NATS 1740 Chapter Notes - Chapter 15: Dwarf Elliptical Galaxy, Inverse-Square Law, Elliptical Galaxy

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Published on 13 Apr 2013
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Chapter 15 Galaxies and the Foundation of Modern Cosmology
15.1 Islands of stars
Cosmology: study of galaxies
What are they 3 major types of galaxies?
Spiral galaxies: like the milky way, look like flat, white disks with yellowish blues at
their centres
The disks are filled with cool gas and dust interspersed with hotter ionized gas and
usually display spiral arms
Elliptical galaxies: redder, rounder and often longer in one direction that then other like
a football
Compared with spiral galaxies, elliptical galaxies contain very little cool gas and dust
thought they contain very hot ionized gas
Irregular galaxies: appear neither disklike nor rounded
Spiral galaxies
- Spiral galaxies have a disk, bulge and halo just like the milky way
Disk component (population I): flat disk in which stars follow orderly and nearly
circular orbits around the galactic centre
Always contains an interstellar medium of gas and dust but the amount may differ
from one spiral galaxy to the next
Spheroidal component (population II): contains little cool gas and dust and stars have
orbits with many different inclination s
Barred spiral galaxies: have a straight bar of stars cutting across the centre with spiral
arms curling away from the ends of the bar
Leticular galaxies (lens shaped): have disk and spheroidal components but lack spiral
arms
Intermediate class between spiral and elliptical
Elliptical galaxies
- Elliptical galaxies differ from spiral galaxies in that they don’t have significant disks
Sometimes called spheroidal galaxies
Some are called massive elliptical galaxies or dwarf elliptical galaxies
Irregular galaxies
- Irregular galaxies appear to be in disarray
Hubbles galaxy classes
- Created a system to organize galaxies into a diagram shaped like a tuning fork (page 489)
How are galaxies grouped together?
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- Spiral galaxies tend to congregate in small groups while elliptical galaxies are primarily
found in large clusters
15.2 Distances of galaxies
How we do measure the distances of galaxies?
Radar ranging: technique to measure AU in which radio waves are transmitted from
earth and bounced off Venus
Standard candles
- We can determine distance by measuring the apparent brightness of an object whose
luminosity we already know and applying the inverse square law for light
Standard candle: light source of a known standard luminosity
- The inverse square law for light tells us how an objects apparent brightness depends on
its luminosity and distance
Distance = square root of luminosity/ 4π x (apparent brightness)
Main sequence fitting
- Need to follow 2 steps to use bright main-sequence stars as standard candles
1.) Identify a star cluster that is close enough for us to determine its distance by parallax
and plot its H-R diagram
Because we know the distances to the cluster stars, we can use the inverse square law
for light to establish their true luminosities from their apparent brightness
2.) We can look at stars in other clusters that are too far away for parallax measurements
and measure their apparent brightness’s
If we assume that main-sequence stars in other clusters have the same luminosities as
their counter parts in the nearby cluster, we can use the inverse square law for light to
calculate their distances
Main sequence fitting: technique for determining distances by comparing main
sequences in different star clusters
Cepheid variables
- Cepheid variable stars are useful for measuring distances because we can determine a
Cepheids luminosity from the period between its peaks of brightness
Cepheid variable stars (Cepheids): alternately become dimmer and lighter and are used
to measure distances between galaxies
Been used for almost a century
Distant standard candles
- White dwarf supernovae are useful for measuring large distance because they are bright
and all have about the same peak luminosity
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Document Summary

Chapter 15 galaxies and the foundation of modern cosmology. Spiral galaxies: like the milky way, look like flat, white disks with yellowish blues at their centres. The disks are filled with cool gas and dust interspersed with hotter ionized gas and usually display spiral arms. Elliptical galaxies: redder, rounder and often longer in one direction that then other like a football. Compared with spiral galaxies, elliptical galaxies contain very little cool gas and dust thought they contain very hot ionized gas. Irregular galaxies: appear neither disklike nor rounded. Spiral galaxies have a disk, bulge and halo just like the milky way. Disk component (population i): flat disk in which stars follow orderly and nearly circular orbits around the galactic centre. Always contains an interstellar medium of gas and dust but the amount may differ from one spiral galaxy to the next. Spheroidal component (population ii): contains little cool gas and dust and stars have orbits with many different inclination s.

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