Protostars

Protostars are stars in the process of forming. Planets are a natural by-product of the formation of stars, so protostars are most likely to be forming both stars and attendant planetary systems simultaneously. Understanding how other stars and planets form provides insight into how our own solar system formed.

Overview

In the Northern Hemisphere, the Orion dominates the early evening sky. If one scans the familiar sword and belt with binoculars or a small telescope, the center star in the sword looks like a small, fuzzy patch that resembles a piece of lint on the lens. This patch is actually the famous Orion and is the closest and, therefore, best-studied star-forming region in the sky. More powerful telescopes reveal many similar nebulae where star formation is currently taking place.

Other stars and our own Sun formed by similar processes. A nebula, which is a cloud of gas and dust in interstellar space, begins to collapse. Once something triggers the collapse, the nebula’s own gravity takes over to continue the collapse. As the cloud collapses, it begins to spin more rapidly, like a figure skater bringing in her arms. The rapid spinning causes the cloud to flatten into a disk, like pizza dough, when it is tossed spinning into the air. Most of the gas and dust in the nebula collapses into the protostar, which will become the central star. The leftover gas and dust in the surrounding disk will coalesce into planets orbiting the star. This process, known as the of solar-system formation, is the most widely accepted explanation of the origins of our solar system. In this way, planets naturally form out of the debris surrounding the protostar. Therefore, astronomers expect that most stars should be surrounded by an entourage of planets.

Astronomers can begin to understand the details of this process by studying the Orion Nebula and similar nebulae. Beginning in the 1950s and 1960s, new regimes, notably radio and infrared, opened up to astronomers. Astronomers were now able to probe the dusty depths of star-forming nebulae by analyzing their radio and infrared emissions. Formerly, these nebulae were “invisible” to astronomers because the dust found in these clouds blocks visible light; however, the nebular dust tends to allow more infrared and radio waves to get through, allowing the emissions to be collected by instruments sensitive to these regions of the electromagnetic spectrum.

When astronomers find a new tool to probe star-forming regions, they usually try using it first on the Orion Nebula and then on other star-forming regions. At infrared wavelengths, astronomers found that the Orion Nebula contains several sources of infrared light. Similar infrared sources are very common in other nebulae associated with star formation. The infrared signature of these sources tells astronomers that the sources are warm but not hot. The dusty cocoon out of which the star is forming still surrounds the protostar. The dust blocks the direct light from the protostar, but the energy from the heats the dusty shell enough that it glows in the infrared. Hence, infrared sources in star-forming nebulae are signatures of a protostar that is still enshrouded in a dusty cocoon.

At radio wavelengths, astronomers found both giant and H II regions associated with star-forming nebulae. The symbol H II is from a notation used by astronomers to indicate that the hydrogen atoms have lost their electrons and are, therefore, ionized. H II regions are regions of ionized rather than neutral hydrogen. H II regions can surround newly formed stars if the stars are hot enough to emit significant amounts of ultraviolet light. The stars falling into this category are massive O and B stars. As the expands around these massive hot stars, the leftover material from the protostar stage is swept away. Hence, H II regions allow massive protostars to get rid of their leftover cocoons of gas and dust.

Molecular clouds are also often associated with protostars. Typically, they contain a few hundred thousand protostars, but giant molecular clouds can contain as much as ten million times the mass of the Sun and extend for hundreds of light-years. They are clouds of molecules in interstellar space. Astronomers think that the molecules form on the surfaces of interstellar dust grains and then escape the dust grains. More than a hundred different types of molecules have been identified in molecular clouds. The first stage in star formation is often a clump starting to collapse within a giant molecular cloud. These clumps will typically form into a small group of half a dozen to a dozen protostars. As these protostars collapse, they heat up enough to ignite nuclear reactions in their cores. They then become stars and warm the surrounding dusty cocoon so astronomers see infrared sources. If the stars are massive stars, they emit enough ultraviolet light to form H II regions. In addition to clearing away the leftover material, the expanding H II regions send a into the giant molecular cloud and trigger the collapse of a new batch of protostars.

Massive stars produce H II regions to clear away the protostellar cocoons of leftover material, but how can less massive stars shed their cocoons? One way is the T-Tauri phase. T-Tauri stars have very strong stellar winds, which are similar to the but much stronger. Younger T-Tauri stars still have shells of surrounding material, while older (often called “naked”) T-Tauri stars do not. If a less massive protostar goes through a T-Tauri stage, the strong stellar winds can blow away its shell of leftover material.

