Download The Milky Way

Reminders
•
Office Hours For Tuesday and Thursday 3:30 – 5pm
•
Final Exam: Monday May 8th 6:30-9:00pm
–
Phys1403 – Introductory Astronomy
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Instructor: Dr. Goderya
•
You can come look at your standing in class
Bring a Green SCANTRON Sheet (make sure its not
crimpled)
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No. 2 Pencil and a scientific Calculator
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You are allowed to bring 1 page (both sides) reference
sheet with equations, constants, conversion factors from
textbook or lecture PowerPoint notes.
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50 Multiple Choice Questions
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About 12-15 questions from each of Exam 1, 2 and 3.
Remaining questions from material not included in exam 3
and material covered till last class.
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No Cell Phones or laptops.
Last set of homework will be due Wednesday May 3rd.
Topics for Today’s Class (Last Class)
Foundations of Astronomy | 13e
Seeds
• Milky Way
– General Shape and Structure
– Globular Clusters
– Stellar Populations
– Black Hole in the Center
Chapter 15 and 16
• Other Galaxies
– Shape of Galaxies and Galaxy Classification
Selected Topics
– Hubble's Law
–
Extragalactic Distance Scale
• Dark Matter and Dark Energy
–
What is Dark Matter?
–
What is Dark Energy?
© Cengage Learning 2016
The Milky Way
The Structure of the Milky Way (1)
Disk
Nuclear Bulge
Sun
Almost everything we see in
the night sky belongs to the
Milky Way
We see most of the Milky
Way as a faint band of light
across the sky
From the outside, our Milky
Way might look very much like
our cosmic neighbor, the
Andromeda galaxy
Halo
Globular Clusters
1
The Structure of the Milky Way (2)
Exploring the Milky Way Using
Clusters of Stars
Two types of star clusters:
1) Open clusters: young clusters of recently
formed stars; within the disk of the Galaxy
Galactic Plane
Open clusters h
and  Persei
Galactic Center
The structure is hard to determine because:
1) We are inside
2) Distance measurements are difficult
3) Our view towards the center is obscured by gas
and dust
Globular Clusters
Globular Cluster M 19
2) Globular clusters: old, centrally concentrated
clusters of stars; mostly in a halo around the
Galaxy
Locating the Center of the Milky Way
• Dense clusters of
50,000 – 1 million
stars
Distribution of
globular clusters is
not centered on
the sun…
• Old (~ 11 billion
years), lower-mainsequence stars
• Approx. 200
globular clusters in
our Milky Way
Globular Cluster M80
The Mass of the Milky Way (2)
Total mass in the disk
of the Milky Way:
Approx. 200 billion
solar masses
Additional mass in an
extended halo:
Total: Approx. 1 trillion
solar masses
Most of the mass is not
emitting any radiation:
Dark Matter!
…but on a location
which is heavily
obscured from direct
(visual) observation
Metals in Stars
Absorption lines almost exclusively from hydrogen: Population
II
Many absorption lines also from heavier elements (metals): Population
I
At the time of
formation, the gases
forming the Milky Way
consisted exclusively
of hydrogen and
helium. Heavier
elements (“metals”)
were later only
produced in stars.
=> Young stars contain more metals than older stars
2
A Black Hole at the Center of Our
Galaxy
Stellar Populations
Population I: Young stars:
metal rich; located in spiral
arms and disk
By following the orbits of individual stars near the
center of the Milky Way, the mass of the central black
hole could be determined to ~ 2.6 million solar masses
Population II: Old stars: metal
poor; located in the halo
(globular clusters) and
nuclear bulge
Other Galaxies in the Universe
Galaxy Diversity
Even seemingly
empty regions
of the sky
contain
thousands of
very faint, very
distant galaxies
Large variety of
galaxy
morphologies:
Spirals
Ellipticals
• Star systems like our Milky Way
Irregular
• Contain a few thousand to tens of billions of stars.
• Large variety of shapes and sizes
10-day exposure on an apparently empty field in the sky
Distance Measurements to Other
Galaxies: The Hubble Law
Galaxy Classification
E0, …, E7
The Hubble Deep Field:
(some interacting)
Sa
E0 =
Spherical
Large
nucleus;
tightly
wound arms
E1
Sb
E. Hubble (1913):
Distant galaxies are moving away from our Milky Way, with
a recession velocity, vr, proportional to their distance d:
vr = H0*d
H0 ≈ 70 km/s/Mpc is the
Hubble constant
Sc
E7 = Highly
elliptical
E6
Small
nucleus;
loosely
wound arms
• Measure vr
through the
Doppler effect 
infer the distance
3
The Extragalactic Distance Scale
What is Dark Matter?
• Many galaxies are typically millions or
billions of parsecs from our galaxy.
• Typical distance units:
Matter that can be seen by its gravitational
effects, but does not emit light.
Mpc = Megaparsec = 1 million parsec
Gpc = Gigaparsec = 1 billion parsec
• Distances of Mpc or even Gpc  The
light we see left the galaxy millions or
billions of years ago!!
• “Look-back times” of millions or billions of years
Dark Matter
Not Dark Matter
Dark Energy Comprises
73% of Universe
What is the Dark Energy?
Dark Energy is an unknown
form of energy that is present
everywhere in the Universe
and is hypothesized to
accelerate the expansion of
the Universe.
Dark Energy
73%
“Normal Matter”
4%
Dark Matter
23%
Wikipedia
Dark Energy is an
Unfinished Story
From The Syllabus
WE DON’T KNOW WHAT IT IS!
But it traces the story of our understanding
of the nature of the universe.
• An ideal setting for illustrating the process of science:
– Science is alive and on-going.
– Our ideas change as the data changes.
– Scientific debate differs from social/political
debate.
– Progress in science results from both individual
and group efforts.
4
That’s It For This Semester
Exam 3
Hi – 86
Low – 0
Average - 65
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
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