11.1: Stellar Origins

Star Formation

There are specific terms in biology, which deal with life: birth, growing up, getting old, death, and even evolution. For the sake of convenience and familiarity, astronomers use similar terminology when dealing with stars. Note that stars are not alive! These are simply terms of familiarity. As we begin our exploration of how stars are formed, let’s review some basics about stars discussed in earlier chapters:

• Main-sequence stars such as our Sun maintain equilibrium by producing energy through nuclear fusion in their cores. The ability to generate energy by fusion defines a star.
• Each second in the Sun, approximately 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning into energy in the process. This rate of hydrogen use means that eventually the Sun, and all other stars, will run out of central fuel.
• Stars come with many different masses, ranging from 1/12 M_Sun to roughly 100–200 M_Sun. There are far more low-mass than high-mass stars.
• The most massive main-sequence stars, spectral type O, are also the most luminous and have the highest surface temperature. The lowest-mass stars on the main sequence, spectral type M or L, are the least luminous and the coolest.
• A galaxy of stars such as the Milky Way contains enormous amounts of gas and dust, enough to make billions of stars like the Sun.

If we want to find stars still in the process of formation, we must look in places that have plenty of the raw material from which stars are assembled. Since stars are made of gas, we focus our attention on the dense and cold clouds of gas and dust that dot the Milky Way .

Molecular Clouds: Stellar Nurseries

The most massive reservoirs of interstellar matter are molecular clouds. These clouds have cold interiors with characteristic temperatures of only 10–20 K Most of their gas atoms are bound into molecules. These clouds turn out to be the birthplaces of most stars in our Galaxy.

The masses of molecular clouds range from a thousand times the mass of the Sun to about 3 million solar masses, or M_Sun. Molecular clouds have a complex filamentary structure, similar to cirrus clouds in Earth’s atmosphere, but much less dense. The molecular cloud filaments can be up to 1000 light-years long. Within the clouds are cold, dense regions called clumps, with typical masses of 50 to 500 times the mass of the Sun. Within these clumps, there are even denser, smaller regions called cores. The cores are the embryos of stars. The conditions in cores, low temperature and high density, are just what is required to make stars. Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The force of gravity, pulling inward, tries to make a star collapse. Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. When a star is first forming, low temperature and low pressure and high density both work to give gravity the advantage. In order to form a star, we need a typical core of interstellar atoms and molecules to shrink in radius and increase in density by a factor of nearly 10²⁰.

Figure \(\PageIndex{1}\): Dust and Dense Globules in M16. These Hubble Space Telescope images of M16, the Eagle Nebula, includes huge columns of cool gas and dust. The very dense globules contain areas where stars are forming. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\) includes two Hubble Space Telescope images of M16, the Eagle Nebula. In image a there are huge columns of cool gas and dust that are of higher density than the surrounding regions. The tallest pillar is about four light-years from top to bottom, and the M16 region is about 7000 light-years away from us. Image b is a close-up view of one of the pillars with some very dense globules, many of which harbor embryonic stars. Astronomers coined the term evaporating gas globules (EGGs) for these structures, in part so that they could say we found EGGs inside the Eagle Nebula.

The Orion Molecular Cloud

One of the best-studied stellar nurseries is in the constellation of Orion, The Hunter, about 1500 light-years away, Figure \(\PageIndex{2}\). In image a, the pattern of the hunter is a roughly human figure with a “belt” of three stars that mark his waist. The Horsehead Nebula is near the leftmost star of Orion's belt. The ancients imagined a sword hanging from the belt. The object near the end of this sword is the Orion Nebula.

Figure \(\PageIndex{2}\): Orion is an asterism named after the hunter in Greek mythology. The infrared image demonstrates the large size of the Orion molecular cloud, including dense regions near Orion's belt and sword. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{2}\).

The Orion molecular cloud is much larger than the star pattern and is truly an impressive structure. In its long dimension, it stretches over a distance of about 100 light-years. The total quantity of molecular gas is about 200,000 times the mass of the Sun. Most of the cloud does not glow with visible light but betrays its presence by the radiation that the dusty gas gives off at infrared and radio wavelengths. In Figure \(\PageIndex{2}\), image b is an infrared view of the same area in Image a collected by the Infrared Astronomical Satellite. Heated dust clouds dominate in this false-color image, and many of the stars in image a are now invisible. An exception is the cool, red-giant star Betelgeuse, which is a yellowish point near Orion’s left armpit. The large, yellow ring to the right of Betelgeuse is the remnant of an exploded star. The infrared image lets us see how large and full of cooler material the Orion molecular cloud really is. The Horsehead and Orion nebulae are much larger and brighter in the infrared image compared to the visible image.

