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Week 8
The Milky Way
Reading:
Chapter 13, Sections 1-5,7 (12 pages)
Homework for Next Time:
www.galaxyzoo.org
Question
Which of these is a picture of the Milky Way?
A
B
C
D
The Milky Way
Andromeda: MW’s Sister Galaxy
Angular Size
Our Galaxy is a spiral galaxy. Here are two other
spiral galaxies, one viewed from the side and the
other from the top, which are thought to resemble
the Milky Way:
From Earth, we see few stars when looking out
of the galaxy (red arrows), many when looking in
(blue and white arrows).
The Milky Way across the sky
An infrared view of our galaxy shows much more
detail of the galactic center than the visible-light
view does, as infrared is not as much absorbed by
gas and dust.
• Nuclear Bulge
• Diameter ~20,000 ly
• Disk
• Diameter ~100,000 ly
• ~2000 ly thick
• Spiral Arms
• Young stars
• Halo
• Very large
• Spherical distribution
• Old stars
Galactic Structure
The galactic halo and globular clusters formed
very early; the halo is essentially spherical. All the
stars in the halo are very old, and there is no gas
and dust.
The galactic disk is where the youngest stars
are, as well as star formation regions – emission
nebulae, large clouds of gas and dust.
Surrounding the galactic center is the galactic
bulge, which contains a mix of older and younger
stars.
Galactic Structure
Stellar orbits in the
disk are in a plane
and in the same
direction; orbits in
the halo and bulge
are much more
random.
Measuring the Milky Way
How do we know about distances and
structures in the Milky Way?
Two ways:
1) Variable Stars
2) Radio Mapping
Variable Stars
on the HR
Diagram
How Variable Stars Work
Slowly pulsate (change in size and temperature)
• Star compressed: heats up, pushes outer
layers out
• Star expands: cools and contracts
• Cycle repeats
Some Types of Variable Stars
RR Lyrae
• Low mass variable stars
• Periods shorter than 1 day
Cepheids
• High mass variable stars
• Longer periods
Periods of Variable Stars
The upper plot is an RR Lyrae star. All such stars have
essentially the same periods (0.5 to 1 day).
The lower plot is a Cepheid variable; Cepheid periods range
from about 1 to 100 days.
The usefulness of these stars comes from their
period-luminosity relation:
Luminosity vs. Brightness
Two stars that appear
equally bright might
be a closer, dimmer
star and a farther,
brighter one:
Distances From Variable Stars
How to get the distance of a variable star:
1) Measure the apparent brightness
2) Measure its period
3) Use the period-luminosity relation to
determine its intrinsic brightness
(luminosity)
4) Use the relationship between apparent
brightness and luminosity to determine
the distance
RR Lyrae stars all have about the same
luminosity; knowing their apparent brightness
allows us to calculate the distance.
• Cepheids have a luminosity that is strongly
correlated with the period of their oscillations;
once the period is measured, the luminosity is
known and we can proceed as above.
Milky Way Scales Tutorial
Radio Mapping
• Radio waves can penetrate clouds of
gas and dust.
• Gas and dust absorb light at specific
wavelengths (e.g. @ 21 cm).
• If clouds are moving, Doppler shifts
cause absorption to be shifted.
• So, can see more than one set of clouds.
Radio Mapping
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.
Spiral Arm Structure from Radio
Mapping
Spiral Arms
4 major arms, some shorter arms
Sun is in shorter Orion arm
Major arms:
1) Sagittarius arm – towards Galactic center
2) Perseus arm – away from Galactic center
3) Centaurus arm
4) Cygnus arm
What Causes the Spiral Arms?
The spiral arms cannot rotate along with the galaxy;
they would “curl up”:
Rather, they appear to be density waves, with
stars moving in and out of
them much as cars move
in and out of a traffic jam:
Spiral Arms as Cosmic Traffic Jams
Globular Clusters
Globular clusters are
spherically distributed in
the halo
Variable stars tell
you distance.
Finding the Galactic Center
How to Find the Galactic Center
1) Locate globular clusters in the halo
2) Determine distances to globular clusters
using variable stars in the clusters
3) Find center of globular cluster
distribution; this is the Galactic center
Our Sun is ~28,000 lt-yrs from the Galactic
Center
The Galactic Center
This is a view towards the
galactic center, in visible light.
The two arrows in the inset
indicate the location of the center;
it is entirely obscured by dust.
These images, in
infrared, radio, and
X-ray, offer a
different view of the
Galactic center.
Sagittarius A*
At Galactic center, there is strong radio and
X-ray emitter called Sagittarius A*.
Near Sag A*, the gas and dust are moving
very fast.
6
Need a mass of ~4x10 solar masses to keep
this material from flying away.
Thus Sagittarius A* is probably a supermassive
black hole at the center of the galaxy.
The Galactic Center
Infrared zoom-in to the Galactic Center
Black Hole at the GC?
These stars are very close to the galactic center. The orbit on
the right is the best fit; it assumes a central black hole of 3.7
million solar masses.
Adaptive Optics
BH Orbits
http://astro.unl.edu/classaction/questions/m
ilkyway/ca_milkyway_bhstarorbit.html
Galactic
Center
Movie
IR Movie
https://www.youtube.com/watch?v=zAtnBzDZvsk
Next Time
Other Galaxies
Reading: 15.1, 15.3
galaxyzoo.org