1 Ch. 10: Star Formation of Planetary Systems A summary of the process by which our solar system formed, according to the nebular theory.
Materials in the solar nebula. 2
3 Temperature differences in the solar nebula led to different kinds of condensed materials, sowing the seeds of two different kinds of planets. About 10 11 (100,000,000,000) stars in a galaxy; also about 10 11 galaxies in the universe Stars have various major characteristics, the majority of which fall into several simple types. These are related to their stages of the life cycles, just like people. The INITIAL MASS is a very important aspect that determines a star s future:
Here is anther similar diagram: 4
5 Life Cycles of the Hot and Massive Young stellar objects (YSOs) The normal life cycle of stars with mass about that of the Sun is as follows: 1) Gas and dust in a cool nebula condense, forming a young stellar object (YSO) Life Sequence of a Star 2) Shrinking, the YSO dispels its remaining birth cloud, and its hydrogen fire ignites 3) As the hydrogen burns steadily, the star joins the main sequence. 4) When the star uses up all of the hydrogen in its core, the hydrogen in the shell (a larger region around the core) ignites. 5) The energy released by the burning of the hydrogen shell makes the star brighter and it expands, which makes the surface larger, cooler, and redder. The star has become a red giant. 6) Stellar winds blowing off the star gradually expel its outer layers, which form planetary nebula around the remaining hot stellar core. 7) The nebula expands and dissipates into space, leaving just the hot core. 8) The core, now a white dwarf star, cools and fades forever. Note that after most H is converted to He in the core, the star becomes a Red Giant. If the star is much more massive, it burns elements in the core all the way up to Iron (Fe), and becomes a super Red Giant, then explodes as a SUPERNOVA, and the inner region forms a neutron star or black hole. The Sun will last about 10 billion years, but a much more massive star may last only a few million years after its birth. Less massive starts may be red dwarfs, and remain so for as very long time. RULE: the bigger (more massive) the star, the shorter is its lifetime.
6 Main-Sequence Stars: A long adulthood Having shed their birth clouds, young stars now shine due to the nuclear engine. Normal stars are main-sequence stars, with roughly the same mass as that of the Sun The vast majority of main-sequence stars are red dwarfs So dim that cannot be seen even the nearest ones----- without a telescope Red giants: Burning out the golden years Much larger than the Sun (sometimes having the size of the orbit of Venus, or even Earth). Occurs in the late life of an ordinary Sun-like star ----
7 Stars MUCH LARGER than the Sun first become Red Giants, then start burning He to C (and perhaps other elements depending on mass), and finally swell up to massive RED SUPERGIANTS. Can become 1,000-times the size of the Sun, big enough to extend past the orbit of Jupiter or Saturn. NO TIME for life to form on orbiting planets!! If the star is large enough, after the core burns all H to He it shrinks and H burns on shell. Red giant. But also, as the core shrinks He can burn to C. So tow layers of burning: in core He to C, and outside H to He. The core outward pressure involves both gas pressure and core degeneracy pressure. So star can go from Red Gant to Red Supergiant! Closing time: Stars at the tail end of stellar evolution Central stars of planetary nebula Little stars at the center of beautiful nebula. The shape and color (sometimes false) depend on condition of the exploding star and configuration of nearby stars and fields (gravitational & magnetic). Like little white dwarfs; white because still hot Gradually the clouds expand and blow away
8 White Dwarfs A diamond (carbon)weighing 10 billion trillion trillion carats is at the heart of a dead white dwarf star nicknamed Lucy in this conception by an artist at the Harvard-Smithsonian Center for Astrophysics Can have the MASS of the SUN, but be as small as EARTH White at first when very hot, but as it cools can transition through yellow, orange, red, brown and black, depending on the temperature. They just slowly fade away (visually)---------- No longer burning, just cooling A TEASPOON of this dead star would have a weight of a TON on EARTH
After the core burns all H to He, the outer shell will burn and red giant develops. If the star is massive enough it will start burning He to C and heavier elements in rings, with the heaviest in the middle. The star will then swell up to a red supergiant. Whether the core eventually becomes a white dwarf, neutron star or black hole depends on the mass of the star. 9
10 Supernovas Enormous explosions!! Before exploding was a RED or BLUE SUPERGIANT Neutron Stars Extremely small!! One in diagram here about 20 km!! Expands only a several miles across, BUT perhaps twice the mass of the Sun. A TEASPOON would WEIGHT about a BILLION TONS on Earth Better known as PULSARS: highly MAGNETIZED, rapidly SPINNING, beams of RADIATION; FLASHING like a BEACON. During rapid shrinkage of the original star to a very small neutron star, conservation of angular momentum occurs, and these objects can spin at rates of up to 700 revolutions per second (faster than the rotation speed of a food blender). Amazing!!
11 Black Holes Much denser and small than neutron stars Not even light can escape; therefore, BLACK Some hypothesize that matter that falls into one leaves our universe (not proven or widely accepted idea) Can not directly see a black hole, BUT the matter around it gets swept into the hole, causing much high energy radiation that we CAN observe There are also, powerful JETS that shoot matter out into space at near the speed of light Another indication of a BLACK HOLE are stars in orbit moving at fantastic speeds due to the extreme gravity of the black hole. Three types of black holes: Stellar-mass black hole: From 3 to 100-times mass of the Sun Result of a Supernova explosion About size of a neutron star; SIZE means diameter to the EVENT HORIZON, the point at which no light can escape. For a mass of 10-times Sun, size = 60 kilometers Sun (would be, but not possible) 6 kilometers Intermediate-mass black holes: Mass of 500 to 1,000-times Sun Super-massive black hole: Mass of 10 5 to 10 9 Sun. Usually located at the CENTER OF A GALAXY; Milky Way has a central black hole, called Sagittarius A, 2.5 million solar masses We orbit about it once every 226 million years.
12 Explosive neighbors: Flare stars Little red dwarfs that suffer big explosions like ultra-powerful solar flares. Nice to novae: Exploding stars NOVAS explode through a build-up process onto a white dwarf in a binary system. The hot, dead white dwarf, rips off hydrogen from its companion. This is like dumping gasoline on a fire. Suddenly the white dwarf once again has fuel to carry out fusion reactions, and the engine is restarted in an extremely dramatic way. Suddenly it undergoes a special type super-nova explosion!! SUPENOVAS: Ejected mater; cores become neutron stars or black holes. HYPERNOVAS: especially bright supernovas. Stellar hide and seek: Eclipsing binary stars Observed brightness changes as stars eclipse each other periodically. Hogging the starlight: Micro-lensing events Microlensing: gravity so large that light is bent around object. When looking deep into space with a large telescope, one finds very strange double images that are sort of twisted out of shape. At first nobody could understand this phenomenon, but eventually it was understood to be gravitational lensing Gravitational lensing occurs when light from a distant object passes a region with extremely high mass and thus gravitational field. This could be a black hole or dark matter. This super mass causes the light traveling towards the telescope
13 to be bent in a focusing manner, like light passing through a convex focusing lens in a telescope. Thus the name gravitational lensing. This phenomenon has provided a way to detect very distant objects. More importantly, it also provided a way to locate otherwise invisible phenomena like black holes and dark matter, and has thus become very important in astronomy. There is also Microlensing, which is like gravitational lensing, but due to much less-massive objects, like foreground stars.