The Birth of Stars


We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened. Jim he allowed they was made, but I allowed they happened; I judged it would have took too long to make so many. Jim said the moon could a laid them; well, that looked kind of reasonable, so I didn't say nothing against it, because I've seen a frog lay most as many, so of course it could be done. We used to watch the stars that fell, too, and see them streak down. Jim allowed they'd got spoiled and was hove out of the nest.

Huckleberry Finn, Mark Twain

In the previous chapter you learned about the distances and basic properties of stars. All of this begs the question - how did the stars form in the first place?

"The Clues:"

Where do the stars come from? Let's consider what a careful investigation of the night sky will reveal by looking at the clues...

Clue #1 - The Interstellar Medium

With the refinement of the telescope and especially with the skillful observations of William and Caroline Herschel in the late 18th century we began to see a richness in the universe that was hitherto unknown. 
Famed for their discovery of many nebulae and clusters the Herschels also noticed, especially when looking in the crowded chaos of the milky way, numerous "empty" patches. They thought that this was just what it seemed - holes in the galaxy - no stars. By the early part of this century (circa 1930), however, the view was much different. Robert Trumpler succeeded in showing that these patches were in fact very dusty regions that were obscuring our view. Evidence for this came primarily by noting that around the periphery of such regions it is not uncommon to find stars that are reddened. That means that while their spectra say they should be blue (for example) they look distinctly reddish (big B-V). This is not unlike what you see as stars approach the horizon.

Figure 9.1 Image of Barnard33 or better known as the Horsehead Nebula, an example of an interstellar dust cloud. Click on image to inspect. The King's University Observatory photo.

This is the interstellar medium (or ISM) which is a collective name that astronomers give to all of the stuff that exists between the stars. On average there is only 1 atom per cubic centimeter making up a tenuous, cold gas having a density of about 10-20 of the density of Earth's atmosphere at sea level. Despite this the total mass contained in the ISM is about 5 billion times the mass of the sun! It has taken astronomers most of this century to piece together a "picture" of what the interstellar medium is but now we think that we have a pretty good understanding of this tenuous but important component of our galaxy.

Cold Atomic Clouds which have densities of 10 - 20 atoms per cubic centimeter and temperatures of about 50 K. Typical clouds measure 10 pc (30 ly) across and have masses of about 1000 suns. On average, these clouds are about 500 ly apart.

Warm Diffuse Atomic Gas clouds have temperatures of about 5000 K but densities of only about 0.1 atoms/cc. The clouds are mostly hydrogen (90%) and along with the cold atomic clouds make up about 50% of the mass of the ISM.
Molecular Clouds are large regions of cold gas with densities from 1000 to 10 000 hydrogen atoms/cc. Dust mixed into these clouds help block out star light and the result is clouds that are dark and very cold (only a few 10's K). It is the combination of high density and cold that makes it possible to form complex molecules in these clouds. Radio astronomy has revealed a wealth of complex and in many cases organic molecules in molecular clouds.
HII Regions are bright glowing regions of ionized hydrogen gas surrounding luminous, hot stars. These regions have temperatures of about 10 000 K and densities of a few 100 to 1000 ions/cc.
Dust is also an important part of the ISM. The dust in the ISM consists of small (less than a micron or millionth of a meter) "pellets" consisting of elements heavier than hydrogen and helium. We think that most of the dust grains in the ISM are being produced in the cool atmospheres of red giant stars. Mass loss from these stars spreads the dust into the ISM.
Table 9.1 The components of the Interstellar Medium

Example 9.1 What components of the ISM can you identify in the following image of the Eagle Nebula (Messier 16) which is located in the constellation Serpens - low in Canadian skies during the summer months?

Solution: Use the mouse to rollover the image on the right to see some of the ISM components revealed. Other components (cold atomic clouds for example) may not be visible in the visible. We will discuss this further in future section.

