Birth of Stars & Discovery of Planets Outside the Solar System

3 August 2005 AST 2010: Chapter 20 1

Birth of StarsBirth of Stars &&

Discovery of Discovery of Planets Planets Outside the Outside the

Solar SystemSolar System

3 August 20053 August 2005 AST 2010: Chapter 20AST 2010: Chapter 20 22

Questions about Star FormationQuestions 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?


Known Basics about StarsKnown Basics about StarsStable (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 starEach 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 onesFor 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 coolestA galaxy of stars, such as the Milky Way, contains huge amounts of gas and dust, enough to make billions of stars like the Sun


Giant Molecular Giant Molecular Clouds Clouds

Vast clouds of gas (and dust) dot the Milky Way GalaxyA 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 starsThe combination of high density and low temperature makes it possible for gravity to overcome pressure

Pillars of high-Pillars of high-density, cool dust density, cool dust and gas in the and gas in the central regions of central regions of the the EagleEagle nebula, nebula, or or M16M16

The colors in this The colors in this image (taken by image (taken by the Hubble Space the Hubble Space Telescope) have Telescope) have been reassigned to been reassigned to enhance the level enhance the level of detail visible in of detail visible in the image the image

Go to Go to websitewebsite

http://hubblesite.org/sci.d.tech/behind_the_pictures/


Dense Globules in Eagle NebulaDense Globules in Eagle NebulaOne of the pillars in M16 appears to have dense, round pockets at the tips of finger-like features protruding from itThese 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

http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/44/video/a


Evaporating Gas Globules Evaporating Gas Globules


EGGs in M16EGGs in M16


Understanding Early Stages of Understanding Early Stages of Star Formation 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 telescopesThe timescale for the initial collapse is estimated to be very short astronomically (thousands of years), implying that stars undergoing the collapse process are relatively fewThe 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


Stellar BirthStellar BirthThe 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


Images from the Hubble Space Telescope

Visible Infrared

Observation of ProtostarsObservation of Protostars

Infrared detectors enable observation of possible protostarsMany stars appear to be forming in the Orion Nebula above and to the right of the Trapezium starsThey can only be seen in the infrared image


Winds & JetsWinds & JetsOnce 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

Protostar JetsProtostar JetsThe jets can collide with the material around the protostar and produce regions that emit lightThese glowing regions are called Herbig-Haro (HH) objects

They allow us to estimate the location of the hidden protostar


True Star Being BornTrue 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


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 coreAt 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

Birth of True StarBirth of True Star


Time to Reach Main-Sequence Time to Reach Main-Sequence StageStageThe 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


H-R Diagram: Analogy to Weight versus H-R Diagram: Analogy to Weight versus Height for PeopleHeight for People


Weight and Height Change as Age Weight and Height Change as Age Increases (Marlon Brando)Increases (Marlon Brando)


Different Paths for Different Body Different Paths for Different Body Types (Woody Allen)Types (Woody Allen)


Evidence that Planets Form Evidence that Planets Form around Other Starsaround Other Stars

It is very hard to see a planet orbiting another starPlanets around other stars may be detected indirectlyOne way is to look for disks of material from which planets might be condensing

A big disk is more visible than a small planetLook for the evolution of disks, evidence for clumping into planets


Disks around ProtostarsDisks around ProtostarsFour disks observed around stars in the Orion NebulaThe red glow at the center of each disk is believed to be a young star, no more than a million years old


Dust Ring around a Young StarDust Ring around a Young StarA 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


Disk around Epsilon ErdaniDisk around Epsilon ErdaniEvidence for a clumpy disk has been found around a nearby star named Epsilon EridaniThe star is surrounded by a donut-shaped ring of dust that contains some bright patchesThe bright spots might be warmer dust trapped around a planet that formed inside the donutAlternatively, the spots could be a concentration of dust brought together by the gravitational influence of a planet orbiting just inside the ring


Planets Beyond the Solar System: Search & Planets Beyond the Solar System: Search & DiscoveryDiscovery

If we can’t directly observe planets, can we indirectly observe them?Kepler’s and Newton’s laws applyIn a star-planet system, both the planet and the star orbit a common center of massThe planet’s motion has an effect on the star’s motion

As a result, the star wobbles a bitFrom 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)


Doppler Method for Detecting Doppler Method for Detecting PlanetsPlanets

The star slightly wobbles due to the motion of the unseen companion planet


To date, more than 150 “planets” have been found in other star systemsSystems of 2, 3, and possibly more planets have been seenThe masses of the planets are measured in Jupiter-masses

DiscoveredDiscovered PlanetsPlanets


Some Properties of First 101 Some Properties of First 101 Extrasolar Planets FoundExtrasolar Planets Found


Explaining the Planets SeenExplaining the Planets SeenNow that we have a large sample of “planetary” systems, astronomers need to refine, perhaps significantly, their current models of planetary formationMost 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 Jupiter in mass, or more massive, and have highly eccentric orbits close to their starsThis is a big surprise and is difficult for the early models to explainThe 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 …

Birth of Stars & Discovery of Planets Outside the Solar System

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