Download 21structure1i

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Cosmic distance ladder wikipedia, lookup

Aquarius (constellation) wikipedia, lookup

Ursa Minor wikipedia, lookup

Corvus (constellation) wikipedia, lookup

Astronomical unit wikipedia, lookup

Perseus (constellation) wikipedia, lookup

Timeline of astronomy wikipedia, lookup

Hipparcos wikipedia, lookup

Observational astronomy wikipedia, lookup

International Ultraviolet Explorer wikipedia, lookup

Flatness problem wikipedia, lookup

Fine-tuned Universe wikipedia, lookup

Physical cosmology wikipedia, lookup

Ultimate fate of the universe wikipedia, lookup

Non-standard cosmology wikipedia, lookup

Lambda-CDM model wikipedia, lookup

Shape of the universe wikipedia, lookup

Hubble Deep Field wikipedia, lookup

Universe wikipedia, lookup

Chronology of the universe wikipedia, lookup

Dark energy wikipedia, lookup

IK Pegasi wikipedia, lookup

Galaxy Zoo wikipedia, lookup

Gamma-ray burst wikipedia, lookup

Hubble Space Telescope wikipedia, lookup

Redshift wikipedia, lookup

Big Bang wikipedia, lookup

History of supernova observation wikipedia, lookup

Wilkinson Microwave Anisotropy Probe wikipedia, lookup

Anthropic principle wikipedia, lookup

Transcript
Structure of the Universe
“The Universe --
Astronomy 315
Professor Lee Carkner
Lecture 21
Size: Bigger than the biggest
thing ever and then some. Much
bigger than that in fact, really
amazingly immense, a totally
stunning size, real "wow, that's big,"
time. ... Gigantic multiplied by
colossal multiplied by staggeringly
huge is the sort of concept we're
trying to get across here.”
--Douglas Adams, The Restaurant at
the End of the Universe
The Universe
One of the earliest models of the universe
had everything outside of the solar system
fixed to a celestial sphere
Everything was the same distance from the
earth
This is how the universe looks
We have no depth perception when viewing the
universe
We have to somehow find the distance to
celestial objects to understand the true
nature of the universe
Early Model of the Universe
The Distance Ladder
There is no single method that can be
used to find the distances to all objects
We use many methods, each building
on the other
Called the cosmic distance ladder
Each method takes us one step further
away, out to the limits of our
observations
Steps on the Distance Ladder
Parallax:
out to ~1000 pc
Spectroscopic Parallax:
out to 100,000 pc
Cepheid Period/Luminosity Relationship:
out to ~5,000,000 pc
Supernova Standard Candle:
out to 4 billion pc
Redshift:
out to limits of universe
Parallax
As we have seen parallax is the
apparent motion of a star as you look at
it from two different points of view
Shift decreases with distance
Shift is only measurable out to 1000 pc
maximum
From space with the Hipparcos satellite
Spectroscopic Parallax
We can use spectroscopy and
photometry to get the spectral type and
the apparent magnitude (m) of a star
We can estimate the absolute
magnitude (M) from the spectral type
With the two magnitudes we can get
the distance:
m-M = 5 log d - 5
Example: We know how bright an A0
should be, so we can find its distance by
how bright it looks
Cepheid Period-Luminosity
Relationship
Cepheids are bright pulsating variable stars
As the star get larger and smaller the
brightness goes up and down in a very
regular way
There is a direct relationship between period
and luminosity
Long period (slow changes) means brighter star
Again we can get the distance from the
luminosity and flux (flux measured directly):
F = L/4pd2
Variation in Cepheid
Properties
P-L Relation for Cepheids
Supernova Standard Candles
Type Ia supernovae are not exploding
massive stars, but rather a white dwarf
that accretes mass from a companion
until it exceeds the Chandrasekhar limit
(1.4 Msun)
When this occurs the WD collapses and
rapidly burns its carbon
All type Ia supernova have the same
absolute magnitude are are very bright
We can use them to find distance to very
distant objects
Most Distant Supernova
Distance Indicator Limitations
All methods have limits where they
can’t be used and problems that can
lead to errors
