1. PSCI 1414 General Astronomy
The Birth of Stars
Part 1: Nebulae, Protostars, and the Main Sequence
Alexander C. Spahn

2. Nebulae and the Interstellar Medium
At first glance, the space between the stars seems to be empty. On closer inspection, we find that it is filled with a thin gas laced with microscopic dust particles. This combination of gas and dust is called the interstellar medium. Any interstellar cloud is called a nebula (plural nebulae) or nebulosity.

3. Nebulae and the Interstellar Medium
You can see evidence for the interstellar medium with the naked eye. Look carefully at the constellation Orion.
While most stars in the constellation appear as sharply defined points of light, the middle “star” in Orion’s sword has a fuzzy appearance.
This “star” is actually not a star at all, but the Orion Nebula—a cloud in interstellar space.

4. Emission NebulaE
The Orion Nebula emits its own light, with the characteristic emission line spectrum of a hot, thin gas. For this reason it is called an emission nebula. Typical emission nebulae have masses that range from about 100 to about 10,000 solar masses.

5. Emission NebulaE
Emission nebulae are found near hot, luminous stars of spectral types O and B. Such stars emit copious amounts of ultraviolet radiation. When atoms in the nearby interstellar gas absorb these energetic ultraviolet photons, the atoms become ionized. Indeed, emission nebulae are composed primarily of ionized hydrogen atoms, that is, free protons (hydrogen nuclei) and electrons.

6. Emission NebulaE
Astronomers use the notation H I for neutral, un- ionized hydrogen atoms and H II for ionized hydrogen atoms, which is why emission nebulae are also called H II regions. H II regions emit red visible light when some of the free protons and electrons get back together to form hydrogen atoms, a process called recombination.

7. Dark Nebulae
There exist larger bits of matter, called dust grains, in the interstellar medium. These dust grains make their appearance in dark nebulae and reflection nebulae. A dark nebula is so opaque that it blocks any visible light coming from stars that lie behind it.

8. Dark Nebulae
These nebulae have a relatively dense concentration of microscopic dust grains, which scatter and absorb light much more efficiently than single atoms. dark nebulae are typically many light-years deep such that they block the passage of light. In the same way, a sufficient depth of haze or smoke in our atmosphere can make it impossible to see distant mountains.

9. Reflection Nebulae
The other evidence for dust in is the bluish haze surrounding some stars and sets of stars. A haze of this kind, called a reflection nebula, is caused by fine grains of dust in a lower concentration than that found in dark nebulae. The light we see coming from the nebula is starlight that has been scattered and reflected by these dust grains.

10. Reflection Nebulae
The grains are only about 500 nm across, no larger than a typical wavelength of visible light, and they scatter short-wavelength blue light more efficiently than long-wavelength red light. Hence, reflection nebulae have a characteristic blue color. Box 5-4 explains how a similar process—the scattering of sunlight in our atmosphere—gives rise to the blue color of the sky.

11. Protostars
In order for interstellar material to condense and form a star, the force of gravity—which tends to draw interstellar material together—must overwhelm the internal pressure pushing the material apart. This means that stars will most easily form in regions where the interstellar material is relatively dense, so that atoms and dust grains are close together and gravitational attraction is enhanced.

12. Protostars
To assist star formation, the pressure of the interstellar medium should be relatively low. This means that the star-forming region of the interstellar medium should be as cold as possible, because the pressure of a gas goes down as the gas temperature decreases. The only parts of the interstellar medium with high enough density and low enough temperature for stars to form are the dark nebulae.

13. Protostars
Other relatively small, nearly spherical dark nebulae are known as Bok globules, after the Dutch-American astronomer Bart Bok, who first called attention to them during the 1940s. A Bok globule resembles the inner core of a dark nebula with the outer, less dense portions stripped away. The density of the gas and dust within a Bok globule is indeed quite high, in the range from 100 to 10,000 particles per cm3. By comparison, most of the interstellar medium contains only 0.1 to 20 particles per cm3.

