Post on 15-Oct-2020
transcript
The Great Debate
! What was being debated at the Great Debate? Why is this a difficult problem? How was the debate resolved?
! What are the basic properties (mass, shape, color, luminosity) of elliptical, spiral and irregular galaxies?
! Why are spiral galaxies (their arms) blue? For the same reason, it’s true that irregular galaxies are blue (young) and ellipticals galaxies are red (old)
! How do rotation curves tell us that most of the mass in galaxies (and so in the Universe) is made of dark matter?
! How do galaxies form? What is one observational clue to why galaxies are so different? What is a giant elliptical galaxy? What is a rich cluster? A poor cluster (or group?)
Learning Objectives
Rosse’s M51 sketch
Modern M51 picture
“Leviathan” Lord Rosse’s 1.8 m reflecting telescope
“Spiral Nebulae”! Dim, diffuse “nebulae”
with spiral patterns! Spiral structures noted
in the mid-1800s by Lord Rosse (Ireland)
! Much, much disagreement:! Kant: Our Galaxy is a spiral
“island universe” and the other spiral nebulae are the same asour Galaxy and far away
! Herschel and others: Our Milky Way Galaxy is the whole Universe, and the other spiral nebulae are nearby
Sun*Galaxy
*Sun wrongly in center as this is pre-Shapley’s globular cluster studies
The Great Debate - How Far Away Are They?
The Great Debate! Think about how difficult a
problem was discussed at the Great Debate
! We see these weird things in the sky…but we see lots of weird things...novae, star clusters, giant clouds!Why should these nebulae
be special?! How might we prove these
things are either quite close to us (within our Galaxy), or really far away from us (outside of our Galaxy)?
The Andromeda Galaxy: The largest “spiral nebula”
Edwin Hubble! 1923: Hubble resolved M31, the
Andromeda “nebula”, into stars! After resolving stars, Hubble went
looking for Cepheid Variables (stars that have a pulsation period-absolute magnitude relation)
! Found M31 was ~300,000pc away (modern estimate is ~770,000pc)
! Andromeda is an “island universe” like our own Galaxy
The Andromeda “Nebula”
Hubble at Mt. Wilson Observatory
The three classes of galaxies
Irregular Galaxies Spiral Galaxies
Elliptical Galaxies
Classes of Galaxies!Spirals (S)
!Structure: disk, halo, bulge!Medium to large galaxies!The disk has the young
blue stars, while the bulge and halo have older red stars
!Ellipticals (E)!Pure bulge, no disk component!Large range in sizes!All older red stars
! Irregulars (Ir)!Well, erm, irregular!Smaller galaxies!Mostly young blue stars
Galaxy Types: Overview
Spirals Ellipticals Irregulars
Mass (MSun) 109 – 1012 105 – 1013 108 – 1010
Luminosity (LSun) 108 – 1010 105 – 1011 107 – 109
Diameter (kpc) 5 - 200 1 - 200 1 - 10
Color
Disk: Blue-to- white
Halo/Bulge: Red-to-yellow
Reddish-yellow Bluish-white
Why do we see Spiral Arms?! They are easily
seen as the arms contain numerous bright, blue O and B stars!These stars
also illuminate dust in the arms
! However, stars in general are evenly distributed throughout the disk
Face-on
Edge-on
Effect of Viewing Angle
Easier to seethe spiral arms
Easier to see the dust in the disk
Rotation Curves‣ A rotation curve is a graph that plots orbital
speed vs. the distance from the center ‣ Example: rotation curve of the planets
Masses of Galaxies! As with the Milky
Way, we measure the speed that stars orbit in a galaxy’s disk!Use (Newton’s
version of)Kepler’s Laws to calculate the mass
! Like the Milky Way, all other galaxies have flat rotation curves
! The mass is mostly in the outer parts of galaxies, not where the stars exist
! Indicates a halo of dark matter
Dark Matter! Normal matter makes up less than
5% of the Universe! Dark matter is spherically
distributed all around galaxies in huge halos
! What is this stuff that emits no light!Neutrinos? Black holes, brown or
red dwarfs, cooled-off white dwarfs?! MACHOs: Massive Compact Halo Objects
!Something exotic – particles as yet unknown?! WIMPs: Weakly Interacting Massive Particles
! We don’t know exactly what dark matter isbut we categorically, absolutely know it’s there
How did our galaxy form?(a) Proto-galactic clouds are
drawn together by gravity; halo stars begin to form
(b) Merged cloud collapses(c) Gas & dust settle into a
spinning disk; halo star formation stops. As of now there are only old and long-lived objects in the halo (red dwarfs, white dwarfs, black holes etc.)
(d) Stars continue to form in the disk. As of now there are still young stars in the disk Warning: This model is
oversimplified!
M87NGC 4414
But why do some galaxies end up looking so different?
Observational Clue: Galaxy Clusters! Galaxies occur in a wide
range of sizes of clusters! “Rich” clusters have many
galaxies, mostly elliptical!At the center of each rich
cluster there is usually a giant elliptical galaxy
! “Poor” clusters contain fewer than ~100 galaxies!Our Galaxy is in a “poor”
group of ~35 galaxies!For some reason “ending up close” to other
galaxies makes galaxies tend to be elliptical
Elliptical galaxies dominate rich clusters of galaxies
Next Time
Objects in the Rear-view Mirror May Appear Closer Than They Are