Star Formation

1. Star Formation • Why is the sunset red? • The stuff between the stars • Nebulae • Giant molecular clouds • Gravitational collapse of molecular cloud • Gravitational contraction of protostars • Which clouds collapse? Demo: 6F40.10 Atmospheric Scattering

2. Interstellar medium • Space between the stars within a galaxy is not empty. • The interstellar medium (ISM) consists of gas and dust. • Gas is mainly hydrogen, but also contains other elements and molecules. • Density is typically around 1 atom per cubic centimeter.

3. Clouds and nebula • The interstellar medium is not uniform, but varies by large factors in density and temperature. • The clumps in the interstellar medium are clouds or nebulae (one nebula, two nebulae). • Three types of nebulae: • Emission nebulae: photons strike atoms and excite electrons to higher energy levels, the atoms then produce emission lines as the electrons return to lower energy levels. • Reflection nebulae: photons scatter off dust, there is no change in the photon energy. Some fraction of the photons get absorbed – depends on wavelength (do demo). • Dark nebulae: photons are absorbed by atoms and/or dust.

4. Emission nebulae Emission nebulae emit their own light because luminous ultraviolet stars (spectral type O,B) ionize gas in the nebula. The gas then emits light as the electrons return to lower energy levels. In this image Red = Hydrogen, Green = Oxygen, Blue = Sulfur.

5. Reflection nebulae Reflection nebulae do not emit their own light. Dust scatters light from nearby stars. Scattering is stronger towards the blue.

6. Dark nebula Dark nebula are so opaque that the dust grains block any starlight from the far side from getting through.

8. Reflection nebulae emit light as a result of • Ultraviolet radiation from O and B stars • Nuclear fusion • Dust scattering light from stars • Ionized gas

9. Molecular clouds • Dark nebula are usually molecular clouds • Molecular clouds are relatively dense and are very cold, often only 10 K. • Giant molecular clouds can contain as much as 104 solar masses (M) of gas and be 10 light years across. • Molecular clouds are the primary sites for star formation.

10. Eagle nebula

11. Eagle nebula in infrared

12. Star birth can begin in giant molecular clouds Carbon monoxide map

13. Protostars form in cold, dark nebulae Visible (left) and infrared (right) views of the Orion nebula show new stars. These new stars can only been seen in infrared because the protostar’s cocoon nebula absorbs most of the visible light.

14. Gravitational collapse Which configuration has more potential energy? A B

15. Potential energy due to gravity

16. Gravitational collapse Which configuration has more potential energy? A B

17. Potential energy due to gravity Sphere of mass M and radius R Gravitational potential energy is released as sphere shrinks

18. Gravitational collapse • How much energy is released when 1 M of material collapses from a radius of 10 R  to 1 R ?

19. Gravitational collapse • About 21041 J of energy is released when 1 M of material collapses from a radius of 10 R  to 1 R . This collapse takes about 10-20 million years. The luminosity is: • Size of cloud? • Temperature of cloud?

20. Evolution of stars • Stars change over their lifetimes (from formation to death). • We can track these changes via motion of the star in the HR diagram. • Where would a collapsing gas cloud appear on an HR diagram?

21. Cloud collapse to star: on HR diagram Cloud is transparent. Protostar is when cloud becomes opaque.

22. Protostars on the HR diagram Hotter

23. Why does temperature increase as star contracts? • Note that luminosity remains constant. • To produce constant luminosity as radius decreases, need increase in temperature

24. More massive stars form faster

25. Which clouds will collapse? • Gravitational force causes objects to collapse. • What keeps objects from collapsing? • In the solar system, the motion of the planets keeps them from falling in to the Sun. • In a gas, the random motions of the gas atoms can support the gas against gravity.

26. Temperature lower T higher T • Temperature is proportional to the average kinetic energy per molecule k = Boltzmann constant = 1.3810-23 J/K = 8.6210-5 eV/K

27. Energy of gas cloud Gravitational potential energy: Sphere of mass M and radius R Kinetic energy of N atoms

28. Energy of gas cloud If E < 0 then gas cloud collapses If E > 0 then gas cloud can support itself Density of gas cloud is n

29. Critical size of gas cloud By increasing the mass, we can always cause the gravity to dominate so that the gas cloud collapses. Critical size and mass are called the Jean’s length and mass T in Kelvin, n in atoms/cm3

30. Critical size of gas cloud If we have a cloud at T = 100 K and n = 1 cm-3, how large pieces does it fragment into? Therefore, such clouds will typically form a group of stars rather than a single star. Stars are generally found in groups, called star clusters or OB associations, depending on the type of stars.

31. Critical size of gas cloud If we have a cloud at T = 30 K and n = 300 cm-3, how large pieces does it fragment into? Therefore, such clouds will typically form a small group of a few stars rather than a single star.

32. Critical size of gas cloud The dense cores can reach n = 300,000 cm-3, how large pieces do they fragment into? Therefore, the dense cores fragment into individual stars.

33. Protostars form by collapse of molecular clouds • Clouds must form dense and cold clumps or cores to collapse • Typically, multiple stars will form from one gas cloud

34. Star cluster

35. An OB association is a group of O and B class stars which are producing ionizing radiation, causing an HII nebula glow (example: Trapezium in Orion Nebula)

36. Star formation Watch for: • Collapse of cloud • Rotation of cloud • Formation of disk near protostar • Show animation

37. As the gas/dust falls in, it picks up speed and energy. It is slowed by friction and the energy is converted to heat. • As long as the protostar is transparent, the heat can be radiated away. • When the protostar becomes so dense it is opaque, then the heat stars to build up, the pressure increases, and the rapid collapse slows.

38. Gas in the cloud keeps falling onto the protostar. • The collapsing gas tends to start rotating around the protostar as it falls in forming a disk and a jet. • Eventually, the protostar develops a wind, like the solar wind but much stronger. This out flowing wind stops the in falling matter. • The protostar keeps contracting under it own gravity. The protostar is powered by gravity via contraction - not by fusion. • The protostar becomes a star when it has contracted so much that it is dense and hot enough to begin nuclear fusion.

39. During the birth process, stars both gain and lose mass Magnetic field lines are pulled toward the protostar as material is attracted to the protostar. The swirling motions of the disk material distort the field into helical shapes and some of in-falling disk material is channeled outward along these lines.

40. Jets, disks form in protostars

41. Disk and jet of a protostar

42. Protostar jet

43. As gas is pulled in towards a protostar which does not occur • the gas starts to rotate more rapidly • some of the gas is ejected in jets • some of the gas forms a disk around the protostar • some of the gas undergoes nuclear fusion

44. Review Questions • What is a protostar’s source of energy? • How does a protostar’s radius and luminosity change as it contracts? • What is the relation between luminosity, radius, and temperature. • How does a protostar’s mass influence its speed of formation? • What is the Jean’s mass?