14. Protostars
A typical Bok globule is about one-tenth as large as a dark nebula. The chemical composition of this material is the standard “cosmic abundance” of about 74% hydrogen, 25% helium, and 1% heavier elements. Within these clouds, the densest portions can contract under their own mutual gravitational attraction and form clumps called protostars. Because dark nebulae contain many solar masses of material, it is possible for a large number of protostars to form out of a single nebula. Thus, we can think of dark nebulae as “stellar nurseries.”

15. The Evolution of a Protostar
At first, a protostar is merely a cool blob of gas several times larger than our solar system. The pressure inside the protostar is too low to support all this cool gas against the mutual gravitational attraction of its parts, and so the protostar collapses. As the protostar collapses, gravitational energy is converted into thermal energy, making the gases heat up and start glowing. Energy from the interior of the protostar is transported outward by convection, warming its surface.

16. The Evolution of a Protostar
After only a few thousand years of gravitational contraction, the protostar’s surface temperature reaches 2000 to 3000 K. At this point the protostar is still quite large, so its glowing gases produce substantial luminosity. The radiated energy comes exclusively from the heating of the protostar as it contracts.

17. The evolutionary Track
In order to determine the conditions inside a contracting protostar, astrophysicists use computers to solve equations similar to those used for calculating the structure of the Sun. The results tell how the protostar’s luminosity and surface temperature change at various stages in its contraction. This information, when plotted on a Hertzsprung-Russell diagram, provides a protostar’s evolutionary track. The track shows us how the protostar’s appearance changes because of changes in its interior.

19. Observing a Protostar
Observing the evolution of a protostar can be quite a challenge. The reason is that protostars form within clouds that contain substantial amounts of interstellar dust. The dust in a protostar’s surroundings, called its cocoon nebula, absorbs the vast amounts of visible light emitted by the protostar and makes it very hard to detect using visible wavelengths. Protostars can be seen, however, using infrared wavelengths.

21. A One-Solar-Mass Protostar
For a protostar with the same mass as our Sun (1 M⊙), the outer layers are relatively cool and quite opaque. Due to frequent scattering or absorption, light does not pass easily through an opaque material. This means that energy released from the shrinking inner layers in the form of radiation cannot easily reach the protostar’s surface.

22. A One-Solar-Mass Protostar
Instead, energy flows outward by the slower and less effective method of convection. The result is that for a contracting 1-M⊙ protostar, the surface temperature stays roughly constant, the luminosity decreases as the radius decreases, and the protostar’s evolutionary track initially moves downward on the H-R diagram.

23. A One-Solar-Mass Protostar
Although its surface temperature changes relatively little, the internal temperature of the shrinking protostar increases. After a time, the interior becomes more ionized, which makes it less opaque. Energy is then conveyed outward by radiation in the interior and by convection in the opaque outer layers, just as in the present-day Sun.

24. A One-Solar-Mass Protostar
This makes it easier for energy to escape from the protostar, so the luminosity—the rate at which energy is emitted from the protostar’s surface— increases. As a result, the evolutionary track for a 1- M⊙ protostar bends upward (higher luminosity) and to the left (higher surface temperature, caused by the increased energy flow).

25. A One-Solar-Mass Protostar
In time, the 1-M⊙ protostar’s interior temperature reaches a few million kelvins, hot enough for thermonuclear reactions to begin converting hydrogen into helium. Eventually, these reactions provide enough heat and internal pressure to stop the star’s gravitational contraction, and hydrostatic equilibrium is reached. The protostar’s evolutionary track has now led it to the main sequence, and the protostar has become a full-fledged main-sequence star.

26. High-Mass Protostars
If its mass is more than about 4 M⊙, a protostar contracts and heats more rapidly, and hydrogen fusion begins quite early. As a result, the luminosity quickly stabilizes at nearly its final value, while the surface temperature continues to increase as the star shrinks. Thus, the evolutionary tracks of massive protostars traverse the H-R diagram roughly horizontally.

