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transcript
July 23, 2004
How do astronomers know the things about the universe that
they claim that they know?
• They observe the universe.• They do observational astronomy.• They use the scientific method to
develop theories that explain what is observed and seek out the laws under which the universe operates.
July 23, 2004
What is involved in doing observational astronomy?
• Detecting and recording photons.• Characteristics of the detected photons are
used to infer physical properties of objects and nature of processes.
• Since humans are a visual life form, detected photons normally presented as an image.
• Collecting photons normally involves a telescope.
July 23, 2004
How is observational astronomy actually done?
There are three broad categories of activities…
• Astrometry
• Photometry (technically radiometry)
• Spectroscopy
July 23, 2004
How is observational astronomy actually done?
There are three broad categories of activities…
• Astrometry– positions, coordinates, motions
• Photometry (technically radiometry)
• Spectroscopy
July 23, 2004
How is observational astronomy actually done?
There are three broad categories of activities…
• Astrometry– positions, coordinates, motions
• Photometry (technically radiometry)– brightness, variability
• Spectroscopy
July 23, 2004
How is observational astronomy actually done?
There are three broad categories of activities…
• Astrometry– positions, coordinates, motions
• Photometry (technically radiometry)– brightness, variability
• Spectroscopy– Spectra, chemical composition, motion
July 23, 2004
How is observational astronomy actually done?
There are three broad categories of activities…
• Astrometry– positions, coordinates, motions
• Photometry (technically radiometry)– brightness, variability
• Spectroscopy– Spectra, chemical composition, motion
July 23, 2004
Some activities illustrating observational astronomy…
as exemplified by the history of GRBs
July 23, 2004
Vela Result Reported to the World
Current Date: 1973
Current Event: Organization of Petroleum Exporting Countries (OPEC) hikes oil prices tremendously in retaliation for Western countries' involvement in Yom Kippur War.
Klebesadel (LANL) publishes a paper in Nature and a presentation to the AAS reporting the discovery of 16 “cosmic” gamma-ray burst like events during the period of 1967-1972.
Klebesadel and Olsen
July 23, 2004
The First Questions• 16 recorded events have occurred.• These events appear to be coming random
locations out in space.
What are some of the questions we scientists should be asking ourselves about these events?
Current Date: July 2 1967
Current Event: Racial violence in Detroit; 7,000 National Guardsmen aid police after night of rioting. Similar outbreaks in New York City's Spanish Harlem, Rochester, N.Y., Birmingham, Ala., and New Britain, Conn. (July 23).
July 23, 2004
The CGRO MissionCurrent Date: April 5, 1991 Current Event: Europeans end sanctions on South Africa (April 15). South African Parliament repeals apartheid laws (June 5).
Compton Gammaray Observatory is launched.
On board are 8 BATSE detectors designed to detect the position of GRB.
July 23, 2004
Alert! A BATSE burst just went off !
• A gamma-ray burst was just detected by BATSE. We have determined the object’s approximate location.
• We need some able bodied scientists to get an exact location of the burst.
Who is up for the challenge?
Current Date: January 20, 1992
Current Event: Yugoslav Federation broken up (Jan. 15). US recognizes three former Yugoslav republics (April 7). UN expels Serbian-dominated Yugoslavia (Sept. 22).
July 23, 2004
Astrometry
• Astrometry is the determination of astronomical objects positions in the sky and how those positions change over time.
• Astrometry involves making a map of objects in sky Hyades from Hipparcos
satellite data
July 23, 2004
Equatorial Coordinate System
• The equatorial coordinate system is the preferred coordinate system of observational astronomers.
• The equatorial coordinate system like latitude and longitude is also based on measures of angular separation from some arbitrary origin.
• The units of these measures are Right Ascension (RA) and Declination (Dec).
RA: 0 – 24 hours, Dec: -90o – +90o
Lon: 0o – 360o, Lat: -90o – 90o
July 23, 2004
ECS Unit Conversion
• RA– 1 hour = 60 minutes– 1 minute = 60 seconds
• Dec– 1o = 60’ (‘ = arcminutes)– 1’ = 60” (“ = arcseconds)
July 23, 2004
Astrometry
• Using the RA and Dec plots given to your team determine the location of the gamma-ray burst to the nearest arcminute.
• The object labeled SAO 75945 has known coordinates.
• The object labeled GRB 910923 indicates the relative location of the detected GRB to SAO 75945.
• See if you can get the GRB’s location.
July 23, 2004
Leap Forward 5 Years to 1996
By December 1996 ~1700 bursts had been detected by BATSE.
What conclusions can we draw based on this map?
July 23, 2004
Enter BeppoSAX• The question
arose, if GRB are not concentrated evenly throughout our galaxy then where are they?
• At the same time CGRO was in the peak of its life an Italian Dutch satellite named BeppoSAX was being launched.
