Download Plotting Supernova Light Curves

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Life Cycle of Stars
Plotting Supernova Light
Curves
Author: Sarah Roberts & Daniel
Duggan
Plotting Supernova Light curves - Faulkes Telescope Project
Plotting Supernova Light curves
Introduction
For stars with masses of more than 15 times the mass of the Sun, their lives end in a
violent explosion called a supernova. Nuclear fusion stops in the core of the star, which
then collapses and bounces back outwards, ejecting most of its matter into space. During
this explosion, the star increases drastically in luminosity, which is the visible supernova.
The matter which was blown out from the star heats up as it travels through the gas and
dust in space, causing it to glow. This is known as a supernova remnant. Depending on
the mass of the star, it either collapses to form a neutron star or, in the case of stars more
than a hundred times the mass of the Sun, it forms a black hole. After the explosion, the
star’s luminosity gradually decreases. A plot of how the luminosity of the star changes
with time as all this happens, is called a light curve.
Instructions
In this activity, you will obtain a supernova light curve using downloaded data of the
galaxy M100, taken with the Faulkes Telescopes. The software package, SalsaJ will be
used to carry out photometry on the supernova in the galaxy and standard star (a star
which has already had its magnitude accurately calculated) close to the galaxy. The
images you will use were taken on different dates, so are ideal for plotting a supernova
light curve.
Life Cycle of Stars
1. Follow the instructions in the document, ‘Photometry with SalsaJ’ in order to define an
aperture radius to use when carrying out photometry on the supernova image. For this,
you only need to open one of your images, and use the comparison star to find a value
for the aperture radius.
To identify the comparison star and supernova in your M100 data files, use the image of
M100 below:
Comparison star
Supernova (the
lowest of the 2 bright
spots in the image)
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Plotting Supernova Light curves - Faulkes Telescope Project
2. Below are the values for the magnitudes of the standard star in each filter. You only
have the M100 data files in the R filter, so you only need to look at the R magnitude for
the comparison star:
Open the photometry spreadsheet, ‘Photometry.xls’. Copy the R magnitudes of the
standard star as given above into the photometry spreadsheet into the relevant cell (R
Mag).
3. The next step is to use SalsaJ to measure the intensity, or pixel counts in your FT
image for the comparison star. To do this, go to:
Analyse > Photometry Settings
Life Cycle of Stars
Enter the aperture radius, as measured earlier, in the star radius box, as shown here
(with an example of 15)
Now go to
Analyse > Photometry
A pop-up box will appear, with the photometry results in it.
4. Click on the comparison star to obtain the intensity value, and then enter this value in
the relevant cell in the photometry spreadsheet (R Count).
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Plotting Supernova Light curves - Faulkes Telescope Project
5. Now. ensuring that you use the same star radius aperture value which you measured
earlier, find the intensity value for the supernova in your image.
When you have the intensity value, enter this into the spreadsheet in order for the
magnitude to be calculated. Make a note of the magnitude value for the supernova in the
table below.
The day of observation was found by counting the number of days from the date that the
first FITS file was taken. The dates for each image can be found in the FITS file header,
which can be viewed in SalsaJ by going to:
Life Cycle of Stars
Image > Show Info...
FITS file Name
Day of Observation
M100_R1.fits
1
M100_R2.fits
3
M100_R3.fits
16
M100_R4.fits
25
M100_R5.fits
59
M100_R6.fits
103
Magnitude of
supernova
6. Repeat steps 4-5 for all the data files, for each date, and make a note of the
magnitudes of the supernova in the table above.
Occassionally, SalsaJ returns zero values for the intensity of objects - if this
happens when you are measuring the intensity of your supernova in any of the
images, close the image and just use the remaining ones for your plot.
7. Open a new worksheet in Excel
File > New
8. Enter the values for the day of observation and magnitudes into this new Excel
worksheet.
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Plotting Supernova Light curves - Faulkes Telescope Project
9. Plot a graph with the day of observation on the x axis and the magnitude values on the
y axis.
To plot a graph in Excel, click on the chart button, and select XY (scatter)
Then, just follow the instructions in Excel for setting up the plot.
A typical supernova light curve looks like the one below:
Light curve of SN 2006X
Time
14.5
15
Magnitude
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0
14
15.5
1
2
3
4
5
6
When you plot your graph,
make sure that the y axis is
reversed, as shown to the
left. To do this, plot the
graph as detailed above,
and then double click on
the y axis values. In the
pop-up window that
appears, select
16
16.5
Scale > Values in reverse
order
17
17.5
18
Select appropriate values
for the minimum and
maximum y values so that
your curve covers the
plotting area.
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