Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System

Are new stars still being born or did star creation stop billions of years ago? Where are new stars being created? Are planets a natural result of star formation or is our solar system unique in the universe? How can we observe planets around distant stars?

These are some of the questions addressed in this chapter. Table 20.1 summarizes the basic facts about stars covered in chapters 15 to 19.

20.1 Star Formation

Stars are composed of hydrogen and helium gas, so it is natural to look for new stars forming in regions where hydrogen and helium gas is readily available. The likely places are in dense clouds of gas, like those discussed in chapter 19. We can pretty much confine ourselves to looking in our own galaxy where such clouds are plentiful, and it is easier for our telescopes to see faint objects (stars still in the process of forming are probably rather faint).

Molecular Clouds: Stellar Nurseries

Molecular clouds have the best properties for star formation. They contain enough gas and dust to make hundreds or even millions of Suns, they are cold, and they are clumpy. The clumpiness provides the high density regions around which the gas can aggregate -- a perfectly smooth cloud wouldn't tend to fall into any particular location and form a star. Also cold gas will more easily aggregate than hot gas. The heating of the star to start the fusion process comes from the energy released in the gravitational collapse of the gas, not in the starting temperature of the gas.

The Orion Molecular Cloud

The closest and most studied stellar nursery is the Orion cloud. The cloud is located in the Orion constellation, along the group of stars that make the "sword" that hangs from "Orion's belt". The existence of the Orion nebula is most obvious in the infrared (fig. 20.3b). Stars are being formed in the nebula (Fig. 20.5 and HST).

It is believed that massive first generation stars heat the surrounding gas, pushing it outward from the star. This causes the pressure to increase in regions away from the first generation star, compressing the surrounding cold gas and initiating regions of collapse where second generation stars are formed. Potentially, this cycle has repeated many times during the life of the universe.

To see the newly formed stars in Orion, we must look through the gas and dust in the infrared. In going from a cloud of gas to a star, the density of the gas must increase by a factor of 1020. Exactly how this occurs is not known. The problem is that the aggregation produces very little light (would be mostly infrared radiation due to the heating that occurs), and occurs inside a cloud of gas and dust which blocks our view.

Before the aggregating gas becomes a true star it is called a protostar. A dense core of material begins attracting surrounding material, building up pressure. When the gravitational pressure overwhelms the gas pressure, the material rapidly collapses, and the protostar is formed. Remaining material around the protostar forms a disk (necessary for "conservation of angular momentum") which may later form into planets around the star.

Observations indicate that after forming, the protostar ejects material with a powerful stellar wind. Jets of ejected material are seen around some young stars (figures 20.9 and 20.10).

20.2 The H-R Diagram and the Study of Stellar Evolution

We can use the H-R diagram to summarize the steps in star formation. When used in this way, we think of the protostar as moving in the H-R diagram, eventually ending up on the main sequence at the time that the star begins its long, stable life of fusing hydrogen. Moving in the H-R diagram doesn't imply that the star is moving in space; the two have nothing to do with each other. The star "moves" in the sense that its temperature and luminosity change as it evolves, and since that is what determines a star's position on the diagram, it will move as temperature and luminosity evolve.

The path followed on the H-R diagram by a protostar is called an evolutionary track. Figure 20.12 shows the evolutionary tracks for protostars with different stellar masses. Along the black curves, the stars are not producing energy by fusion; their energy comes from gravitational contraction of the gas. The time it takes for the gas to contract depends on the (final) mass of the star. Very high mass stars contract "relatively" quickly, requiring about 10,000 years (we'll see that high mass stars tend to do everything quickly). Low mass stars take their good time. Our star took around 10 million years to contract, and the smallest stars can take over 100 million years to contract. This is still relatively short compared to the time spent on the main sequence (the time spent fusing hydrogen into helium). The state of the protostars after various periods of time is indicated on the black curves.

The general trend is that as protostars collapse, their temperatures increase. On the H-R diagram, the stars "move" from above and to the right of the main sequence towards the main sequence. During this time the protostar's energy comes from the gravitational contraction of the gas. (Note that gravitational contraction can create a good deal of energy, as indicated by the luminosity of the protostar, however the length of time that this can be sustained is much less than the billion year lifetimes of most stars.) The larger the star, the shorter the time of contraction. When the star begins producing energy by fusing hydrogen, then it has reached the main sequence ("is born").

