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REQUIRED READINGS: Text, Ch. 18 & 19, and all that follows here. Pay particular attention to Section 4 (Extra-Solar Planets); it is not covered in your text.

This gas has a wide range of physical conditions. Most of interstellar space is filled with diffuse interstellar gas, having atomic density ranging from about 1 - 100 atoms/cm³. Most of the diffuse gas has temperature of about 100 K. We can see it best by observing radio emission from hydrogen atoms, which emit radio waves at the famous 21 cm line. We call such regions HI regions, the symbol HI indicating that the hydrogen atoms are neutral and not bound to other atoms. The atomic hydrogen image in the Multiwavelength Milky Way was observed through this radiation. Be sure to read the captions there. When illuminated by a nearby hot star, the interstellar hydrogen gas becomes hot (10⁴ K) and ionized (the electrons are stripped off the atoms), and we can see optical emission lines from the gas. We call such regions HII regions.

A substantial fraction of interstellar space in the Milky Way is occupied by coronal gas, which, like the solar corona, has temperatures ranging from 10⁶ - 10⁷ K. This gas is heated by the explosions of massive stars -- supernovae -- which we shall discuss later in this course.

A smaller fraction of the interstellar volume is filled with molecular clouds (also called dense clouds, or dark clouds), having density ranging from 10³ atoms/cm³ up to 10⁵ atoms/cm³ or greater. We call this gas "dense interstellar gas", but it is still rarefied by Earth standards. The highest vacuum ever achieved in a laboratory on Earth has density of about 10⁴ atoms/cm³. We call the dense clouds molecular clouds because most of the hydrogen there is in the form of H₂ (molecular hydrogen, in which two hydrogen atoms are bound together chemically), in contrast to the hydrogen atoms (HI) in the diffuse interstellar gas, which are unbound. Even though these molecular clouds occupy a relatively small fraction of the volume of interstellar space, the clouds comprise a substantial fraction (perhaps 1/2) of the mass of interstellar gas, because they are so much denser than the diffuse interstellar gas.

In addition to H₂ molecules, the molecular clouds contain many other chemical compounds, ranging from relatively simple molecules such as carbon monoxide (CO) and cyanide (HCN) to more complex molecules such as ethyl alcohol (CH₃CH₂OH). Hundreds of such molecules have been found in molecular clouds and identified by comparing their radio emission line spectra with radio spectra of the same molecules in laboratories on Earth. Since H₂ molecules have no radio spectral lines, we observe the molecular clouds best through the radio emissions of other molecules. The easiest one to see is CO. In fact, the image labeled "molecular hydrogen" in the Multiwavelength Milky Way is actually an image of radio emission from CO, not H₂ molecules (which neither emit nor absorb radio waves). Note that the "molecular hydrogen" image is much thinner than the atomic hydrogen image there, illustrating that the molecular clouds occupy less volume than the atomic hydrogen and are confined more closely to the equatorial plane of the Milky Way than the diffuse gas.

In addition to hydrogen and helium, the interstellar medium contains interstellar dust. The mass of dust is about 1% of the mass of gas. The dust consists of tiny grains of silicates (much like ordinary beach sand or volcanic ash) and soot (very similar to the black soot in the exhaust of a diesel engine). We know this because we see that the infrared emission spectra from dust in interstellar space is almost identical to the laboratory spectra of tiny grains of sand and soot. The first pair of graphs below is a comparison of the infrared spectrum emitted by dust in the Orion Nebula and that emitted by the exhaust of a diesel truck. Both spectra show prominent emission features at 6.2 and 7.6 micrometers that we can identify with the laboratory spectra of "polycyclic aromatic hydrocarbons", or "PAHs" -- toxic chemicals that are known carcinogens. The second pair of graphs is a comparison of the infrared spectrum from dust in the atmosphere of a red giant star with the laboratory spectrum of coronene, one such PAH. Observations with infrared spectrometers also show that the dust grains in the dense molecular clouds are often covered with ices, such as H₂O and CO₂ (dry ice).

The interstellar dust grains are very effective absorbers of optical and ultraviolet radiation, so we can't see through the dense interstellar clouds in these wavelength bands. The obscuration due to dust accounts for the dark regions we see in the optical image in the Multiwavelength Milky Way. But we can see right through the dark clouds at infrared (and radio, X-ray, and gamma ray) wavelengths, so these dark clouds are not evident in the images in these bands.

