The last days of massive stars

3. The last days of massive stars • Outer layers expand as helium core contracts • Helium fuses to form carbon, carbon fuses with helium to make oxygen, and heavier and heavier nuclei get built until iron (Fe) is made • Iron does not fuse because it takes energy to fuse Fe to heavier elements (star doesn’t have enough energy to do so) • Whole things stops…equilibrium breaks down and something bad is about to happen

4. Supernovae • Now that there is no pressure pushes out….gravity wins the long-lasting battle • In less than 1 second, the entire star collapses in on itself, hits the iron core and bounces off to release an EXTREME amount of energy • As bright as an entire galaxy of 10 billion stars • Only high-massive stars go supernovae (must be at least 3x the mass of Sun) • Can emit as much energy in this explosion as the Sun does in its entire lifetime • All of the material is explodes out up to 10% the speed of light (30,000 km/s) • No supernova has been witnessed since 1604 (Kepler) • On average, this event occurs 3 times every century in the Milky Way “Crab Nebula” remnant of supernova noted by Chinese, Japanese, and Korean astrologers in 1054

5. Black Holes • After some massive stars (greater than 10 suns) explode as supernovae, they will retain a mass of 2 to 3 solar masses in their cores • Nothing in the universe is strong enough to hold up the remaining mass against the force of gravity, so it collapses into a black hole • Matter that falls into a black hole disappears from contact with the rest of the Universe….not even light can escape the gravitational pull of a black hole • The “escape velocity” exceeds the speed of light • Black holes are a consequence of Einstein’s theory of gravity which is called General Relativity • Black holes are insanely dense…. • Think of a star ten times more massive than the Sun squeezed into a sphere approx. the size of New York City!

6. Escape Velocity • Think about throwing a tennis ball up • The faster the ball is traveling when it leaves your hand, the higher the ball will go before turning back • If you throw the ball hard enough it will never return, the gravitational attraction will not be able to pull it back down (e.g. a rocket into space is moving this fast) • The velocity that the ball must have to escape is known as the escape velocity (you must be traveling approx. 25,000 mi/hr to escape Earth’s gravitational attraction) • The more massive or denser a planet (or star), the larger the escape velocity • Black holes are so massive that the escape velocity is greater than the speed of light

7. Recent History of Black Holes 1915, just after Einstein published his theory of gravity, Karl Schwartzchild showed that black holes could theoretically exist • Until the 1907’s, most physicists believed that black holes wouldn’t happen in nature • In 1977, Stephen Hawking & Roger Penrose proved that black holes are a generic feature in Einstein’s theory of gravity, and cannot be avoided in some collapsing objects • The expression “black hole” was coined by theoretical physicist John Wheeler in a public lecture in 1967

8. Black Holes • Scientists can’t directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation • We can infer the presence of black holes by detecting their effect on other matter nearby • If a black holes passes through a cloud of interstellar matter, it will draw matter inward in a process known as accretion • A similar process can occur if a normal star passes close to a black hole • As the attracted matter accelerates and heats up, the black hole “eats too much at one time” and emits x-rays that radiate into space and powerful gamma-ray bursts

9. Event Horizon • 1916 K. Schwartzchild showed that if an object was smaller than a characteristic radius “the Schwartzchild radius” then even light moving straight out would not escape • This imaginary radius is known as the “event horizon” • If matter (or light) falls within this radius, it is doomed to be pulled in

10. Frequently Asked Questions About Black Holes How is time changed in a black hole? • Einstein proved that time is relative (my time may not be the same as your time depending on our different velocities) • Faster you travel, the slower time “ticks” for you when observed by someone who is not going as fast as you are (you would observe time as “ticking” at its normal rate) • If you were just outside the event horizon, time would be slowed time dramatically for you while your friends back on Earth would go through time as normal • If you stayed around this event horizon long enough and returned back to Earth some number of years later, you would find that your friends became old (maybe even dead) while you had barely aged at all How big can a black hole get? • There is no limit to how large a black hole can be. However, the largest black holes we think are in existence are at the centers of many galaxies, and have masses equivalent to about a billion suns. Their radii would be a considerable fraction of the radius of our solar system (150,000 light years)