Star Birth

1. Star Birth What are the raw materials that stars are made from?How do stars form?

2. What are the raw materials that stars are formed from? • Cold interstellar gas (H and He) • Cold complex molecules • Interstellar dust grains (heavier elements)

3. Interstellar Gas • Very low-density hydrogen and helium gas exists between the stars. • There are only about 300,000 atoms per cubic meter (1 atom in every 3 cubic centimeters!) • These atoms are cold, about 70 K (-200oC), and therefore very slow-moving.

4. Interstellar Gas • Even though the atoms are far apart, gravity slowly causes them to clump into nebulae (Latin for “clouds”). • As the atoms draw closer together, the density of the cloud increases. This causes the atoms to speed up and to heat up.

5. Emission Nebulae • The H atoms heat up enough that they begin to emit light. The cloud “lights up” from the inside and begins to glow with an eerie reddish light. • The most famous emission nebula is the Orion Nebula (but there are many others also.)

7. Emission Nebulae • Emission nebulae also glow for another, more important, reason: hot, bright O & B class stars inside them. • Recall that O & B stars are very large bluish-white stars. They emit much of their light in the X-ray and UV portions of the spectrum.

8. Hot O & B stars • Close to the star, the UV light ionizes the Hydrogen gas (separates the proton and electron in the H atom). • This area around the star is called the HII (H-two) region. • A little farther from the star, the UV light merely excites the H gas, causing it to glow red. • This region is the HI (H-one) region.

9. HI region outside – H is excited& glowing red HII region inside – H is ionized: H H+ and e-

10. Emission Nebulae • Astronomers use the visible red glow from nearby emission nebulae to locate the places where new stars are forming. • Hydrogen also “glows” in another light: radio waves.

11. H radio emissions • Astronomers can also locate nebulae by the radio waves they give off. • When H atoms are excited, sometimes the electron will spin in the same direction as the proton (called parallel spins). • When the electron flips back (anti-parallel), it gives off a radio wave with a wavelength of 21 centimeters.

12. http://zebu.uoregon.edu/~js/ast122/images/21cm_radiation.gif

13. Christine Jones/Smithsonian Astrophysical Observatory This is how our galaxy looks in 21 cmradio waves. It’s easy to see the nebulaewhere new stars are being formed.

14. Interstellar Molecules • Besides cold H and He, there are more complex molecules found in space. • One of the most common is carbon monoxide (CO). • Other molecules found include water (H2O), carbon dioxide (CO2), methane (CH4), formaldehyde (CH2O), hydrogen cyanide (HCN), and even ethanol (CH3CH2OH).

15. Molecular Clouds • Like much of the interstellar gas, more complex molecules also exist in clumps, known as molecular clouds. • The most famous & well-studied molecular cloud is near the Orion Nebula.

16. The OrionMolecular Cloud

17. Interstellar Dust • How do we know dust is out there? • Is space dust like earth dust?

18. Dust – It’s what’s for dinner • Dust forms thick clouds out in space which extinguish or completely block out star light. (The process is called extinction.) • The Horsehead Nebula near Orion’s belt is a good example. • So is the Eagle Nebula.

20. Notice how the dust blocks starlight.

21. Dust – It’s what’s for dinner • Dust also produces another effect – it reflects starlight. • The Pleiades star cluster is moving towards a dust cloud. This dust cloud reflects the bluish light from the bright O-type stars. • The lit-up cloud is a reflection nebula.

23. M15 star cluster with reflection nebula(in Pegasus).

24. Exactly what is dust? • Interstellar dust isn’t anything like the dust under your sofa. • Space dust is more like a tiny sand grain coated with tar. • Like a mini-comet, it has a nucleus, mantle, and crust.

25. Here’s a large interstellar dust grain – about 15 millionths of a meter long. Most dust grains are only 0.1 to 1.0 micrometers in diameter.

26. The core is made of silicates – rock.The mantle is made of frozen gases.

27. Over time cosmic rays, UV light, and heat cause chemical changes in the dust grain’s mantle. • The gases combine into more complex molecules, including sugars and amino acids. • It appears that dust grains may be the chemical factories of molecular clouds and of some of the chemicals of life!

29. Whew! Any questions so far? Let’s change topics.

30. The Process of Starbirth • Rotating, collapsing nebula • Protostar • Pre-main sequence star • Main-sequence star

31. How is star born from a nebula? • Start with a nebula (gas, dust, molecules) several light years wide. The nebula is probably rotating slowly. • Gravity begins to pull the nebula inward.

33. As the nebula contracts, collisions between the gas molecules increase in frequency. The gas pressure increases, and so does the temperature of the cloud. • The interior of the cloud contracts the quickest because it’s closest to the center of gravity, so the cloud gets hottest in the interior.

35. Material from the outer regions continues to pile onto the hot knot in the center, heating it further. • Eventually, the interior pressure gets high enough that it balances the inward pull of gravity. A stable core is formed, with a temperature of several hundred oC. • This is hot enough to glow in infrared (IR) light, but not at visible wavelengths.

36. This cloud of hot gas is opaque to visible light, which helps to trap heat and increase the temperature build-up. • The hot cloud is several times larger than the sun and up to 10x to 1000x more luminous than the sun (but all in IR light!) • The hot cloud is called a protostar.

37. A protostar – glowing in IR. To our eyes, it would be a dark, hot cloud.

38. Slowly, over millions of years, the protostar continues to contract and its interior grows slowly hotter. • When the interior of the protostar reaches 8 million Kelvin, nuclear fusion “switches on”, and the star begins to shine with visible light.

40. Eventually, either all the extra gas and dust falls onto the surface of the protostar, or stellar wind blows it away. • When it becomes visible, it’s called a pre-main-sequence star. It’s a little dimmer and cooler than it will be later in its life.

41. Here, solar winds have begun to blow away the excess gas and dust.

43. As the nuclear fusion inside the pre-main-sequence star stabilizes, the star grows a little brighter and hotter. (Think of how a streetlight starts out orange and dim, and eventually grows brighter and hotter.) • At this point, the star becomes a main-sequence star. • The whole starbirth process takes 10 to 100 million years.

45. How does rotation affect all this? • If the initial nebula is slowly rotating, the cloud particles don’t just fall inwards towards the center of mass. • The gas & dust particles force one another into a disk around the equator of the protostar. (Think Saturn’s rings)

46. If there’s enough “stuff” in this disk, it may clump up into planets, moons, comets, etc. • For this reason, we call these dusty disks protoplanetary disks.

47. The protoplanetary disk around the equator of the new star produces an unusual effect! • When the protostar starts blowing away the excess gas and dust around it, the disk at its equator confines most of this stellar wind to the star’s north and south pole. • These high-velocity polar winds are called bipolar outflows.

48. www.umanitoba.ca /faculties/science/ astronomy/courses/ astro280/98R/ Steve/evo.htm

49. Bipolar Outflows • Any young star that shows bipolar outflows is called a T-Tauri star, after the first such star observed (star T in the constellation Taurus). • One of the most famous T-Tauri stars is Beta-Pictoris, about 50 light years from earth.