1. Module : Activity 2: From Joining the Party Module 7: Evolving onto the Main Sequence Swinburne Online Education Exploring Stars and the Milky Way © Swinburne University of Technology

2. Summary In this Activity we will examine how a star evolves from a cloud of cool gas into a hydrogen-burning star. To begin the Activity, however, we will take a quick look at how astronomers observe star-forming regions. We will then follow a star’s evolution on the H-R diagram.

3. In the Activity Starbirth we saw that star formation involves the collapse of a cool molecular cloud. How to observe star formation? In the final stages of this collapse, the cloud’s core becomes sufficiently hot and the gas is under enough pressure for fusion reactions to begin: this is a young stellar object (YSO), a baby star. Obviously, astronomers are very interested in the stages of the cloud collapse that occur before the star is born. The protostellar cloud is not very luminous, however, since it is not generating energy by fusion reactions. So our first question is this: what kind of observations can astronomers make to observe the early stages of the star formation process?

4. In some cases, microwaves are emitted by protostellar clouds in an intense, narrow, single-frequency beam. 2x10 -12 10 -10 10 -12 10 -14 10 -8 10 -6 10 -4 10 -2 110 2 10 4 10 6 10 810 10 -16 10 -14 10 -18 10 -20 10 -22 10 -24 10 -26 10 -28 10 -30 10 -32 10 -34 Visible light Radio Microwave Infra-redUltra-violet X-rays Gamma rays Wavelength of photon (metres) Energy of photon (J) How in space can this happen? Is there an extraterrestrial out there with a nebula-sized malfunctioning oven or remote control? Celestial Microwaves

5. Lasers, Masers Humans are able to make intense, narrow single-frequency beams of visible light, called lasers.This stands for Light Amplification by Stimulated Emission of Radiation. The microwave version is called a maser, and it is probably not a sign of extraterrestrial intelligence. It happens naturally. Hey, why the microwaves all of a sudden? It’s not my fault! It’s these idiots beside me It’s not my fault! It’s these idiots beside me molecular cloud Click hereClick here to learn more about MASERs Click hereClick here to learn more about MASERs

6. Sources of maser emission are: newly-forming stars and old stars losing mass. In each case it seems that the maser emission comes from extremely dense clouds or disks of material containing the right molecules. These clouds surround sources of excitation (such as bright young stars, shockwave energy from exploding stars, or radiation from centre of an active galaxy) which provide the radiation to trigger the maser emission. Masers detected in our Galaxy typically extend over only a few thousands of an arcsec.

7. Real-life masers The photo shows part of the Orion Nebula, where since 1963 hydroxyl (OH) masers have been detected in the hot, thick gas surrounding the bright central area of new stars. The location of the first known maser is marked by the yellow dot. We’ll see more observations of star forming regions and young stars throughout this Activity. For now, let’s take up the story of protostellar formation using the H-R diagram...

8. Deep in a cloud We left our study of the birth of stars (in the Activity Starbirth) at the time when the star was just becoming visible through the remains of the molecular cloud. 28,000 6,000 3,500 Surface temperature (K) 28,000 6,000 3,500 Surface temperature (K) Luminosity (compared to the Sun) 10 -4 1 10 -2 Sun A A B B C C Answer: None of the above. Question: Where do you think such a molecular cloud would be on this H-R diagram? Near A? B? C?

9. young protostar The contracting protostar is initially very cold and faint, deep within a molecular cloud. Because it is cold, it is way over to the right of the H-R diagram. Temperature Luminosity Main Sequence Because it is faint, it is near the bottom of the luminosity scale.

10. Gravity, gravity The cloud then contracts under its own gravity, but becomes red-hot in the process and begins to glow. Question: Can you work out what this change should look like on the H-R diagram? Temperature Luminosity Main Sequence young protostar

11. Answer: the protostar moves up and to the left as it becomes brighter and hotter. young protostar Temperature Luminosity Main Sequence hotter brighter

12. Smaller and …? The protostar in its cocoon of gas and dust continues to collapse under gravity. It becomes smaller, and therefore hotter. Hotter = brighter? Smaller = dimmer? Question: Will the luminosity of the protostar increase, or decrease? Luminosity depends on both the size of an object and its temperature...

13. Temperature Luminosity Main Sequence Answer: It turns out that although the protostar does get hotter, its size is significantly reduced. So the protostar actually becomes a bit dimmer during this contraction. young protostar Hotter and dimmer

14. T-Tauri stars How do we know all this? This is the cluster RCW38 in the constellation Vela. Images at visual wavelengths show only a murky molecular cloud, but this photograph, taken by sensing infra-red radiation, reveals a multitude of young stars and protostars. The evidence is the existence of T-Tauri stars: stars of 0.2 to 2 solar masses embedded in small dark molecular clouds. These stars have not yet quite reached the main sequence, but they are getting there. At least, we suspect so: these small stars evolve so slowly that we won’t be around long enough to actually see them arrive!

