On July 4, 1054, a “guest star” appeared in the sky, as noted by astronomers in China, Japan and possibly Europe and North America. The “guest star”, however, was not an actual star. It was a supernova. A supernova is a tremendous explosion that occurs when a massive star runs out of nuclear fuel and collapses. You can see the gas and dust left over from this explosion – the supernova remnant – if you point a telescope just a few degrees north and east of Aldebaran, the “eye” of Taurus, the bull. This supernova remnant is a faintly glowing cloud of cooling debris, known as the Crab Nebula. In the core of the supernova remnant, spins what remains of the original star – the Crab pulsar. Whirling around 33 times per second, this cosmic lighthouse sends out jets of particles and light that can be seen in videos made from images taken by NASA’s Chandra X-ray observatory.
Not all “guest stars” originate in massive stars, however. There is another type of supernova that was first envisioned by Indian scientist Subrahmanyan Chandrasekhar – after whom NASA’s Chandra X-ray Observatory was named. When smaller stars, like our Sun, run out of nuclear fuel, instead of exploding as a supernova, they eventually become a collapsed, cool object known as a white dwarf. If the white dwarf is fortunate enough to orbit another star, the white dwarf can gain mass by gravitationally attracting matter from its partner. And if it gains too much matter, growing heavier than the theoretical limit that was calculated by Chandrasekhar, it will collapse, ultimately creating a supernova explosion. These Type 1a supernovae all have similar properties, and hence predictable brightnesses. Each shines as bright as a `10^36`-watt lightbulb, and appears fainter at greater distances from Earth. Thus, by measuring the supernova’s brightness at Earth, we can determine how far away it is.
In this exercise, you will learn how to measure the light from an example supernova. By measuring its brightness, you can determine how far it is from Earth. And by measuring the distances from a collection of similar supernovae, you can even measure the expansion of the universe!
The goal of this procedure is to obtain the relative brightness of a target star with respect to two (or possibly more) comparison stars. Below are two windows: the window on the left shows an image of the target supernova and two comparison stars. You will use the window on the right to use Js9 to perform the image analysis. First, you will select a region that includes the target star, and measure the brightness in that region. Then you will select each of the two comparison stars and measure their brightnesses. The comparison stars were chosen because they are known to have a constant brightness. By comparing the brightness measurement of the target to the average brightness of the two comparison stars, you will be able to tell if the target supernova has changed in brightness.
However, there is a limitation with this data analysis approach. – it cannot measure the true brightness of the target, only the relative brightness. Relative brightness is a number which can be close to 0.0 (zero, target is very faint), near 1.0 (one, target is near the average brightness of the comparison stars), or greater than 1.0 (target is brighter than the average brightness of the comparison stars). Measuring relative brightness for a series of images creates a plot called a light curve. Follow these steps to use Js9 to create a light curve for the example supernova, which occurred in 2014 in the galaxy M82
1. Press the T1 (Time 1) Button to load the first Supernova image
2. Press the Zoom Button to zoom into the correct spot of the picture.
3. Press the LOG button to make it easier to see the stars. If you click and drag the mouse across the JS9 window you will be able to adjust the contrast of the image.
4. On the finder chart below, your target Supernova is in the small green circle closest to the bottom of the chart compared to the other circles.
5. Open the Magnifier Box by pressing the Magnifier Button.
6. Move the Magnifier Box so it doesn’t block buttons or boxes.
7. Press the Add Region Button to load in a circle that you will use to measure the brightness of the supernova as well as the brightness of the different comparison stars. Once you have clicked on the circle, use the arrow keys on the keyboard to move the circle more precisely. If you end up with more than one circle click your extra circle and press the delete key on your keyboard.
8. Drag the circle to your target supernova and click the Target Button.
9. Drag the circle to comparison star 1, as shown circled and labeled on the finder chart and click the Comp 1 Button.
10. Drag the circle to comparison star 2, as shown circled and labeled on the finder chart and click the Comp 2 Button.
11. Click the Plot Button to plot the relative brightness, which is the regional pixel count of the target supernova divided by the average of the regional pixel counts of the comparison stars. The plot is drawn under the finder chart and Js9 window.
12. Repeat these steps for each of the available Supernova images taken at different times, (T1-T5).
Region Pixel Counts:
You are about to compare your relative brightness calculations for this Supernova in the M82 galaxy with those calculated by astrophysicists for many more images taken as the supernova brightened and then dimmed. Your relative brightness calculations will remain on the plot below in blue. The astrophysicists’ calculations will show in red.