SN 1987 A - Supernova Analysis Lab

Introduction

During the night of February 23, 1987, several diligent astronomers were looking through their telescopes at stars in the Large Magellanic Cloud when they noticed a bright star that was not there previously. Word quickly spread across the globe that a rare supernova event had occurred in the LMC. The top picture shows the event at maximum brightness, and the lower pair of pictures compares the star before the explosion, and 10 days afterward. Click on the image to learn more about it.

Supernova events are exceedingly rare. A supernova event has not happened in our galaxy since October 9, 1604 when Johannes Kepler noticed a new star in the sky. While this exploding star was not in our own galaxy, it was pretty close. The Large Magellanic Cloud is only 179,000 light years away, and that is close enough that astronomers could see the new star with their naked eyes. Over the course of the next 12 months, astronomers pointed their telescopes at the LMC and studied the explosion, gleaning important clues to the manner in which stars of this type die. The Hubble Space Telescope took the pair of pictures seen below in 1994. Certainly, a lot has happened in the 7 years since the star first exploded!

This final image below depicts the theoretical lifecycle of this star, Sanduleak -69 degrees 202. While this particular image is based on what is believed to be the typical lifecycle of this particular star, the past 20 years of research into the explosion that caused SN1987a supports this model. It is interesting to see how this particular star lived its life relative to the HR Diagram, and how quiclky this star died once the core was exhausted of its supply of Hydrogen.

Lab Purpose

As I write this lab, we are 20 years past that exciting year, but astronomers continue to observe the expanding rings of stellar debris. The purpose of this lab is to review some of the images that were taken as the star brightened more and more, and then slowly dimmed back to naked-eye invisibility. As you study the images and measure the apparent magnitiude of sn1987A, you will collect enough data to create the "light curve" for the star. From this curve, you will be able to determine the maximum apparent magnitude (mv). From this determined value, and knowing the distance to the LMC, you can determine the absolute magnitude (Mv). By looking at the pattern of brightening as well as the maximum Mv, you ought to be able to determine what kind of supernova this particular event was.

The reason your determination of this event is part of this course is because astronomers discovered 161 supernova in 2006, and many more continue to be found. A quick look at the long list in the linked document shows different kinds of supernovae, and these different kinds are all related to the type of star at the moment of the explosion. One particular class of supernovae, the Type 1a events, are exceedingly important to astronomers because they appear to brighten and dim, following the same pattern, and they always reach the same Mv at maximum brightness. These close similarities among the Type 1a events make them excellent "standard candles," and thus they are the most important tool that astronomers have to measure the distance to the most remote galaxies. Type 1 supernovae are a class of events that are believed to result from a White Dwarf star suddenly exceeding its upper mass limit (known as the Chandrasekhar Limit). The star explodes violently, with energy levels greater than all of the stars in the entire Universe lighting up at one time (Mv values reach -19 in just a few days). The light curve (below) will brighten very quickly. Then, the brightness quickly fades for a few more days, and over the course of a year will more slowly fade into relative invisibility.

Supernova of the Type 2 variety are the explosions of giant stars whose core fuses Silicon into Iron so fast, and without any outward energy from the fusion event, that the core collapses. The collapse of the core triggers a violent outward shockwave surge that literally blows the rest of the star out into space, while the inner core continues to collapse into a Neutron Star or even a Black Hole! These objects brighten to a level that is most often significantly less than Type 1 events (mV = -15 or -16). Their light curves are not consistent as the Type Ia events. They brighten to their peak levels quickly, but star remains relatively bright much longer (plateau times of several weeks is common), and their pattern of fading is more erratic than the smooth fading light curve of a Type I event.

So, on to the lab work, and your opportunity to determine whether this was a Type 1 or a Type 2 event.

Instructions and Procedures

Since the Large Magellanic Cloud is visible only in the southern hemisphere, people here in the US could only study it if they were "down under." The Royal Observatory of the UK operates a 1.2 meter telescope in Coonabarabran, New South Wales, Australia. A series of photographs of the event were taken from the Schmidt Telescope in Australia, and you will be asked to look at these images. There are 3 pages that have 4 images on each page. The pages were constructed with the photographs that I cut out and pasted to a page for your analysis. The images are the photo negatives instead of real color or regular black & white because it is easier to actually measure the star as it brightens and dims. Each small photograph shows the bright nebula known as 30 Doradus. This is the large, black, and very irregular feature on each negative, and it does not change in size or brightness. As you look on your computer screen, 30 Doradus will be in the upper and slightly right region of each image. If you turn your computer sideways so that the writing is are the bottom, 30 Doradus will be in the upper left of each image. The star that you are measuring is in the lower part of each image, and slightly to your right as you look at it on your computer screen. Once again, it you tilt the screen so that the writing is on the bottom, the star can be seen in the lower right of each image. Over the span of 10 months of images (12 actual photographs), this star changes in brightness (size), and typically has 4 small points that indicate it is a star and not a nebula.

As SN1987A exploded, it got brighter and brighter, and as the explosion died of, the stellar remnant got dimmer and dimmer. Your task is to measure the brightness of SN1987A in each image and associate that measurement with a value for apparent magnitude (mv). A scale of the relative apparent magnitudes is included on each page, and with that scale as a reference, it is possible for you to determine the apparent magnitude of SN1987A in each of the 12 images found on the ensuing 3 pages. (Remember, just because the photograph shows the star to be brighter does NOT mean that the star is getting bigger).