2. STAR-FORMING REGIONS

The brightest, hottest stars must be very young compared to the Sun, because (as you will see later in this course) they have very short lifetimes (less than 100 million years) compared to the Sun's age (about 5 billion years). When we see such hot stars, we always see them in clusters containing hundreds or thousands of stars (see text, Fig. 17.19). The most prominent such cluster in the sky is the Pleiades in the constellation Taurus. (The Japanese name for this cluster is Subaru; every Subaru car has a chrome Pleiades.) Moreover, we always find these young clusters near to molecular clouds. Often, we find that such hot stars are buried within the molecular clouds. When the clouds contain such stars, we see that the radiation from the stars is causing the dense gas to disperse into interstellar space. Clearly, the dense clouds are the birthplaces of new stars.

More than half the stars in the sky are found in close binary systems, in which two stars orbit each other. Single stars like the Sun are in the minority. This fact turns out to be an important clue to understanding how stars form.

Stars are born inside the dark interstellar clouds. In fact, the nearest star-forming region is the Rho Ophiuci cloud. Figure 18.12 of your text shows an optical image of this cloud. The infrared image of the same region (Figure 18.15) shows that the dark cloud is in fact bright at infrared wavelengths because the dust in the cloud absorbs the optical light and re-radiates the energy at infrared wavelengths. Recently, the Infrared Space Observatory obtained a sharper image of the Rho Ophiuci Cloud that shows many newborn stars within this dark cloud.

In the planetarium I showed several slides of star-forming regions in the Milky Way. The most famous (about 1000 light years from the Sun) star-forming region is the Orion Nebula (also called M42, which means it is object number 42 in the famous catalogue of nebulae compiled by Charles Messier in 1782. M42 is the middle object in the "sword" of the constellation Orion. With binoculars, you can see that it is a fuzzy object, not a star. Much of what we have learned about star formation comes from observations of this nebula.

Your text, Fig. 19.21, shows two images of the Orion Nebula, the one on the left in optical light and the one on the right in infrared light. The optical emission is caused by the action of several hot blue stars on the near side of a dense molecular cloud, which have heated and ionized the hydrogen gas so it glows red (the wavelength of the most prominent emission line of hydrogen gas is in the red part of the optical spectrum). The Hubble Space Telescope has obtained a spectacular optical image of the inner part of the Orion Nebula. You can see several more HST close-ups, with captions, on Doug Johnstone's Orion Page. Several of these images were obtained by Prof. John Bally of our dept.

The optical emission nebula that you see in the above images of Orion is caused mostly by ultraviolet light from a group of four bright blue stars called the "Trapezium". (See Fig. 5 of Doug Johnstone's page). These stars, and the optical nebula, lie on the near side of a giant dark molecular cloud (called Orion Molecular Cloud 1, or OMC-1) that contains many newborn stars. At infrared wavelengths, we can see these embedded stars. The best such view that we have gotten to date is from the new infrared camera (called NICMOS) that was recently installed on the Hubble Space Telescope. In May 1997, the Space Telescope Science Institute posted these results on a web page called NICMOS Captures the Heart of OMC-1. Be sure to look at this page and to read the captions. It has wonderful pictures and movies. It will take you a long time to download all the pictures and movies if you are logged in on a modem. You would be better off using one of the campus computers, which have fast ethernet connections.

By observing Orion at specific radio and infrared wavelengths, we can see emission due to particular atoms or molecules that are present at specific locations inside the OMC. For example, have a look at the composite image of the Orion Bar's photo-dissociation region, where you can see that the emission from warm carbon monoxide (red) is separated from the hot molecular hydrogen (green) and the warm dust grains (blue). UV radiation from hot stars to the upper right of this image are heating and destroying the dust grains and molecules. The CO is easiest to destroy, so it shows up furthest from the stars. The H₂ is somewhat more durable and is seen closer to the stars, and the grains, closer yet.

At the highest magnification of the HST optical images of Orion, we can see little comet-shaped gas clouds, called "cometary globules", and little dark disks, called "proto-planetary disks", or "proplyds" (see Doug Johnstone's Orion Page). These are dense disks of gas and dust swirling around newly forming stars. We suspect that planetary systems will form from the dust in these disks. In some cases, the objects have tails of luminous gas pointing away from nearby hot stars, just as comet tails point away from the Sun. But these objects are not just comets -- they are whole planetary systems in the making. Their radii are a few hundreds of AU (1 AU is the distance from the Earth to the Sun). We believe that planets form in these swirling disks of gas and dust.