We need to know another piece of information in order to determine where these GRBs are in the Universe and ultimately how much energy they expended during their short lived lives.
July 23, 2004
Alert: BeppoSAX has Just Detected a GRB!
• We have to move quickly - this GRB won’t be around for long.
• We need to confirm the GRB’s location and use a ground-based optical telescope with a CCD camera and spectrometer to determine how bright and how far away this GRB is.
July 23, 2004
BeppoSAX X-ray Image
Designation: GRB 970228Burst Time (UTC):05:00 amBurst Date (UTC): 28 February
1997Time Since Burst: 16 hoursCoordinates:
RA: 5h 01m 44s, Dec: +11o 46.7’Error Circle: 12’Equinox: J2000
Constellation: Orion
July 23, 2004
Step One: study the GRB at different wavelengths
• We are trying to identify the GRB by studying its emission at optical wavelengths. We hope to find another object at the same location that we can recognize (star, galaxy, etc.)
• We’ve been granted time on the 4.5m William Herschel Telescope the La Palma Observatory on the Canary Islands Spain.
• We need to analyze whatever data we get and determine the brightness as well as the spectral characteristics.
July 23, 2004
Photometry• Photometry is
the study of astronomical object’s brightness as a function of time
• One standard result of photometry is
the light curve
This is a lightcurve of GRB 910923.
July 23, 2004
Photometry - Lingo• Magnitude is a logarithmic
measure of the brightness of an object.
• Magnitude: The measured brightness of a celestial body.
– Dim objects have magnitudes of high numbers.
– Bright objects have magnitudes of low or even negative numbers.
– Applies to visible, IR and near UV light.
• If you don’t like the magnitude system send all hate mail to Ptolemy c/o some graveyard in Alexandria, Egypt.
• Deference in magnitude equation.
– m1-m2 = -2.5 log (l1/l2)
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
July 23, 2004
Photometry - Lingo• 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.
• 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 square-meter sized detector.
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
Luminosity = Flux (4d2)
where d =distance to source
July 23, 2004
Determining BrightnessIn this activity you will determine the brightness of GRB 970402 using a clay star field, dissecting that star field, and quantifying each piece.
A star field is fancy name for an image, taken with a camera, on a telescope, of a "field of stars".
July 23, 2004
Procedure
• You now have a clay representation of a GRB 970402. The big clump of clay is the GRB and the clay sheet is background noise in the image.
• The other object is in the field has a known magnitude of 0.
• Can you figure out a way to subtract the background noise out to determine the brightness of the burst in relationship to the other object in the field?
July 23, 2004
Your Tools
• 1 triple beam balance• 1 sharp pencil• 1 12-inch ruler• 1 plastic knife (Safety first)
Determine the difference in magnitude between the two objects.
July 23, 2004
Rules
• You can not lift the red thing off of the black thing.
• Daryl do you get this?
July 23, 2004
Conclusion
What did you discover?
How bright is the object?
Great news, with the measurements you made our La Palma Technician says that the GRB is bright enough to take optical spectra.
Hopefully the optical spectra will be able to tell us more about these objects.
July 23, 2004
Spectroscopy• Spectroscopy is the study of how
bright things are at discretely small intervals of wavelengths of light.
July 23, 2004
What can Spectroscopy Tell Us?
• Spectroscopy can tell us what nuclear and electromagnetic reactions are taking place in an astronomical object.
• Spectroscopy can tell us what elements are in present in an object.
• Spectroscopy can tell us how far an object is away from us!!! (How???)
July 23, 2004
Spectroscopy of GRB 970228• On your desk is a jar of Jelly Belly® beans.• This candy represents the spectral energy
distribution of the GRB optical afterglow.• Separate the Jelly Belly® beans into their
corresponding wavelengths.• Then graph the results on the graph paper
provided.– Graph number vs. color with red on the left and
blue on the right.
July 23, 2004
What did you find out?
GRB 970228 Spectra
6
10
54
9
32
0
2
4
6
8
10
12
Red Orange Yellow Green Blue Indigo Violet
July 23, 2004
Now Compare Your Results to the Laboratory SpectraGRB 970228 and Laboratory Spectra
0
2
4
6
8
10
12
Red Orange Yellow Green Blue Indigo Violet
July 23, 2004
BREAK
• What differences do you see between your graph and the known source?
• What do you think is going on?
THINK GRB SPECTRUM
MYSTERY
July 23, 2004
Review: Astrometry
• Positions• Coordinates (right ascension and declination,
RA and DEC, galactic longitude and latitude)• Motion
• From where on the sky do the photons appear to originate?
July 23, 2004
Review: Photometry
• Brightness• Flux or magnitudes or number of photons• Colors• Variability• Lightcurves
• How many photons are actually detected from an object in a specified interval of time?
July 23, 2004
Review: Spectroscopy
• Spectrum• Spectral energy distribution (SED)• Emission lines, absorption lines• Chemical composition• Temperature• Radial velocity (red shifts, blue shifts)
• What is the relative distribution of detected photons of different wavelengths, frequencies, or energies?