15. young protostar Temperature Luminosity Main Sequence The contraction continues, with an increase in the core temperature of the protostar. Hydrogen fusion! Suddenly one day the density and temperature of hydrogen nuclei in the core are enough to overcome their electrostatic repulsion, and...

16. ZAMS The protostar is no longer a protostar, but now is a real hydrogen- burning star. It has joined the ZAMS: that is, the Zero Age Main Sequence. As for the time all this took: it varies according to how large the star is. What do you think? Would nine months be enough? googoo baba “Nine months”? More like MILLIONS of years!

17. Temperature Luminosity Main Sequence 0.5 M S 100 1 M S 30 2 M S 8 5 M S 0.7 15 M S 0.16 It took our own Sun 30,000,000 years to reach the main sequence! Joining the ZAMS Here you can see tracks of how different-sized stars reach the main sequence, according to how their mass compares to that of our Sun (M s ). The time estimates are in millions of years. Time to reach main sequence Mass of star

18. Larger goes faster Huge, massive stars form very quickly, while little stars take a long time to form. This is because the first thing that happens is the contraction of a cloud due to gravity. The more massive the cloud, the stronger the gravitational force and the quicker the cloud will contract. Mass (in solar masses) Millions of years Zip! Dum de dum... snore

19. Temperature Luminosity Main Sequence A star isn’t allowed into the main sequence club if its mass is too high or too low: these stars have another fate in store. Let’s start by considering stars with insufficient mass to reach the main sequence. Membership is exclusive No way, Tiny Tot! mutter grumble

20. Electrostatic loathing As you learned earlier, the core of a protostar is a seething mass of incredibly hot hydrogen in an ionised state. In other words, it consists of bare protons. These protons have the same charge. So they loathe each other, and the closer they get the worse it gets. Fusion zone Yeuchhh! Your charge is positive! Eeeeek! So is yours! Eeeeek! So is yours! Stop mucking about. Get in there and fuse! Wild horses couldn’t drag me in there... Gravity could … but there just ain’t enough However, for fusion to start the protons have to be very close indeed. So unless there is sufficient mass in the protostar for gravity to overcome this repulsion, fusion can never begin.

21. From gravity to P&T to fusion We’ll fill in a bit of detail here because it’s not quite that simple. You start off with a large cool cloud (which we’ll call a protostellar cloud), and gravity collapses it into a smaller, hotter cloud. In a larger, cooler cloud, the protons are moving more slowly and tend to be further apart. That is, the gas is at a low temperature and a low pressure. The protons have enough time, and space, to avoid each other. Yeuchhh! Eeeeek!

22. Not quite there Gliese 623a Gliese 623b a brown dwarf In protostars with about 0.08 M , the core never reaches a high enough temperature to trigger fusion. The only energy source of these stars is gravitational contraction of the outer stellar layers. They are called brown dwarf stars.

23. Gliese B 40 times heavier than Jupiter Jupiter Little Gliese B This little brown dwarf star is about the same size as Jupiter but has about 20 times the mass. It orbits Gliese A at 40 AU. Our Sun Gliese A 5.2 AU 40 AU Not to scale

24. Not so hot We can tell Gliese B isn’t a normal star because it has methane absorption lines in its spectrum. Methane (CH4) is pretty stable on Earth, but not inside a star, even a cool red dwarf. At temperatures higher than 2 500 Kelvin, all the methane molecules would be destroyed. So Gliese B can’t be star; it must be just a warm ball of gas. Hello there YOW!

25. Rare birds? When we examine the skies we don’t find many brown dwarfs. While it is true that their low luminosity means they are hard to detect, it is unclear whether brown dwarfs are really uncommon or whether our observations are just not powerful enough. As you will see much later in the Unit - in the Activity Dark Matter - an accurate count of brown dwarfs is extremely important for resolving the mystery of missing mass in the Universe. A Palomar and an HST image of another brown dwarf candidate, Gliese 229B.

26. Monster stars Let’s leave the protostars that are too light to join the main sequence and look at the other possibility. Temperature Luminosity Main Sequence Aaaargh! Too bright! heavy sigh... What about stars that are too massive to ever reach the main sequence?

27. Stars with very high mass have extremely high temperatures & pressures in their cores. When the mass is greater than about 100 times that of our Sun, the core temperature is so great that the radiation pressure due to fusion reactions can be stronger than the gravitational forces. Pressure can win gravity pressure

28. Pardon me! When that happens, the star will belch out a plume of hot gas, like a solar prominence only much, much larger and more violent. Ooops GO TO YOUR ROOM!

29. Untidy stars What mess? CLEAN UP THAT MESS! Stars with these extremely high masses are therefore not stable. They can spread huge gusts of hot matter through surrounding space. Because the matter is so hot, it glows and we can see it. You wouldn’t want to live near one!

30. Blast it! A good example is the star Eta Carinae in the Keyhole Nebula. Buried in this vast cloud of hot gas and dust is a star of about 100 solar masses. About 100 years ago, pressure won for a short time and the star expelled several huge gusts of gas and dust. These explosions are likely to keep on happening, but we can’t predict when.