In star-forming regions, we also see the remarkable phenomenon of protostellar jets. Be sure to check the page Hubble Observes the Fire and Fury of a Stellar Birth, and to read the captions of the figures there. (See also text, pp. 398-9.) These jets are narrow streams of gas that are flowing out of newly forming stars (called T-Tauri stars) at velocities of a few hundred kilometers per second. Often we see them as bipolar outflows, meaning that there are two jets oppositely directed from the T-Tauri star, but sometimes we see the jet on only one side, probably because the jet on the opposite side is hidden within a dust cloud. The physical mechanism responsible for these jets is still a mystery.

Besides the Orion Nebula, the Milky Way galaxy contains many more star-forming regions and some of the pictures are spectacular. The best one I know of is the Hubble Space Telescope Picture of the Eagle Nebula (M16). Check it out, and be sure to read the caption about the "eggs" of the Eagle. You can also find infrared images of several star-forming regions in the IPAC Gallery.

3. FORMATION OF STARS AND PLANETS

Inertia: massive objects resist changes of their motion. Fast-moving objects will not be deflected easily by gravity. For example, the Sun's gravity attracts comets. However, most comets do not fall into the Sun, but fly away again after passing close to the Sun. In this case (and in describing the orbits of planets), we call the effect of inertia centrifugal force. In the case of interstellar gas clouds, the inertia due to turbulent motions of the gas tends to counteract gravity.

Heat pressure: Warm gas tends to expand under its own pressure. If interstellar gas is warm enough, this pressure will prevent gravity from causing fragmentation and collapse. This accounts for the fact that we see star formation occurring only in the very cold dense clouds, never in the relatively warm diffuse interstellar gas.

The never-ending battle: Most of the action we see in the universe, from planetary motions to star formation to the motion of the universe itself, results from a never-ending battle between the attractive force of gravity and the various forces that counteract gravity. Often, gravity wins -- as we can see, for example, in star-forming regions. But this doesn't happen immediately. In the case of star formation, some subtle things must occur to help gravity in its ultimate victory. I have already mentioned one: the fission of rotating collapsing clouds.

Interstellar molecules and star formation: Gravity can win over heat pressure only when the density is high and the temperature is very low, i.e., in the dense clouds. But at such low temperatures, most atoms cannot radiate when they collide. Only molecules can become internally excited during a low temperature collision. Therefore, a gas cloud without molecules couldn't radiate enough, and would not be able to collapse to form a star. The molecules are necessary for star formation. The radio and infrared emission lines which we see coming from molecules in star-forming regions are the very photons that are carrying off the heat energy and permitting collapse.

The importance of interstellar grains: In the dark clouds, the interstellar dust grains block the UV starlight, permitting the molecules to survive. Moreover, gas atoms will stick to the surfaces of the dust grains and combine with other atoms there to form new molecules, which evaporate from the grains and return to the gas. (By the same process, catalytic converters in automobiles convert toxic exhaust gases into CO₂ and water.) Thus, the grains increase the concentration of molecules in the gas, both by facilitating their formation and by preventing their destruction. Astronomers who hate dark interstellar clouds because they make it hard to see stars should think again: without those dark clouds, there would be no stars to see! The role of grains in star formation is one of the many subtle processes that determine the ecology of the Milky Way Galaxy.

Magnetic pressure: Interstellar gas is pervaded by a magnetic field, and this magnetic field tends to become stronger when compressed and to counteract gravity. This pressure tends to resist collapse and retard star formation. Interstellar gas can move through magnetic fields, but only very slowly. Therefore, magnetic fields tend to retard star formation, but they cannot stop it. Typically, it takes about 10⁷ (ten million) years for stars to form in a molecular cloud. Figure 19.3 and this Simulation of Star Formation by the National Center for Supercomputer Applications illustrates how magnetic fields influence the flow of gas in a collapsing gas cloud. We also suspect that magnetic fields play a key role in causing the jets of gas that flow out of newly forming stars, but we do not understand the mechanism very well yet.

