars are even small and denser than white dwarfs.
• A neutron star may contain as much as three times the mass of the sun but be only about 25 km in diameter (the size of a city).
• In 1967, a student named Jocelyn Bell detected an object in space that appeared to give off regular pulses of radio waves.
• Astronomers later concluded the source of the radio waves came from rapidly spinning neutron stars.
• Spinning neutrons are called pulsar which is short for pulsating radio sources.
• Some pulsars spin 100 times a second.
• Stars that are more than 10 times the mass of the sun may become black holes when they die.
• A black hole is an object with gravity so strong that nothing, not even light, can escape.

Check Point

Will our sun become a black hole? Explain.

Answer

Our sun is too small and has too little mass to become a black hole.

Color Temperature

• A star’s color indicates its surface temperature.
• The coolest stars have a temperature of about 3,500 K and look red.
• The sun that is yellow has an average temperature of about 5,500 K
• The hottest stars have temperatures ranging from 30,000 K to 60,000 K and appear bluish.

Size

• Most stars are about the size of the sun.
• A small amount of stars are much larger and are called giant stars or supergiant stars.
• Some stars are smaller than the sun.
• White dwarf stars are about the size of the Earth.
• Neutron stars are even smaller, about 25 km in diameter.

Chemical Composition

• The chemical composition of most stars is about 73% hydrogen, 25% helium, and 2% other elements.
• Astronomers use spectrographs to determine the elements found in stars.
• A spectrograph breaks light into colors and produces an image of the resulting spectrum.

Brightness

• A larger star tends to be brighter than a smaller star.
• A hotter star tends to be brighter than a cooler star.
• Astronomers use light-year to measure the distance of stars.
• A light-year is the distance that light travels in one year (9.46 trillion km).
• Brightness of a star depends on distance from Earth and true brightness of the star.
• A star’s apparent brightness is the brightness as seen from Earth.
• Apparent brightness is measured with electronic devices.
• Astronomers can’t tell how much light a star gives off just format he star’s apparent brightness.
• A star’s absolute brightness is the brightness the star would have if it were at a standard distance from Earth.
• Absolute brightness is more complex.
• An astronomer must first fin out both the star’s apparent brightness and its distance from Earth.
• The astronomer can then calculate the star’s absolute brightness.

Check Point

Our sun is an average-sized star, yet appears brighter than others we can see. Explain why.

Classifying Stars

• Henry Norris Russell in the United States and Ejnar Herzsprung in Denmark made graphs to help them determine whether the temperature and the absolute brightness of stars are related.
• H-R diagrams are used to classify stars and to understand how stars change over time.