Birth of Stars & Discovery of Planets Outside the Solar System

Birth of Stars&Discovery of Planets Outside the Solar System AST 2010: Chapter 20

Questions about Star Formation • Are new stars still forming, or did star formation cease a long time ago? • If new stars are still being created, where is this occurring? • Are planets a natural result of star formation, or is our solar system unique in the universe? • If there are planets around distant stars, how can we observe them? AST 2010: Chapter 20

Known Basics about Stars • Stable (main-sequence) stars, such as the Sun, maintain equilibrium by producing energy through nuclear fusion in their cores • The ability to generate energy by fusion defines a star • Each second in the Sun, about 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning to energy in the process • This rate of hydrogen use implies that eventually the Sun (and all other stars) will run out of central fuel • Stellar masses range from 1/12 MSun to ~200 MSun • Low-mass stars are far more common than high-mass ones • For main-sequence stars, the most massive (spectral type O) are also the most luminous and have the highest surface-temperature, whereas the least massive (spectral type M or L) are the least luminous and the coolest • A galaxy of stars, such as the Milky Way, contains huge amounts of gas and dust, enough to make billions of stars like the Sun AST 2010: Chapter 20

Giant Molecular Clouds • Vast clouds of gas (and dust) dot the Milky Way Galaxy • A giant molecular cloud is • an enormous, dense cloud of gas • so cold (10 to 20 K) that atoms are bound into molecules • The masses of giant molecular clouds range from about 1,000 MSun to about 3 million MSun • Within the clouds are lumps, regions of high density and low temperature • These conditions are believed to be just what is required to make new stars • The combination of high density and low temperature makes it possible for gravity to overcome pressure AST 2010: Chapter 20

Pillars of high-density, cool dust and gas in the central regions of the Eagle nebula, or M16The colors in this image (taken by the Hubble Space Telescope) have been reassigned to enhance the level of detail visible in the image Go to website

Dense Globules in Eagle Nebula • One of the pillars in M16 appears to have dense, round pockets at the tips of finger-like features protruding from it • These pockets have been termed evaporating gas globules (e.g.g.s) • They may harbor embryonic stars • Video: zooming in to e.g.g.s in M16 AST 2010: Chapter 20

Evaporating Gas Globules AST 2010: Chapter 20

EGGs in M16 AST 2010: Chapter 20

Understanding Early Stages of Star Formation • The early stages of star formation are still shrouded in mystery because they are almost impossible to observe directly for three reasons: • The dust-shrouded interiors of molecular clouds where stellar births are thought to take place cannot be observed with visible light, but only with infrared and radio telescopes • The timescale for the initial collapse is estimated to be very short astronomically (thousands of years), implying that stars undergoing the collapse process are relatively few • The collapse of a new star occurs in a region so small that in most cases it cannot be observed with sufficient resolution using existing techniques • The current understanding of how star formation occurs is the result of theoretical calculations combined with the limited observations available • This implies that the present picture of star formation may be changed, or even contradicted, by future observations AST 2010: Chapter 20

Stellar Birth • The first step in the process of creating a star is the formation of a dense core within a clump of gas and dust • The process of core formation is not yet fully understood, but gravity can be expected to play an important role • Gravity causes the core to accumulate additional matter from the surrounding cloud material • The turbulence created inside a clump tends to cause the core and its surrounding material to spin • When sufficiently massive material has accumulated, gravity causes the core to collapse rapidly, and its density increases greatly as a result • During the time a dense core is contracting to become a true star — but before the fusion of protons to produce helium begins — the object is called a protostar • When the protostar is still accreting material from the surrounding cloud, dust and gas envelope the protostar, making it observable only in the infrared AST 2010: Chapter 20

Visible Infrared Images from the Hubble Space Telescope Observation of Protostars • Infrared detectors enable observation of possible protostars • Many stars appear to be forming in the Orion Nebula above and to the right of the Trapezium stars • They can only be seen in the infrared image AST 2010: Chapter 20

Winds & Jets • Once almost all of the available material has been accreted and the protostar has reached nearly its final mass, it is called a T Tauri star • after one of the best studied members of this class of stars • Upon reaching this stage of its development, the protostar starts producing a powerful stellar wind, consisting mainly of protons and electrons streaming away from its surface at speeds of a few hundred kilometers per second • The wind tends to emerge more easily in the direction of the protostar’s poles • The disk of material around its equator blocks the wind in this direction • Consequently, two jets of outflowing material appear in opposite directions from the protostar poles AST 2010: Chapter 20

