Introduction

SN 1987A is the name of a famous supernova. The first part of its name refers to the type of event – a supernova, then to the year in which it was first observed (1987) and finally the “A” denotes that it was the first supernova discovered that year.

Supernovae

A supernova is an explosion that signals the death of certain types of stars. There are basically two types of supernovae, but here we will only deal with the so-called Type II supernovae – massive stars that come to the end of their lives in a very spectacular fashion. SN 1987A was the explosion of one such massive star.

Figure 2: The Large Magellanic Cloud (LMC)
The LMC is a small irregular galaxy, one of the nearest of the Milky Way’s neighbouring galaxies. It is filled with stars, dust and gas and is teeming with star formation. SN 1987A appeared here in the LMC.
This image was taken with the Schmidt telescope at the European Southern Observatory’s (ESO) La Silla Observatory.

A massive star (typically more than five solar masses) may end its life in an explosion after a few million years. During the explosion most of the star’s material is blown violently out into space. The velocity of the ejected material can reach 10⁷ m/s (3% of the speed of light). The expanding shell of debris remains visible in interstellar space for thousands of years before it eventually fades into the interstellar medium, leaving a visible residue known as a supernova remnant. Within the surrounding nebula the central part of the original star is compressed to a neutron star.

All supernovae are very bright, with a brightness equivalent to the total emitted light of billions of Suns. They are believed to be among the brightest objects in the entire Universe. This makes them visible over large distances. However, there are very few supernovae and so the sky is not constantly lit by the spectacular deaths of stars. The rate at which supernovae occur is estimated to be only a few per century per galaxy.

Figure 3: Apparition of SN1987A
In the left-hand image you can see the Tarantula Nebula after the supernova exploded. An arrow points to the supernova. The right-hand image shows the Tarantula Nebula in the LMC before the explosion of Supernova 1987A on February 23rd 1987.

Supernova 1987A

On February 23^rd 1987 a supernova visible to the naked eye appeared in the Large Magellanic Cloud (LMC). The LMC is one of the nearest of the Milky Way’s neighbouring galaxies. This was one of the most exciting events in the history of astronomy. SN 1987A was the first supernova visible to the naked eye for almost 400 years.

The Distance to the Large Magellanic Cloud

The determination of distances in the Universe is one of the most fundamental problems in astronomy. An accurate measurement of the distance to SN 1987A, situated within the LMC, can be used to determine the distance to the LMC itself.
All stars in the LMC are at approximately the same distance from us. If we can find the distance, D, to SN 1987A, we then simultaneously find the distance to all the other types of stars found in the LMC. Several other types of objects found in the LMC and in other more distant galaxies can also be used for distance measurements, so a more precise distance to the LMC would be a stepping-stone to more precise distance measurements for other, more distant galaxies.

Figure 4: Supernova 1987A
SN 1987A in the centre (scaled up in the insert) left behind a residue of three rings of glowing gas in the LMC. In this exercise the small central ring is used to measure the distance to the supernova and thus to the LMC.
Many young – 12 million years old – blue stars are seen in the area as well as dust and gas (in dark red). This shows that the region around the supernova is still a fertile breeding ground for new stars.

The Ring

The first images of SN 1987A taken by the NASA/ESA Hubble Space Telescope were made using the ESA Faint Object Camera (FOC) on day 1278 after the outburst. Hubble was first launched in 1990 and then had to be set up in space, so that it was not possible to take images earlier. As well as being of great intrinsic interest, SN 1987A challenged even Hubble’s very high resolution. The pictures of SN 1987A show three circular nebulae surrounding the supernova – an inner ring and two outer rings. In this exercise we use the inner ring only. The ring is too far from the supernova to be material ejected in the explosion. It must have been created earlier, probably as material from the dying star was carried out by the stellar wind during the last few thousand years of its life. It is not clear how the material was shaped into such a well-defined thin ring, but once formed, the material of the ring began to glow rapidly when a flash of ultraviolet light from SN 1987A reached it.

Figure 5: Measuring the distance between galaxies
If the distance of the LMC can be measured more accurately, then more precise distance measurements can also be made for other more distant galaxies.

It is important to realise that the ring was present before the star exploded as a supernova. We will assume that the ring is a perfect circle, but inclined at an angle to a line joining Earth and the supernova so that we see an ellipse. If the ring were facing the observer all parts of the ring would have lit up simultaneously when the flash of light from the supernova reached it. However, as the ring is inclined, the nearer rim appeared to light up first (due to the finite speed of light) and then the light seemed to move around the ring, lighting the farthest point last (see "The Ring Lights Up"). Note that the whole ring was actually illuminated at the same time, but that on Earth we saw the nearer rim light up first.
Since the gas continued to glow and only faded slowly after the light flash passed by, the total light emitted by the ring reached a maximum roughly when the whole circumference had been illuminated. This fact can be used to calculate the distance to SN 1987A.

The questions in the following tasks outline the steps to be taken to calculate the distance to the supernova using the angular size of the ring and a light curve that shows the evolution of the ring brightness with time after the explosion.

Quicktime movie (courtesy STScI/NASA)

Figure 6: The Ring Lights Up As this animation illustrates, the light from SN 1987A reaches the ring of matter around it and the ring lights up. The ring reached a maximum brightness around 400 days after the outburst. Note that even though the light reaches the different parts of the ring at the same time, we see the closest parts light up first (due to the finite speed of light). By measuring the observed time delay it is possible to derive the distance to SN 1987A. The images are taken from an animation sequence made by STScI/NASA.