Where Stars Are Born
Outline
1. The Interstellar Medium
* Gas
> Mainly hydrogen & helium
> Avg density = 1 - 10 atoms/cm3
* Dust
> Tiny grains ('smoke'): silcates, carbon, ice ?
> 1 grain/million m3
* Gas & dust in galaxies
* Looking for Gas
> Emission nebulae
- UV radiation ionizes atoms
- Visible light when electrons rejoin ions
> Interstellar molecules
- In dark, dusty clouds.
* Looking for Dust
> Reflection nebulae
- Short wavelengths preferentially scattered by dust
> Dark nebulae
- dark, dusty clouds
2. Star Formation
* Collapse of molecular cloud by several mechanisms
* Fragmentation of interstellar cloud
* Contraction/collapse into protostars
> Ignition of H fusion stops collapse (Temp = 107 K)
> Arrival on main sequence
* Star birth in the Eagle & Trifid Nebulae
* Formation of accretion disk
> Bipolar Flow
* 'Evolutionary tracks' on the HR diagram
> Contraction time to MS depends on mass
3. Limits to Star Formation
* M < 0.08 Msun
> No H ignition
> 'Brown Dwarfs'
* M > 100 Msun
> photon pressure too high
Questions
1. What is the composition of interstellar gas? Of interstellar dust?
2. If space is a near-perfect vacuum, how can there be enough dust in it to block light?
3. What is an emission nebula?
4. How is interstellar dust detected?
5. Why is dust found in the neighborhood of some stars (as in the Pleiades star cluster) blue?
6. If our Sun were surrounded by a cloud of gas, would this cloud be an emission nebula? Why or why not?
7. What are some mechanisms that can initiate star formation in a molecular cloud?
8. Why do you suppose star formation apparently occurs only in interstellar clouds that are very cold?
9. What must occur in its interior in order for a protostar to end up on the main sequence?
10. What is a 'brown dwarf?' Under what circumstances do these objects form?
11. What determines the final position on the main sequence of a contracting protostar?
Answers
1. The interstellar gas consists mainly of hydrogen and helium. The dust consists of various solids: silicates (rocky material), carbon and ice.
2. Although the density of dust in space is (on average) extremely low, the volume of space is enormous. So we find lots of dust in space.
3. An emission nebula is a region of gas surrounding one or more hot stars; the gas is energized to glow at visible wavelengths by the UV photons emitted by the hot star(s).
4. We detect dust by (1) looking for instances in which the dust blocks background light; and (2) looking for dust the neigborhood of stars, where the dust emits pale blue light. This light results from scattering of photons of starlight by dust particles; the scattering operates in such a way that short wavelength (blue) light is scattered more efficiently that long wavelength (red) light.
5. See Ques. 4 for an answer.
6. No emission nebula would be found around the Sun (which is in fact surrounded by gas) because the Sun is too cool to produce the large numbers of UV photons required to energize the gas of an emission nebula.
7. i) Collision between clouds. ii) Compression of a cloud by a nearby emission nebula (due to expansion of the hot nebular gas). iii) Compression of a cloud by a nearby supernova explosion (which seems to have been the trigger that initiated the Sun's formation 5 billion years ago). iv) Galactic density waves compress some clouds. (These are waves that move through a galaxy; we'll discus these waves later in the course.)
8. In the cloud, we must be able to create a condition in which the gravitational attraction between the molecules of the cloud (which tends to cause the cloud to shrink) overcomes the cloud's gas pressure (which tends to resist shrinkage). As temperature goes down, gas pressure goes down, so the cooler the better. The temperature in
the interiors of the coolest clouds is perhaps only 10 K.
9. The central temperature of the protostar must rise to the ignition temperature for hydrogen fusion (at least 10 million K) in order for the star to arrive on the main sequence.
10. A brown dwarf results when a shrinking protostar's interior cannot get hot enough to initiate hydrogen fusion. The Jupiter-size objects glow faintly at deep red wavelengths, thus look a bit 'brown' in color.
11. Final position on the main sequence is determined by the contracting protostar's mass.