Star Formation: Major Achievements and Observations

1. Astronomy 2Overview of the UniverseWinter 20069. Lectures on Star Formation.

2. Star formation and evolution of pre-main-sequence stars

3. We now know that stars have been forming since about 200 million years after the Big Bang, so throughout the history of our Universe. • The evidence that star formation is a continuous process has been supported by both theory and observations

4. What were the major achievements that contributed to our understanding of star formation? 1- The understanding of the structure, composition and evolution of stars, 2- Observing the short-lived bright O and B stars that the stellar evolution models predicted, clearly proved that star formation has been going on in our “present age”, as these stars only have main-sequence lives of a few million years. 3-The discovery of the interstellar medium. The space between the stars is filled with clouds of gas and dust.

5. The vast space between the stars is not empty, but filled with gas and dust, which, by our terrestrial standards, is very thin: the interstellar medium. This is the raw material out of which new stars are born.

6. The observation of star formation is, however, a very difficult task, since in the very early stages of their lives, stars are hidden from our view by the dust and gas clouds in which they are born. We can’t see them in visible light, and have to rely on IR, radio radiation that penetrates dust. If star formation goes on in our galaxy, where then do we have to look for it? The raw material is interstellar material, gas and dust, so we should look where this is concentrated.

8. Betelgeuse Bellatrix Orion nebula Rigel

9. The Orion nebula with the Trapezium cluster near the center of the image. This young cluster contains about 2000 stars.

10. Viewing objects at different wavelengths gives us information about their structure and composition, and is thus a very powerful diagnostic tool. Here you can see a region of the Orion nebula in optical (left) and in near IR (right). While the optical image reveals the gas and dust structure in the nebula, the IR-image shows the embedded proto-stars

12. Effects of interstellar dust grains and gas • Presence of dust not recognized until ~1930. • Produces dimming of light and reddening of light. • Explained the existence of O stars that were red • Amount of reddening related to amount of dimming, so correction for dust dimming could be made • Dust surrounding stars can reflect the light of the stars and make reflection nebula (nebula=cloud) • Very dense cloud of dust can hide stars behind it and is called “dark nebula.” • Warm dust ( a few 100 degrees) can be observed glowing in the IR • Gas produces interstellar absorption lines in stars, but not many elements in proper state of ionization to produce absorption lines in the visible part of the spectrum • Gas near very hot stars (O stars) can be ionized and excited by the stars and made to fluoresce or emit light. Ionized hydrogen is called “H II” and these emission regions are called “H II regions.”

14. Interstellar matter is concentrated in the disk of our galaxy.

15. COBE infrared edge-on image of our galaxy

17. Effects of interstellar dust grains and gas • Presence of dust not recognized until ~1930. • Produces dimming of light and reddening of light. • Explained the existence of O stars that were red • Amount of reddening related to amount of dimming, so correction for dust dimming could be made • Dust surrounding stars can reflect the light of the stars and make reflection nebula (nebula=cloud) • Very dense cloud of dust can hide stars behind it and is called “dark nebula.” • Warm dust ( a few 100 degrees) can be observed glowing in the IR • Gas produces interstellar absorption lines in stars, but not many elements in proper state of ionization to produce absorption lines in the visible part of the spectrum • Gas near very hot stars (O stars) can be ionized and excited by the stars and made to fluoresce or emit light. Ionized hydrogen is called “H II” and these emission regions are called “H II regions.”

18. The Horsehead Nebula viewed by the AAT

19. Close-up view of the Horsehead Nebula as seen by HST (left) and AAT (right)

20. Tarantula Nebula in the Large Magellanic Clouds is the brightest star forming region in the Local Group. Hundreds of O-stars are forming there. The young cluster 30 Doradus that is over-exposed in the center of the upper image, viewed here by HST

21. A young cluster of stars, formed about 2 million years ago are illuminating a cloud of hydrogen gas and dust. The UV radiation from these O and B stars is ionizing the hydrogen atoms. The subsequent recombination of electrons and protons, with the electrons in excited states leads to the emission of visible light photons, mostly the red Balmer H-alpha transition. This is an example of an emission nebula, but we can also see dust lanes, which are places where the dust is dense enough to block off the starlight. Eagle Nebula