  Figure 9.2 Hubble image of part of the Eagle Nebula

Clue #2 - Nebulae

One particularly striking way in which we see the Interstellar Medium and begin to understand where stars come from is through nebulae - a name derives from the Latin name for cloud. Nebulae come in different types:

Emission Nebulae: hot glowing gases, characterized by emission spectra. HII regions are emissions of ionized hydrogen and are often seen around extremely bright stars. What powers the nebula? Is it just a coincidence that we see bright (and as we shall soon see YOUNG) stars in the presence of hot regions of gas?






Figure 9.3 Hubble image of the Cone Nebula

Reflection Nebulae: light reflecting from dust grains surrounding bright stars. This light is often polarized which gives indirect evidence for the dust grains as well as the presence of interstellar magnetic fields.







Figure 9.4 Hubble image of NGC 1999

Dark Nebulae and Bok globules: dark, dusty regions. Bok globules are several pcs in diameter and can have masses ranging from 10's to 1000's of solar masses. Their compactness is very suggestive. What observation would you consider to be the "smoking gun" that would implicate Bok globules as significant players in star formation?






Figure 9.5 Hubble image of the star forming region IC 2944

  Clue #3 Bright Stars in Dusty Places!

Eagle Nebula (M16) Look at Figure 9.6. This is an earth-based image of the entire region around the Eagle Nebula (Figure 9.2). It sows a commonly occurring theme - lots of bright, young stars embedded in regions rich in dust and other parts of the ISM.
Figure 9.6 Earth-based image of M16. Image courtesy of David Malin Anglo-Australian Observatories.  

Clue #4 - What Hubble Showed Us!

Researchers using the Hubble Space Telescope are producing exquisite images of star formation. These images support the basic ideas that we are developing here while others are extending our understanding in surprising and un-anticipated ways.

Figure 9.7 is shows what we think are "jets" of extremely hot gases being ejected from newly forming stars. Sometimes we see the jets from a side view and see two very symmetric jets. These have been described as "cosmic blow torches" since they represent gases heated to thousands of degrees K.!

Associated with jets are bright knots of light called Herbig-Haro objects. We think that these are the result of shock-wave heating produced by supersonic matter ejected along the jets. As this matter plows into dust and gas it heats the gas and produces these bright, fast moving "blobs".

Figure 9.7 HST images of jets or bi-polar flows from newly forming stars.

Figure 9.10 shows another fascinating image taken in the heart of the Orion Nebula. Small condensations called proplyds are thought to be actual images of protostars surrounded by their accretion disks.
  Figure 9.10 Protoplanetary systems or Proplyds


Putting the Clues Together - A Multi-Wavelength View of a Star Forming Region

The following images provide a "visual essay" that illustrates the ideas discussed thus far. The region of the sky being investigated is the "Swan Nebula" or M17 and is a star forming region located about 5000 ly's away in the direction of Saggitarius. Click on the images to get enlarged views.

Spectral Window


X-ray spectral window : showing very energetic (high temperature) parts of the star forming region. In this case the blue area represents very hot gases (about 1.5 million degrees C) flowing away from newly formed stars. This image was taken by the Chandra X-ray Space Telescope.

Figure 9.11 X-ray image of M17

Image of M17 (omega nebula) as it appears at visible wavelengths. This spectacular Earth-based image was produced by the Canada-France-Hawaii Telescope.

Figure 9.12 Image of M17 in the visible region of the spectrum

Infrared image of the star forming region emphasizing hot, dusty areas. Image produced by the Spitzer Space Telescope.

Figure 9.12 Infrared image of M17

Microwave image of the star forming region - bright central condensation is a characteristic signature of dense molecular clouds that will soon collapse to form stars.

Figure 9.13 Microwave region image of M17


Example 9.2 The Swan Nebula is about 20 light years across. Estimate the mass of this nebula. A typical density for a nebula is 103 atoms per cubic cm. Is there enough mass there to produce a star like our sun?

Solution: Start with what you know:

  1. typical density 1000 atom per cubic cm.
  2. most common atom in the ISM is Hydrogen
  3. Swan nebula can be approximated as a sphere 20 ly's across

Begin with the density. It will be easier to work in cubic meters so first convert to atoms/m3. Since there are 100 cm in a meter, there are 100 x 100 x 100 or 1 million cm3 in a m3. This means that there are about 1000 million or 109 atoms/m3. Each hydrogen atom has a mass of 1.6 x 10-27 kg, so 1 cubic meter of the ISM should have a mass of (1.6 x 10-27 kg/atom) X (109 atoms/m3) = 1.6 x 10-18 kg/m3.

