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Summary Measure the brightness of sn1994i from astronomical images of the spiral galaxy Messier 51. Background and Theory

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The Light Curve of SN1994i in Messier 51

Summary Measure the brightness of SN1994i from astronomical images of the spiral galaxy Messier 51.

Background and Theory

Supernovae are stupendous explosions of stars that brighten suddenly, and then fade from view over weeks or months. While the most recent supernova in our own Milky Way Galaxy was seen in the 17th century, astronomers find many supernovae each year by searching other galaxies. At least two supernovae have been detected in recent years in the nearby Whirlpool Galaxy, Messier 51. M51 is located at a distance of about 31 million light years (about 10 megaparsecs) in the direction of the constellation Canes Venatici. In the pair of images of the galaxy Messier 51 to the right, you can find one of the two recent supernovae to be spotted in M 51, SN2005cs, to the left of the galaxy's nucleus.

Figure 1: SN2005cs appeared in Messier 51, the Whirlpool Galaxy, in 2005. Can you find it in the images above? (R. J. GaBany;
tars can explode as supernovae in different ways, and the different explosion mechanisms produce characteristically different behavior of brightness with time, as the supernova brightens and then fades again. The two dominant types of supernovae are Type Ia supernova, which are thought to be exploding white dwarfs, and Type II supernovae, which are explosions of massive stars that run out of nuclear fuel.
Figure 2: Light curves of Type Ia and Type II supernovae.( Hands-On-Universe)
he light curves of supernovae of Type Ia and Type II are compared in Figure 2. Type Ia supernova rise to maximum and then fall steadily in brightness, fading in just a few weeks. Type II supernovae remain bright for a longer period of time as energy is provided by the decay of radioactive products produced in the explosion.

From observations of the light curve of SN1994i in M51, you can determine whether the explosion is a Type Ia (exploding white dwarf) or a Type II (massive star) supernova.

Obtaining the Data. Twelve images of M51 were obtained in the spring of 1994 with the Leuschner Telescope of the University of California at Berkeley. These images are available on your thumbdrive or can be downloaded from The images are in a special format ("fits" files) that preserve quantitative information about the brightness of the pixels so that numerical measurements can be made. Load the images onto your computer. Thumbnails of the twelve images are shown below, and the dates of the observations are provided in the table below. These images have been cropped to show the region close to the supernova. SN1994i is the object with variable brightness below and to the right of the nucleus (the fuzzy object).






2 April 1994


22 April 1994


7 April 1994


23 April 1994


11 April 1994


27 April 1994


13 April 1994


28 April 1994


14 April 1994


3 May 1994


21 April 1994


6 May 1994

The Sky Image Processor (SIP)1. SIP is available from the Virginia Technical Institute to measure the magnitudes of SN1994i and of either of the objects labeled "star A" or "star B' in the image on the next page. SIP is a Java-based image processing system that can measure the brightness of stars in astronomical images.
To start SIP, open a browser and link to the website The tool works best with Microsoft Internet Explorer and can be buggy with other browsers. Click the "start SIP" button to begin a session. A SIP image window will open on your computer.
Using the SIP file menu, select "Open Image File from User's Machine" and browse to select one of the .fits images on the thumbdrive. The image should load into the display window. (Sometimes, only a partial image is displayed. If this is the case, select "Conventional FITS Image Orientation" under the "view" menu to display it again. You may need to toggle between these two settings, but note that the image will flip when you do. It may be easier to see the star if you click “Automatic Contrast Adjustment” under the “View” file menu.)
To measure the magnitude of a star in the image, use the "Determine Centroid or Instrumental Magnitude of an Object" in the "analyze" menu. Two squares will appear in the image (a red target box and a green background box), and a control panel will appear. The boxes may be moved with sliders in the control panel. Select a box size of 11 pixels for the green "object" box and a size of 23 pixels for the red "background" box. Use the sliders in the control panel to center the boxes over the star or supernova to be measured. Adjust the background annulus box to encompass a representative area around the star, but to avoid including any light from the star itself. Because the supernova is near the nucleus of M51, you should shift the background box to be a bit further from the supernova and not centered on the green box, as shown below.

The applet will add up the light in the target box, and subtract an estimate of the background flux based on the light included in the background annulus. The applet automatically corrects for the different areas of the two boxes.

In the table on the next page, record the "instrumental magnitude" shown at the bottom of the control panel for each image. These instrumental magnitudes will be negative numbers.

Note: If the program should freeze up, you may need to exit your web browser, then reopen it and go back to the website. This should free it up.
Correcting for sky variability. The apparent magnitudes that SIP will measure will vary because of different sky conditions on different nights. The atmosphere may be more or less transparent, and the quality or sharpness of the images may also vary due to "seeing," which is a defocusing effect due to motions in the Earth's atmosphere.
To correct for sky variability, we will compare the measured brightness of the supernova to standard stars in the same image. Astronomers refer to this technique as "differential photometry." Both the supernova and the standard star should vary in the same way due to changes in the Earth's atmosphere. Assuming that the intrinsic brightness of the standard star (star A or star B) is constant, the difference in instrumental magnitude between the supernova and the standard is a reliable measure of the relative brightness of the supernova. Compute and record the difference in instrumental magnitude between the supernova and the standard star you have chosen in the table below.


Inst. Mag of Standard (A or B - choose one)

Inst. Mag. of SN

Difference in Magnitude Standard - SN

Number of Days

























Graph the difference in magnitude between the supernova and your standard (on the y-axis) versus the number of days since the first observation (on the x-axis) to plot a light curve for SN1994i.

On what date was the supernova most likely at maximum brightness?

Is SN1994i a Type Ia or a Type II supernova?

This exercise is based on and makes use of data provided by Hands on Universe from the Lawrence Hall of Science and has been developed for use in the "Exploring the Dark Universe" Workshop for teachers at Indiana University.

1 The Sky Image Processor (SIP) was prepared by John H. Simonetti of the Virginia Technical Institute

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