The stars in Orion’s belt are typically about 5 million years old, whereas the stars near the middle of the “sword” hanging from Orion’s belt are only 300,000 to 1 million years old. The Orion Nebula, at the end of the sword, is where star formation is still taking place. About 2200 young stars are found in this region, which is only slightly larger than a dozen light-years in diameter. In Figure \(\PageIndex{3}\), image a is the Orion Nebula in visible light. In image b, with near-infrared radiation, astronomers detect more detail within the dusty nebula and more than 2000 stars.

Figure \(\PageIndex{3}\): The visible light in Image (a) of the Orion nebula reveals its structure, but infrared light in Image (b) can reveal significantly more details. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{3}\).

The Orion Nebula also contains a tight cluster of stars called the Trapezium. The Trapezium cluster includes four very bright stars that provide much of the energy that causes the nebula to glow so brightly. In Figure \(\PageIndex{4}\), image a is a visible light image of the four bright stars of Trapezium. In image b, infrared light reveals many more stars which are completely hidden by dust in the visible image. Studies of Orion and other star-forming regions show that star formation is not a very efficient process. In the region of the Orion Nebula, about 1% of the material in the cloud has been turned into stars. That is why we still see a substantial amount of gas and dust near the Trapezium stars. The leftover material is eventually heated, either by the radiation and winds from the hot stars that form or by explosions of the most massive stars.

Figure \(\PageIndex{4}\): The four stars in the visible image (a) make up the Trapezium cluster. The same image in the infrared reveals many more stars surrounding Trapezium. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{4}\).

Whether gently or explosively, the material in the neighborhood of the new stars is blown away into interstellar space. Older groups or clusters of stars can now be easily observed in visible light because they are no longer shrouded in dust and gas. In Figure \(\PageIndex{5}\), the young cluster of stars known as Westerlund 2, which formed within the Carina star-forming region about 2 million years ago. Stellar winds and pressure produced by the radiation from the hot stars within the cluster are blowing and sculpting the surrounding gas and dust. The nebula still contains many globules of dust. Stars are continuing to form within the denser globules and pillars of the nebula. This Hubble Space Telescope image includes near-infrared exposures of the star cluster and visible-light observations of the surrounding nebula. Colors in the nebula are dominated by the red glow of hydrogen gas, and blue-green emissions from glowing oxygen.

Figure \(\PageIndex{5}\): This Hubble image includes new stars in Westerlund 2 that are no longer surrounded by dust. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{5}\).

Although we do not know what initially caused stars to begin forming in Orion, there is good evidence that the first generation of stars triggered the formation of additional stars, which in turn led to the formation of still more stars, as depicted in Figure \(\PageIndex{5}\). Star formation can move progressively through a molecular cloud. The oldest group of stars lies to the left of the diagram and has expanded because of the motions of individual stars. Eventually, the stars in the group will disperse and no longer be recognizable as a cluster. The youngest group of stars lies to the right, next to the molecular cloud. This group of stars is only 1 to 2 million years old. The pressure of the hot, ionized gas surrounding these stars compresses the material in the nearby edge of the molecular cloud and initiates the gravitational collapse that will lead to the formation of more stars.

Figure \(\PageIndex{6}\): Propagating Star Formation. As new stars form, they are surrounded by hot, ionized gases that compress regions of the molecular cloud leading to the formation of more stars. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{6}\).

The basic idea of triggered star formation is this: when a massive star is formed, it emits a large amount of ultraviolet radiation and ejects high-speed gas in the form of a stellar wind. This injection of energy heats the gas around the stars and causes it to expand. When massive stars exhaust their supply of fuel, they explode, and the energy of the explosion also heats the gas. The hot gases pile into the surrounding cold molecular cloud, compressing the material in it and increasing its density. If this increase in density is large enough, gravity will overcome pressure, and stars will begin to form in the compressed gas. Such a chain reaction, where the brightest and hottest stars of one area become the cause of star formation “next door,” seems to have occurred not only in Orion but also in many other molecular clouds.