31. A closer view Here is another picture of Eta Carinae. It was taken by the Hubble Space Telescope, using higher resolution which only imaged the very centre of the nebula, around the star itself. It shows two clear lobes of expelled gas, a lot of dark dust and some mysterious streaks and lumps.

32. Quick overview Let’s finish this Activity with a review of what you have learned: Temperature Luminosity Main Sequence The large, cool, dark molecular cloud begins to contract It becomes hotter and brighter … but also smaller If it has enough mass fusion begins and it reaches the ZAMS If it is too massive it becomes unstable If it’s not massive enough it won’t be a star at all If it’s not massive enough it won’t be a star at all

33. Summary This Activity has shown you how a star evolves from a molecular cloud until it joins the Zero Age Main Sequence. You have also learned what happens when a molecular cloud has far too much mass, or not enough mass, to end up as a star on the Main Sequence. In the next Activity we will follow a main-sequence star through its youth and middle age.

34. Image Credits MSSSO © M. Bessell (used with permission) Large Magellanic Clouds, Eta Carinae AAO © D. Malin (used with permission) Star trails in the Southern Cross Gliese 623B http://antwrp.gsfc.nasa.gov/apod/image/9911/gl623_hst_big.jpg http://antwrp.gsfc.nasa.gov/apod/image/9911/gl623_hst_big.jpg Gliese 229B http://antwrp.gsfc.nasa.gov/apod/image/gl229b_hst.gif http://antwrp.gsfc.nasa.gov/apod/image/gl229b_hst.gif Eta Carinae http://antwrp.gsfc.nasa.gov/apod/image/etacarinae_hst2.gif http://antwrp.gsfc.nasa.gov/apod/image/etacarinae_hst2.gif

35. Hit the Esc key (escape) to return to the Module 7 Home Page

36. Stimulated emission When atoms or molecules get into excited states, they don’t always decay back to the ground state quickly.

37. For some transitions, they can wait quite a while in what is called a “metastable state” to decay naturally, or until another photon (with the same energy as the one they should be emitting) reminds them of what they should be doing. This is called stimulated emission. Hey, you in the metastable state! Wake up! Aren’t you supposed to be emitting a photon like me? Oh, yeah! Thanks for reminding me!

38. The maser process If a molecular cloud releases maser radiation due to water molecules, it is called a water (H 2 O) maser, and that’s the example we’ll study. As far as we understand it (or think we do), there are a few steps in this process and it differs according to the particular molecule you are “stimulating”. In molecular clouds, such molecules include silicon oxide (SiO), hydroxyl (OH) and water molecules (H 2 O).

39. 1. Take a molecular cloud containing water molecules 4. Let the water be irradiated with some of the infrared photons 3. Let the dust absorb most of the gamma rays from the hot young stars and reradiate infrared radiation 2. Add a newly-forming nice and hot OB star (or group of stars) in a cocoon of dust

40. The rest of the recipe The water molecule has one of these “sleepy” metastable energy levels. When the molecular cloud is bathed in infrared photons from the nearby dust- enshrouded young stars, an unusually large number of water molecules can end up in the metastable energy levels. 5. The water molecule is excited by an infrared photon into a higher energy level 6. The molecule decays to a meta-stable state ground state meta-stable state excited state It does this by absorbing an infrared photon to move up to an excited state, then releases a photon and drops to the metastable state, where it would normally stay for an extended period of time. infrared

41. Usually most molecules would be in the ground state, with only a few in the metastable excited state. However in these circumstances an unusually large proportion of the population of molecules is in the metastable state - what we call a “population inversion”. ground state meta-stable state excited state 5. The water molecule is excited by an infrared photon into a higher energy level 6. The molecule decays to a meta-stable state infrared The way that the infrared photons aid in populating the meta-stable state in the water molecules is called pumping.

42. 7. … until a “reminder” microwave comes past Molecules in the metastable state can be stimulated to drop to the ground state early, emitting a microwave photon, if a photon of exactly the right energy comes past and reminds the molecule to emit an identical photon. ground state meta-stable state excited state Get moving! Release that microwave! 5. The water molecule is excited by an infrared photon into a higher energy level 6. The molecule decays to a meta-stable state microwave The thermal distribution of photons coming from the dust surrounding the young stars will peak in the infrared, but will include higher and lower energy photons, including some in the microwave region.

43. 7. … until a “reminder” microwave comes past ground state meta-stable state excited state This is essentially the same process of stimulated emission that makes lasers work, except there it is visible light photons that are emitted, not microwave photons. 5. The water molecule is excited by an infrared photon into a higher energy level 6. The molecule decays to a meta-stable state microwave

44. Copy-cats So instead of the one incoming photon, you now have two, which trigger another two, and so on, all through the cloud. This can amplify the relevant molecular emission line by factors of up to 10 billion (10 10 ). Repeated all through cloud Heaps of micro- waves These molecules are wimps, with no initiative. All of the outgoing microwave photons have the same wavelength (or frequency), and are headed in the same direction as the original trigger microwave.

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