The Life and Death of Stars The Universe The Life and Death of Stars

What is a Star?

WHAT IS A STAR? STAR: sphere of hot gas & plasma emits light & energy the sun is a main sequence star & nearest to Earth LIGHT-YEAR: distance that light travels in one year - about 9.5 trillion kilometers Earth is 8 light-minutes from the Sun

POWERING STARS WITH STAR FUEL NUCLEAR FUSION REACTIONS: the nuclei of hydrogen (H) atoms combine into helium (He). When two particles fuse, energy is released. Takes place in the core of a star Core = extremely hot, extremely dense, and under extreme pressure (perfect for fusion)

At 15,000,000 ºC in the core of the star, fusion ignites! NUCLEAR FUSION BASICS At 15,000,000 ºC in the core of the star, fusion ignites! 4 hydrogen atoms smash into each other --> 1 helium atom + particles + ENERGY Where does the energy come from? Mass of four 1H atoms > Mass of one 4He atom Extra mass in converted to energy E = mc2 (Energy) = (mass) x (speed of light)2

OUR STAR – THE SUN Converts 700 million tons of hydrogen to helium every second Converts 4.7 million tons of matter to energy every second We receive two billionths of that energy here on Earth - each second Enough to power 100 light bulbs for 5 million years The sun formed about 5 billion years ago. 1 Solar mass = size of the Sun Example: 3 solar masses = size of 3 Suns

WE STUDY STARS BY STUDYING LIGHT Brightness of a Star Depends on the star’s temperature, size, and distance from Earth. The brightest star in the night sky = Sirius (Because relatively close to Earth, 9 light years) Stars produce not only visible light but all types of electromagnetic radiation waves Gamma Rays, X-rays, Ultraviolet, Infrared, Microwaves, and Radio waves. Scientists use special telescopes and satellites to see the different types of radiation coming from the stars

STUDYING STARS TEMPERATURE A star’s color is related to its temperature. Hotter objects glow with blue light that has shorter wavelengths. Cooler objects glow with red light that has longer wavelengths

Types of Stars

The Hertzsprung- Russell Diagram (H-R Diagram) 1911: Danish astronomer Ejnar Hertzsprung compared and plotted magnitude of stars with their color (temperature) 1913: American astronomer Henry Norris Russell plots stars’ spectral class against magnitude

Together, the plots show a relationship between temperature and luminosity Shows that stars fall into distinct groups

Life and Death Depends on the Mass of Star Main Sequence Stars (Sun) Massive Stars

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

STAR LIFE CYCLE: STAGE 1 – BIRTH MAIN SEQUENCE AND MASSIVE STARS All stars begin their life the same They form from a Stellar Nebula = Cloud of gas and dust at beginning of star life cycle Steps that lead to the birth of a star Gravitational attraction causes the gas and dust to accumulate (this is called accretion) A protostar forms (not yet a star) Temperature and pressure increase until nuclear fusion begins (now a star) Nuclear fusion beginning means that a star has been born

Omega Nebula - Emission Nebula

Orion Nebula - Diffuse Nebula

“Pillars of Heaven” from the Eagle Nebula

Horsehead Nebula - Dark Nebula

The Pleiades - Reflection Nebula

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

STAR LIFE CYCLE: STAGE 2 – MIDDLE AGE MAIN SEQUENCE AND MASSIVE STARS Nuclear Fusion has begun and continues… Hydrogen fuses into helium and releases lots of energy The fusion reactions in the core of the star produce an outward force that balances the inward force due to gravity. Massive Stars = higher fusion rates More gravity = hotter cores = uses hydrogen faster Over time, the percentage of the star’s core that is helium becomes larger. After millions of years, core will run out of hydrogen and star will begin to die.

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

STAR LIFE CYCLES: STAGE 3 - DYING MAIN SEQUENCE STAR LIFE CYCLE As fusion slows, the outer layers of the star will cool and expand Becomes a red giant RED GIANT: 10 to 100 times larger Converts helium to carbon and oxygen When the star runs out of helium, fusion stops, outer layers will expand, and leave the star’s orbit. MASSIVE STAR LIFE CYCLE As fusion slows, the outer layers of the star will cool and expand Becomes a supergiant SUPERGIANT: 200 to 1500 times larger Converts helium to carbon, oxygen, and… Has enough mass (gravity) so heavier elements can fuse into iron

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

STAR LIFE CYCLES: STAGE 4 – DEATH MAIN SEQUENCE STAR LIFE CYCLE Fusion stops at helium Outer gas layers from red giant stage are released to form planetary nebula PLANETARY NEBULA: Cloud of gas and dust at end of stars life cycle Sends enriched elements into space When planetary nebula clears, a white dwarf is left (part of Stage 5 for a Main Sequence Star) MASSIVE STAR LIFE CYCLE Supergiants can fuse heavier elements beyond helium Fusion stops at iron Develop layers of different elements with iron at the core Iron will not fuse Force of gravity becomes greater than force of fusion Eventually the core collapses and then explodes in a Type II supernova. SUPERNOVA: a gigantic explosion in which a massive star collapses and throws its outer layers into space

Cat’s Eye Nebula - Planetary Nebula

Ring Nebula - Planetary Nebula

Crab Nebula from a supernova

Layers of heavy elements in a supergiant

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

STAR LIFE CYCLES: STAGE 5 – AFTER DEATH MAIN SEQUENCE STAR LIFE CYCLE When planetary nebula clears, a white dwarf is left WHITE DWARF: a small, hot dim star that is the leftover carbon center of a main sequence star. Very dense - 1 ton in 1cm3 Will cool until it becomes a black dwarf (no more light, big lump of coal) MASSIVE STAR LIFE CYCLE After a Type II supernova, either A. neutron star or B. black hole forms. If the remaining core has a mass of 1.4 to 3 solar masses = neutron star (densest star). If the core has a mass that is >3 solar masses = black hole. BLACK HOLE: an object so massive and dense that not even light can escape its gravity NEUTRON STAR: composed of neutrons, 1 teaspoon would weigh 5 x 1012 kg

Crab Nebula from a supernova

Crab Pulsar

STAGE 1 STAGE 2 STAGE 3 STAGE 4 STAGE 5 Main Sequence Star

WHY DO WE CARE ABOUT STARS? We get all elements lighter than Iron from fusion in stars We get all elements heavier than Iron from supernovae Without stars, we wouldn’t have: Elements which make up our body Energy needed to sustain life