Recall that the volume of a sphere is , and r = 10 ly (r is 1/2 the diameter!). Convert this to meters by using the conversion 1 ly = 9.46 x 1015 m. So...

Multiply the density by the volume:

Since the sun has a mass of 2 x 1030 kg, you can convert the mass into solar units to get:

The swan nebula contains a lot of matter - nearly 3000 solar masses. Nebulae like M17 certainly contain enough mass to "build stars"!

Weighing the Evidence

A picture is beginning to emerge. We think that new stars form (and are forming) from the vast supply of dust and gas that we find spread throughout the galaxy. This seems to be a ubiquitous mechanism for, as you shall learn later, you find similar structures in other (spiral) galaxies. In very simple terms, then, we believe that stars form by the condensation and collapse of huge interstellar clouds. We can describe this process in four steps:

Step One: The Initial Collapse

  • Something triggers the collapse of a huge nebula. The nebula measures many light years across and contains several thousand solar masses of material. Average densities are on the order of 103 particles/cc and typical temperatures are a "chilly" 10 K - 50 K! gravitational potential energy is released - heats up the cloud. This turns out to be a trick that a star will use many times during its life. Material piles up at the cloud center, density rises in the center.
  • the core begins to collapse faster than the outer envelope.

Step Two: Fragmentation and Further Collapse

  • After several million years the collapsing cloud begins to fragment into numerous smaller clouds that each continue to collapse. The following descriptions apply to each of these perhaps thousands of "cloudlets".
  • the increasing frequency of collisions between H2 molecules is the equivalent of heating up the cloud. The dust grains also heat up and begin to re-radiate this energy in the Infrared region.
  • the core is warmed up to several hundred degrees K, the collapse slows down dramatically. Densities are now approaching 106 particles/cc and the cloud is about 100 times the size of our solar system.
Figure 9.14 Schematic depiction of collapse of a large nebula into smaller "cloudlets" which eventually form stars.

Step Three - It's Heating Up!

  • material from the envelope continues to rain onto the core causing it to collapse slowly under the added weight.
  • the core continues to heat.
  • the contracting cloud is also spinning. In order to conserve angular momentum (the "figure skater effect") the spin rate increases and the cloud flattens out into a disk. Rapid spinning begins to inhibit collapse.
  • the magnetic field of the collapsing cloud becomes more concentrated as the cloud collapses and begins to "stiffen" - pushing back and further inhibiting collapse.
  • when the core reaches 2000 K the H2 molecules break apart. This takes a lot of energy - in order to pay the bill - the core collapses again. Eventually the core reaches approximately 1 million K, the surface about 3000 K and the cloud begins to resemble a star. At about 100 times the size of our Sun the cloud has become a Protostar.

Step Four: A STAR IS BORN!

  • the core collapses from a diameter of about 4 AU to the size of the sun, heats up considerably and becomes hot enough for nuclear fusion to begin.
  • the outer envelope - like a womb - shields the entire event from view at optical wavelengths. The dust in the envelope heats up and becomes a very large region glowing brightly in the infrared part of the spectrum.
  • the outer envelope is blown away and a pre-main sequence star emerges. This is a violent process - a rapidly spinning and still collapsing outer disk collides with a now expanding shell from the newly formed star. Motion above and below the rotational plane of the nebula is easier and hence material "squirts" away at right angles to the disk. Figure 9.15 illustrates the "textbook" idea of what this should look like. Compare this with the Hubble image shown in Figure 9.16. This image confirms that the model of star formation described here is, essentially correct.
Figure 9.15 An astrophysicist's model of what a protostar "should" look like.


Figure 9.16 Hubble space telescope image of the protostellar system HH30 which is found in the constellation Taurus.