With your 12 measurements, you can then plot the apparent magnitude value (mv) relative to time on a piece of graph paper. You will be constructing the graph, but there is no need to turn the graph in to the CDE office or return it to me via Moodle. It is an assignment that you will need to do in order to determine the type of supernova event. The plotted points represent the "Light Curve" for SN1987A, and developing light curves is something that members of the AAVSO (American Association of Variable Star Observers) spend their time doing. I wish I had more time to talk about this group, but not here, even though what they do is of critical importance, and this group is comprised of a large number of amateurs who make truly important contributions for the professionals. In fact, it was a member of the AAVSO who first saw SN1987A on February 23 and promptly alerted the professionals who have bigger telescopes.

Step 1 - Get a measurement of the size of SN1987A from each of the 12 photographs.

The 12 photographs are found on the following 3 pages:

Page 1 (images A, B, C, and D)

Page 2 (images E, F, G, and H)

Page 3 (images I, J, K, and L)

Go to each page and determine the apparent magnitude of SN1987A by comparing the size of the star to the scale on the side of each page. Astronomers who study variable stars compare the brightness of their "target" star with a stars whose brightness is well-established and does not change. Those who are very good can make a visiual determination within 0.1 magnitudes of precision. Those who are not as good, or who have aging eyes simply take a photograph and physically the size of the images on the picture. You are doing the latter method, even if you have the eyes as sharp as a hawk.

Take a ruler and measure the diameter of SN1987A in the image. Make 2 or 3 measurements for each image and determine an average value. Then compare that diameter to the scale at the side of the page. The particular "dot" that is closest in size to your measurement will give you an estimate of the apparent magnitude of SN1987A for that particular day. Write down this value, and go on to the next image and repeat the process.

Write your measurements for A-L on a separate page of paper. Below is a chart that gives the photographic dates.

Photograph
Date
Days Since Event
A
27 February, 1987
4
B
10 March, 1987
15
C
29 March, 1987
34
D
8 April, 1987
44
E
24 April, 1987
60
F
5 May, 1987
71
G
25 May, 1987
91
H
4 July, 1987
131
I
15 August, 1987
173
J
17 October, 1987
236
K
9 December, 1987
289
L
11 February, 1988
353

Step 2 - Plot the value of each of the 12 measurements relative to the number of days since the event on a graph

Create a graph that has time on the X-axis and Apparent Magnitude (mv) on the Y-axis. Plot each of the 12 values of apparent magnitude relative to the corresponding "day since the event) on the graph. Remember, the Y-axis that measures magnitude must have the higher number on the bottom, and lower numbers ascending to the top because the lower numbers correspond to higher levels of brightness. (A star that is mv = 6 is much more dim than a star that is mv = 3).

Step 3 - Create the Light Curve

Draw a smooth curve through the 12 plotted points on your graph. The date of the maximum brightness may not have been recorded because of the actual timing of the photograph, since cloudy weather is scourge of the astronomer, and it does get cloudy from time to time in Australia. Use the smooth curve to estimate the date when peak brightness occurred.

Step 4 - Calculate the Absolute Magnitude of SN1987A at peak brightness

Use the magnitude-distance formula to determine the Absolute Magnitude (Mv) of SN1987A. The formula is:

Mv = mv + 5 - 5logr

Mv = Absolute Magnitude, mv = Apparent Magnitude, r = the known distance to the LMC (50,000 pc*). The (*) symbol denotes a generalized value for the distance to the LMC. In my searching of the internet Astronomy sites, and various textbooks, I cannot find a consistently agreed upon value for the distance to the LMC. Distances vary from 160,000, to 163,000, to 170,000, to 179,000 light years. A value of 50,000 pc is 163,000 light years, and just makes problem-solving easier.

Step 5 - Record all of the results in the boxes below and answer the questions I am asking.

When you have filled in the boxes, be sure to press "Submit" or else nothing will arrive at my desk, and you will not get the credit for this lab

Before you do anything else for this exercise, fill in the 2 blanks immediately below this, and then read on.

Your Name:

E-mail Address (Required):


Photograph - Date - Days Since the Event - Your Measured Apparent Magnitude (mv) goes into the textbox

A - 27 Feb - 4 -

B - 10 Mar - 15 -

C - 29 Mar - 34 -

D - 8 Apr - 44 -

E - 24 Apr - 60 -

F - 5 May - 71 -

G - 25 May - 91 -

H - 4 Jul - 131 -

I - 15 Aug - 173 -

J - 17 Oct - 236 -

K - 9 Dec - 289 -

L - 11 Feb - 353 -

1) After plotting the graph and drawing the light curve, what is your estimate of the date of peak brightness?

2) According to your graph and measurements, what was your estimate of the Apparent Magnitude at peak brightness?

3) Using the formula Mv = mv + 5 - 5logr, calculate the value of Absolute Magnitude at peak brightness? r = 50,000 pc

4) Based on your analysis, what kind of supernova event was SN1987A? Type 1 or Type 2

5) Why did you make this conclusion?

 

Link to AAVSO Explanation of SN1987A for a more detailed account of this event.

Remember, all of your efforts and filling in of these boxes is moot unless you press the submit button at the bottom of this page.***


       

The links below only take you to content area of my teaching, but will not return you to the Moodle site.


| Home | Course Information | Assignments | Teacher Bio | Course Units | Syllabus | Links |