22. Rosette nebula The bright stars in the center have carved out a cavity in the nebula through their intense radiation and winds

23. The dark clouds seen in this image are known as “globules” and are associated with star forming regions, so-called HII regions that glow red due to the emission of the red hydrogen line H-alpha. The largest globule on top is actually 2 separate clouds that overlap along our line of sight. Each cloud is about 1.4 ly across and they contain material to make more than 15 stars like our own. This region is 5900 ly away, in Centaurus.

24. How do stars form?

25. The process of star and planet formation begins with the collapse of an unstable cloud of gas and dust.

26. The birth process of a star can be divided into two main distinctive stages: The proto-star-phase. Protostars are still in the process of attaining star-like structure. Protostars are accompanied by strong outflows and jets, and are surrounded by accretion disks. The disk is pouring more mass onto the protostar. The protostar is hidden within the cocoon of the birth cloud and cannot be seen in visible light. For a low mass star this phase lasts about 100,000 years. The pre-main-sequence phase. The star’s mass remains largely constant, and the stellar object has become visible in optical and near-infrared light. Accretion is ongoing but at a much lower rate. After about 5 million years from the start, the disk is mostly gone, and the Jovian planets are largely formed. During this phase the star can be placed on an HR-diagram

28. The collapse of The cloud core leads to the formation of a protostellar disk surrounding a central protostar. The disk rapidly grows to the size where it becomes self-gravitating and hence unstable. The collapse occurs from the inside out and not everywhere at once.

29. At the center, there is a proto-star, and in the disk, planets may form. High mass stars may not likely form disks, because their intense radiation and winds can carve the disks away. 2-8 times the diameter of our solar system

30. These disks are made up of about 99% gas and 1% dust. Even this tiny percentage of dust is sufficient to make all the planets that we have in our solar system. The disks appear dark, because they are viewed against the bright background of the Orion nebula. The reddish glowing object in the middle is a proto-star: A star that didn’t yet reach the main sequence where it will power itself fully from nuclear fusion of hydrogen into helium. These stars are only about 150,000 years old.

31. Here the star is covered, so that its light does not overpower the much fainter reflected light from the dust

33. In the standard theory of planet formation, the disk first accretes most of its mass onto the central star. Then, in the cold outer regions of the disk, icy bodies grow into a planetary core of several Earth masses. The core then accretes remaining gas from the disk to form a gas giant planet like Jupiter.

34. Sources of energy GRAVITY 1) Kelvin-Helmholtz contraction. The stellar object keeps contracting slowly as a result of gravity. The gravitational energy released leads to an increase of the core’s temperature, which in turn leads to an increase in pressure. The star is not in balance. Instead, gravity always stays half a step ahead of the pressure force. This is because energy is leaking out of the star in form of radiation. 2) The accretion disks also contribute to the total luminosity. They are sources of gravitational energy as the gas spirals deeper into the potential well of the forming star.

35. Ultimately, the core becomes hot enough and dense enough to ignite fusion of H into He. The star is born (becomes a certified star), when its luminosity is provided fully by fusion.

36. This reflection nebula resides in Orion. It shines only because the light from the newly born star visible just to the left of its center reflects off the dust that is part of the nebula. The star is known to have T=10,000K and is about 3 times more massive than the Sun.

37. A young cluster of stars, formed about 2 million years ago are illuminating a cloud of hydrogen gas and dust. The UV radiation from these O and B stars is ionizing the hydrogen atoms. The subsequent recombination of electrons and protons, with the electrons in excited states leads to the emission of visible light photons, mostly the red Balmer H-alpha transition. This is an example of an emission nebula, but we can also see dust lanes, which are places where the dust is dense enough to block off the starlight. Eagle Nebula

38. The pillars are about 1 ly tall and somewhat broader than our solar system. The lower image is a detail of the top of the leftmost pillar. Everything is part of a larger complex, 65 ly across, the Eagle Nebula.