Transcript Click Here to Powerpoint Presentation

High Energy Outreach
An Introduction to Photometry for
Educators and Beginning
Astrophysicists
Written and Created by: Tim Graves and the Sonoma
State NASA Education and Public Outreach Team
What instruments are available to
measure the brightness of stars?
•
•
•
•
Your Eyes
Photographic Cameras
Photomultipliers
Charged Coupled Devices
What units do we use to describe
the brightness of a star?
•
•
•
•
•

The astronomical magnitude system
Magnitude is a logarithmic
measure of the brightness of an
object.
Magnitude: The measured
brightness of a celestial body.
– Apparently dimmer objects have
magnitudes that are higher
numbers.
– Apparently brighter objects have
magnitudes that are low or
negative numbers.
– Applies to visible, IR and near
UV light.
Difference in magnitude equation:


m2 m12.5 log


Poigson’s rule:
m2.5 log K

1 

2 
Object
Magnitude
Sun
-26.8
Full Moon
−12.6
Maximum brightness of Venus
−4.4
Maximum brightness of Mars
-2.8
Brightest star: Sirius
−1.5
Second brightest star: Canopus
−0.7
The zero point by definition: Vega
0
Faintest stars visible in an urban
neighborhood
+3.0
Faintest stars observable with
naked eye
+6.0
Brightest quasar
+12.6
Faintest objects observable with
GORT
+20.0
Faintest objects observable
with HST
+30
What is a digital image?
Objects
Point source objects
Extended sources
Noise
Sky noise
Dark noise
Readout noise
Where does sky noise come
from?
• Artificial light
pollution
• Environmental
light pollution
• Integrated light
from faint and
distant stars
What does a star look like in a
digital image?
Cookie Cutter Photometry
You will be provided with a clay model of a
star field. This model represents an
image of a small part of the sky. Your
model will have two or more mounds of
clay that represent stars. The higher the
mound, the more light reached the
image at that point. The sheet of clay on
which the mounds are located
represents the brightness of the
background.
Procedure
• Figure out a way to subtract out the
background noise so that you can
determine the brightness of the star in
relationship to the other object in the
field.
Your Tools
•
•
•
•
1 triple beam balance
1 sharp pencil
1 12-inch ruler
1 plastic knife (Safety first)
Use the worksheet provided to solve the
problem.
Rules
• You can not lift the red thing off of the
black thing.
What is a CCD camera?
• A CCD imaging camera is made of regularly spaced
pixels.
• Pixels are aligned in rows.
• Each pixel accumulates charge by using the
photoelectric effect to liberate electrons upon the
incidence of light.
– There are two major types of CCDs: front illuminated and back
illuminated
– Back illuminated have a much higher efficiency than front
illuminated but they are much more difficult to manufacture.
What does a CCD camera look
like?
• They come in many shapes and sizes.
– Total number of pixels - 4008 x 2672 pixels = 11 Mega Pixels
– Pixel size – Anywhere between 5 and 25 microns.
– Imager size – Anywhere between 5 and 36 mm per axis
How does a CCD camera work?
•
•
•
•
•
Four roles a CCD must
perform
–
Charge generation
–
Charge collection
–
Charge transfer
–
Charge detection
Charge is generated when
photons hit the pixel surface
liberating electrons. Thank
you Einstein!
Charge collection occurs when
the electrons fall into quantum
wells at each pixel site.
Charge transfer occurs when
the electrons are moved along
the rows towards the readout
register.
Charge detection occurs when
the electrons from each pixel
are sent through an amplifier
What advantage does a CCD have over other
methods of measuring star brightness?
• Much more efficient than film.
– CCDs can be greater than
95 percent efficient.
– Film is at best 10% efficient.
– With a CCD you can image
9 times deeper in the same
amount of time.
– Your CCD efficiency is
directly proportional to your
budget!
• Digital images can easily be
reproduced and shared with
others.
• Easier to use.
• Immediate quantifiable
feedback.
• More accurate than the trained
human eye.
• Much lower risk of severe