Final note: the dashed line on the plot indicates the time at which the protostar will push away the surrounding gas and dust and become visible to our telescopes.

20.3 Evidence That Planets Form Around Other Stars

We are very interested to learn if there are planets around other stars. The search for planets is extremely difficult. It's been compared to finding a mosquito around a search light! Let's first look at evidence that planets may exist around other stars, then discuss the direct searches for planets.

Disks Around Protostars: Planetary Systems in Formation?

There is observational evidence that many (all?) young stars are surrounded by disks of material. The disks are large, and can be readily seen in a number of Hubble pictures (Figure 20.13). The existence of the disks means that material exists out of which planets might form. But does it form planets?

Evolution of Proto-Planetary Disks

Do planets form in the disks? There is indirect evidence that it does sometimes occur. First, astronomers have seen that the disks change according to the age of the star (remember this is seen, not by watching the evolution of a single star, but by measuring the properties of many stars and disk systems).

Second, in older stars, the disk seems to thin out into a ring (Figure 20.14). This is a strange observation because calculations show that a ring of dust around a star is not stable, but should collapse into the star. That is, unless there is a planet about the size of Jupiter circling inside the ring. (This is somewhat reminiscent of the "sheperd" moons that keep some of Saturn's rings stable.)

20.4 Planets Beyond the Solar System: Search and Discovery

How can we directly observe a planet and prove that they exist?

Search for Orbital Motion

Evidence for other planets has been obtained by looking for the effect of the orbital motion of the planet on its star. Recall how in a binary star system, two stars orbit about their common center of mass. Well, this is also true for planets orbiting stars, but since planets are so much smaller than stars, the common center of mass is at almost the same location as the center of the star. But it isn't exactly the same, so the star "wobbles" a bit, meaning that the center of the star moves about a point that is not far away -- probably still inside the star itself.

The star wobbles with the same period as the orbit of the planet. For the Sun, there is a small wobble with a period of 12 years due to the motion of Jupiter. There are also small perturbations produced by all the other planets, though they are smaller due to the smaller mass of the other planets.

Search for Doppler Shifts

We could look for small motions of stars in the sky, but the motions are too tiny to detect with present instruments. So instead we look for a doppler shift of light coming from the stars. The basic idea is summarized in Fig. 20.16.

The Discovery of Planets

The doppler shifts are small and the observations are painstaking, yet in 1995 a group in Europe succeeded in observing an effect. They found a planet near the star 51 Pegasi. The planet orbits the star in 4.2days, and has a mass about half the mass of Jupiter. (The technique doesn't determine the mass of the planet but rather a minimum mass for the planet.) This planet must be very large, at least half the mass of Jupiter, yet very close to the star, about 1/10 the distance of Mercury from the Sun.

Within a week, the U.S. team of Butler and Marcy (Marcy grew up in Detroit) verified the discovery. Now more than 100 planets outside our solar system are known [show up to date list].

Explaining the Planets We Found

Most of the planets are not like what we see in our own solar system. They are very large, typically several times Jupiter's mass, and they orbit very close to their star, often closer than Mercury is to the Sun. This is not at all what astronomers expected to be "typical".

But then again, the doppler technique used to find most of the planets is most efficient to find large planets that cause the greatest motion of their stars and thus the largest doppler shift, and planets with short orbital periods so that the change in motion can be detected in a short time. Recall that the search for planets is only about a decade old, so a planet like Jupiter that orbits in 12 years will require patience. [Show radial velocity curves.]

Habitable Planets Circling Other Stars

Are there habitable planets orbiting other stars? Philosophically this is an interesting issue. The technical problems to find other habitable planets are daunting. There are a number of prejudices that may lead researchers in different directions: should one first try to find evidence of Earth-sized planets, or rather concentrate on finding evidence for molecules indicative of a "habitable" planet, water and oxygen. Both are prejudiced by our own experience.

© Robert Harr 2002