week 8 galaxies reading: chapter 15, sections 1, 3 (9 pages)
TRANSCRIPT
Week 8Galaxies
Reading:
Chapter 15, Sections 1, 3 (9 pages)
Galaxy/Not a Galaxy?
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765
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8 9 1011
Types of Galaxies
The Hubble Classification System:• Spirals• Barred Spirals• Ellipticals• Irregulars
Spiral GalaxiesHave nuclear bulges and spiral arms.
Spiral arms have clouds that are forming new stars.
Three Sub-types:
Nuclear Bulge
Spiral Arms
Sa Big Tightly wound
Sb Moderate Moderately wound
Sc Small Loosely wound
Spiral galaxies are classified according to the size of their central bulge and the tightness of their arms.
Spiral Galaxies (Face-on)
Sa Sb Sc
Tight arms, big bulge
Loose arms, small bulge
Spiral Galaxies (Edge-on)
Sa Sb Sc
Tight arms, big bulge
Loose arms, small bulge
Face-on
Edge-on
Barred Spiral Galaxies• Like normal spirals, but have central bars
• Bars are extended, linear bulges
• Half as common as “normal” spirals
SBa SBb SBc
Spiral Density Waves
Spirals arms like traffic jams – jam always there, but different cars.
Stars and gas clouds rotate around galaxy faster than the spiral density wave.
Clouds get compressed when passing through wave; new star formation is triggered.
Spiral Density Waves
Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars.
Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation.
Elliptical Galaxies
Elliptical Galaxies
Elliptical in shape
Mostly older stars (note yellowish color)
Seven Sub-types: E0, E1, E2, E3, E4, E5, E6, and E7
E0 – almost perfectly round
E3 – somewhat elongated
E7 – very elongated
Elliptical Galaxies
E0 E3 E6
Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated). Which class depends on angle; E0 can look like E7 if seen “edge-on”.
Giant Elliptical Galaxies
~20x as big as the Milky Way
Dwarf Elliptical Galaxies
So few stars that you can see right through them.
S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas:
Irregular galaxies have a wide variety of shapes. Both these galaxies appear to be undergoing interactions with other galaxies.
Irregular Galaxies
Irregular Galaxies
LMC = Large Magellenic Cloud
SMC
Galaxy Zoo
How We Know that Galaxies are Far Away
Cepheid variables
1912 – Henrietta Leavitt’s Cepheid Period-Luminosity Relation
Cepheid Variables in Andromeda
1923 – Edwin Hubble discovers Cepheids in the Andromeda “Nebula”, M31. It is 2.2 million light-years beyond the Milky Way
The Local Group
The Local Group is the cluster of galaxies to which the Milky Way belongs.
• LG is relatively poor: ~40 galaxies
• Andromeda (M31): largest and most massive
• More than 1/3 are dwarf ellipticals
• 3 are spirals (MW, M31, M33)
• New dwarf ellipticals being discovered
Here is the distribution of galaxies within about 1 Mpc of the Milky Way.
The Local Group
A nearby galaxy cluster is the Virgo cluster; it is much larger than the Local Group, containing about 3500 galaxies.
Most galaxies are too far away to pick out their Cepheid variables.
How do we measure distances to them?
Using atomic spectra, Doppler shifts and the Hubble law.
Very Distant Galaxies
Cosmic Fingerprints
Spectral lines are like fingerprints – they identify the element that produces them.
We use these fingerprints to study the chemical composition and distances of objects in space.
Doppler Shifts
A moving source of light appears to produce different lines (fingerprints) than a stationary source of light.
This effect is called a Doppler Shift.
That is, a moving object’s fingerprints will be shifted with respect to the fingerprints from a stationary object.
The Doppler Effect
Depends only on the relative motion of source and observer.
If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer.
Light moving towards you is blueshifted.
Light moving away from you is redshifted.
Most galaxies are too far away to pick out their Cepheid variables. Their distances are instead found through their Doppler shifts.
All galaxies seem to be moving away from us, with the redshift of their motion correlated with their distance:
Very Distant Galaxies
These plots show the relation between distance and recessional velocity
Hubble’s Law
The relationship (slope of the line) is characterized by Hubble’s constant H0:
The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc.
Note that the Universe is expanding. More on this in the last week of class.