Stars Goal: Compare star color to star temperature.

What is a star? A star is a ball of gases that gives off a tremendous amount of electromagnetic energy. Stars energy is from nuclear fusion (Hydrogen atoms fusing to form Helium)

Analyzing Starlight Astronomers determine the composition and temperature of a star by analyzing the light that a star emits. Spectrographs separate light into different colors or wavelengths.

Analyzing Starlight Bands of color crossed by dark lines (dark-line spectra) tell what the star is made of and its temperature. Different elements absorb different wavelengths of light.

Composition of Stars Every element has a different spectrum. Most stars are made up of Hydrogen and helium.

Temperatures of Stars Red – Coolest White – Medium Blue – Hottest Red star 3,000 K Yellow/white star 5,000 K Blue star 10,000 K

Masses and Sizes of Stars Most stars are smaller and less massive than the sun. The sun is a medium-sized star like most of the stars you see in the sky. Our sun is a “yellow” star.

Stellar Motion Apparent motion – motion as it appears from Earth, caused by the movement of Earth. – 1. As Earth rotates, the stars appear to move across the sky. – 2. On Earth, stars appear in a different place in the sky than they were the night before. This is caused by Earth revolving around the sun.

Stellar Motion STELLAR MOTION Actual Motion – stars can move away from us, toward us, or may revolve around another star (a binary star is a pair of stars held together by gravity).

Doppler Effect (Doppler Shift) The apparent shift in the wavelength of light emitted by a source moving toward or away from an observer. Colors moving toward Earth are blue-shifted. Colors moving away from Earth are red shifted.

Distances to Stars Light year – The distance light travels in one year (9.46 trillion km) The sun is 8 light minutes from Earth. Nearest star to Earth is Proxima Centauri (4.2 light years away).

Determining Distance For close stars, scientists use parallax to determine distance from Earth. This works within 1,000 light-years from Earth.

Stellar Brightness Apparent magnitude – The brightness of a star as it appears from Earth. – This depends on how much light the star emits and how far the star is from Earth. – The lower the number on the scale, the brighter the star appears to Earthlings.

Stellar Brightness Absolute magnitude – The true brightness of a star. (luminosity) – The brighter a star actually is, the lower the number of its absolute magnitude.

Section 2: Classifying Stars Astronomers graph temperature and luminosity of stars on the Hurtzsprung-Russell diagram (H-R). – Highest temperatures are on the left and highest luminosities are at the top. Most stars fall diagonally through the middle. These stars are called main-sequence stars.

Stage 1: Star Formation Star begins in a nebula – a cloud of gas and dust.

Stage 1: Star formation A protostar is formed when gravity causes a region to begin to flatten into a disk and form a central concentration of matter.

Stage 1: Star formation A star is born when the temperature in a protostar increases until 10,000,000 degrees Celsius and nuclear fusion begins. Outward pressures of radiation resist the inward pull of gravity to stabilize the star.

Stage 2: The Main-Sequence Stage This is the longest stage in a star’s life. A star the size of our sun stays on the main- sequence for approximately 10 billion years. More massive stars fuse hydrogen more rapidly, so they only stay for 10 million years.

Stage 3: Leaving main sequence Leaves when 20% of the hydrogen have fused into helium. The outer shell of the star expands to form a giant (for a sun-like star) or a supergiant (for more massive stars).

Final Stages of sun-like star A planetary nebula forms when fusion stops. Gravity will cause the remaining matter to collapse and form a white dwarf.

Final Stages in more massive stars A more massive star will end it’s main sequence stage by becoming a supernova, a star that has a tremendous explosion that blows itself apart.

Final Stages in more massive stars After supernova, the core contracts into a very small, dense ball of neutrons called a neutron star. Neutron stars rotate rapidly. If they emit pulses of radio and optical energy are called pulsars.

Final Stages in more massive stars Sometimes, a core will contract and become a black hole instead of a neutron star. Gravity of a black hole is so great that nothing, not even light, can escape it.

Life Cycle of Stars