George Observatory The Colorful Night Sky

A Stars Lifespan depends on its Mass Massive Stars live shorter lives. Low mass stars live longest. 1.Gravity contracts the Hydrogen gas 2. Gas Spins 3. Gas Heats 4. Protostar Stage 5. Fusion begins in the clouds core 6. Cloud glows brightly 7. Main Sequence Star A star's life cycle is determined by its mass. The larger the mass, the shorter the life. Over time, gravity pulls the hydrogen gas in the nebula together and it begins to spin. As the gas spins faster, it heats up and is known as a protostar. Eventually the temperature reaches 15,000,000 °C and nuclear fusion occurs in the cloud's core. The cloud begins to glow brightly. At this temperature, it contracts a little and becomes stable. It is now called a main sequence star and will remain in this stage, shining for millions or billions of years to come

Star Birth Hydrogen collects in the center of the swirling disk . Gravity pulls the densest pockets of hydrogen gas inward The Gas spins faster, and heats up. The cloud begins to shine brightly, a young star is born in the cloud

Star Birth

Sun Like Star – Long Lifetime The protostar is now a stable main sequence star . Gravity pulls in – Pressure pushes out Star is in balance Neither shrinks or expands Yellow shining mass While gravity pulls inward trying to squeeze the star’s material tighter, pressure of the hot gases inside energized by nuclear reactions acts as a counterbalance & the star neither shrinks or expands, the Star is a yellow shining mass.

The Sun is a Main Sequence Star It fuses hydrogen gas into helium Lifetime: 10 billion years. Near the end - hydrogen fuel is depleted and the star begins to die. The fuel after 10 billion years is converted into helium in its core. The flames of this furnace burns out – the core is inactive. Only a shell of hydrogen burns surrounding the dormant helium core. Our Sun is considered to be an ordinary star with a spectral classification of G2 V, a yellow dwarf main sequence star.

Sun Like Stars how do they do it? In the star’s core protons collide and stick together with a strong nuclear bond. A chain reaction occurs, 4 protons weld together to make 2 protons & 2 neutrons. Hydrogen converts to Helium through nuclear fusion. Every second the Sun through thermonuclear reaction converts 600 million tons of hydrogen into Helium within its core and emits a tiny fraction of energy E=MC2, the radiation escapes into space bathing the star’s surroundings in heat and light. This is what warms our solar system

Red Giant Phase As the Sun ages, Eventually, the Supply of hydrogen in the core ends, and a shell of hydrogen surrounds the helium core. The Sun’s core becomes unstable The helium core contracts and gets hotter. As the main sequence star glows, hydrogen in the core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. All stars evolve the same way up to the red giant phase.

Red Giant star seen from a planet The Sun’s hydrogen shell expands The Sun is now a Red Giant Hydrogen in the shell around the core continues to burn Its core temp continues to increase As a old star expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. Throughout the red giant phase, the hydrogen gas in the outer shell continues to burn and the temperature in the core continues to increase

Red Giant Phase Now the Helium core contracts When the Hydrogen shell ignites: The shell continues to push outward Sun becomes enormous It goes from 1 million to 100 million miles in size The quiet helium core contracts and ignites the hydrogen shell. The shell generates energy that pushes the outer envelope out. The star goes from 1 million miles to nearly 100 million miles in size. The stars temperature drops in the greatly expanded surface layers turning to a cooler shade of color, it is a red giant.