July 23, 2004
What is the GTN?• The GTN is a network of individuals and
organizations interested in supporting several NASA space observatories by obtaining ground-based observations.
• The GTN has been designed to allow students, educators, and others to become involved in doing real science and to contribute to the science goals of several NASA missions.
• Here is the GTN paradigm…Picture of mike ford’s students goes here
July 23, 2004
The Science of the GTN
• Observational astronomy with small telescopes.• Science objectives focused on obtaining synoptic surveillance of
objects which will be observed by the NASA missions we support.
• Program objects are variable and can change in brightness unexpectedly.
• Program objects are the blazars and the polars.• Obtain CCD images of these objects and analyze the images.
– Determine magnitudes– Do photometry– Obtain light curves
July 23, 2004
So you want to do some observing?
We are interested in digital images obtained using CCDs with small to moderate size telescopes.– The costs are not… impossible.– It can be fun.– It can be exciting.– The images can readily be used to produce
significant scientific results.
July 23, 2004
What can you do relatively easily with CCD images
obtained with a telescope?• You can enjoy obtaining detailed images of those
exotic deep sky objects you have heard about.• You can do photometry.
– Variability– Lightcurves– Discovery
• You can also do astrometry (easy!) and even spectroscopy (more challenging).
July 23, 2004
Your Mission… if you choose to accept it
You have been invited to submit an observing proposal for the robotic telescope system GORT (GLAST Optical Robotic Telescope)
If your proposal is accepted, you will enter the specifications for the objects you wish to observe into the telescope control system.
The system will acquire the images you specify.
You get to see (and keep) the images!
July 23, 2004
The image fields you will specify for the activity…
1. An image of some field or object in the sky that is of interest to you (fun).
– Messier object, for example– Other resources are available– Use your imagination
2. An image of a GTN program object (science)
– A GTN blazar– A GTN polar
July 23, 2004
What will you need to specify in your observing proposal?
• Object name or designation• Object coordinates (RA and DEC)
– Nearest second of time for RA, neatest second of arc for DEC, equator and equinox for 2000.0
– Need to be accurate!
• Exposure times– Seconds or minutes– Not too long (saturated) or too short (not there)
• Filters (clear, V, R, I)• Justification for selecting this object
– What do you think you will see?
July 23, 2004
The tools you will have available for the activity…
1. Lists of sky objects– Catalogs– Web links
2. Sky simulation software3. List of instrumental limitations and
recommendations4. Tutorials and resource materials
– What do you need to know?– What would you like to know?
July 23, 2004
Instrumental LimitationsGuidelines for Using GORT
1. Angular field of view of an image is 10 arcminutes2. Maximum exposure time for an untrailed image is 1
minute3. A 1 minute exposure can probably record objects
as faint as 18th magnitude4. Objects brighter than 8th magnitude are likely to be
saturated for exposure times longer than 1 second5. Exposure times as short as 0.001 second may be
specified.
July 23, 2004
An expanded description and an online version of this activity is available for classroom use.
• Expanded description– The expanded description includes questions to engage the
audience and some background material.– Expanded Activity Description
• Online version– The online version contains extensive links to resource
materials.– URL…
July 23, 2004
Presentation of Results
• Groups will have an opportunity to present results. (Monday Night)– You should be able to tell us about
the objects in your images.– You should be prepared to answer
questions about what you have done.
• Prizes for the best science and for the most interesting images
July 23, 2004
What things do you need to know or worry about for a
successful proposal?
• It must be dark– After twilight (sunset), before dawn (sunrise)
• Object must be above your telescope horizon at the time you are able to observe
• Object must not be too large or too small for the field of view of your images
• Object must not be too faint or too bright• Exposure times must not be too long or too short
July 23, 2004
How can you determine when an object is up at night?
• Ask an amateur astronomer.• Professional astronomers tend not to know these
things… unless…• Ask someone who operates a telescope on a regular
basis.• Use sky simulation software.
– TheSky– Voyager– Stary Night– YourSky on the web
July 23, 2004
How do you determine a magnitude??
• Aperture photometry
• All magnitudes are really relative– With some agreed zero point– Comparison stars or sequence stars
• Magnitude equationm1-m2 = -2.5 log (l1/l2)
• Image analysis software systems
July 23, 2004
Image Analysis Software
• Maxim
• CCDSoft
• AIP (Astronomical Image Processor)
• HOU (Hands On Universe)
• Mira
• IRAF
• SIP (Sky Image Processor, on the web)
July 23, 2004
How do you determine what the magnitude should be?
• You do not know… – variable objects– but you should determine your measurement errors
• The GTN will maintain statistical summaries.• The AAVSO maintains database of current
magnitude estimates and measurements (also lightcurves)
• Outbursts, flares, declines, flickering