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HR Diagram and Stellar Fusion http://www.astro.ubc.ca/~scharein/app lets/#Sim http://www.unm.edu/~astro1/101lab/l ab10/HRdiagram.html H-R Diagram named for… • …Ejnar Hertzsprung and H. N. Russell, graph (see illustration) showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent characteristic such as spectral class or color) decreases to the right on the horizontal axis. It is found that the majority of stars lie on a diagonal band that extends from hot stars of high luminosity in the upper left corner (blue supergiant) to cool stars of low luminosity in the lower right corner. This band is called the main sequence. Stars called white dwarfs lie sparsely scattered in the lower left corner. The giant stars—stars of great luminosity and size (red giant)—form a thick, approximately horizontal band that joins the main sequence near the middle of the diagonal band. Above the giant stars, there is another sparse horizontal band consisting of the supergiant stars. The stars in the lower right corner of the main sequence are frequently called red dwarfs, and the stars between the main sequence and the giant branch are called subgiants. The significance of the H-R diagram is that stars are concentrated in certain distinct regions instead of being distributed at random. This regularity is an indication that definite laws govern stellar structure and stellar evolution. Play with H-R Diagram • http://astro.unl.edu/naap/hr/animations/hr.ht ml • http://rainman.astro.uiuc.edu/ddr/stellar/arc hive/suntrackson.mpg H-R Diagram • There are more lower main sequence stars than any other luminosity classes • Stars are classified into five main luminosity classes. These are the five classes: • I Supergiants • Very massive and luminous stars near the end of their lives. They are subclassified as Ia or Ib, with Ia representing the brightest of these stars. These stars are very rare - 1 in a million stars is a supergiant. The nearest supergiant star is Canopus (F0Ib) 310 light years away. Some other examples are Betelgeuse (M2Ib), Antares (M1Ib) and Rigel (B8Ia). luminosity classes • II Bright Giants • Stars which have a luminosity between the giant and supergiant stars. Some examples are Sargas (F1II) and Alphard (K3II). • III Normal Giants • These are mainly low-mass stars at the end of their lives that have swelled to become a giant star. This category also includes some high mass stars evolving on their way to supergiant status. Some examples are Arcturus (K2III), Hadar (B1III) and Aldebaran (K5III). luminosity classes • IV Subgiants • Stars which have begun evolving to giant or supergiant status. Some examples are Alnair (B7IV) and Muphrid (G0IV). Note also Procyon which is entering this category and therefore is: F5IV-V. • V Dwarfs • All normal hydrogen-burning stars. Stars spend most of their lives in this category before evolving up the scale. Class O and B stars in this category are actually very bright and luminous and generally brighter than most Giant stars. Some examples are the Sun (G2V), Sirius (A1V), and Vega (A0V). How does it all work? • Good question. Here’s what this whole thing means. • Ready? • Ok, click into the next slide and before you do, open the Astronomy Today textbook at your computer workstation and open it to Chapter 20 so you can see it’s H-R Diagram and this powerpoint at the same time—believe me, it will help you follow along. How it all works • As stars are born, they find a place on that line called the main sequence. If born from a big cloud of gas and dust (nebula), a star will find itself in the upper left—the domain of the socalled blue supergiants. If a star is born from a modest sized nebula like our Sun, it will be somewhere in the middle and on down the line to really small red dwarf stars. How it all works • The big stars or, “upper main sequence” stars are very luminous (check their place on the yaxis) and are hot (check their place on the top x-axis). Such stars are given the classification “O” and “B” type stars—their spectral classification. • And so on, down the main sequence. Our Sun is a G class star. How it all works • Notice that there are nine divisions within each spectral class—to get a more detailed classification. • Our Sun is a G2 class yellow star. Any other class G2 star anywhere in the Universe would look like the Sun, be as hot as the Sun, have the same color as the Sun, live as long as the Sun and read the same books as the Sun. How it all works • While our Sun is an average star, most stars in our Milky Way galaxy and Universe are K and M class red dwarfs, small and um, red. • The lower on the main sequence a star starts out its life, the longer it lives. Nuclear fusion energy conserving dwarfs may live 40 billion years, our Sun will live 10 billion and blue supergiants will live fast and die young at just 100 million years. How it all works • As our Sun like all stars age, helium “ash” builds up in the core like the remains of a log in the fireplace. This is because hydrogen atoms have been fusing into helium, right? • Ok, so hydrogen is the fuel of stars. But, there is a price to pay. Even stars do not live forever. • Eventually, the hydrogen is converted to helium and the star begins to die. How it all works • As the nuclear furnace in the star’s core begins to dim, the star’s own gravity squeezes the helium ash core. • The force of gravity begins to win out against the ever more feeble energy rush outward. • But wait…if gravity begins to win…doesn’t that mean the core gets squeezed? • And, that means it’s temperature goes up and… How it all works • …that means the ash heats up. • At 100 million degrees F, the ash begins to fuse and the star has a new lease on life! Energy rushes outward and the star begins to swell larger and larger. • It is now much hotter inside than it ever was as a young star and this late middle age star must give up this extra outpouring of heat. That’s why it gets bigger. How it all works • It gets bigger to increase its surface area and like a car radiator, that extra surface area is able to disperse the extra heat out into