The document summarizes key information about stars and stellar evolution: - Proxima Centauri is the nearest star to our sun at about 270 km away, while the next closest stars are much farther. - A star's apparent brightness depends on both its luminosity and distance from Earth. Luminosity is measured by absolute brightness or total power radiated. - The Hertzsprung-Russell diagram plots stars' temperatures and luminosities and helped show stages of stellar evolution. - Stars evolve through stages as giants, dwarfs, white and brown dwarfs, neutron stars, pulsars, planetary and diffuse nebulae as their cores fuse hydrogen and heavier elements.

1. Stars

2. The Solar Neighborhood Nearest star to the Sun: Proxima Centauri which is a member of a 3-star system: Alpha Centauri complex Model of distances : Sun is a marble, Earth is a grain of sand orbiting 1 m away Nearest star is another marble 270 km away Solar system extends about 50 m from Sun; rest of distance to nearest star is basically empty

3. The Solar Neighborhood The 30 closest stars to the Sun:

4. Luminosity and Apparent Brightness Apparent brightness is how bright a star appears when viewed from Earth; it depends on the absolute brightness but also on the distance of the star. Absolute brightness, or luminosity , is a measure of the total power radiated by a star. Therefore , two stars that appear equally bright might be a closer, dimmer star and a farther, brighter one:

5. Luminosity and Apparent Brightness Apparent brightness is measured using a magnitude scale , which is related to our perception. It is a logarithmic scale; a change of 5 in magnitude corresponds to a change of a factor of 100 in apparent brightness. It is also inverted – larger magnitudes are dimmer.

6. Luminosity and Apparent Brightness Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. We can see objects up to 6th magnitude without a telescope. This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C

7. Stellar Temperatures The color of a star is indicative of its temperature : Red stars are relatively cool, while blue ones are hotter.

8. Stellar Sizes Stellar radii vary widely: Giant stars have radii between 10 and 100 times the Sun’s. Dwarf stars have radii equal to, or less than, the Sun’s. Supergiant stars have radii more than 100 times the Sun’s.

9. Mass and Other Stellar Properties So the most massive stars have the shortest lifetimes – they have a lot of fuel but burn it at a very rapid pace. On the other hand, small red dwarfs burn their fuel extremely slowly, and can have lifetimes of a trillion years or more. Mass is related to stellar lifetime :

10. Characterizing Stars

11. Hertzsprung-Russel Diagram The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution or "the lives of stars". Plots stars on a graph measuring the star's absolute magnitude or brightness against its temperature and color.

12. Giants and Dwarfs Giant stars have larger radii and luminosity than a main sequence star. Giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Dwarf star alone generally refers to any main sequence star. Lower mass than giants. Create energy through the nuclear fusion of hydrogen atoms into helium.

13. Yellow Dwarf A yellow dwarf is a main-sequence star in the process of converting hydrogen to helium in its core by means of nuclear fusion. A yellow dwarf has about 0.8 to 1.2 solar masses and a surface temperature of between 5,300 and 6,000 K. Yellow dwarves actually range in color from white to only slightly yellow When the hydrogen is exhausted the star expands to many times its previous size and becomes a red giant. Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core cools and contracts into a compact, dense white dwarf.

14. Red Giants A red giant is a luminous giant star of low mass in a late phase of stellar evolution. They have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. Formerly main sequence stars. Will eventually become white dwarves.

15. White Dwarf A white dwarf is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Luminosity comes from the emission of stored thermal energy. Over a very long time, a white dwarf will cool to temperatures at which it will no longer be visible, and become a cold black dwarf.

16. Brown Dwarf Brown dwarfs are objects with mass too low to sustain stable hydrogen fusion.

17. Blue Giant A blue giant is a massive star that has exhausted the hydrogen fuel in its core. Blue giants have a surface temperature of around 30 000 K and a luminosity some 10 000 times that of the Sun. Blue giants are extremely luminous, reaching absolute magnitudes of -5, -6 and even higher. A sizable fraction of their energy output is in the ultraviolet range, thus invisible to our eyes. As they grow older they expand and cool, eventually becoming red giants.

18. The biggest star VY Canis Majoris (VY CMa) is a red hypergiant star. 1800 to 2100 solar radii The largest known star and also one of the most luminous known. It is located about 4,900 light years away from Earth.

19. Binary Stars Two stars orbiting their common center of mass. Brighter star: primary Dimmer star: companion Most stars are binary

20. Binary Stars and Supernovae If the companion star is a white dwarf The white dwarf can steal mass away from the primary When the mass of the white dwarf reaches the Chandrasekar limit… Type 1a supernova!

21. Neutron Stars and Pulsars Giant stars end in Type II supernovae The compressed core makes a star of all neutrons : a neutron star Neutron stars are highly magnetized and rotate rapidly The visible manifestation are called pulsars.

22. Nebula

23. Nebulae “ Nebula ” is a general term used for fuzzy objects in the sky. Two Types: Diffuse nebula Planetary nebula

24. Diffuse nebula Larger than planetary nebulae Make up most nebulae Two types: Emission nebulae Reflection nebulae

25. Emission Nebulae Some diffuse nebulae occur near an extremely hot, bright star Intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light

26. Emission Nebulae Emission nebulae generally glow red – this is the H α line of hydrogen . The dust lanes visible in the image are part of the nebula, and are not due to intervening clouds.

27. Emission Nebulae Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines:

28. Emission Nebulae Emission nebulae mostly consist of hydrogen, helium, and trace components. Oxygen is responsible for the greenish color in the Orion nebula:

29. Reflection Nebulae These selections illustrate a reflection nebula and how it forms. Reflection nebulae are usually blue, due to scattering of light.

30. Planetary Nebulae Planetary nebulae are ball-like clouds of dust and gases that surround certain stars. They form when a star begins to collapse and throw off the outer layers of its atmosphere. Planetary nebulae are very faint None visible with the naked eye