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: Poigson’s rule: ObjectMagnitude 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: Vega0 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 Faintest objects observable with HST +30

What is a digital image? Objects Point source objects Extended sources Noise Dark noise Readout noise Sky 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 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 square- meter 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. ObjectLuminosity Sun4 x W Sirius8 x W Betelgeuse2 x W Accreting X-ray binary W Supernova at peak10 37 W Bright quasar10 38 W Gamma-ray Burst peak 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

PulsatingPulsating 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.

e r up t i v 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.

E c li p si n g 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.

IrregularIrregular 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 - MaximDL - CCDSoft - Mira - Iris - Canopus - 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…

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