1. Copyright © 2010 Pearson Education, Inc. Life Cycle of the Stars

2. Copyright © 2010 Pearson Education, Inc. Interstellar Matter  The interstellar medium consists of gas and dust.  Gas is atoms and small molecules, mostly hydrogen and helium.  Dust is more like soot or smoke; larger clumps of particles.  Dust absorbs light, and reddens light that gets through.  This image shows distinct reddening of stars near the edge of the dust cloud.

3. Copyright © 2010 Pearson Education, Inc. Interstellar Matter Reddening  Stars appear redder than they really are  Stars in the cloud are dimmer and redder than those around it  Spectral lines do not change Allows true class and color to be determined

4. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions Central section of the Milky Way Galaxy Showing several emission nebulae (areas of star formation).

5. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions These nebulae are very large and have very low density Their size means that their masses are large despite the low density.

6. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions “Nebula” is a general term used for fuzzy objects in the sky. Dark nebula: dust cloud Emission nebula: glows, due to hot stars

7. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions Emission nebulae are among the most spectacular objects in the universe, yet they appear only as small, undistinguished patches of light when viewed in the larger context of the Milky Way. Perspective is crucial in astronomy!

8. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions An emission nebula results when ultraviolet radiation from one or more hot stars ionizes part of an interstellar cloud. The reddish color is produced as electrons and protons recombine to form hydrogen atoms.

9. Copyright © 2010 Pearson Education, Inc. Star-Forming Regions Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines.  These lines give us a wealth of information about the nebulae Emission nebulae are referred to as HII regions  they are made primarily of ionized hydrogen Regions of space containing neutral hydrogen atoms are referred to as HI regions

10. Copyright © 2010 Pearson Education, Inc. Dark Dust Clouds  More than 99% of space is simply dark!  Average temperature of dark dust clouds is less than 100 K  These clouds absorb visible light (left), and emit radio wavelengths (right).  These clouds can be distinguished by their extreme densities

11. Copyright © 2010 Pearson Education, Inc. Dark Dust Clouds This cloud is very dark, and can be seen only by its obscuration of the background stars.

12. Copyright © 2010 Pearson Education, Inc. Dark Dust Clouds The Horsehead Nebula in Orion is a particularly distinctive dark dust cloud.

13. Copyright © 2010 Pearson Education, Inc. Dark Dust Clouds Interstellar gas emits low-energy radiation, due to a transition in the hydrogen atom. This allows radio astronomers to study areas undetectable by other means.

14. Copyright © 2010 Pearson Education, Inc.  Star formation happens when part of a dust cloud begins to contract under its own gravitational force  As it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core. The Formation of Stars Like the Sun

15. Copyright © 2010 Pearson Education, Inc. When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion. The Formation of Stars Like the Sun

16. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun As the number of atoms increases, their gravitational attraction increases too.  Eventually the gravitational force is great enough to prevent the clump from re-dispersing. For this to happen, the clump must of a mass comparable to the Sun! This type of clumping must be triggered by an external event.

17. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stars go through a number of stages in the process of forming from an interstellar cloud. These numbers are only valid for stars like our sun

18. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stage 1: An Interstellar Cloud cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces.

19. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Depending on the conditions, an interstellar cloud can produce either a few hundred stars much larger than our sun or a collection of hundreds of stars comparable or smaller than our sun. Stars are not born in isolation! Stage 1 takes a few million years to complete

20. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stage 2 and 3: A Contracting Cloud Fragment Stage 2 Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation. Stage 3 The interior of the fragment has begun heating, and is about 10,000 K. For the first time, the fragement is beginning to look like a star. The dense, opaque region at the center is called a protostar

21. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun The Orion Nebula is thought to contain interstellar clouds in the process of condensing, as well as protostars.

22. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stage 4 The core of the cloud is now a protostar, and makes its first appearance on the H–R diagram. Very high luminosity Luminosity due entirely to the release of gravitational energy

23. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stage 5: Planetary formation has begun, but the protostar is still not in equilibrium – all heating still comes from the gravitational collapse. Luminosity is decreasing and protostar is now only 10 times the size of the sun

24. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun The last stages can be followed on the H–R diagram: The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.

25. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun Stage 6: The core reaches 10 million K, and nuclear fusion begins. The protostar has become a star. The star continues to contract and increase in temperature, until it is in equilibrium. Stage 7: The star has reached the main sequence and will remain there as long as it has hydrogen to fuse in its core. The journey from interstellar cloud to star occurs over the course of 40 – 50 million years

26. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun These jets are being emitted as material condenses onto a protostar.

27. Copyright © 2010 Pearson Education, Inc. The Formation of Stars Like the Sun These protostars are in Orion.

28. Copyright © 2010 Pearson Education, Inc. Stars of Other Masses This H–R diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun. The shape of the paths is similar, but they wind up in different places on the main sequence. Stars do not evolve along the main sequence!

29. Copyright © 2010 Pearson Education, Inc. Stars of Other Masses If the mass of the original nebular fragment is too small, nuclear fusion will never begin. These “failed stars” are called brown dwarfs.

30. Copyright © 2010 Pearson Education, Inc. Star Clusters Because a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.

31. Copyright © 2010 Pearson Education, Inc. Star Clusters This is a young star cluster called the Pleiades. The H–R diagram of its stars is on the right. This is an example of an open cluster.

32. Copyright © 2010 Pearson Education, Inc. Star Clusters This is a globular cluster – note the absence of massive main-sequence stars, and the heavily populated red giant region.

33. Copyright © 2010 Pearson Education, Inc. Star Clusters These images are believed to show a star cluster in the process of formation within the Orion Nebula.

34. Copyright © 2010 Pearson Education, Inc. Star Clusters The presence of massive, short-lived O and B stars can profoundly affect their star cluster, as they can blow away dust and gas before it has time to collapse.