Date post: | 19-Dec-2015 |
Category: |
Documents |
View: | 222 times |
Download: | 1 times |
ASTR100 (Spring 2008) Introduction to AstronomyDiscovering the Universe
Prof. D.C. RichardsonSections 0101-0106
How can we know what the universe was like in the past?
Thus, we see objects as they were in the past:The farther away we look in distance,
the further back we look in time.
Destination Light travel time
Moon 1 second
Sun 8 minutes
Sirius 8 years
Andromeda Galaxy 2.5 million years
Light travels at a finite speed (c = 300,000 km/s).
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?
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 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)
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.
New Year’s Day: The Big Bang
Milky Way forms
Sun & planets form
Oldest known life (single-celled)
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 inOur Solar System?
The Earth rotates around its axis once every day.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
How is Earth Moving inOur Solar System?
The Earth orbits the Sun (revolves) once every year.
How is Earth Moving inOur 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 CThe 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.DEFCorrect
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
Universeexpands
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
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
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 SphereNorth 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.
Celestial equator
Your horizon
A circumpolar star never sets
This star never rises
Ended Here 1/31/08
(lots of questions during lecture, plus organizational
stuff)