Red Giant Phase Helium ignites, it starts to fuse into Carbon and Oxygen. The core collapses. The outer layers are expelled. It becomes a brilliant cool variable star for thousands of years like Betelgeuse in Orion. The hydrogen shell near the core fuses helium, and helium pores onto the core compressing and heating it. The helium ignites in a helium flash, and starts to fuse into carbon and oxygen. The star shrinks as it adjusts to its new fuel mixture. As the Helium burns steadily the star expands again. It becomes a brilliant star for tens of thousands of years, like Aldebaran in Taurus. The core can’t get hot enough now to ignite carbon fusion, so the carbon oxygen core is a dead end for stars like the Sun. Toward the end of helium burning the intense nuclear reaction moves to the stars surface generating winds which whisk away the red giant’s transparent envelope into the interstellar medium. Actual photograph of Betelgeuse

Red Giant becomes a White Dwarf star Eventually all of the hydrogen gas in the outer shell of the Red Giant is blown away by stellar winds to form a ring around the core. This ring is called a planetary nebula. The core is now a hot white dwarf star. Red Giant becomes a White Dwarf star At 200,000,000 °C the helium atoms in the core fuse into carbon atoms. A white dwarf star is left in the center of the dying red giant star, surrounded by the red giant’s expanded atmosphere

Death of a Sun like star White dwarf to black A White dwarf star is a dense stable star about the size of the Earth weighing three tons per cubic centimeter. It radiates its left-over heat for billions of years. When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star

A star like our Sun will become a white dwarf when it has exhausted its nuclear fuel. As helium atoms in the core are fused into carbon atoms, the Sun size star begins to die. Gravity causes the last of the star's matter to collapse inward and stage. At this stage, the star's matter is extremely dense. White dwarfs shine with a white hot light. Once all of their energy is gone, they no longer emit light. The star has now reached the black dwarf phase in which it will forever remain

Death of a Massive Star

Massive Stars When massive stars ( At least 5 times larger than the Sun) reach the red giant phase, their core temperature increases because carbon is formed from the fusion of helium. Gravity pulls carbon atoms together. The core temp goes higher forming oxygen, then nitrogen, and eventually iron.

SuperNova Explosion The core becomes iron, fusion stops. No energy. Iron is the most stable element and requires the most energy of any element to fuse. So, the core heats to 100 billion degrees, the sudden lose of energy causes the core to collapse The iron atoms in the core are crushed. The core becomes rigid. In falling layers of the star strike the core, then recoil in a Shockwave. The shockwave hits the surface and the star explodes. When the core contains essentially just iron, fusion in the core ceases. This is because iron is the most compact and stable of all the elements. It takes more energy to break up the iron nucleus than that of any other element. Creating heavier elements through fusing of iron thus requires an input of energy rather than the release of energy. Since energy is no longer being radiated from the core, in less than a second, the star begins the final phase of gravitational collapse

Supernova

SuperNova Explosion

If the core of a massive star collapses when it is 1 If the core of a massive star collapses when it is 1.5 to 3 times as massive as our Sun’s core. It ends up as a neutron star. The protons and electrons are squeezed together by gravity, leaving a residue of neutrons, creating a neutron star. Neutron Stars Neutron stars spin rapidly giving off radio waves. If the radio waves are emitted in pulses (due to the star's spin), these neutron stars are called pulsars Neutron stars (right) are about ten miles in diameter. Spin very rapidly (one revolution takes mere seconds!). Neutron stars are fascinating because they are the densest objects known except for black holes. A teaspoon of neutron star material weighs 100 million tons.

Extremely Massive Stars Massive Stars (8 times or more larger than the Sun. Core remains massive after the supernova. Fusion is stopped. Nothing supports the core. The core is swallowed by its gravity. It becomes a black hole Black holes are detected by X-rays given off matter that falls into the black hole. become black holes The core of a massive star that has 8 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core So it is swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it. Black holes are not visible. They are detected by the X-rays which are given off as matter falls into the hole.

Black Holes Black holes are objects so dense that not even light can escape their gravity and, since nothing can travel faster than light, nothing can escape from inside a black hole. Nevertheless, there is now a great deal of observational evidence for the existence of two types of black holes: those with masses of a typical star (4-15 times the mass of our Sun), and those with masses of a typical galaxy. This evidence comes not from seeing the black holes directly, but by observing the behavior of stars and other material near them!