Self-regulating star formation: When enough stars form in a dense cloud, they will terminate the process of star formation. The UV light from the hot stars will penetrate into the dark clouds, destroying the molecules and heating the gas. That will cause the rest of the gas cloud to disperse. The beautiful optical emission nebulae (e.g., the Eagle Nebula) that you see in star forming regions are bright just because of the dense gas that is being driven away from the dense clouds by the UV light of newborn stars.

Propagating star formation: Gravity is not the only force that can compress interstellar gas. The supernova explosions of massive stars can compress diffuse interstellar gas into dense clouds. The dense clouds can then cool enough that gravity can take over. The result is that the deaths of stars can stimulate the birth of new stars, as illustrated by Figure 19.19 of your text.

4. EXTRA-SOLAR PLANETS AND BROWN DWARF STARS

Be sure to read this section carefully. This is one of the hottest subjects in astronomy today, and it is not mentioned in your textbook. You can also find excellent summaries of this topic in this article from Scientific American and this article from Microsoft's Encarta encyclopedia.

The formation of planetary systems: the current theory for the origin of the solar system and other planetary systems has its origins in early (1755) ideas by the philosopher Immanuel Kant. The basic idea is that planets will form out of the protoplanetary disk of gas and dust that swirls around a newborn star. If the disk is dense enough, the dust grains will tend to stick to each other, bonded by ices, and eventually accumulate into planetesimals. (Astronomers are interested in comets because they are thought to be similar to planetesimals. The planetesimals keep growing, eventually becoming massive enough to attract and capture gas from the nearby disk and build up gaseous atmospheres. The giant planets capture the little ones, and sometimes fling them out of the planetary systems. This process is illustrated here.

When astronomers developed this theory mathematically, they found that giant gaseous planets like Jupiter could only form at distances greater than about 5 AU - where Jupiter lies. There were two reasons. The first is that tidal forces would prevent such a planet from forming, and the second is that the light from the star would heat the gas, preventing accumulation. According to this theory, only smaller rocky planets like the Earth, Venus, Mars, Mercury, could form so close to the star. When such planets formed, they would fling out many other planets, leaving just a few behind, as illustrated here. But as you will shortly see, when astronomers did discover planets around other stars, they were massive gaseous planets like Jupiter and they were closer to their companion stars than Mercury is to the Sun. So much for that theory!

The discovery of extrasolar planets: Four years ago, we wouldn't have much to say about planets around other stars because none were known. The first planet orbiting a normal star was discovered in 1995 by two Swiss astronomers. Today, astronomers have discovered 20 nearby stars like the Sun that have planets or brown dwarf stars orbiting around them. A brown dwarf star, like the Sun, is a dense ball of hydrogen and helium gas; but unlike the Sun, it doesn't ever become hot enough in its center to burn hydrogen into helium. We believe that if a star has mass greater than about 80 times Jupiter's mass, it will begin to burn. So, brown dwarf stars must be less massive than that. On the other hand, Jupiter is also a dense ball composed mostly of hydrogen and helium, and we call it a planet, not a brown dwarf. The distinction between planets and brown dwarfs is subtle. It turns out that if the object weighs between 13 and 80 times Jupiter, it will be able to burn a rare isotope of hydrogen, called deuterium. But after a few million years, the deuterium is used up and the object ceases to burn. We call such a star a brown dwarf, but otherwise it's no different from a planet. Let's not be picky -- call it a planet if it has mass less than 80 times Jupiter. Click here for an up-to-date catalog of extrasolar planets. You will see that astronomers have found planets with masses ranging from about half Jupiter's mass to more than 60 times Jupiter.

Where are the stars with planets? They are all relatively near to the Sun, at distances less than 60 light years. Of course, there must be many more, but our present technology only allows us to find them within this distance.

In addition to these planets, the catalog lists two pulsars with planets orbiting them. One pulsar has four planets: two of them have masses comparable to Earth, one comparable to the Moon, and one more massive than Jupiter. A pulsar is a very weird kind of collapsed star called a neutron star that emits pulsed radio signals. Its mass is greater than the Sun but its diameter is only about 20 km! We'll talk about pulsars and neutron stars later in this course. In fact, the first extrasolar planet to be discovered (in 1992) was orbiting a pulsar. If you want to learn more about the discovery of planets around pulsars, check the pulsar planets site. But you don't have to; we'll come back to this subject later.

