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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 • For Chapter 11, skip sections 11.9, 11.11, 11.14, 11.16, 11.17, 11.18, 11.19 • For Chapter 12, sections 12.1-12.7 • Tuesday, May 7: wrap-up and review • Tuesday May 14, Final 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.) • Chapter 12 homework Due May 7: Question 5 (What observations led Harlow Shapley to conclude we are not at the center of the Galaxy?) 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. 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 After the Main Sequence: High Mass • A massive star (more than about 10 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. 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 and neutron degeneracy pressure can support. 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. Where it Stops • For large masses (initial mass greater than about 30 solar masses): – The core ends up with a substantially more than 1.4 solar masses. The temperature gets hot enough to fuse elements all the way up to iron. – The fusion of iron takes energy rather than liberating it. The core collapses, but it is too massive to be supported by electron degeneracy pressure and neutron degeneracy pressure. No known force can halt the collapse, and the core collapses to a point. A black hole is born. A Black Hole • At this point, the density, and hence the gravitational force, are quite large. • Newton’s gravitational theory no longer accurately describes gravity, one must use Einstein’s more complex theory…. Einstein’s Theory • In Newton’s theory of gravity, gravity is a force between two objects. – The “force” travels instantly through space by some unspecified mechanism. – Space is the ordinary 3 dimensional “Euclidean space.” • In Einstein’s theory: – Nothing travels faster than light, and the speed of light is the same for all observers. – Matter causes space to “warp”, and gravity is a manifestation of curved space. Einstein’s Theory • The speed of light is the same, regardless of the motion of the source or observer. Einstein’s Theory • The length of an object decreases in the direction of its motion as its speed increases. • The mass of an object increases as it moves faster. • Clocks in motion run slower than ones at rest. Einstein’s Theory • Time slows down near matter. Clocks run slower in gravitational fields compared to clocks in empty space. Einstein’s Theory • Matter alters the geometry of space. Empty space is “flat”, whereas it is curved near massive bodies. Einstein’s Theory • The curvature of space depends on the mass and density. • The tendency of material and of light is to take the shortest path between two points. • Large bodies can alter the path of light. Image from Nick Strobel’s Astronomy Notes (http:www.astronomynotes.com) Einstein’s Theory • The tendency of material and of light is to take the shortest path between two points. • Large bodies can alter the path of light. Einstein’s Theory • Light loses energy as it leaves the surface of an object. The higher the gravity, the more energy it loses. • A black hole is an object with an infinite “redshift”. Black Holes • A black hole is an object with a gravitational field so strong that nothing, not even light, can escape. • All of the matter is compressed to a point. • There is no physical surface. However, one can define a radius within which nothing can escape: this is called the “event horizon” or the “Schwarzchild radius” . • Once matter or light crosses the event horizon, it is gone forever. Black Holes • A black hole is an object with a gravitational field so strong that nothing, not even light, can escape. Black Holes • Since it is so compact, the tidal force near a black hole is extremely strong: matter is stretched lengthwise, and compressed in the perpendicular direction. Black Holes • A black hole is an object with a gravitational field so strong that nothing, not even light, can escape. • Black holes have only three properties: – Mass – Angular momentum (if it is spinning) – Electric charge (not astrophysically important since macroscopic objects are neutral) • Black holes cannot have magnetic fields, or a temperature, or a color, etc. Detecting a Black Hole • If light cannot escape from a black hole, how do we detect them? By looking at material close to the black hole, before it disappears… Detecting a Black Hole • If the black hole is close to another star, it can pull material off that star. As the matter falls into the black hole, it gets very hot, and emits X-rays. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Detecting a Black Hole • If the black hole is close to another star, it can pull material off that star. As the matter falls into the black hole, it gets very hot, and emits X-rays. Detecting a Black Hole • If the black hole is close to another star, it can pull material off that star. As the matter falls into the black hole, it gets very hot, and emits Xrays. • Jets of matter can also be produced. The X-ray Sky from HEAO I • There are a few hundred bright X-ray sources in the sky, and most are powered by accretion of matter onto a compact object. What’s Next? • After the source is identified, what happens next? • If the X-rays “turn off”, the companion star can be seen: take and measure its “radial velocity curve.” • Use Kepler’s laws to deduce mass limits. If the mass exceeds the maximum mass for a neutron star, the source must be a black hole. Recent Results from SDSU (and elsewhere): The Massive Black Hole in the Spiral Galaxy M33 http://www.nature.com/nature/journal/v449/n7164/full/nature06218.html The Massive Stellar Black Hole in M33: M33 • SA galaxy in Triangulum • d = 840 +/- 20 kpc • M33 X-7 discovered by Einstein in 1981 M33 • X-ray source localized with Chandra and optical counterpart found with HST by Pietsch et al. (2004) • Pietsch et al. also showed that M33 X-7 is an eclipsing binary with P=3.453014 days M33 • Top: Chandra X-ray “light curve” • Bottom: Radial velocity curve obtained from Gemini North 8.2m telescope. M33 • The optical spectrum indicates the companion is an Ostar with T=35,000 K and a radius of R=19.6 solar radii M33 X-7 Results: • Combine the radial velocity curve, the light curves, the eclipse width, the rotational velocity, and the radius (from temperature, apparent magnitude, and distance): • MBH = 15.65 +/- 1.45 solar masses • MSEC = 70.0 +/- 6.9 solar masses • This is the most massive known stellar mass black hole. • The secondary is among the most massive stars with a secure mass determination. M33 X-7 Results: • Links to press releases: http://chandra.harvard.edu/press/07_releases/press_101707.html http://newscenter.sdsu.edu/sdsu_newscenter/news.aspx?s=70814 Cyg X-1 Results: • Links to press releases: • http://chandra.si.edu/press/11_releases/press_111711.html • http://newscenter.sdsu.edu/sdsu_newscenter/news.aspx?s=73292 Results • There are 21 cases where there is good evidence that there is a black hole that came from a massive star: – Strong X-ray sources (usually flares). – Optically dark objects (that is, only one star is seen in the spectrum, and it is the mass-losing one). – Masses too large to be a white dwarf or a neutron star. Supermassive Black Holes • There is evidence that most (if not all) galaxies have black holes with masses 106-109 the Sun’s mass at their centers. These BHs don’t come from single stars. Recap • Before a massive star “dies”, it loses much of its initial mass: – If the initial mass is less than about 8 solar masses, the mass loss is in a gentle “planetary nebula”. – If the initial mass is more than about 8 solar masses, the mass loss is in a violent explosion called a “supernova”. • The universe started only with hydrogen and helium. Thus all of the heavier elements were made in stars. Recap • When a star “dies”, it leaves behind a remnant: – A white dwarf if the initial mass is less than about 8 solar masses. – A neutron star if the initial mass is between about 8 and 30 solar masses. – A black hole if the initial mass is more than about 30 solar masses. • Although white dwarfs, neutron stars, and black holes have strange properties, examples of each are observed. NEXT: Our Galaxy and Other Galaxies