PSCI 1414 General Astronomy The Birth of Stars Part 2: Young Stars, Molecular Clouds, and Star Birth Alexander C. Spahn

Young star clusters Dark nebulae contain tens or hundreds of solar masses of gas and dust, enough to form many stars.

Young star clusters As a consequence, these nebulae tend to form groups or clusters of young stars. As a cluster ages, intense radiation transforms the dark nebula, producing a reddish H II region.

Young star clusters All the stars in a cluster may begin to form nearly simultaneously, but they do not all become main-sequence stars at the same time. As you can see from their evolutionary tracks, high-mass stars evolve more rapidly than low- mass stars. The more massive the protostar, the sooner it develops the central pressures and temperatures needed for steady hydrogen fusion to begin, thus joining the main sequence.

Young star clusters Star clusters tell us still more about how high-mass and low-mass stars evolve. This image shows the young star cluster NGC 2264 in a reddish H II region. Stars within the cluster provide the ultraviolet light that powers this emission nebula.

Young star clusters Note that the hottest and most massive stars, with surface temperatures around 20,000 K, are on the main sequence. Stars cooler than about 10,000 K, however, have not yet quite arrived at the main sequence. These are less massive stars in the final stages of pre–main-sequence contraction and are just now beginning to ignite thermonuclear reactions at their centers.

Young star clusters This image shows another young star cluster called the Pleiades. The photograph shows gas that must once have formed an H II region around this cluster and has dissipated into interstellar space, leaving only traces of dusty material that forms reflection nebulae around the cluster’s stars. This implies that the Pleiades must be older than NGC 2264.

Young star clusters In contrast to the H-R diagram for NGC 2264, nearly all the stars in the Pleiades are on the main sequence. The cluster’s age is about 50 million years (compared to ~2 million years for NGC 2264), which is how long it takes for the least massive stars to finally begin hydrogen fusion in their cores.

Open clusters A loose collection of stars such as NGC 2264 or the Pleiades is referred to as an open cluster. Open clusters possess barely enough mass to hold themselves together by gravitation.

Where does star birth begin? We have seen that star formation takes place within dark nebulae. But where within our Galaxy are these dark nebulae found? Does star formation take place everywhere within the Milky Way, or only in certain special locations? The answers to such questions can enhance our understanding of star formation and of the nature of our home Galaxy.

Where does star birth begin? Dark nebulae are a challenge to locate simply because they are dark—they do not emit visible light. Nearby dark nebulae can be seen silhouetted against background stars or reddish H II regions, but sufficiently distant dark nebulae are impossible to see in contrast with background visible light because of interstellar extinction from dust grains. They can, however, be detected using longer-wavelength radiation that can pass unaffected through interstellar dust. In fact, dark nebulae actually emit radiation at millimeter wavelengths.

Where does star birth begin? Hydrogen is by far the most abundant element in the universe. Unfortunately, in cold nebulae much of it is in a molecular form (H2) that is difficult to detect. In contrast, molecules such as carbon monoxide (CO) are easily detectable at these wavelengths. The ratio of carbon monoxide to hydrogen in interstellar space is reasonably constant. As a result, carbon monoxide is an excellent “tracer” for molecular hydrogen gas. Wherever astronomers detect strong emission from CO, they know molecular hydrogen gas must be abundant.

Giant Molecular Clouds In mapping the locations of CO emission, astronomers in 1974 discovered huge clouds, now called giant molecular clouds, that must contain enormous amounts of hydrogen. These clouds have masses in the range of 105 to 2 ×106 solar masses and diameters that range from about 15 to 100 pc (50 to 300 ly). The density of these clouds is several thousand times greater than the average density of matter in the disk of our Galaxy. Astronomers now estimate that our Galaxy contains about 5000 of these enormous clouds.

Giant Molecular Clouds A radio telescope was tuned to a wavelength of 2.6 mm to detect emissions from carbon monoxide (CO) molecules in the constellations Orion and Monoceros. The result was this false-color map, which shows a 35° × 40° section of the sky. The Orion and Horsehead star-forming nebulae are located at sites of intense CO emission (shown in red and yellow), indicating the presence of a particularly dense molecular cloud at these sites of star formation.

Giant Molecular Clouds By using CO emissions to map out giant molecular clouds, astronomers can find the locations in our Galaxy where star formation occurs. These investigations reveal that molecular clouds clearly outline our Galaxy’s spiral arms. This arrangement resembles the spacing of H II regions along the arms of other spiral galaxies. The presence of both molecular clouds and H II regions shows that spiral arms are sites of ongoing star formation.

Star Formation in Spiral Arms Spiral arms are locations where matter “piles up” temporarily as it orbits the center of the Galaxy. You can think of matter in a spiral arm as analogous to a freeway traffic jam. Just as cars are squeezed close together when they enter a traffic jam, a giant molecular cloud is compressed when it passes through a spiral arm. When this happens, vigorous star formation begins in the cloud’s densest regions.

Star Formation in Spiral Arms Once star formation has begun and an H II region has formed, the massive O and B stars at the core of the H II region induce star formation in the rest of the giant molecular cloud. Ultraviolet radiation and vigorous stellar winds from the O and B stars carve out a cavity/bubble in the cloud, and the reddish H II region, powered by the stars, expands into it.

Star Formation in Spiral Arms These winds travel faster than the speed of sound in the gas—that is, they are supersonic. A shock wave forms where the expanding H II region pushes at supersonic speed into the rest of the giant molecular cloud. This shock wave compresses the gas through which it passes, stimulating more star birth.

Star Formation in Spiral Arms

Supernova Remnants and Star Formation Spiral arms are not the only mechanism for triggering the birth of stars. Presumably, anything that compresses interstellar clouds will do the job. The most dramatic is asupernova, caused by the violent death of a massive star after it has left the main sequence.

Supernova Remnants and Star Formation Astronomers have found many nebulae across the sky that are the shredded funeral shrouds of these dead stars. Such nebulae are known as supernova remnants. Many supernova remnants have a distinctly circular or arched appearance, as would be expected for an expanding shell of gas. This wall of gas is typically moving away from the dead star faster than sound waves can travel through the interstellar medium.

Supernova Remnants and Star Formation Such supersonic motion produces a shock wave that abruptly compresses the medium through which it passes. When a gas is compressed rapidly, its temperature rises, and this temperature rise causes the gas to glow. When the expanding shell of a supernova remnant slams into an interstellar cloud, it compresses the cloud, stimulating star birth.

For next time… Read Chapter 19 Homework 21: questions 27, 28*, and 38 (due Wednesday, April 20th) *We really only discussed two in class. The book briefly gives more but you can always look online too.