There are many molecular clouds that form mostly low-mass stars. Because low-mass stars do not have strong winds and do not die by exploding, triggered star formation cannot occur in these clouds. There are also stars that form in relative isolation in small cores. Therefore, not all star formation is originally triggered by the death of massive stars. However, there are likely to be other possible triggers, such as spiral density waves and other processes we do not yet understand.

The Birth of a Star

So how are stars ‘born’? That is, what are the steps to a stellar birth? First, hydrogen, helium, and dust, the interstellar medium has to accumulate. The interstellar medium starts to collapse due to gravity; these various atoms and dust particles attract each other, forming larger and larger clumps, attracting more gas and dust. As the interstellar medium collapses, the density, pressure, and temperature within the interstellar medium increase. As the density increases, more particles are striking each other, generating more energy and heat.

Although regions such as Orion give us clues about how star formation begins, the subsequent stages are still shrouded in mystery. There is an enormous difference between the density of a molecular cloud core and the density of the youngest stars that can be detected. Direct observations of this collapse to higher density are nearly impossible for two reasons. First, the dust-shrouded interiors of molecular clouds where stellar births take place cannot be observed with visible light. Second, the timescale for the initial collapse, thousands of years, is very short, astronomically speaking. Since each star spends such a tiny fraction of its life in this stage, relatively few stars are going through the collapse process at any given time. Nevertheless, through a combination of theoretical calculations and the limited observations available, astronomers have pieced together a picture of what the earliest stages of stellar evolution are likely to be.

The first step in the process of creating stars is the formation of dense cores within a clump of gas and dust, Figure \(\PageIndex{7}\). It is generally thought that all the material for the star comes from the core. Part (a) of Figure \(\PageIndex{7}\) is a core that is 5000 AU across. Part b is when the gravitational force of the infalling gas becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores. The material then undergoes a rapid collapse, and the density of the core increases greatly as a result. During the time a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostar.

Figure \(\PageIndex{7}\): Formation of a Star. This diagram demonstrates the four stages of star formation. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{7}\).

The natural turbulence inside a clump tends to give any portion of it some initial spinning motion. As a result, each collapsing core is expected to spin. According to the law of conservation of angular momentum, a rotating body spins more rapidly as it decreases in size. In other words, if the object can turn its material around a smaller circle, it can move that material more quickly, like a figure skater spinning more rapidly as she brings her arms in tight to her body. This is exactly what happens when a core contracts to form a protostar: as it shrinks, its rate of spin increases.

But all directions on a spinning sphere are not created equal. As the protostar rotates, it is much easier for material to fall onto the poles, which spin most slowly, than onto the equator, where material moves around most rapidly. Therefore, gas and dust near the protostar’s equator do not fall into the star due to the rotation and form an extended disk around the equator, Figure \(\PageIndex{7}\) part (b). However, gas and dust can fall onto the protostar easily from directions away from the star’s equator. At the same time, stellar winds break out but are confined by the disk to flow out along the two poles of the star, Figure \(\PageIndex{7}\) part (c).

The protostar and disk at this stage are embedded in an envelope of dust and gas from which material is still falling onto the protostar. This dusty envelope blocks visible light, but infrared radiation can get through. As a result, in this phase of its evolution, the protostar itself is emitting infrared radiation and so is observable only in the infrared region of the spectrum. Once almost all of the available material has been accreted and the central protostar has reached nearly its final mass, it is given a special name: it is called a T Tauri star, named after one of the best studied and brightest members of this class of stars, which was discovered in the constellation of Taurus. (Only stars with masses less than or similar to the mass of the Sun become T Tauri stars. Massive stars do not go through this stage, although they do appear to follow the formation scenario of smaller stars.

Eventually, wind sweeps away the cloud material and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes observable, as in Figure \(\PageIndex{7}\) part (d). These sketches are not drawn to the same scale. The typical diameter of the disk around a new star is about 100 AU or slightly larger than the diameter of the orbit of Pluto.