electrocution!
What is the brightness of a star?
•
•
•
Photon Flux: the number of
photons emitted per second
that are detected in a square
meter-sized detector.
Energy flux: the energy
emitted per second (luminosity)
that is deposited in a squaremeter sized detector.
Luminosity: The measured
energy emitted each second by
a celestial body.
– Applies to all wavelengths of
light.
•
•
Fluence: the integrated
luminosity over some specified
time duration.
Image brightness: Depends
on the type of detector. In CCD
astronomy we refer to the
brightness of a star as the
integrated photon flux of the
object minus any noise.
– Applies to arbitrarily chosen
wavelength ranges.
Object
Luminosity
Sun
4 x 1026 W
Sirius
8 x 1027 W
Betelgeuse
2 x 1031 W
Accreting X-ray
binary
1031 W
Supernova at peak
1037 W
Bright quasar
1038 W
Gamma-ray Burst
peak
1045 W
How can we use what a star looks like in an
image to determine its brightness?
Analysis Aperture
Gap
Annulus
How can we use what a star looks like in an
image to determine its brightness?
• The method of determining
the brightness of the two
stars in the activity is known
as differential photometry.
• Differential photometry uses
two or more objects in an
image.
– The more objects that are
used the better the
statistical errors become.
– Easier to tell if one of
your comparison stars is
variable
Why is it important to be able to
determine the brightness of stars?
• Being able to determine the brightness of an object
allows us to:
– Understand how that object is changing with time.
– Determine the distance to objects.
– Determine properties of objects.
• This allows us to deduce what processes are at work
in the system.
What types of variable objects
are there?
• There are four basic types of variable
objects.
– Pulsating
– Eruptive
– Eclipsing
– Irregular
Pulsating
• These are stars that actually pulsate.
• They change in size and surface temperature.
• When they become large their surfaces cool, and when they
become smaller their surfaces become hotter.
• One sub-type of pulsating variable is a Long Period Variable
(LPV).
– These objects can vary by six magnitudes or more over
periods of hundreds of days.
tiv
erup
e
• These are stars for which explosive events occur.
• The most extreme case of an eruptive variable is the supernova.
• Internal instabilities generate an explosive event that can totally
destroy the star.
• A supernova can increase in brightness by more than 10
magnitudes in a matter of hours.
• A slow decline follows which can take many months, or even
years.
Eclipsing
• These are binary star systems for which the orbital plane is
aligned with the direction toward the sun and the solar system.
• As these objects orbit they can undergo mutual eclipses as
one component passes directly in front of its companion.
• When an eclipse occurs the observed brightness will
decrease.
• Eclipsing systems tend to remain relatively constant until an
eclipse occurs.
Irregular
• Variable stars often appear to be periodic and repeat
their light variation at some specific period.
• Some variable objects do not appear to be any kind of
periodic behavior.
• The light variations can appear to be random.
Digital Image Analysis
Software
•
•
•
•
•
•
•
AIP4WIN - http://www.willbell.com/aip4win/AIP.htm
MaximDL - http://www.cyanogen.com/
CCDSoft - http://www.bisque.com/
Mira - http://www.axres.com/mira_ap.html
Iris - http://www.astrosurf.com/buil/us/iris/iris.htm
Canopus - http://www.minorplanetobserver.com/
SIP - http://www.phys.vt.edu/~jhs/SIP/
Additional Information
• An activity is available that is designed to introduce students to
designing an observing program and collecting image data using
a remote robotic telescope. You can examine this activity at…
http://gtn.sonoma.edu/activity/oa/
Credits
•
•
•
•
•
Hubble Space Telescope – Space Telescope Science Institute
AAVSO – American Association of Variable Star Observers
Chandra Observatory – Harvard Smithsonian Institute
Eclipsing binary movie - Copyright © 1997 Richard W. Pogge, All Rights
Reserved.
Astronomy Picture of the Day