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Swinburne Online Education Exploring Stars and the Milky Way Module : 8: Module Life on the Main Sequence Activity 2: From After © Swinburne University of Technology the ZAMS Summary In this Activity you will learn about • the life of stars after they reach the ZAMS, • how the time they spend there depends on their mass, • whether stars gain or lose mass and why, and • open clusters and how they have taught us so much about the life stories of stars. Our own Sun Luminosity Our Sun will spend about 80% of its entire life on the main sequence. 10 billion years … What will happen then? that’s a nice long middle age ... You’ll find out in the next Module! While on the main sequence, though, the Sun ZAMS will very slowly and quietly (Zero Age Main Sequence) keep on turning hydrogen into helium. Temperature Middle Age of a 1-solar-mass star Our Sun’s part of the main sequence Luminosity Helium is of course more compact than the four protons from which it was made. So during this phase the core shrinks, which increases its temperature. The radiated heat swells the outer layers: the star grows slowly larger and its surface becomes cooler. Sun moves a little on the H-R ZAMS Temperature Watch out! Many people get the wrong idea about stars and H-R diagrams, because of the term main sequence. The sequence is actually a sequence of mass, e.g. more massive stars appear on the upperleft side and low mass stars on the lower right. It’s got nothing to do with the history of any single star. I reckon stars move like this... Luminosity They think that stars move up the main sequence, but that isn’t so. WRONG!! Temperature You’ll shortly see a rough map of how a star might move on the H-R diagram. Its actual development will depend on its mass, its surroundings and its history. During the animation, keep an eye on the chart below as well. Luminosity What really happens The important thing for you to remember is that stars don’t move up and down the main sequence. If anything, they move across it! protostar main sequence star Temperature old star Gaining mass? Er ... Do stars gain or lose mass while they are on the main sequence? You might think that because they are so incredibly massive that they would trap any gas or dust in the vicinity. Come and have some turkish delight ... Losing mass A nice idea, but it forgets the stellar winds. Stars are constantly losing a bit of mass because of the matter-to-energy conversion in their cores. However the stellar winds are a much more serious source of mass loss. These winds commence as soon as a molecular cloud starts to heat up. But the effect really becomes important when fusion starts in the core. A big problem AAAARGHHHHH! This means that anything near a star will be blown away, or eroded. If it moves anywhere, it will be most likely away from the star, not towards it. But far greater than this is the loss of material from the star’s outer layers: it is just swept into space by the pressure of radiation from the core. A large star can lose up to 60% of its mass in stellar wind. All of the really light elements in its outer atmosphere can be blown away. You mean I’m leaking? Low mass stars The less mass a star has, the longer it takes to evolve onto the main sequence and the longer it will spend there. sigh protostar main sequence star old star The Universe simply isn’t old enough for many very low mass stars to have made it onto the main sequence, let alone off it! This slowness is however of great benefit to us on Earth, with a Sun which has evolved not terribly fast. The Big Ones Very massive stars evolve very quickly, in stellar terms, and so we can see a lot of these at different ages and stages. Remember too about look-back time: the further away a star is, the longer its light takes to reach us. So our cameras may be receiving light (and therefore images) which left the star thousands of years ago. A 40 solar mass O5 star in the nearby galaxy Messier 31 will have evolved along the Main Sequence in 1/2 the time it takes light to travel to us! For these reasons, we have a much better idea of the entire life story of the larger stars than we do of the smaller ones. This is Betelgeuse, as seen recently by the HST. As Betelgeuse is a red supergiant, it will probably explode fairly soon. But as it is 520 light years away, we won’t know for 520 years! Features of a Big One Live fast, die young ... A massive star is characterised during its time on the main sequence by a few clear markers. Its time on the main sequence is very short The star turns hydrogen into helium more through the CNO cycle than the p-p cycle There is likely to be carbon fusion and nucleosythesis of heavier elements in the core The stellar wind can be so very fierce that it strips the outer layers of their lighter elements. There are also profound differences in the star’s old age (but they’ll be covered in a later Module). A measure of age While you can guess the age of a human by looking at their skin, hair, body shape and so on, with a star one of the best leads that you have is the proportion of hydrogen to helium and other elements. I’m a B-class, but I haven’t got much helium in my core yet You’re just a baby! I’ve been on the main sequence so long my core’s just full of it ... A bit about Open Clusters You’ll be studying globular clusters in a later Module, but we are now going to have a look at open clusters as they can tell us so much about life on the main sequence. A cluster is open if its stars are pretty thinly scattered: usually 100 to 1000 stars in a space of a few tens of light years. This is the Jewel Box cluster. N330 Here is N330, a young cluster in the Lesser Magellanic Clouds. Lovely, isn’t it? Let’s look at it a bit more closely. Blue = lots of UV (very hot) Yellow-white = A-type supergiants Colour Orange = red supergiants To tell the truth, the colours in this photo aren’t realistic. They have been enhanced to indicate each star’s strongest radiation. Red = H (Balmer series) Pink = blue stars with lots of H All in the family Here’s a family like many human families: you can see the resemblances. Looking at the parents allows you to predict what might happen as the children grow up. It’s pretty likely that as they get older both kids will get darker hair. It is probably less likely that the boy will wear lipstick and the girl will grow a moustache, but you never know .... Stellar families The stars in open clusters are usually much like our own Sun: younger stars that are relatively rich in heavier elements being recycled from older, exploded stars. The larger the star, the faster it will evolve. So a photo of an open cluster is like a family photograph. It will let us predict to some extent where a star is headed by looking at its faster-developing relatives. Astronomers have learned a great deal about stars in general by studying such clusters, but we have to make a few assumptions. 