Eagle Nebula Star Formation Region AST 303: Chapter 17 1 The Formation of Stars (2) • A cloud of gas and dust must collapse if stars are to be formed. • The self-gravity of the cloud will tend to cause it to collapse. • Radiation pressure from nearby hot stars may do the same. • The passage of a shock wave from a nearby supernova blast or some other source (such as galactic shock waves) may do the same. – Note: The “sonic boom” of a jet plane is an example of a shock wave. • When two clouds collide, they may cause each other to collapse. AST 303: Chapter 17 2 Trifid Nebula AST 303: Chapter 17 3 Trifid Nebula Stellar Nursery Revealed AST 303: Chapter 17 4 Young Starburst Cluster Emerges from Cloud AST 303: Chapter 17 5 The Formation of Stars (3) • The gas in the collapsing cloud probably becomes turbulent. • This would tend to fragment the collapsing gas, producing condensations that would be the nuclei of new stars. • There is abundant evidence that shows that the stars in a cluster are all about the same age. For a young cluster, many stars have not yet reached the main sequence: ! Isochron Luminosity "Temperature AST 303: Chapter 17 6 The Formation of Stars (4) • The evolutionary paths of young stars on the H-R diagram look like this. Note the T Tauri stars, long thought to be young stars. • Theory says that these stars use convection as the main method of transporting energy to their surfaces. ! T Tauri Stars Luminosity "Temperature AST 303: Chapter 17 7 The Search for Stellar Precursors • Astronomers have long been fascinated by very dark, dense regions seen outlined against bright gas, called globules. • These may be the first visible evidence of star formation. • When a star begins to collapse, it will heat up, but it will still be fairly low temperature. Furthermore, it may be wrapped up in dust that makes it impossible for us to see optically. • For this reason, astronomers turn to infrared observations to look for stellar precursors and very young stars. AST 303: Chapter 17 8 Stellar Nurseries AST 303: Chapter 17 9 Stellar Nurseries AST 303: Chapter 17 10 The Orion Nebula • The Orion Nebula (M 42) is the nearest (1500 light years) and most prominent region of star formation. • It contains regions of emission and absorption, and is a strong microwave (OH, H2O) and infrared emitter. Several regions of strong infrared emission are the BN (Becklin- Neugebauer) object and the KL (Kleinman-Low) nebula. • It’s center is dominated by 4 very bright stars (the Trapezium), including an O4.5 star, ϑ1C Orionis, mass about 50 solar masses. This star is extremely hot (emits a lot of ultraviolet) and very young. The ultraviolet emission from this star is largely responsible for exciting the gas of the Orion Nebula to glow. • This glowing gas is an H II region. AST 303: Chapter 17 11 Orion Nebula (WFPC-2 Portrait) AST 303: Chapter 17 12 The Orion Nebula (2) • A model of the Orion Complex: Dark molecular cloud Visible nebula Trapezium AST 303: Chapter 17 13 Star Formation in Orion Nebula AST 303: Chapter 17 14 Closeup of Star Being Born AST 303: Chapter 17 15 Evolution of the Sun • Eventually, the Sun’s core will become depleted of hydrogen and turned entirely into helium. • At this point, no energy can be generated in the core. The core will contract under gravity and heat up. Ironically, the atmosphere swells up and the star becomes a red giant Helium core Hydrogen envelope (Not to scale) AST 303: Chapter 17 16 Evolution of the Sun (2) • Sometime later, hydrogen will start to burn into helium in a shell around the helium core. This happens when the contracting core gets hot enough at the boundary between the helium and hydrogen layers. Helium core Hydrogen-burning shell Hydrogen envelope (Not to scale) AST 303: Chapter 17 17 Evolution of the Sun AST 303: Chapter 17 18 The Fate of the Earth AST 303: Chapter 17 19 Evolution of the Sun (4) • The position of the point on the H-R diagram that represents the luminosity and surface temperature of a star will trace out an evolutionary track on the H-R diagram with time. 106 104 D 2 E 10 F B 1 C A: Main sequence A 10-2 B: Shell burning C: Fully convective Solar luminosities -4 D: Helium flash 10 E, F: Later evolution 10-6 40000 20000 10000 5000 2000 Surface temperature (Kelvin) AST 303: Chapter 17 20 Evolution of the Sun (5) • Eventually the core becomes so hot that helium can fuse into carbon. • The process that burns helium into carbon is called the triple alpha process. It requires the near-simultaneous collision of three helium nuclei, and since the electrical repulsion of helium nuclei is much stronger than the electrical repulsion of hydrogen nuclei, it requires much higher temperatures (100 million K) and pressures. 