From Black Clouds to White Dwarfs

Along with stars, stellar remnants, and planets, spiral galaxies like the Milky Way are populated by vast dusty clouds of molecular hydrogen. These may extend over an area of many parsecs. Gravitational interactions within such clouds cause clumping, and clumps that become dense enough may eventually collapse to form newborn stars. The image above shows the Coalsack, a dark cloud located at a distance of about 150 parsecs (489 light years), just beyond the bright stars of the constellation Crux. Infrared observations of the Coalsack reveal a complex filamentary structure that extends for about 15 parsecs (50 light years), weighing in at about 3500 times the mass of our Sun (Lada et al. 2004). Although it is currently empty of stars, the Coalsack may represent the earliest phase in the evolution of stars and planetary systems. Coalsack photographed by Yuri Beletsky. Image credit: NASA/courtesy of nasaimages.org

Stars form in clusters through the collapse of dense clouds of molecular hydrogen. Most newborn clusters contain at least 100 stars at a wide range of masses and spectral types (Lada & Lada 2003). The image above represents the Iris Nebula (NGC 7023), a newborn cluster still embedded in its native cloud, located at a distance of about 300 parsecs (975 light years) in the constellation Cepheus. The central region shown here is less than 2 parsecs in diameter; the associated cloud complex contains about 730 Solar masses (Kirk et al. 2009). At the heart of the cluster is its brightest and most massive star, HD 200775, a pre-main sequence ("Herbig AeBe") binary that is evolving into spectral type A or B (Kun et al. 2008). Stellar winds and radiation from this binary system are dispersing the remaining gas, a process that requires only a few million years. The newly formed stars will then pursue their own orbits around the Galactic Core in the main sequence phase of their evolution. Image credit: Wikimedia, courtesy Hunter Wilson

Perhaps 80% of young stars are surrounded by dusty clouds of gas, variously known as nebulae, protoplanetary disks, or proplyds (Haisch et al. 2001). These clouds rapidly flatten into extended rotating disks and disperse on time scales of 1 to 10 million years. In most disks, solids condense out of the gases and begin to assemble rocky or icy bodies. The viscous gobs in this photograph are proplyds surrounding newborn stars in the Orion Nebula. Image credit: NASA/C.R. O'Dell/Rice University

The proplyd flattens as it spins, and the central star begins to shine through the dust. The outer regions of the nebula cool first, with volatiles freezing out of the gases as the ice line moves inward. Image credit: NASA/JPL-Caltech.

As the primordial nebula flattens into a disk, solids begin clumping together, first as pebbles or snowballs and eventually as planetesimals, which are rocky or icy bodies a kilometer or more in diameter. This artist's impression depicts a protoplanetary disk at the epoch when planetesimals begin to form. Image credit: NASA/JPL-Caltech

Planetesimals collide as their orbits cross, forming larger and larger objects. When the resulting planetary embryos reach a minimum mass several times that of Earth, they rapidly accrete gas from the protoplanetary disk. This phase of "runaway growth" opens a gap in nebula and breaks it into rings. Planetary cores that manage to accrete large quantities of gas and achieve stable orbits evolve into gas giant planets. Image credit: NASA/JPL-Caltech.

Eventually most of the original disk is dispersed, with fields of dust and debris remaining in belts beyond the influence of planetary perturbations. Debris rings analogous to the Asteroid and Kuiper Belts have been detected in many estrasolar systems. Image credit: T. Pyle/SSC/NASA/JPL-Caltech.

Under favorable conditions the evolutionary process results in a system of stable planets. Depending on the mass and spectral type of the host star, a planetary system can endure anywhere from several hundred million years (for hot, massive A-type stars) to several hundred billion years (for cool, low mass M-type stars). Artist's view of the 55 Cancri system, courtesy NASA/JPL-Caltech

Given an appropriate planetary environment, the high luminosity and long lifetimes of Sun-like stars (spectral types F, G, and K) provide the conditions necessary for the evolution of carbon-based life such as we find on Earth. Simon Taylor photographed this forest of tree ferns in New Zealand. Image credit: Simon Taylor / Creative Commons

When a Sun-like star burns through all the hydrogen in its core, it begins a new evolutionary phase. Nuclear fusion consumes first the hydrogen in its outer layers and then the helium in its core. Its diameter expands by a factor of 10 or more as it radiates far more heat and light. The star is now said to have left the main sequence and entered the giant or red giant phase. Among the brightest red giant stars in the Solar neighborhood are Arcturus and Aldebaran. The image above depicts such a star as seen from a desolate planetary companion. Image credit: Jeff Bryant

Eventually, when the star has exhausted all its nuclear fuel, it throws off mass in a series of explosive contractions. In some cases the result is a roughly spherical gas cloud known as a planetary nebula surrounding the stellar core. One of the best-known examples of such an object is the Cat's Eye Nebula, shown above in a photograph by the Hubble Space Telescope (courtesy NASA/STScI). The distance to the Cat's Eye Nebula has recently been calculated as approximately 1000 parsecs (3,260 light years). Planetary nebulae are extremely short-lived phenomena, at least in astronomical terms, since they dissipate in a matter of millennia to leave behind a white dwarf.

The image above shows the white dwarf G29-38, which is surrounded by a debris ring created by the disintegration of one of its surviving comets or asteroids. G29-38 is located at distance of 13.6 parsecs (44 light years) in the direction of Pisces (Debes et al. 2005). Image credit: NASA/JPL-Caltech