1. Ageist assumptions We have to assume that all the stars in the cluster are of about the same age. If we didn’t do that, we’d be unable to say as definitely as we do that larger stars evolve faster. Having seen clusters forming in molecular clouds, though, supports the idea that stars in a cluster were “born” at about the same time. This is the Lagoon Nebula, an active region in which clusters of stars and protostars abound. 2. Compositionist assumptions We have to assume that all the stars in the cluster are made of much the same kind of stuff: that they formed from the same molecular cloud, and it was wellmixed. other If we didn’t make this assumption, we’d be unable to predict what will happen to one star in the cluster by looking at others in the same cluster that are further along in their development. He 26% 2% Composition of our Sun H 72% four 3. Groupist three assumptions two hup Third, we have to assume that all the stars fourin the cluster are staying with the group and moving roughly in the three same direction. The stars within the group may move around a bit, but the group as a whole heads in a common direction.two hup four Within the group three two hup Click to see that animation again and watch four just one of the stars: you’ll notice that the stars within this particular three group are circling each other. If we record the proper motion of the stars over a period of two time, we can work out both the direction that the cluster is heading, and the motion of the stars within the cluster. hup What clusters tell us If you make those three assumptions, you can really get down to business. It is like having an entire family photo album so that you can study and predict the changes in a standard member over time. A sort of time-line In this family, the hair apparently gets darker as middle-age approaches, then goes gray, then white. At about the same time, the top lip disappears, sags appear under the eyes and the cheeks start to drop. All through life the nose, ears and chin get bigger (but that’s true of all humans). 6 months 7 years 28 years 51 years 74 years Stretching the truth The trouble with these nifty analogies is that sooner or later they fall apart. In a human family, people are born at different times... but (usually) age at about the same rate. Born 1925 Born 1948 Life expectancy about 80 years Life expectancy about 80 years Born 1971 Life expectancy about 80 years Born 1992 Life expectancy about 80 years Born 1999 Life expectancy about 80 years A cluster, on the other hand … In a stellar family, a cluster, the stars do the reverse. They are born at the same time ... Time on main sequence (millions of years) Born 1642 Born 1642 2000,000 Born 1642 15,000 Born 1642 Born 1642 3 500 3,000 … but they age at very different rates: the bigger, the faster. (We chose 1642 for this example because that’s when Isaac Newton was born) Star family time-lines Astronomers treat photos of clusters as if they were family photos, and use them to look for changes in particular types of stars with time. However, human family members have their own peculiarities (like wearing lipstick or moustaches), and so do the stars in a cluster. They are of different masses (and therefore at different stages), for a start. They are subject to different stellar winds from neighbouring stars, different gravitational pulls, and different accidents (such as running into a nebula). But we can still see a lot of patterns. The Pleiades Spotty bit = Pleiades luminosity This is the Pleiades cluster: a well-known, bright and beautiful group of stars in the constellation Taurus. If you plot an H-R diagram for the Pleiades cluster, you can see that different stars are at different stages of their evolution … because they are of different mass. Red line = ZAMS temperature The Pleiades H-R At the bottom, there are the small, newly-formed stars and protostars: cool and not very luminous, and very slow to reach the ZAMS. great huge bloated stars that have already left the ZAMS Once a star joins the ZAMS, it usually becomes a bit brighter and cooler, so the middle-sized stars drift up and right of the ZAMS. The very large stars, with the highest temperature and luminosity, are so mature that they are leaving, or have already left, the ZAMS. (The next Module looks at those.) PLEASE NOTE: all of the stars in this diagram are the same age! middle-sized stars small stars protostars Summary This Activity has shown you how stars of different masses continue to evolve very slowly after joining the Zero Age Main Sequence. The evolution of the star will be controlled mostly by its mass. Open clusters give us a sort of “family album” that lets us see stars of the same age and type (but different mass) at different stages of their evolution. Image Credits MSSSO: Michael Bessell © (used with permission) The Jewel Box cluster (NGC 4755), containing Kappa Crucis N330 Lagoon Nebula RCW 38: http://antwrp.gsfc.nasa.gov/apod/image/9812/RCW38_vlt_big.jpg AAO: Pleiades cluster © David Malin (used with permission) http://www.aao.gov.au/local/www/dfm/image/uks018.jpg Now return to the Module home page, and read more about stars on the main sequence in the Textbook Readings. Hit the Esc key (escape) to return to the Module 8 Home Page