Parallax -- Motion has to be large
enough to resolve
Even from space can’t resolve parallax
beyond 1000 pc
Spectroscopic Parallax -- Have to be
able to resolve star and it must be
bright enough to get a spectrum
Exact spectral type is uncertain
Standard Candle Problems
Cepheids and supernova have to be
bright enough to see
Can see supernova further than Cepheids
but, supernova are transient events (have to
wait for one to occur)
Largest source of error is extinction
along the line of sight
Makes things appear more distant
Red Shift
The spectral lines from distant galaxies
are greatly shifted towards longer
wavelengths
The galaxies are moving away from us
very quickly
The degree to which the lines are
shifted is represented by z
High z = large red shift = high velocity
We can find the velocity with the
Doppler formula:
z = v/c
The Hubble Flow
Spectra of all distant galaxies are red shifted
This means that everything in the universe is
moving away from everything else
This in turn means that he universe is expanding
Objects can have other motions as well, but
the motion due to expansion is called the
Hubble flow
The Hubble flow velocity is related to the
object’s distance
The Hubble Law
If a plot is made of recession velocity
versus distance, the result is a straight
line
Larger distance, larger velocity
The two are related by the Hubble
Constant H, through the Hubble law:
V = Hd
We can always get V from the red shift,
so if we know d or H we can find the
other
The Hubble Constant
The Hubble constant is found by
plotting velocity versus distance and
finding the slope
Need accurate distance over a range of
distances
Use the distance ladder methods
H is given in units of kilometers per
second per megaparsec (km/s/Mpc)
Megaparsec is one million parsecs
Our best determination for H is about
70 km/s/Mpc
The Hubble Law
Look Back Time
Light is the fastest thing in the universe, but
its speed is finite
c = 3 X 108 m/s
When we look at distant objects we are
seeing them the way they were when the light
left them, not the way they are now
For other galaxies we can see things as they
were billions of years ago, when the universe
was young
Distance in light years gives the look back time
Using the Distance Ladder
We can use the distance ladder to map the
structure of the universe
Parallax and Spectroscopic Parallax
Use to find the dimensions of our galaxy
Cepheid variables
Use to find the distance to near-by galaxies
Supernova
Use to find distances for very distant galaxies
Local Neighborhood
Our galaxy is about 100,000 light years
in diameter
We are surrounded by near-by, smaller
companion galaxies
LMC and SMC are two examples
These companions are a few hundred
thousand light years away
Companions tend to be dwarf
ellipticals
Local Group
The Milky Way is in a cluster called the
Local Group
The local group extends out over
several million light years
Group is dominated by the two largest
spirals: M31 and the Milky Way
Most other galaxies are small
companions to these two
The Local Group
Beyond the Local Group
If we photograph the sky, we clearly
see places where galaxies are grouped
together
The universe is full of clusters
Clusters tend to be millions of light
years across and 10’s of millions of light
years apart
Clusters gathered into superclusters
Supercluster size ~ 100 million light years
Large Scale Structure
The Virgo Cluster
One of the nearest clusters is the Virgo
cluster
More than 2000 galaxies and covers 100
square degrees in the sky
15 Mpc or 50 million light years away
Centered on giant ellipticals larger than
the entire local group
Local group is a poor cluster, Virgo is a
rich one
The Virgo Cluster
Hubble Deep Field
The Distant Universe
It is hard to see into the distant
universe
Things are very far away and so are faint
We can see powerful things like
quasars
Can see other objects in the 10 day long
exposure of the Hubble Deep Field
Can see back to when the universe was
only 1 billion years old
See things that may be protogalaxies
Next Time
Read the rest of Chapter 19
Question of the Day:
How did the universe form and how will it
end?
List 3 due Friday
Quiz 3 on Monday