Most of the known extrasolar planetary systems have been discovered by two astronomers from San Francisco State University, Goeff Marcy and Paul Butler. You should check Goeff Marcy's Home Page , which has a very nice illustration comparing these planetary systems to the solar system. If you are enjoying this, you might also want to check Extrasolar visions , which presents the same information in a very attractive format with some speculative paintings of what the planets might look like. It is striking that most of these systems have massive planets in orbits comparable to or smaller than the orbit of Mercury. In one sense this result is not surprising; the heavier the planet is and the closer it is to its star, the easier it is to discover. With existing techniques, it is still impossible to detect a planet like the Earth around a normal star (except pulsars). Moreover, the larger the planet's orbit, the longer it takes to discover.

Detecting extrasolar planets: How did astronomers detect planets around nearby stars? The technique is called Doppler spectroscopy, and you first saw it in the film clip about the Doppler effect that I showed on the first day of class. The idea is this: we can see whether a star is moving toward or away from us by observing its spectrum and seeing whether the spectral lines are shifted to the blue or red, respectively, compared to the wavelength of the line as measured in Earth laboratories. If two stars are orbiting each other, then during part of the orbit, the first star will be moving away from us while the second star is moving toward us, while during the rest of the orbit, the motions are reversed. You can see this in the Binary star simulation, which also shows that the bigger the mass ratio (mass of heavy star/mass of lighter star), the less the heavy star moves compared to the lighter star. If the lighter star is faint, we may only be able to see the spectrum of the heavy star. We call such a system a single-line spectroscopic binary, in contrast to a double-line spectroscopic binary, where you can see the moving spectral lines from both stars. A star with a massive planet is just like a single-line spectroscopic binary: you can't see any light from the planet but you can see that some invisible object is orbiting the star because you can see the spectral lines from the star shifting periodically from red to blue and back again.

The mass of the planet can be inferred from this periodic variation of the star's Doppler shift through Kepler's 3rd Law (as revised by Newton -- see text, p. 48). You will soon have the opportunity to see how this works in Homework 3. You will see, however, that the observations of the Doppler shift do not give quite enough information to tell the mass exactly. The uncertainty arises from our inability to measure the inclination, or tilt, of the planet's orbit. If the inclination angle, customarily denoted i, is 90^o, that means we see the orbit edge-on, while i = 0^o means pole-on. From the observations we can only infer the value of the product M sin i, and that is what is listed in the catalog of extrasolar planets. Since sin i is always less than 1, the mass of the planet must be greater than the number listed in the catalog.

Direct Imaging: Why not just take a picture of a planet? Well, that's very difficult. Consider, for example, the problem of imaging one of the planets listed in the catalog. For example the planet around the star 51 Peg. It has roughly the mass of Jupiter and an orbital radius of 0.05 Astronomical Units (1 AU is the distance from the Earth to the Sun). The star is at a distance of 15.4 parsecs, or about 47 light years. From these data I calculate that the maximum angle between the planet and the star is about 0.003 arcseconds. But the angular resolution of the Hubble Space Telescope is only about 0.1 arcsec, 30 times fuzzier! The problem is even worse than that: in optical light, the planet is roughly a million times fainter than the star. With existing technology, it's impossible to detect such a faint object so close to such a bright star.

But if the planet is brighter and more distant from the star, there's a chance. And indeed, astronomers have succeeded in imaging a brown dwarf star named Gliese 229B (the B means that the object is the binary companion of star number 229 in a catalogue of nearby faint red stars compiled by an astronomer named Gliese). GL229B has a mass more than 40 times Jupiter's mass and an orbital radius of about 40 AU -- roughly equal to the orbit of Pluto. In this case the angular separation between the planet (brown dwarf) and the star is about 6 arcseconds and the brightness contrast is not so great. In fact, this planet was discovered first in images taken from the ground, although the image taken subsequently by the HST is better. Check the images and the caption.

How about detecting Earth-like planets? That would be fantastic, because it would be a big step on the way to finding out whether there is life on other worlds. It's the highest scientific priority of NASA. But the task is very difficult; it will require NASA to develop new technologies and to spend more money -- tens of $Billions -- than they ever have on a scientific project. (Maybe not as much as the Apollo project to land men on the Moon, but the primary mission of Apollo was not science.) NASA does in fact have a long-range plan, called the Exploration of Neighboring Planetary Systems (ExNPS) to do this, and the European Space Agency (ESA) has a plan for a space observatory called Darwin to detect Earth-like planets.

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Last modified May 1, 1998
Copyright by Richard McCray