Transcript Unit 1
Units to cover: 66, 67, 68, 69 Homework Unit 64 Problems 15, 17, 20 Unit 65, Problems 12, 17, 18 Unit 66 Problems 18, 19, 20 Unit 67 Problems 19, 20, 21 The Chandrasekhar Limit and Supernovae • • • If mass is added to a white dwarf, its gravity increases If the white dwarf mass exceeds 1.4 solar masses (the Chandrasekhar Limit), the end of the white dwarf is near. The additional gravity squeezes the degenerate material in the white dwarf, causing it to compress by a small amount • This compression causes the temperature to soar, and this allows carbon and oxygen to begin to fuse into silicon • The energy released by this fusion blows the star apart in a Type 1a supernova Type 1a Supernova – Another standard candle! • The light output from a Type 1a supernova follows a very predictable curve – Initial brightness increase followed by a slowly decaying “tail” • All Type 1a supernova have similar peak luminosities, and so can be used to measure the distance to the clusters or galaxies that contain them! Formation of Heavy Elements • Hydrogen and a little helium were formed shortly after the Big Bang • All other elements were formed inside stars! • Low-mass stars create carbon and oxygen in their cores at the end of their lifespan, thanks to the higher temperatures and pressures present in a red giant star • High-mass stars produce heavier elements like silicon, magnesium, etc., by nuclear fusion in their cores – Temperatures are much higher – Pressures are much greater • Highest-mass elements (heavier than iron) must be created in supernovae, the death of high-mass stars The Lifespan of a Massive Star Layers of Fusion Reactions • As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace • Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core • Carbon is left behind until it too starts to fuse into heavier elements. • A nested shell-like structure forms. • Once iron forms in the core, the end is near… Core Collapse of Massive Stars • Iron cannot be fused into any heavier element, so it collects at the center of the star • Gravity pulls the core of the star to a size smaller than the Earth’s diameter! • The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core • The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “bounce” • The “bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction. The star then explodes. • This is a supernova! Before and After – a Supernova Light Curve for a Supernova • The luminosity spikes at the moment of the explosion, and gradually fades, leaving behind a… The Crab Nebula Stellar Corpses • A type II supernova leaves behind the collapsed core of neutrons that started the explosion, a neutron star. • If the neutron star is massive enough, it can collapse, forming a black hole… A Surprise Discovery Pulsating radio sources were discovered, and were named “pulsars” All pulsars are extremely periodic, like the ticking of a clock. – But in some cases, much, much faster… An Explanation • An idea was proposed that solved the mystery • A neutron star spins very rapidly about its axis, thanks to the conservation of angular momentum • If the neutron star has a magnetic field, this field can form jets of electromagnetic radiation escaping from the star • If these jets are pointed at Earth, we can detect them using radio telescopes. • As the neutron star spins, the jets can sweep past earth, creating a signal that looks like a pulse. • Neutron stars can spin very rapidly, so these pulses can be quite close together in time! Slowing Down? • Over time, the spin rate of a pulsar can decrease at a small but measurable rate • Sometimes the pulsar’s diameter shrinks slightly, causing a momentary increase in the pulsar’s rotation • These “glitches” are short lived, and the spin rate begins to decrease again. Interior Structure of a Neutron Star High-Energy Pulsars • Most pulsars emit both visible and radio photons in their beams • Older neutron stars just emit radio waves. • Some pulsars emit very high energy radiation, such as X-rays – X-ray pulsars – Magnetars • Magnetars have very intense magnetic fields that cause bursts of x-ray and gamma ray photons. The Escape Velocity Limit • Recall that the velocity necessary to • Also recall that nothing can travel avoid being gravitationally drawn faster than the speed of light, c, or back from an object (the escape 3108 m/s velocity) is: Vesc 2GM R 2G M RS c2 Mass Warps Space • Mass warps space in its vicinity • The larger the mass, the bigger “dent” it makes in space • Objects gravitationally attracted to these objects can be seen as rolling “downhill” towards them • If the mass is large enough, space can be so warped that objects entering it can never leave – a black hole is formed. Black Holes • It takes for a test particle infinite time to fall onto a black hole. How can black holes grow in mass? Viewing a black hole • You may be asking, “If light cannot escape a black hole, how can we see one?” • If a black hole is in orbit around a companion star, the black hole can pull material away from it. • This material forms an accretion disk outside of the event horizon and heats to high temperatures • As the gas spirals into the black hole, it emits X-rays, which we can detect! Light curves from a black hole binary system General Relativity • Einstein predicted that not only space would be warped, but time would be affected as well • The presence of mass slows down the passage of time, so clocks near a black hole will run noticeably slower than clocks more distant • The warping of space has been demonstrated many times, including by observations of the orbit of Mercury • The slowing of clocks has been demonstrated as well! Gravitational Redshift • Photons traveling away from a massive object will experience a gravitational redshift. – Their frequency will be shifted toward the red end of the spectrum Star Clusters • Stars form in large groups out of a single interstellar cloud of gas and dust • These groups are called star clusters • Open clusters have a low density of stars – there is lots of space between the cluster’s members • They can contain up to a few thousand stars in a volume 14 to 40 light years across • The Pleiades is a very familiar open cluster Globular Clusters • Some clusters are much more densely packed than open clusters. • These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across! A snapshot of stellar evolution • Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars • We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants We can deduce the age of a cluster by finding this turnoff point. Finding a Cluster’s Age