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Stars form from condensing clouds of gas & dust. Sometimes triggered by an exploding supernova or colliding clouds of gas. Nebula. From the Latin for “Cloud” Once any fuzzy-looking object in the sky was called a “nebula” (we now know that many of these were galaxies or star clusters)
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Stars form from condensing clouds of gas & dust • Sometimes triggered by an exploding supernova or colliding clouds of gas
Nebula • From the Latin for “Cloud” • Once any fuzzy-looking object in the sky was called a “nebula” (we now know that many of these were galaxies or star clusters) • Now applied to a cloud of interstellar gas • Plural: “nebulae”
Types of Nebulae • Reflection Nebula • seen by the light of nearby stars • Emission Nebula • emits light of its own, especially H • Dark Nebula • invisible, except that it blocks light of stars behind it • Planetary Nebula • shell of gas ejected by a dying star
New hot, blue stars are forming in the center of the Rosette Nebula (NGC 2237)
Visible Infrared
Star formation Cloud of gas & dust collapses by gravity Protostar heats up as it is compressed Accretion stops as radiation prevents more material from falling into the protostar Nuclear fusion begins
Not all protostars can become stars • Stars less than about 0.08 M are too small to become stars, and are brown dwarfs • Stars bigger than about 100 M are unstable and shed mass until small enough
Plotting the stars of a cluster on an H-R diagram NGC 2264, a young cluster Not all stars have reached the main sequence
Plotting the stars of a cluster on an H-R diagram M55, an old globular cluster Larger stars have left the main sequence
The balance between gravity and radiation pressure is called hydrostatic equilibrium • As long as there is available hydrogen in the core of a star, it will be stable • When the hydrogen runs out, the balance is broken, and the star will move to the next stage in its life
Stars spend most of their life cycle on the main sequence • Main sequence stars are in hydrostatic equilibrium • outward thermal pressure is exactly balanced by the inward force of gravity • Main sequence stars are those stars fusing hydrogen into helium in their cores • Zero-age main sequence (ZAMS) is the location where a pre-main-sequence star fusing hydrogen in its core first becomes a stable object
The more massive a star, the faster it goes through its main sequence phase
Red Giants • When hydrogen in a star’s core has been used up, shell fusion, then He fusion begins • Hotter core • Larger radius • Cooler surface • More luminous overall
Plotting the stars of a cluster on an H-R diagram M55, an old globular cluster Larger stars have left the main sequence
Composition of Stars • Young, metal-rich stars like the sun are called Population I stars • Older stars formed from material that had not been enriched by heavy elements are Population II stars
Next week: Telescope observations • 8 pm Tuesday, May 6, last hour of class (telescopes on the roof) • Weather permitting • Attendance not required, but strongly recommended and will be for credit • If you can’t make it, see me for an alternate assignment
Fremont Peak Observatory Association • Public viewing starting 8 pm most Saturdays • Website http://www.fpoa.net • Info: (831) 623-2465
