Physics: Lecture 20
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Definition of Galaxy
- how big
- what does it contain besides stars
a very large collection of stars held together by gravity
- > 107 - 1010
- usually contains gas and other matter as well
What are some important aspects of stars
- Distance + Flux = Luminosity
- Structure (shape, morphology (form))
The Milky Way
- How many stars
- why, in pictures of the galaxy, does light appear to be smooth, how transparent?
- elements of the galaxy
- what type of galaxy
- - 100 billion (1011) stars
- - stars are so far away and so close together that their light overlaps and looks smooth (continuous emission of light from billions of stars) --> opaque
- - bulge (direct center), disk, spiral arms, halo (actually a "stellar halo"- a very sparse, faint collection of very old metal poor stars, globular clusters
- - spiral galaxy
Distances in the Galaxy
- from sun to center - on what arm of galaxy?
- length of disk
- - 28,000 ly, 8 kpc - sagittarius arm
- - 1,000 ly or 3000 pc
- - 100,000 ly or 30, 000 pc
The different components of a galaxy have different ....
- colors (sizes --> blue = massive)
- chemical compositions
- - flat disk with bulge at center
- - size of bulge and prominence of spiral patters vary
- - disk contains inteterstellar medium (ISM) of cool gas and dust
- - larger bulge, less ISM, and therefore less star formation
- - dust lanes --> gas --> active star formation
Shape depends on
orientation (somewhat), formation
Disk Component vs Spiral Component of Spiral Galaxies
- disk component - flat disk where stars follow orderly almost circular orbits around center.
- - Always contains an interstellar medium (gas and dust)
- Spheroidal component - bulge and halo together.
- - Bulge rotates, but not much, more like stars moving back and forth,
- - usually little ISM
- - those with larger bulges (the Sombrero Galaxy) usually contain less interstellar gas and dust, and less current star formation
- - bulge dominates, but still has a clear disk.
- - spiral pattern is visible, but tightly wound.
How are massive stars produced
gas in the disk collapses
Rotation of the whole galaxy takes how long
- 100 million years,
(bulge rotates, but not much, more like stars just moving back and forth)
Elliptical Galaxies (or just "ellipitcals")
- - all bulge
- - very little gas
- - mostly low-mass stars
- - "late type"- has evolved to the point where there is no gas left for making new stars!
- - random motion
Irregular Galaxies - in Between Elliptical and Spiral
- - "lenticualrs"or S0 Galaxies
- - like ellipticals but usually a bit fatter, usually lack spiral arms
- - tend to have less cool gas than normal spirals but more than ellipticals
- - bulgeless, low mass --> more chaotic
- - very high star formation rate
- - no bulge or spiral patterns
Barred Spiral Galaxies
- - many galaxies have "bars" - linear arrangements: straight bar cutting across the center, with spiral arms curling away from ends of bar
- - the Milky Way - b/c our bulge is somewhat elongated
The Hubble Sequence
- - Hubble invented a system for classifying galaxies (diagram looks like a tuning fork)
- - Ellipticals or spheroids (more massive, gas poor, older) on handle then split into Spirals on to (normal or barred) and Disks (Both are less massive, gas rich, ongoing star formation)
- - most numerous type of galaxy in universe
- - most giant galaxies probably made up of merged dwarf galaxies
Mathematical Insight 20.1: Standard Candles
- The apparent brightness of a light source decreases with the square of its distance away from us.
- (apparent brightnesss = luminosity / 4π x (distance)2)
- We can always measure and objects apparent brightness (flux)
- If an object is close enough to measure its distance through parallax, then we can calculate the luminosity using inverse square law (above)
But if the object is a standard candle
(already know luminosity) - we can solve the inverse square law to find distance
distance = √(luminosity / 4π x (apparent brighness))
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