27. High-Mass Protostars
Greater mass means greater pressure and temperature in the interior, which means that a massive star has an even larger temperature difference between its core and its outer layers than the Sun. This difference causes convection deep in the interior of a massive star. By contrast, a massive star’s outer layers are of such low density that energy flows through them more easily by radiation than by convection.

28. Low-Mass Protostars
The internal structure is also different for main-sequence stars of very low mass. When such a star forms from a protostar, the interior temperature is never high enough to fully ionize the interior. The interior remains too opaque for radiation to flow efficiently, so energy is transported by convection throughout the volume of the star.

29. Main Sequence Mass Limits
The theory of how protostars evolve helps explain why the main sequence has both an upper mass limit and a lower mass limit. Protostars less massive than about 0.08 M⊙ can never develop the necessary pressure and temperature to start hydrogen fusion in their cores. Instead, such “failed stars” end up as brown dwarfs, which shine faintly by Kelvin-Helmholtz contraction.

30. Main Sequence Mass Limits
Protostars with masses greater than about 200 solar masses also do not become main-sequence stars. Such a protostar rapidly becomes very luminous, resulting in tremendous internal pressures. This pressure is so great that it overwhelms the effects of gravity, expelling the outer layers into space and disrupting the star. Main-sequence stars therefore have masses between about 0.08 and 200 M⊙, although the high-mass stars are extremely rare.

31. T Tauri Stars
Much of the material of a cold, dark nebula is ejected into space and never incorporated into stars. As it is ejected, this material may help sweep away the dust surrounding a young star, making the star observable at visible wavelengths. Mass ejection into space is a hallmark of T Tauri star. These objects are protostars with emission lines as well as absorption lines in their spectra and whose luminosity can change irregularly on timescales of a few days.

32. T Tauri Stars
T Tauri stars have final masses less than about 3 M⊙ and ages around 106 years. The emission lines show that these protostars are surrounded by a thin, hot gas. The protostars eject gas at speeds around 180,000 mi/h and eject about 10–8 to 10–7 solar masses of material per year.

33. T Tauri Stars
The T Tauri phase of a protostar may last 107 years or so, during which time the protostar may eject roughly a solar mass of material. Thus, the mass of the final main-sequence star is quite a bit less than that of the cloud of gas and dust from which the star originated. Young protostars that are more massive than about 3 M⊙ do not vary in luminosity like T Tauri stars. They do lose mass, however, because the pressure of radiation at their surfaces is so strong that it blows gas into space.

34. Herbig-Haro Objects
In the early 1980s, it was discovered that many young stars, including T Tauri stars, lose mass by ejecting gas along two narrow, oppositely directed jets—a phenomenon called bipolar outflow. As this material is ejected into space at speeds of several hundred kilometers per second, it collides with the surrounding interstellar medium and produces knots of hot, ionized gas that glow with an emission-line spectrum. These glowing knots are called Herbig-Haro objects.

35. Herbig-Haro Objects
Observations suggest that most protostars eject material in the form of jets at some point during their evolution. These bipolar outflows are very short-lived by astronomical standards, a mere 104 to 105 years, but they are so energetic that they typically eject into space more mass than ends up in the final protostar.

36. Accretion Disks
Protostars slowly add mass to themselves at the same time that they rapidly eject it into space. As a protostar’s nebula contracts, it spins faster and flattens into a disk with the protostar itself at the center. Particles orbiting the protostar within this disk collide with each other, causing them to lose energy, spiral inward onto the protostar, and add to the protostar’s mass. This process is called accretion, and the disk of material being added to the protostar in this way is called a circumstellar accretion disk.

37. Accretion Disks
What causes some of the material in the disk to be blasted outward in a pair of jets? One model involves the magnetic field of the dark nebula in which the star forms. As material in the circumstellar accretion disk falls inward, it drags the magnetic field lines along with it. Parts of the disk at different distances from the central protostar orbit at different speeds, and this can twist the magnetic field lines into two helix shapes, one on each side of the disk. The helices then act as channels that guide initially infalling material away from the protostar, forming two opposing jets.

39. For next time…
Read Chapter 18 sections 6-8
Homework 20 due Monday