4 He 4 2 2 He ! ray 8 ( Be)* 4 12 4 6 C 2 He AST 303: Chapter 17 21 Helium Burning Phase AST 303: Chapter 17 22 Pulsating Stars • The point representing the Sun moves back down and to the left on the H-R diagram. This time it enters a region called the instability strip. At this point, the star becomes somewhat unstable and pulsates. It has turned into a Cepheid variable. 106 Instability 104 Strip 102 1 10-2 Solar luminosities 10-4 10-6 40000 20000 10000 5000 2000 Surface temperature (Kelvin) AST 303: Chapter 17 23 Pulsating Stars (2) • A Cepheid Variable star pulsates very regularly, with a period of days to weeks. At first, the star gets smaller, heating up its interior. It overshoots the equilibrium point (like a child on a swing overshooting the bottom of the swing), so eventually the pressure in the interior starts to make it expand again. Again it overshoots the equilibrium point. This happens as long as the star is in the instability strip. ! Luminosity Time ! AST 303: Chapter 17 24 A Cepheid Variable AST 303: Chapter 17 25 Light Curve of Cepheid Variable AST 303: Chapter 17 26 Pulsating Stars (3) • Other types of pulsating stars are δ Scuti stars, whose masses are similar to the Sun, and RR Lyrae stars, of lower mass than Cepheids and with periods about a day. • All of these stars are important for measuring the distance scale in the Universe, because their luminosities can be related to the pulsational period. This makes them useful standard candles. AST 303: Chapter 17 27 Dying Stars • Eventually, a star can no longer burn any kind of nuclear fuel. • It dies as a nuclear energy source. Most stars end up as white dwarf stars. • White dwarfs are about the size of the Earth, but have mass comparable to that of the Sun. • Their densities are about a million times that of the Sun. Surface of Sun Earth White dwarf AST 303: Chapter 17 28 Evolution to White Dwarf AST 303: Chapter 17 29 Structure of White Dwarf AST 303: Chapter 17 30 Dying Stars (2) • The first white dwarf discovered was a companion to the star Sirius. Friedrich Bessel discovered that Sirius “wobbled” back and forth in the sky. The wobble was not due to parallax. Bessel concluded that Sirius had a companion, but he couldn’t see it. • Visually detected in 1862. It was only 10-4 as luminous as Sirius, and after the orbit was calculated, it turned out to be as massive as the Sun. • A single cubic centimeter of matter from this star would weigh over a ton! • White dwarfs have no internal nuclear reactions; they simply cool off, emitting heat that is left over from when they were a star. Their surface area is very small. AST 303: Chapter 17 31 Dying Stars (4) • S. Chandrasekhar showed that there is a relationship between the mass and diameter of white dwarfs. No white dwarf can have a mass over 1.44 times the mass of the Sun (the Chandrasekhar Limit). 30,000 ! 20,000 Chandrasekhar Limit 10,000 Diameter (km) 0 0.0 0.5 1.0 1.5 Mass (solar masses) ! AST 303: Chapter 17 32 Planetary Nebulae • In the later stages of evolution (red giants, planetary nebula formation, novae and supernovae), stars may lose large amounts of mass. This mass is recycled into the interstellar medium. • Since new stars are formed from the recycled material, and since planets like the earth are formed form the heavier elements in it, this means that we ourselves are made of stuff that used to be in the interior of stars. AST 303: Chapter 17 33 Planetary Nebula AST 303: Chapter 17 34 Planetary Nebula AST 303: Chapter 17 35 Planetary Nebula AST 303: Chapter 17 36 Egg Nebula AST 303: Chapter 17 37 Hourglass Nebula AST 303: Chapter 17 38 NGC 6543 AST 303: Chapter 17 39 Planetary Nebulae (3) • As the nebula dissipates, the star cools down. The star becomes a white dwarf; here is the evolutionary track. 106 4 G: Central star of 10 G planetary nebula H: White dwarfs 102 1 H 10-2 Solar luminosities 10-4 10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin) AST 303: Chapter 17 40 Evolution of Massive Stars • Massive stars (say more massive than 5 solar masses) have a different history. • They produce energy through the CNO Cycle, in which carbon, nitrogen and oxygen catalyze the conversion of hydrogen into helium. • Their cores are convective, and energy is transported through their envelopes by radiation.