Star Formation and the HR Diagram

In the final stages of collapse, a 1.0 solar mass protostar will have a diameter of about 100 times that of the sun and a surface temperature of about 3000 K. Where would you plot this star on the HR diagram? If you were to follow a collapsing cloud from nebula to protostar to eventually becoming a star and plotted its temperature and luminosity as a function of time you would produce an evolutionary track on the HR diagram.

Example 9.3 Use the applet HRexplorer which was introduced in Chapter 8 to estimate the absolute magnitude of a protostar of radius 100 Ro and surface temperature 3000 K. Would you expect this object to be bright or "just detectable"?

Solution: Protostars are intrinsically very bright! The huge surface area of the protostar means that it will be hundreds of times more luminous than the sun (even though it is cooler). The position of the protostar is shown on the HR diagram in Figure .17. The absolute magnitude of such a protostar would be approximately -4 making it very bright.
  Figure 9.17 HR plot showing location of a 1 solar mass protostar.

Figure 9.18 illustrates the evolutionary track for a solar mass star from collapse to becoming a main-sequence star. The entire process takes about 30 million years.

    Figure 9.18 Animation showing the evolutionary track for a solar mass protostar. To advance through animation press the green play button

Table 9.2 shows how the time required to form a star is related to the mass of the star

Mass (M/Mo)
Time to Contract to Star
(Million Years)
Table 9.2 Contraction times from collapse to main-sequence

Example 9.4 In example 9.3 you showed that protostars are expected to be hundreds of times more luminous than our sun. This implies that they are very bright. However in the visible wavelength region we really don't see any extremely bright protostars. Why is this?

Solution: Protostars are shrouded by the dust and gas from which they collapse. As the protostar heats up and brightens this obscuring shroud of gas and dust is blown away from the newly forming star by the intense stellar wind created by the protostar. We see the protostar emerge from its cocoon of dust as it crosses the "birth line" on the HR diagram. Prior to this we observe the protostar primarily in the radio region of the spectrum. Figure 9.19 shows how the birth line is positioned in relation to the main-sequence.

Figure 9.19 also shows a series of evolutionary tracks for stars of varying masses as they collapse onto the main-sequence. This helps emphasize a critical new feature of the HR diagram - it is dynamic and allows you to visualize how stars will evolve over time.

  Figure 9.19 The birth line for stars of different masses.


  1. In several paragraphs (and possibly using simple sketches) explain the sequence of events that lead to the formation of stars
  2. Explain why the protostellar phases of star formation are better studied in the radio and infrared parts of the spectrum rather than the visible.
  3. A protostar is just entering the birth line on the HR diagram and has a surface temperature of 3500 K and an absolute magnitude of -2. Estimate the size of the star. (Hint: use an applet!)
  1. You are a science writer for a newspaper - please write a concise, 1 paragraph description of the following Hubble image:
  2. Identify two possible mechanisms that could trigger the collapse of interstellar clouds.
  3. Which process takes longer: Formation of a 0.8 solar mass star or formation of a 8.0 solar mass star? Provide rough numerical estimates for this.



To understand how stars are born

Chp 10 (all)

Chp 11.1








A quick way to "take a star's temperature" is to measure its magnitude in the Blue region of the spectrum and compare this with its magnitude in the Visual part of the spectrum. The difference between these two magnitudes or B-V is a good indicator of stellar temperature. For example, hot stars have negative B-V values while cool stars have large , positive B-V values.






The process that causes interstellar reddening is the same process that makes our sky blue and produces brilliant sunsets - the scattering of photons in our atmosphere by molecules and tiny dust particles.















































































































The Swan Nebula is also referred to as the "Omega" or "Horse Shoe" nebula and is easily seen in binoculars or a small telescope.























































































an evolutionary track is a collection of luminosity-temperature points for a given star that show how the star's temperature luminosity, radius etc change as it evolves. When plotted this appears as a continuous curve or "track"





































The evolutionary tracks in Figure 9.19 depict collapse onto the main-sequence and are called Hayashi tracks after the Japanese astronomer Chushiro Hayashi who carried out ground breaking research into star formation in the 1960's