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Proto-stars with tails. The tails point away from Trapezium. Debris disk around young star Stellar Wind and Radiation pressure • Stellar wind is comprised of charged particles which are shot out of a star and move with very high speeds out into space • When stellar wind particles hit gas outside the star, it causes the gas move rapidly away from the star. • Radiation (light) being emitted by stars can also effect material outside of a star. When dust absorbs the light, dust particles respond by moving away from the star. Proto-planetary Survivors New star in the vicinity of a high mass O-star. The same effect occurs with comets and our Sun Comets • Comets form far outside of the normal solar system. They sometimes fall in and pass close to the and then move back out into the depths of the solar system. • They have a composition that is the same as the gas cloud (the solar nebula) that the Sun formed out of. • They are mostly frozen water, frozen carbon dioxide, dust and small rocks. • Their typical size is about the size of Lexington. • Radius ~ 10 miles. • When a comet approaches the Sun the volatile compounds (water, CO2) begin to evaporate. • Jets of gas knock off dusty particles as well. • As the comet approaches the Sun it starts to grow a tail. Often times two distinct tails. One composed of gas molecules and one composed of dust. Halley’s comet and gas jets Gas tail Dust tail When the comet goes around and then leaves the Sun, will it look like this…. • 1) Or this? • 2) Please make your selection... 1. Choice One 2. Choice Two It’s this one • 2) • In the vacuum of space the only thing pushing on the tail is the solar wind and the light coming from the Sun. The tail has to always point away from the Sun, regardless of how the comet moving. Star pushing back with its own, weaker wind Direction of intense light and stellar wind from high mass stars Eagle Nebula (M 16) Self-sustained star formation • Very massive O-type stars and drive away all the gas and dust near them. When the material collides with other gas and dust in the molecular cloud, the gas and dust becomes compressed and new star formation begins. • This is why most star forming regions have stars forming, and nearby a cluster of stars with hot Ostars that are driving the new formation. Perseus Double Cluster • This is also why, not all of the star forming regions in a galaxy are found in the spiral arms. Some are in between the spiral arms. Because of self-sustained star formation. M 51 The Whirlpool Galaxy Triggers to star formation • Over density of gas and dust in spiral arms can initiate star formation in a molecular cloud. • It is also possible for two molecular clouds to collide and set off star formation. This happens when galaxies collide. • Once star formation is initiated in a molecular cloud it is possible to continue the process through self-sustained star – Either from Hot O and B type stars – Or from supernova explosions Cloud-cloud collisions Cloud-cloud collisions Cloud-cloud collisions The spiral arms in a galaxy are density waves. These waves are nearly stationary. They do not move like the stars, gas and dust do. The sun for instance orbits the galaxy at 220 km/s. Since the spiral arms are barely moving, the Sun passes in and out of spiral arms several times each orbit. Gas clouds also orbit at the same speed as the Sun. But when gas clouds pass through the spiral arms they become compressed. The spiral arms in galaxies are star forming factories. M 51 The Whirlpool Galaxy Spiral Density waves initiate star formation A molecular cloud passing through the Sagittarius spiral arm A molecular cloud passing through the Sagittarius spiral arm Spiral arm Gas outflows from super supernova or O/B star winds Initiation of star formation • It takes the Sun about 250 million years to orbit the Galaxy once. (250,000,000 years) • The Sun formed about 5 billion years ago. • That’s 5,000,000,000 years ago. • How many times has the Sun orbited the Galaxy? How many times has the Sun orbited the Galaxy? 60 30 1. 2. 3. 4. 2 times 20 times 200 times It has made one orbit yet 0% 01 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 12 1 0% 13 14 15 2 0% 16 17 3 0% 18 19 20 4 • 5 billion years/250 million years for each orbit. • 5,000,000,000/250,000,000 • Or 500/25 = 20 times. • The Sun has orbited the Galaxy about 20 times since it formed. • It takes the Sun about 250 million years to orbit the Galaxy once. • The circumference of the orbit is about 200,000 light years. Let just call it 250,000 light years. (for ease of calculation) • If an O-star forms that has the same orbit as the Sun but has a total lifetime of 1 million years. • How far does this star make it from the spot that it formed? How far does the O-star move in its orbit before it dies? 60 30 1. 2. 3. 4. 1 light year 100 light years 1000 light years 250 million light years (one complete orbit) 0% 01 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 12 1 0% 13 14 15 2 0% 16 17 3 0% 18 19 20 4 • If it takes 250 million years to go around the Galaxy once, then the O-star will only make it 1/250 the way around. • IF the circumference is 250,000 light years, then it will travel 250,000 ly/250. • Distance from the spot of origin is 1,000 light years. Then it dies. Supernova triggered star formation • The largest mass stars, form, use up their fuel and die in an enormous explosion before they ever move away from their formation site. • The shock waves from the explosion helps to compress the molecular cloud in which they formed and set off new star formation. Supernova bubble Supernova bubble Molecular cloud Bubble moving outward from supernova Star formation in a compressed cloud • A region of the molecular cloud becomes dense. • This pocket of over density is much bigger than a single star. • This over dense region is not uniform, but has within it other, smaller regions of high density. • As the over density begins to be drawn together by gravity, it fragments into smaller pockets of gas which go on to form individual stars. • The result is a star cluster. The more massive pockets from massive stars, the less massive form smaller stars, like the Sun Spinning stars and disks • As material falls into a newly forming star it begins to spin rapidly. • This is due to another conservation law. It is the conservation of angular momentum. • Angular momentum is similar to regular momentum in a straight line. Angular momentum is just the momentum that keeps things spinning. Angular momentum is constant • L = mass X velocity X radius • Where L is angular momentum and it is constant in a system. • L = mvr • Let’s examine this by first holding the mass, m, constant. L = mvr So, what happens if the radius decreases? 30 30 1. The velocity will increase 2. The velocity will decrease 3. The velocity will stay the same 4. L will decrease 0% 01 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 12 1 0% 13 14 15 2 0% 16 17 3 0% 18 19 20 4 • If the radius were to decrease, then the velocity has to increase. Causing the object to speed up its rotation. • L = mvr and L is constant. So if r gets smaller and m is constant, there is no choice but for v to increase in such a way as to keep mvr constant.