The Hubble has helped astronomers understand other ways less massive protostars can rid themselves of their shells of leftover material. Close-up photographs of the Orion Nebula made with the Hubble Telescope reveal protoplanetary disks (or proplyds for short). These proplyds look like small, dusty disks. The protostars are collapsing and spinning quickly, so they flatten into a disk. The central regions of these proplyds will become the stars, and the disks will coalesce into the planets. Nearby newly formed O and B stars can ionize the remaining hydrogen in these proplyds.

In 1995, the Hubble Space Telescope took a high-resolution photograph of the Eagle Nebula, also known as M16. This strikingly beautiful photograph shows a large number of what astronomers call evaporative gaseous globules (EGGs). These EGGs are most likely the same thing as proplyds but seen from a different angle. In M16, there is a cluster of young hot stars just above the EGGs. Ultraviolet light from these stars helps rid the low-mass protostars forming in the EGGs of their excess material in a manner similar to the way massive protostars do with their own ultraviolet light. Other Hubble Space Telescope pictures of star-forming nebulae show similar phenomena, so it is believed that nearby massive stars provide the ultraviolet light that less massive stars need to shed their protostellar cocoons.

Knowledge Gained

The earliest studies of star-forming regions, such as the Orion Nebula and similar nebulae, were at visible wavelengths only. Protostars are still enshrouded in the dust out of which they formed, and this dust blocks visible light. Hence, it is difficult to learn much about protostars by studying them only with visible light. A probe that penetrates dust is needed. The advent of radio and infrared provided the needed probe. Infrared light and radio waves have wavelengths that are longer than typical interstellar dust grains. Hence, the dust grains do not block them as they do visible light. In the late 1960s, when these tools became readily available to the astronomical community, the study of protostars and star formation blossomed.

Most astronomers thought that molecules could not form in interstellar space, but the discovery of giant molecular clouds with radio telescopes proved them wrong. Because giant molecular clouds are the site of the initial stages of star formation, their discovery helped astronomers understand protostars. The study of radio H II regions also helped astronomers understand how very hot massive stars rid themselves of the gas and dust shells left over from their formation.

Infrared astronomy also gave astronomers important clues for understanding protostars. Just as red stars are cooler than blue stars, objects that are brightest at infrared wavelengths are cooler than stars. Dust shells surrounding protostars are warm but not as hot as stars, so helped astronomers find stars and protostars in these early stages.

The launch of the Hubble Space Telescope gave astronomers the ability to take pictures of unprecedented resolution. High-resolution photographs of the Orion Nebula, M16, and other star-forming regions gave astronomers direct views of proplyds and EGGs, which are solar systems in the process of forming. A next-generation space telescope, the James T. Webb Space Telescope, produced similar dramatic advances.

Astronomers have long thought that the planets in our solar system formed from the material left over from the Sun’s formation and that planets are a natural by-product of star formation. The discovery of proplyds and EGGs supports this theory, and if this theory is correct, planets around other stars should be very common. In 1995, the discovery of the first extrasolar planet was announced. Since that time, over 5,000 extrasolar planets have been detected. The discovery of these extrasolar planets helps confirm astronomers’ ideas that the solar system and other stars formed by the same process.

Context

One of the questions that virtually all young children ask is, “Where did I come from?” Parents usually interpret this question in terms of reproductive biology. However, at a much deeper level, this question concerns origins: the origin of life, the origin of Earth, the origin of the solar system, and the origin of the universe. Studying protostars helps us understand the origin of stars and, because the Sun is a star, the origin of the solar system that includes Earth. Hence, studying protostars and star formation relates to the universal human quest to understand where we came from. The study of origins has been a strong focus for the National and Space Administration (NASA).

Because protostars are one of the earliest stages in the life cycle of stars, studying protostars is part of the larger study of stellar evolution. To completely understand the entire life cycle of stars, it is necessary to understand each of the stages a star goes through, including the protostar stage. Only the lightest elements on the periodic table were made during the Big Bang. The carbon, oxygen, and other atoms important to life were made later in stars and blasted back into space to be recycled by the next generation of protostars. Hence, the study of , in general, relates to the question of human origins and the origin of the atoms in our bodies.

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