Protostar Jets • The jets can collide with the material around the protostar and produce regions that emit light • These glowing regions are called Herbig-Haro (HH) objects • They allow us to estimate the location of the hidden protostar

True Star Being Born • Eventually, the stellar wind sweeps away the obscuring envelope of gas and dust, leaving behind the protostar and its surrounding disk • The protostar still continues to undergo gravitational contraction • This generates heat inside it and slowly increases its interior temperature AST 2010: Chapter 20

Birth of True Star • If the protostar is sufficiently massive, its central temperature will continue to increase to about 10 million K when nuclear fusion of hydrogen into helium begins inside its core • At this stage, the (proto)star is said to have reached the main sequence • It is now more or less in (hydrostatic) equilibrium and generates energy mainly through nuclear fusion inside its core • Thus astronomers say that a (true) star is born when it can sustain itself through nuclear reactions • Stars devote an average of 90% of their lives on the main sequence AST 2010: Chapter 20

Time to Reach Main-Sequence Stage • The development of contracting protostars can be tracked on the H-R diagram • The time to reach the main sequence is • short for high-mass stars • as low as 10,000 years • long for low-mass stars • up to 100 million years AST 2010: Chapter 20

H-R Diagram: Analogy to Weight versus Height for People AST 2010: Chapter 20

Weight and Height Change as Age Increases (Marlon Brando) AST 2010: Chapter 20

Different Paths for Different Body Types (Woody Allen) AST 2010: Chapter 20

Evidence that Planets Form around Other Stars • It is very hard to see a planet orbiting another star • Planets around other stars may be detected indirectly • One way is to look for disks of material from which planets might be condensing • A big disk is more visible than a small planet • Look for the evolution of disks, evidence for clumping into planets AST 2010: Chapter 20

Disks around Protostars • Four disks observed around stars in the Orion Nebula • The red glow at the center of each disk is believed to be a young star, no more than a million years old AST 2010: Chapter 20

Dust Ring around a Young Star • A debris disk has been found around a star called HR 4796A • The star has been estimated to be young, about 10 million years old • If there are newly formed planets around the star, they will concentrate the dust particles in the disk into clumps and arcs AST 2010: Chapter 20

Disk around Epsilon Erdani • Evidence for a clumpy disk has been found around a nearby star named Epsilon Eridani • The star is surrounded by a donut-shaped ring of dust that contains some bright patches • The bright spots might be warmer dust trapped around a planet that formed inside the donut • Alternatively, the spots could be a concentration of dust brought together by the gravitational influence of a planet orbiting just inside the ring AST 2010: Chapter 20

Planets Beyond the Solar System: Search & Discovery • If we can’t directly observe planets, can we indirectly observe them? • Kepler’s and Newton’s laws apply • In a star-planet system, both the planet and the star orbit a common center of mass • The planet’s motion has an effect on the star’s motion • As a result, the star wobbles a bit • From the observed motion and period of the wobble, the mass of the unseen planet can be deduced using Kepler’s laws • It is a planet if its mass is less than 1/100 the Sun’s mass (or about 10 times Jupiter’s mass) AST 2010: Chapter 20

Doppler Method for Detecting Planets • The star slightly wobbles due to the motion of the unseen companion planet AST 2010: Chapter 20

DiscoveredPlanets • To date, more than 150 “planets” have been found in other star systems • Systems of 2, 3, and possibly more planets have been seen • The masses of the planets are measured in Jupiter-masses AST 2010: Chapter 20

Some Properties of First 101 Extrasolar Planets Found AST 2010: Chapter 20

Explaining the Planets Seen • Now that we have a large sample of “planetary” systems, astronomers need to refine, perhaps significantly, their current models of planetary formation • Most of the extrasolar “planets” found are not at all like the ones in our own solar system • Many of the extrasolar planets are similar to Jupiterin mass, or more massive, and have highly eccentric orbits close to their stars • This is a big surprise and is difficult for the early models to explain • The very massive planets orbiting close to their stars are sometimes called hot Jupiters • There are other surprises … • The formation of planetary systems is more complex and chaotic than we thought • Intensive search continues … AST 2010: Chapter 20

Birth of Stars & Discovery of Planets Outside the Solar System