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ASTRO 101 Principles of Astronomy Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: • Telephone: 594-7118 • E-mail: [email protected] • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00 Homework/Announcements •Chapter 10 homework due April 30: Question 15 (Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s age.) •For Chapter 11, skip sections 11.9, 11.11, 11.14, 11.16, 11.17, 11.18, 11.19 •Tuesday, May 7: wrap-up and review •Tuesday May 14, Final Stellar Evolution • Observational aspects – Observations of clusters of stars • Theory – Outline of steps from birth to death Stellar Models Stellar Evolution • There are several distinct phases in the life cycle of a star. The evolutionary path depends on the initial mass of the star. • Although there is a continuous range of masses, there are 4 ranges of masses that capture all of the interesting features. Stellar Evolution Stellar Evolution • The basic steps are: Gas cloud Main sequence Red giant Rapid mass loss (planetary nebula or supernova explosion) Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass. Points to Remember: • How to counter gravity: – Heat pressure from nuclear fusion in the core (no mass limit) • Gas pressure proportional to the temperature. – Electron “degeneracy” pressure (mass limit 1.4 solar masses) – Neutron “degeneracy” pressure (mass limit 3 solar masses) • Stars experience rapid mass loss near the end of their “lives”, so the final mass can be much less than the initial mass. Points to Remember: • Sources of energy: – Nuclear fusion: • needs very high temperatures • about 0.7% efficiency for hydrogen into helium. – Gravitational “accretion” energy: • Drop matter from a high “potential” • About 10% efficient when falling onto massive bodies with very small radii. Stellar Evolution Star Formation • The starting point is a giant molecular cloud. The gas is relatively dense and cool, and usually contains dust. • A typical cloud is several light years across, and can contain up to one million solar masses of material. • Thousands of clouds are known. Condensation Theory Image from Nick Strobel’s Astronomy Notes (http://www.astromynotes.com) The Protostar • This diagram shows the steps as computed using a computer model. The Protostar • This diagram shows how a star “moves” through the temperature-luminosity diagram as it forms. The Protostar • This diagram shows how a star “moves” through the temperature-luminosity diagram as it forms. The Protostar • High mass stars simply get bluer, whereas the lower mass stars contract and become dimmer. The Protostar • An external disturbance can cause the cloud to collapse: The material collapses to a rotating disk, and friction drives material into the center, where it builds up. The central object heats up as the cloud collapses. Eventually, the temperature gets hot enough for nuclear fusion to occur. • We are left with a newly born star surrounded by a disk of material. Young Star Systems • Many stars in the Orion nebula are surrounded by disks of material. Young Star Systems • Many stars in the Orion nebula are surrounded by disks of material. Young Star Systems • A collapsing cloud can form hundreds of stars. Stars with small masses (less than a solar mass) are much more common than massive stars (stars more than about 15 to 20 solar masses). The highest mass stars are very hot and luminous, and can alter the cloud environment. Young Star Systems • Infrared images are useful since the infrared light penetrates deeper into the dark clouds, allowing one to see what is inside. Often one sees young stars. Young Star Systems • Infrared observations often reveal hundreds of newly-formed stars embedded in molecular clouds. Young Star Systems • Infrared observations often reveal hundreds of newly-formed stars embedded in molecular clouds. • In this particular case, many of the stars have not arrived on the main sequence. Star Formation Summary Stellar Evolution Stellar Evolution • The basic steps are: Gas cloud Main sequence Red giant Rapid mass loss (planetary nebula or supernova explosion) Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass. The Main Sequence • A star that is fusing hydrogen to helium in its core is said to be on the main sequence. • A star spends most of its “life” on the main sequence; the time spent is roughly proportional to 1/M3, where M is the initial mass. Hydrostatic Equilibrium • The Sun (and other stars) does not collapse on itself, nor does it expand rapidly. Gravity and internal pressure balance. This is true at all layers of the Sun. • The energy from fusion in the core ultimately provides the pressure needed to stabilize the star. Stellar Evolution Stellar Evolution • The basic steps are: Gas cloud Main sequence Red giant Rapid mass loss (planetary nebula or supernova explosion) Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass. After the Main Sequence • On the main sequence, the star is in hydrostatic equilibrium where internal pressure supports the star against gravitational collapse. Nuclear fusion (hydrogen to helium) is the energy source. • What happens when all of the hydrogen in the core is converted to helium? The details depend on the initial mass of the star… Points to Remember: • Sources of energy: – Nuclear fusion: • needs very high temperatures • about 0.7% efficiency for hydrogen into helium. – Gravitational “accretion” energy: • Drop matter from a high “potential” • About 10% efficient when falling onto massive bodies with very small radii. • After a stage of nuclear fusion is complete in a stellar core, it will collapse and get hotter. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • Fission of elements heaver than iron can release energy (leads to higher BE’s). More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • As you fuse heavier elements up to iron, higher and higher temperatures are needed since more and more electrical charge repulsion needs to be overcome. – – – – Hydrogen nuclei have 1 proton each temperature ~ 10,000,000 K Helium nuclei have 2 protons each temperature ~ 100,000,000 K Carbon nuclei have 6 protons each temperature ~ 700,000,000 K ….. • After each stage of fusion is complete, the core collapses and heats up. • More mass in the core --> higher core temperature --> fusion of heavier elements … • For a given core mass, there is a limit to how hot it can become. After the Main Sequence: Low Mass • After the core hydrogen is used up, internal pressure can no longer support the core, so it starts to collapse. This releases energy, and additional hydrogen can fuse outside the core. • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. After the Main Sequence: Low Mass • The red giants are stars that just finished up fusing hydrogen in their cores. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) After the Main Sequence: Low Mass • Some red giants are as large as the orbit of Jupiter! • The Sun will reach approximately to the orbit of the Earth After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further. After the Main Sequence: Low Mass • Helium fusion starts in a “shell” around the core, then after a “helium flash” the helium fusion starts in the core. After the Main Sequence: Low Mass • Helium fusion starts in a “shell” around the core, then after a “helium flash” the helium fusion starts in the core. After the Main Sequence: Low Mass • As core hydrogen fusion stops, low mass stars become more luminous and red (e.g. cooler), higher mass stars tend to just get redder while keeping the same luminosity. • In all cases, the star gets larger in size. Next: • The “deaths” of stars. After the Main Sequence: Low Mass • After the core hydrogen is used up, internal pressure can no longer support the core, so it starts to collapse. This releases energy, and additional hydrogen can fuse outside the core. • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further. After the Main Sequence: Low Mass • The core of a star like the Sun will not get hot enough to fuse carbon. After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further. The outer layers eventually may be ejected to form a “planetary nebula”. After the Main Sequence: Low Mass • After hydrogen fusion is completed, the core collapses, and the outer parts of the star expand. • The core may fuse helium into carbon for a short time, after which the core collapses further. • The outer parts of the star expand by large amounts, and eventually escape into space, forming a planetary nebula. Matter is recycled back into space. Planetary Nebulae • These objects resembled planets in crude telescopes, hence the name “planetary nebula.” • They are basically bubbles of glowing gas. Planetary Nebulae • They are basically bubbles of glowing gas. • The ring shape is a result of the viewing geometry. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Planetary Nebulae • The red light is the Balmer alpha line of hydrogen, and the green line is due to oxygen. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Planetary Nebulae • This HST image shows freshly ejected material interacting with previously ejected material. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Planetary Nebulae • The outer layers of the star are ejected, thereby returning material to the interstellar medium. What about the core? The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses. The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses. • To what? The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses. • To what? • But first: a historical mystery involving the brightest star in the sky: Sirius (the “dog” star). Sirius • This bright star is relatively close to the Sun. The spectral type is A1V, and its mass is about twice the Sun’s mass. • In the 1830s it was discovered that Sirius moves in the plane of the sky (roughly 1 arcsecond per year). However, the motion was not in a straight line: Sirius has a binary companion. Sirius • From the size of the wobble, it was estimated that the companion star had a mass roughly equal to the Sun’s mass. • However, this object was extremely faint, and observers tried for decades to spot it without success. • The famous telescope maker Clark spotted the faint companion in the 1870s when testing out his latest refracting telescope. Sirius • Clark discovered the faint companion was roughly 10,000 times fainter than Sirius but bluer. • Here is a modern image, early on it was relatively hard to study the faint star owing to the high contrast. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius! The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius! • The density is roughly 1.4 million grams per cubic centimeter! The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius! • The density is roughly 1.4 million grams per cubic centimeter! ???? Degenerate Matter • The nature of Sirius B was solved in the 1920s and 1930s. It has to do with what happens to the star when pressure can no longer support it… Degenerate Matter • Once the internal pressure stops, the gravitational collapse begins. • Eventually, the gas becomes supercompressed so that the particles are touching. The the gas is said to be degenerate, and acts more like a solid. • For a star with an initial mass of less than about 8 solar masses, the final object has a radius of only about 1% of the solar radius, and is extremely hot (and therefore blue). Degenerate Matter • Once the internal pressure stops, the gravitational collapse begins. • Eventually, the gas becomes supercompressed so that the particles are touching. The the gas is said to be degenerate, and acts more like a solid. • For a star with an initial mass of less than about 8 solar masses, the final object has a radius of only about 1% of the solar radius, and is extremely hot (and therefore blue). These are the white dwarf stars. After the Main Sequence: Low Mass • The red giants are stars that just finished up fusing hydrogen in their cores. • The white dwarfs are the left over cores of red giants that have shed their mass in planetary nebulae. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Planetary Nebulae and White Dwarfs • The central star is a white dwarf. Planetary Nebulae and White Dwarfs • This particular planetary nebula is nearly spherical. • The central star is a white dwarf. Planetary Nebulae and White Dwarfs • More central white dwarfs… Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) After the Main Sequence: Low Mass • The core collapses until the gas is “degenerate”, at which point it acts like a solid. It becomes a white dwarf: – The density is more than 1 million times that of water. – The source of support is the “electron degeneracy” pressure. The maximum mass that can be supported is 1.4 solar masses. – There is no internal source of energy, and the white dwarf cools down slowly over time. Initially, the white dwarf is relatively hot (several times the solar temperature). White Dwarfs • If a white dwarf accretes matter from a close binary companion, a huge explosion on the white dwarf’s surface can be triggered… White Dwarfs • If a white dwarf accretes matter from a close binary companion, a huge explosion on the white dwarf’s surface can be triggered. These events are called novae. White Dwarfs • If a white dwarf accretes matter from a close binary companion so that its mass exceeds the Chandrasekhar limit, the white dwarf itself can explode. White Dwarfs • These events, called “Type Ia supernovae”, can be up to 10 billion times as luminous as the Sun at their peak. Stellar Evolution Next: Evolution of High Mass Stars After the Main Sequence: High Mass • A massive star (more than about 10 to 15 solar masses) will use up its core hydrogen relatively quickly. The core will collapse. • The core heats up, and helium is fused into carbon. After this, carbon and helium can fuse into oxygen since the high mass gives rise to very high temperatures. • Eventually elements up to iron are formed in successive stages. After the Main Sequence: High Mass • A high-mass star will develop an onion-like structure near its core. The central iron core will not have nuclear fusion, so it will collapse. After the Main Sequence: High Mass • A high-mass star will develop an onion-like structure near its core. The central iron core will not have nuclear fusion, so it will collapse. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • Fission of elements heaver than iron can release energy (leads to higher BE’s). • Fission or fusion of iron does not give energy. After the Main Sequence: High Mass • Eventually elements up to iron are formed in successive stages. • Iron fusion does not produce energy, so there is no energy source to halt the gravitational collapse…. Points to Remember: • How to counter gravity: – Heat pressure from nuclear fusion in the core (no mass limit) • Gas pressure proportional to the temperature. – Electron “degeneracy” pressure (mass limit 1.4 solar masses) – Neutron “degeneracy” pressure (mass limit 3 solar masses) • We have used up fusion, and there is a limit to how much mass electron degeneracy pressure can support. After the Main Sequence: High Mass • Eventually elements up to iron are formed in successive stages. • Iron fusion does not produce energy, so there is no energy source to halt the gravitational collapse. • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure. After the Main Sequence: High Mass • Eventually elements up to iron are formed in successive stages. • Iron fusion does not produce energy, so there is no energy source to halt the gravitational collapse. • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure. The collapse continues. After the Main Sequence: High Mass • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure. The collapse continues. • Protons and electrons are fused to form neutrons and neutrinos. The core collapses to a very tiny