Download ASTR100 Class 01 - University of Maryland Department of

ASTR100 (Spring 2008)
Introduction to Astronomy
Discovering the Universe
Prof. D.C. Richardson
Sections 0101-0106
How can we know what the universe was
like in the past?
 Light travels at a finite speed (c = 300,000 km/s).
Destination
Light travel time
Moon
1 second
Sun
8 minutes
Sirius
8 years
Andromeda Galaxy 2.5 million years
 Thus, we see objects as they were in the past:
The farther away we look in distance,
the further back we look in time.
Example:
This photo shows the Andromeda Galaxy as it looked about
2½ million years ago.
Question: When will be able to see what it looks like now?
M31, The Great Galaxy
in Andromeda
Definition: A light-year
 The distance light can travel in one
year.
 About 10 trillion km (6 trillion miles).
Distance
= Speed x Time
= (300,000 km/s) x (1 yr) x (31,557,600 s/yr)
= 9,500,000,000,000 km!
= 9.5 x 1012 km
• At great distances, we see objects as they were
when the universe was much younger.
• Can we see the entire universe?
Why can’t we see a galaxy 15 billion light-years away?
(Assume the universe is 14 billion years old)
A. Because no galaxies exist at such a
great distance.
B. Galaxies may exist at that distance,
but their light would be too faint for
our telescopes to see.
C. Because looking 15 billion light-years
away means looking to a time before
the universe existed.
Why can’t we see a galaxy 15 billion light-years away?
(Assume the universe is 14 billion years old)
A. Because no galaxies exist at such a
great distance.
B. Galaxies may exist at that distance,
but their light would be too faint for
our telescopes to see.
C. Because looking 15 billion lightyears away means looking to a
time before the universe existed.
How do our lifetimes compare to the age
of the universe?
 The Cosmic Calendar: a scale on which we
compress the history of the universe into 1 year.
Milky Way
forms
New Year’s Day:
The Big Bang
Oldest known life
(single-celled)
Sun & planets
form
First multi-cellular
organisms
How do our lifetimes compare to the age
of the universe?
 The Cosmic Calendar: a scale on which we
compress the history of the universe into 1 year.
Spaceship Earth
How is Earth Moving in
Our Solar System?
The Earth
rotates
around its
axis once
every day.
How is Earth Moving in
Our Solar System?
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
The Earth orbits the Sun (revolves)
once every year.
How is Earth Moving in
Our Galaxy?
The Sun moves randomly relative to other
nearby stars, and orbits the galaxy once
every 230 million years.
More detailed study of the Milky Way’s rotation
reveals one of the greatest mysteries in
astronomy…dark matter!
Most of Milky Way’s
light comes from disk
and bulge …
…. but most of the
mass is in its halo
How do Galaxies Move Within
the Universe?
Galaxies are
carried along
with the
expansion of
the universe.
Part C
The following statements describe ways in which the analogy might apply to the real
universe. Which statements are correct?
A. Both the raisin cake and the universe have a well-defined inside and outside.
B. Raisin 1 is near the center of the cake, just as our galaxy is near the center of the
universe.
C. The temperature starts low and ends high in both the raisin cake and the universe.
D. The raisins stay roughly the same size as the cake expands, just as galaxies stay
roughly the same size as the universe expands.
E. The average distance increases with time both between raisins in the cake and
between galaxies in the universe.
F¡. An observer at any raisin sees more distant raisins moving away faster, just as an
observer in any galaxy sees more distant galaxies moving away faster.
Enter the letters of all correct statements in alphabetical order (without spaces). For
example, if statements C and E are correct, enter CE.
DEF
Correct
Like any scientific model, the raisin cake analogy has limitations, but it gives us a good
overall picture of how the universe is expanding.
Are we ever sitting still? No!
Earth rotates on axis: > 1,000 km/hr
Earth orbits Sun: > 100,000 km/hr
Solar system moves among stars: ~ 70,000 km/hr
Milky Way rotates: ~ 800,000 km/hr
Milky Way moves
in Local Group
Universe
expands
Patterns in the Night Sky
What are constellations?
A constellation
is a region of
the sky.
88 constellations
fill the entire sky
(North & South).
Thought Question
The brightest stars in a constellation…
•
•
•
all belong to the same star cluster.
all lie at about the same distance from
Earth.
may actually be quite far away from
each other.
Thought Question
The brightest stars in a constellation…
A. all belong to the same star cluster.
B. all lie at about the same distance from
Earth.
C. may actually be quite far away
from each other.
The Celestial Sphere
Stars at different
distances all appear
to lie on the
celestial sphere.
The ecliptic is the
Sun’s apparent path
through the
celestial sphere.
The Celestial Sphere
The Milky Way
A band of light
making a circle
around the
celestial sphere.
What is it?
Our view into
the plane of our
galaxy.
The Milky Way
How do we locate objects in the sky?
An object’s altitude (above horizon) and direction
(along horizon) specify its location in your local sky.
We measure the sky in angles...
blank
Angular Measurements
• Full circle = 360º
• 1º = 60 (arcminutes)
• 1 = 60 (arcseconds)
Thought Question
The angular size of your finger at
arm’s length is about 1. How many
arcseconds is this?
• 60 arcseconds.
• 600 arcseconds.
A. 60  60 = 3,600 arcseconds.
Thought Question
The angular size of your finger at
arm’s length is about 1. How many
arcseconds is this?
A. 60 arcseconds.
B. 600 arcseconds.
C. 60  60 = 3,600 arcseconds.
Why do stars rise and set?
Earth rotates west
to east, so stars
appear to circle
from east to west.
What moves? The Earth or the sky?
Celestial Sphere
Zenith: Point
directly
overhead
Horizon:
Where
the sky
meets the
ground
Celestial Sphere
North
Celestial Pole:
Point on
celestial sphere
above North
Pole
Celestial
Equator:
Line on
celestial
sphere
above Equator
Our view from Earth
• Stars near the north celestial pole are circumpolar
and never set.
• We cannot see stars near the south celestial pole.
• All other stars (and Sun, Moon, planets) rise in east
and set in west.
A circumpolar
star never sets
Celestial equator
This star
never rises
Your horizon
Ended Here 1/31/08
(lots of questions during lecture,
plus organizational stuff)