Winds and Jets

Recent observations suggest that T Tauri stars may actually be stars in a middle stage between protostars and hydrogen-fusing stars such as the Sun. High-resolution infrared images have revealed jets of material as well as stellar winds coming from some T Tauri stars, proof of interaction with their environment. The stellar wind consists mainly of protons, or hydrogen nuclei, and electrons streaming away from the star at speeds of a few hundred kilometers per second. When the wind first starts up, the disk of material around the star’s equator blocks the wind in its direction. Where the wind particles can escape most effectively is in the direction of the star’s poles.

Astronomers have actually seen evidence of these beams of particles shooting out in opposite directions from the polar regions of newly formed stars. In many cases, these beams point back to the location of a protostar that is still so completely shrouded in dust that we cannot yet see it. Figure \(\PageIndex{8}\) includes images of HH 34 a Herbig-Haro (HH) object. The star is about 450 light-years away and only about 1 million years old. Light from the star itself is blocked by a disk, which is larger than 60 billion kilometers in diameter and is almost edge-on as observed from Earth. Jets emerge perpendicular to the disk. The material in these jets is flowing outward at speeds up to 580,000 kilometers per hour. The series of three images shows changes during a period of 5 years. Every few months, a compact clump of gas is ejected, and its motion outward can be followed. The changes in the brightness of the disk may be due to motions of clouds within the disk that alternately block some of the light and then let it through. This image corresponds to the stage in the life of a protostar shown in part (c) of Figure \(\PageIndex{7}\).

Figure \(\PageIndex{8}\): Gas Jets. Gas jets flowing out of HH34 from 1995 to 2000. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{8}\).

On occasion, the jets of high-speed particles streaming away from the protostar collide with a somewhat-denser lump of gas nearby, excite its atoms, and cause them to emit light. Herbig-Haro (HH) objects allow us to trace the progress of the jet to a distance of a light-year or more from the star that produced it. Figure \(\PageIndex{9}\) includes two spectacular images of HH objects. These Hubble Space Telescope images include jets flowing outward from newly formed stars. In the HH47 image, a protostar 1500 light-years away, invisible inside a dust disk at the left edge of the image, produces a very complicated jet. The star may actually be wobbling, perhaps because it has a companion. Light from the star illuminates the white region at the left because light can emerge perpendicular to the disk, along with the jet. At right, the jet is plowing into existing clumps of interstellar gas, producing a shock wave that resembles an arrowhead. The image of HH1 and HH2 includes a double-beam jet emanating from a protostar, hidden in a dust disk in the center, in the constellation of Orion. These jets are more than 1 light-year long. The bright regions are places where the jet is a slamming into a clump of interstellar gas and causing it to glow.

Figure \(\PageIndex{9}\): Jets of gas up to 1 light year long flow from Herbig Haro objects 47, 1, and 2. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{9}\).

The wind from a forming star will ultimately sweep away the material that remains in the obscuring envelope of dust and gas, leaving behind the disk and protostar, which can then be seen with visible light. We should note that at this point, the protostar itself is still contracting slowly and has not yet reached the main-sequence stage on the H–R diagram. The disk can be detected directly when observed at infrared wavelengths or when it is seen silhouetted against a bright background Figure \(\PageIndex{10}\). These Hubble Space Telescope infrared images include disks around young stars in the constellation of Taurus, in a region about 450 light-years away. In some cases, the stars are visible. In other cases, the dark, horizontal bands indicate regions where the dust disk is so thick that even infrared radiation from the star embedded within it cannot make its way through. The brightly glowing regions are starlight reflected from the upper and lower surfaces of the disk, which are less dense than the central, dark regions. 

Figure \(\PageIndex{10}\): Disks around Protostars. Images of multiple protostars demonstrate that as stars form, they are surrounded by a disk of gas and dust. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{10}\).

This description of a protostar surrounded by a rotating disk of gas and dust sounds very much like what happened in the Solar System when the Sun and planets formed. Indeed, one of the most important discoveries from the study of star formation in the last decade of the twentieth century was that disks are an inevitable byproduct of the process of creating stars.

To keep things simple, we have described the formation of single stars. Many stars, however, are members of binary or triple systems, where several stars are born together. In this case, the stars form in nearly the same way. Widely separated binaries may each have their own disk; close binaries may share a single disk.

Further Exploration: Interactive Activity

To better understand the structure and star-forming regions of the Orion Nebula, watch Hubble: Journey Through the Orion Nebula (video by Toney Burkhart). The visualization provides a virtual tour through one of the nearest and most actively studied stellar nurseries in the Milky Way.