Fig. 10.1 . An artistís impression of the nebular hypothesis, in which the Sun and planets were formed at the same time during the collapse of a rotating interstellar cloud of gas and dust that is called the solar nebula. The center collapsed to ignite nuclear reactions in the nascent Sun, while the surrounding material was whirled into a spinning disk where the planets coalesced. (Courtesy of Helmut K. Wimmer, Hayden Planetarium, American Museum of Natural History.)
Fig. 10.2 . The central, rapidly rotating Sun is connected to an ionized, slowly rotating disk by magnetic fields (side view). The magnetic field is twisted into a spiral shape (top view), and acts as a brake on the Sunís rotation, transferring angular momentum from the Sun to the proto-planetary disk.
Fig. 10.3 . The infrared heat radiation of hundreds of embryonic stars (white/yellow) and windblown, star-forming clouds (red), detected from the Spitzer Space Telescope. The intense radiation and winds of a nearby massive star, located just above the image frame, probably triggered the star formation and sculpted the cool gas and dust into towering pillars. (Courtesy of NASA/JPL-Caltech/Harvard-Smithsonian CfA/ESA/STScI.)
Fig. 10.4 . This multiple-wavelength portrayal combines infrared (red), visible light (green) and x-ray (blue) images of the bright, star-forming region designated NGC 346. It is located in the Small Magellanic Cloud that orbits our Milky Way Galaxy at a distance of about 210,000 light-years. Both wind-triggered and radiation-induced star formations are revealed, primarily by the infrared emission of the cold dust (red), detected from the Spitzer Space Telescope. Young stars enshrouded by dust appear as red spots with white centers. The pressure of intense radiation from massive stars in the central regions of NGC 346 has pushed against nearby gas, causing it to expand, and created shock waves that have compressed nearby dust and gas into small new stars. Red-orange filaments surrounding the center of the image show where this process has occurred. The supernova explosion of a very massive star apparently triggered the formation of even younger stars, seen as a pinkish concentration at the top of the image. Strong winds from this exploding star pushed dust and gas together about 50,000 years ago, compressing it into new stars. The x-rays (blue), observed from ESAís XMM-Newton orbiting telescope, reveal very warm gas. The visible light (green) radiation was detected using the European Southern Observatoryís New Technology Telescope. (Courtesy of NASA/JPL-Caltech/ESA/MPIA.)
Fig. 10.5 . Clusters of relatively young stars, about one million years old, are found throughout this infrared image of the North America Nebula, also designated as NGC 7000. The infrared detectors aboard the Spitzer Space Telescope have penetrated the dark clouds seen in optically visible light, viewing young stars in many stages of formation, including gas and dust cocoons, disks and jets. The North America Nebula is about 1,500 light-years from Earth and spans about 50 light-years. (Courtesy of NASA/JPL-Caltech.)
Fig. 10.6 . The collapse of an interstellar cloud of gas and dust (left), compresses the cloud and heats it (right). When the cloud shrinks, gravitational potential energy is converted into heat as the gas particles fall inward and collide with each other. This also produces radiation that can carry off some of the energy. The velocities of the gas atoms are denoted by arrows that point in the direction of atomic motion and have lengths that increase with the speed of motion. Higher speeds occur in the compressed cloud, where the gas atoms move faster and in all directions.
Fig. 10.7 . Evolutionary tracks of protostars of various masses in the Hertzsprung-Russell diagram, ending with their arrival on the main sequence when stars have begun burning hydrogen in their core. The absolute luminosity, L, is given in units of the Sunís absolute luminosity, denoted L?. The star mass is given in units of the Sunís mass, designated M?. The mass values are specified along the main sequence, from upper left to lower right. High-mass stars, which have greater luminosity than low-mass stars, are found at higher points on the main sequence and take a shorter time to arrive there. The protostar lifetimes are given above the relevant track. During star formation, transportation of a protostarís internal energy is dominated by either radiation (horizontal lines) or by convection (vertical lines). Stars with lower mass end up having larger convective zones inside.
Fig. 10.8 . Instruments aboard the Hubble Space Telescope have obtained these images of the visible starlight reflected from thick disks of dust around two young stars that might still be in the process of forming planets. Viewed nearly face on, the debris disk surrounding the Sun-like star known as HD 107146 (right) has an empty center large enough to contain the orbits of the planets in our solar system. Seen edge-on, the dust disk around the reddish dwarf star known as AU Microscopii (left) has a similar cleared-out space in the middle. HD 107146 is 88 light-years away, and is thought to be between 50 million and 250 million years old, while AU Microscopii is located 32 light-years away and is estimated to be just 12 million years old. [Courtesy of NASA/ESA/STScI/JPL/David Ardila Ė JHU (right), and John Krist Ė STScI/JPL (left).]
Fig. 10.9 . An exoplanetís orbital motion, denoted by the white elliptical line, was imaged from an adaptive optics instrument attached to the Very Large Telescope in Chile. The small white spot at the center shows the location of the host star, Beta Pictoris. Observations in 2003 are at the left side of the planetís orbital ellipse and those in 2009 are on the right side. The larger dust disc surrounding the host star is also shown by the large flattened blue image at the left and right. (Courtesy of ESO/A. M. Lagrange.)
Fig. 10.10 . An unseen planet exerts a gravitational force on its visible host star. This force tugs the star in a circular or oval path, which mirrors in miniature the planetís orbit. As the star moves along this path, it approaches and recedes from Earth, changing the wavelength of the starlight seen from Earth through the Doppler effect. When the planet pulls the star toward us, its light waves pile up in front of it slightly, shortening or ďblueshiftingĒ the wavelength we detect. When the planet pulls the star away from us, we detect light waves that are stretched or redshifted. During successive planet orbits, the starís spectral lines are periodically shortened and lengthened, revealing the presence of the planet orbiting the star, even though we cannot see the planet directly.
Fig. 10.11 . Discovery data for the first planet found orbiting a normal star other than the Sun. The giant, unseen planet is revolving around the solar-type star 51 Pegasi, located 50 light-years away. The radial velocity of the star, in units of meters per second, designated m s-1, has been measured from the Doppler shift of the starís spectral lines. The velocity exhibits a sinusoidal variation with a 4.23-day period, caused by the invisible planetary companion that orbits 51 Pegasi with this period. The observational data (solid dots) are fit with the solid line, whose amplitude implies that the mass of the companion is roughly 0.46 times the mass of Jupiter. The 4.23-day period indicates that the unseen planet is orbiting 51 Pegasi at a distance of 0.05 AU, where 1.00 AU is the mean distance between the Earth and the Sun. [Adapted from Michael Mayor and Didier Queloz, A Jupiter-mass companion to a solar-type star, Nature 378, 355-359 (1995).]