1. All stars form in clouds of dust and gas

2. Balance of pressure: outward from core and inward from gravity

3. We see different stars at different stages of their lives

5. Sirius is the brightest star in the entire sky. Only slightly larger than the, it is only 8 light years away

6. STELLAR LUMINOSITY Luminosity of a star – the total amount of power it radiates into space (expressed in watts) The Sun’s luminosity is 3.8 x 10 (26) watts A star’s luminosity cannot be measured directly. Sun and Alpha Centauri A have the same luminosity, but the Sun is much, much brighter. Alpha Centauri A is about 270,000 times farther from the earth than the Sun

7. Light obeys the inverse square law

8. Luminosities of stars are compared to the Sun’s luminosity (L-Sun = 3.8 x 10 (26) watts) Proxima Centauri is 0.0006 L-Sun; Betelgeuse is 38,000 L-Sun A star’s apparent brightness is measured with a detector, such as a CCD, that records how much energy strikes its light-sensitive surface each second. Total luminosity and total apparent brightness take into account all photons across the entire electromagnetic spectrum. Once a star’s apparent brightness has been measured, the next step in determining it’s luminosity is to measure its distance.

11. The most direct way to measure the distances to stars is with stellar parallax. This is the small shift in a star’s apparent position caused by the Earth’s motion around the Sun. Astronomers measure stellar parallax by comparing observations of a nearby star made 6 months apart. The nearby star appears to shift against the background of more distant stars because we are observing it from 2 opposite points of the earth’s orbit. Parallax can only be used to measure the distances to stars within a few hundred light years away. (our “local” solar neighborhood) Proxima Centauri, the closest star to us, has a parallax angle of only 0.77 arc-second The distance to an object with a parallax angle of 1 arc-second is one parsec (pc) 1 pc = 3.26 light-years = 3.09 x 10(13) km

12. Enough stars have measurable parallax to give us a fairly good example of the many different types of stars. 300 stars within 33 light-years (10 parsecs) of the Sun. Angular distance in the sky: Width of stretched out hand ~ 20 degrees Width of fist ~ 10 degrees Finger width ~ 1 degree 60 arcminutes per degree; 60 arc-seconds per arc-minute

14. If the Sun were a distance of 10 parsecs from the Earth, it would have an absolute magnitude of 4.8. It would be visible, but not conspicuous on a dark night.

16. STELLAR SURFACE TEMPERATURE Surface temperature is determined directly by the star’s color or spectrum. A stars surface temperature determines the color of light it emits. The naked eye can distinguish colors only for the brightest stars. Colors of stars become more evident when viewed through binoculars or telescope. Betelgeuse, a cool, red star would look much brighter through a red filter than a blue filter.

18. Most important property of a star. Most dependable method of “weighing” a star is to use Newton’s version of Kepler’s Third Law - Universal law of Gravitation. Stellar masses can only be measured in binary star systems in which the orbital properties of the two stars have been determined.

19. Mizar, the second star in the handle of the Big Dipper is actually two stars – a visual binary system

22. THE HERTZSPRUNG-RUSSELL DIAGRAM Most stars fall along the main sequence – upper left to lower right. These stars fuse hydrogen into helium in their cores and have a wide range of life spans, which depend on their mass. Higher mass stars on main sequence have shorter life spans. A star has a limited supply of core hydrogen and therefore can remain as a hydrogen-fusing main sequence star for a limited time - the star’s main sequence lifetime. Our Sun’s main sequence lifetime is about 10 billion years. A 30 x M-Sun star has 30 times more H than the Sun, but burns it with a luminosity that is 30,000 times greater. It’s lifetime is 30/30,000 = 1/10,000 as long as the Sun – corresponding to a lifetime of only a few million years. This is a very short time, cosmically speaking. This is one reason why massive stars are so rare. Most of the massive stars that have ever been born are long since dead.

23. Betelgeuse Orion the Hunter

24. Supergiants are very large in addition to being very bright. Giants are somewhat smaller in radius and lower in luminosity, but still much brighter than main sequence stars of same spectral type. The hot, white, small radius stars near the lower left are called white dwarfs. Giants and Supergiants are stars nearing the ends of their lives because they have already exhausted their core hydrogen. Surprisingly, they grow more luminous when they begin to run out of fuel. If a supergiant were in the same spot as the Sun, it would engulf all of the planets through Jupiter!

26. STAR CLUSTERS All stars are born from giant clouds of gas. Because a single interstellar cloud can contain enough material to form many stars, stars almost inevitably form in groups. Many stars still congregate in the groups in which they formed. The two basic types of groups: open clusters and globular clusters. Open clusters are always found in the disk of a galaxy and can contain up to several thousand stars and typically span about 30 light years (10pc). The most famous open cluster is is the Pleiades (also called “the seven sisters”) in the constellation Taurus.

27. The Pleiades (to the right of Taurus the Bull in the sky) is an open star cluster

28. Globular clusters are found in both the halo and disk of our galaxy. They can contain more than a million stars, concentrated in a ball typically 60-150 light years across. The innermost region of a globular cluster can have 10,000 stars packed within just a few light years! M-13 Globular cluster in Hercules

29. What would the view be like from a planet in the midst of a globular cluster?

30. Star clusters are useful to astronomers for two key reasons: 1. All the stars in a cluster lie within the same distance from Earth 2. All the stars in a cluster formed at about the same time (within a few million years of each other)