space. • It is now also a redder star because with that extra surface area, it becomes really quite good at getting rid of heat and while its core temperature keeps rising, it keeps its cool on the outside. • The star “moves” off the main sequence and takes on a new identity. How it all works • When I say it moves, I mean it gets more radiant, more luminous as its absolute magnitude increases because it is now radiating light from a much larger surface area. • The star “moves” off the main sequence and over to the right on a path called a luminosity class. Depending on how big at birth, the dying star is a red supergiant or just a red giant. How it all works • For any star life over in the upper right of the H-R Diagram will constitute about 10% of its life. • Inevitably, it’s time for the final curtain call. Having exhausted its hydrogen fuel, the star fuses helium into carbon to create the outrush of energy in a desperate attempt to stave off the crush of its own gravity. • But carbon ash buildup is a nasty thing indeed and for our Sun, that will be the end. How it all works • What happens at that point is that the Sun cannot squeeze its core enough to ignite carbon and gravity wins. • As the now hugely bloated red giant Sun turns off its nuclear furnace once and for all, gravity gets the upper hand and shrinks the Sun and all other middle and lower main sequence stars down into the lower right corner of the H-R diagram—a small, hot cinder of carbon called a white dwarf. How it all works • No longer technically a star—because it is not fusing or “burning” a nuclear fuel—the Sun and all stars like it cool for tens of billions of years. Eventually, the white dwarf becomes a black dwarf, as cold as Pluto and the size of Earth destined for every to orbit the galaxy as a really big hard sphere of carbon—some astronomers think that because diamonds are just dense lumps of carbon the Sun will be a diamond forever. How it all works • Another interesting little tidbit of trivia is that the Universe is not yet old enough for any black diamond dwarfs to exist! • But the Universe is plenty old to know how upper main sequence stars die and leave behind a corpse. • So, let’s start with a star a bit larger than our own Sun… How it all works • Such a star is big enough to gravitationally squeeze its carbon core to make it fuse into iron and release energy—thus keeping it on life support. • But its all just a zombie experience for a giant O or B star and we know the end is nigh as carbon fusion is a hideously hot 600 million degrees and happens so very fast. • So, as the carbon-into-iron furnace shuts down the final hurrah proceeds… How it all works • …as gravity wins, the extremely large red supergiant suffers a sudden collapse. • Millions of miles of un-fused hydrogen, helium, carbon and other elements catastrophically collapse down onto the beyond white hot, hot iron core. • In real time that lasts only a matter of Earth hours, a massive star’s outer layers suddenly ignite in the most incredible nuclear display in the Universe—a SUPERNOVA. How it all works • Visible from across not just a galaxy’s spiral arm but from across the Universe, a supernova does not destroy the whole star. In fact, most of the mass survives this detonation—the core of the former star remains to collapse even further upon itself. • Depending on how massive the core is, it will collapse into one of two fates: pulsar or black hole? Pulsar/Neutron Star • The core crushes even protons into electrons, cancelling each other out and all that is left is a Denver-sized sphere of neutrons. No longer iron or carbon or hydrogen or any element, a core of rapidly spinning neutrons spewing energy out like a lighthouse along lines of intense magnetic force. Real picture from Hubble telescope ultimate corpse: the black hole • So large is the gravitational force of a massive star that in the last seconds of the core collapse, the remaining matter of the star is pushed into zero volume. • Let that sink in… • Zero volume. • But it weighs quadrillions of tons and takes up no space at all. How to find a black hole • Like Lewis Carroll’s Cheshire Cat, you have to look for its smile. Because even light cannot escape it, the black hole must be seen as stuff falls into one say, a black hole in orbit around another star or, vice versa. What lies at the center? There are of course, various hypotheses. Scientific testing of these would be obviously, tricky. • nothingness a time travel portal oblivion Another Universe, perhaps? Get too close and everything falls in and nothing, ever, comes out. the real thing • Lurking at the core of most spiral galaxies are really big black holes—like the ones you see here. galaxies • Electrons orbit atoms and atoms make up molecules and molecules make up your body and you are on a planet that orbits a star and your star orbits your galaxy and your galaxy is riding the crest of an expanding and accelerating wave of spacetime into the future. Why should your galaxy be doing this sort of thing? • We’re not really sure. But we do know there are three types of galaxies out there. • We think the types are related to a) how much stuff (hydrogen, etc) formed them and b) how many times any particular galaxy had a gravitational run-in with another galaxy. Galactic types • • • • Now, don’t get the idea that galaxies morph one kind into another. All this diagram does is show you how galaxies are different. btw, galaxies are big. really big. vastly, hugely, mindbogglingly big. 100+ billion stars in an average one and 100+ billion galaxies in the greater metropolitan Universe. family photo galaxy luv Head-on collision Our Milky Way galaxy is headed for this one called, Andromeda • No, it won’t be pretty. Galactic razzies like, wow • If we could leave our galaxy and look back on it, it would look something like this. You are here Forever 21 is here Starbucks is here home, sweet galaxy Wal-Mart is here Now put it all into motion! http://planetquest.jpl.nasa.gov/milkyway/milky_way.html And if you liked that interactive you’ll love the rest of them at http://planetquest.jpl.nasa.gov/gallery/gallery_i ndex.cfm