1. Stars Earth Science – Mr. Foster

2. Why do stars exist? Stars exist because of gravity Two opposing forces in a star are – Gravity – contracts – Thermal nuclear energy – expands, preventing the star from collapsing.

3. How big is BIG? Star size video

4. Luminosity of a Star Luminosity is the amount of light an object radiates each second. A stars luminosity measures how fast it consumes its fuel, which can tell us how old the star is. Depends on 2 factors – Temperature Hotter = brighter – Radius Bigger = brighter

5. Luminosity Inverse Square Law: Relates an object’s luminosity to its distance and its apparent brightness (how it appears from Earth) The idea behind the law is that as light energy leave the source of light, they spread out in all directions. The farther away a light source is the dimmer it appears to be because less photons reach our eyes.

6. Apparent Magnitude The measure of a star’s brightness as seen from Earth. The smaller or more negative the number, the brighter the star.

7. Absolute Magnitude The brightness a star would measure a set distance from Earth. 10 parsecs (1 Parsec = 3.26 light years) Absolute magnitude ranges from -8 (brightest stars) to 15 (dimmest stars)

8. Spectra of Stars: Composition of Stars Based on the energy spectrum from a star, we can determine its composition. Stars with the spectra like type A are hotter and younger than Sun. Stars with the spectra like type C are cooler and older than the Sun. Hotter than the Sun Like the Sun Cooler than the Sun

9. Categorizing Stars Annie Jump Canon, who was the astronomer doing the classification of stars for the Draper Catalog discovered that stars fell into a series of classes. These classes are given the letters

10. Spectral Classes Hottest Coolest

11. H-R DIAGRAMDIAGRAM

12. HR-Diagram Shows the relation between stellar – Brightness (absolute magnitude) and – Temperature Diagram is made by plotting (graphing) each star’s – Luminosity (brightness) and – Temperature

13. HR-Diagram The cooler (red) stars are on the right side. The hotter (blue) stars are on the left. The brighter stars are near the top. The dimmer stars are near the bottom. The diagonal line of stars in the middle is known as the main sequence. Above this line you have the giants. Below this line you have the dwarfs.

14. Parts of HR-Diagram Main-sequence stars 90 percent of all stars Band through the center of the H-R diagram Sun is in the main-sequence Giants (or red giants) Very luminous Large Upper-right on the H-R diagram

15. Parts of HR-Diagram Giants (or red giants) Very large giants are called supergiants Only a few percent of all stars White dwarfs Fainter than main-sequence stars Small (approximate the size of Earth) Lower-central area on the H-R diagram Not all are white in color Perhaps 10 percent of all stars

16. Life Cycle - Birth In dark, cool, interstellar clouds Gravity contracts cloud and temperature rises Radiates long-wavelength (red) light Becomes a protostar

17. Protostar Gravitational contraction of gaseous cloud continues Core reaches 10 million K Hydrogen nuclei fuse Become helium nuclei Process is called hydrogen burning Energy is released Outward pressure increases Outward pressure balanced by gravity pulling in Star becomes a stable main-sequence star

18. Main Sequence Star Stars age at different rates Massive stars use fuel faster and exist for only a few million years Small stars use fuel slowly and exist for perhaps hundreds of billions of years 90 percent of a star’s life is in the main sequence Main Sequence stars burn hydrogen, which is converted to helium to release energy. Once hydrogen fuel is depleted, the star will burn helium and become unstable.

19. Red Giant Hydrogen burning migrates outward Star’s outer envelope expands Surface cools Surface becomes red Core is collapsing as helium is converted to carbon Eventually all nuclear fuel is used Gravity squeezes the star

20. Death of Star Final stage depends on mass Possibilities Low-mass star – 0.5 solar mass – Red giant collapses – Becomes a white dwarf

21. Death of Star Final stage depends on mass Possibilities Medium-mass star – Between 0.5 and 3 solar masses – Red giant collapses – Planetary nebula forms – Becomes a white dwarf

22. Death of a Star Final stage depends on mass Possibilities Massive star – Over three solar masses – Short life span – Terminates in a brilliant explosion called a supernova » Supernova gives us all elements that are heavier than iron. – Interior condenses – May produce a hot, dense object that is either a neutron star or a black hole

24. Stellar Remnants White dwarf – Small (some no larger than Earth) – Dense Can be more massive than the Sun Spoonful weighs several tons Atoms take up less space – Electrons displaced inward – Called degenerate matter – Hot surface – Cools to become a black dwarf

25. Stellar Remnants Neutron star – Forms from a more massive star Star has more gravity Squeezes itself smaller – Remnant of a supernova – Gravitational force collapses atoms Electrons combine with protons to produce neutrons Small size

26. Stellar Remnants Neutron star – Pea size sample Weighs 100 million tons Same density as an atomic nucleus – Strong magnetic field – First one discovered in early 1970s Pulsar (pulsating radio source) Found in the Crab Nebula (remnant of an A.D. 1054 supernova)

27. Stellar Remnants Black hole – More dense than a neutron star – Intense surface gravity lets no light escape – As matter is pulled into it Becomes very hot Emits X-rays