size, liberating a huge amount of energy. The outer layers are blown off in a supernova explosion. Supernovae • A supernova can be a billion times brighter than the Sun at its peak. Supernovae • Several solar masses of material is ejected into space by the explosion. • Many “supernova” remnants are known. Supernovae • Several solar masses of material is ejected into space by the explosion. • Many “supernova” remnants are known. Supernovae • Supernovae are rare events. One occurred in a relatively nearby galaxy in 1987. Supernovae • Supernovae are rare events. One occurred in a relatively nearby galaxy in 1987. • It has been closely studied since with the Space Telescope and other telescopes. Supernovae • Material is returned to the interstellar medium, to be recycled in the next generation of stars. • Owing to the high temperatures, lots of exotic nuclear reactions occur, resulting in the production of various elements. All of the elements past helium were produced in supernovae. Supernovae • Material is returned to the interstellar medium, to be recycled in the next generation of stars. • Owing to the high temperatures, lots of exotic nuclear reactions occur, resulting in the production of various elements. All of the elements past helium were produced in supernovae. • Most of the atoms in your body came from a massive star! The Remnant: High Mass • What happened to the core? Next: Neutron Stars Black Holes Next: Neutron Stars Black Holes but first: A Bit on the Evolution of Binary Stars The Evolution of Binary Stars • In a binary system, the stars start to evolve independently: the most massive star evolves first! • If the separation between the stars is larger than the maximum size of each star, then no problem. • If, however, the most massive star becomes bigger than the distance between the two stars, then the two stars will interact… The Evolution of Binary Stars • The dashed line represents the maximum size the star is allowed to be when inside the binary. • Here is just one example of the many different possibilities (e.g. the stars move apart, or move closer, or merge). The Evolution of Binary Stars • There are many known examples where a star loses mass onto a white dwarf. Lots of energy is liberated when the mass hits the white dwarf. Remnants of High Mass Stars • In many cases, the remnants of high mass stars will appear in close binaries… The Remnant: High Mass • What happened to the core? Gravity overcame the electron degeneracy pressure, so the collapse continued. Protons and electrons form neutrons, and the gas is compressed so that the neutrons become degenerate (i.e. they are basically touching). The resulting remnant has a radius of about 10 km, and a typical mass of 1.4 solar masses. This is a neutron star. The density is 6.4 x 1014 grams/cc. The surface gravity is 1011 times that of Earth. Points to Remember: • How to counter gravity: – Heat pressure from nuclear fusion in the core (no mass limit) – Electron “degeneracy” pressure (mass limit 1.4 solar masses) – Neutron “degeneracy” pressure (mass limit about 3 solar masses) Neutron Stars • According to model computations, a neutron star should be very small (radius of about 10 km), and very hot (temperatures more than 1 million degrees). Neutron Stars • Note that the central density is about 1 quadrillion times the density of water! Neutron Stars • According to model computations, a neutron star should be very small (radius of about 10 km), and very hot (temperatures more than 1 million degrees). • Is there any hope of observing them? • Yes: there are some exotic phenomena that are best explained by neutron stars. Neutron Stars • A radio pulsar is a source of extremely modulated radio waves. • The best model for a radio pulsar is a rapidly rotating neutron star with a strong magnetic field. Neutron Stars • The spinning neutron star acts like a “light house”, leading to pulsed radiation being observed on Earth. Neutron Stars • The spinning neutron star acts like a “light house”, leading to pulsed radiation being observed on Earth. Neutron Stars • If a neutron star is in a close binary, matter from the companion falls onto it, liberating a huge amount of energy, including pulsed X-ray beams in some cases. Neutron Stars • If a neutron star is in a close binary, matter from the companion falls onto it, liberating a huge amount of energy, including pulsed X-ray beams in some cases. Neutron Stars • If a neutron star is in a close binary, matter from the companion falls onto it, liberating a huge amount of energy. If the conditions are right, this matter can explode, much like a hydrogen bomb. Neutron Stars and HST • This object is relatively nearby (the parallax gives about 100 pc). • Nevertheless, it is so faint it is at the HST detection threshold. • However, its temperature is a few million degrees. • ??? Neutron Stars and HST • The radius is only about 10 km. • The temperature and radius are what one expects for a young neutron star. Where it Stops • White dwarfs and neutron stars are pretty strange objects. Does it get any stranger? Where it Stops • White dwarfs and neutron stars are pretty strange objects. Does it get any stranger? • Yes: consider the fate of the most massive stars (about 30 to 100 times the mass of the Sun). Einstein’s Relativity and Black Holes