Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.
As I mentioned recently, a team of astronomers using the Hubble Space Telescope has discovered a gravitationally lensed supernova. I'd like to spend a bit of time today explaining
In short, a supernova is the titanic explosion which occurs when a star blows up.
Don't Panic! Our sun is not going to blow up -- not in the near future, and, as far as we can tell, not ever. Most ordinary stars like our Sun will go through some sort of evolution over billions of years, but they remain largely intact.
Under two different unusual circumstances, however, a star may indeed go Kabloowie.
Finally, the star ends up with an iron core. The trouble with iron is that when one tries to fuse together iron nuclei, the reaction does not release energy; instead, it sucks energy from the surroundings. As a result, an iron core cannot heat itself up and push back against the gravitational force. The core collapses: protons and electrons are crushed together to form neutrons and a giant flood of anti-neutrinos, which trigger a shock wave flying out away from the core. The shock wave blasts the outer layers of the star out into space at high speed -- KABOOM!
Astronomers call this a core collapse supernova. You can see the remains of this type of explosion in our own Milky Way: the Crab Nebula is an expanding cloud of gas created when a star exploded in the year 1054.
The bulk of the star is pulled inward due to gravitational forces, so that it shrinks and shrinks and shrinks, from the size of the Sun to the size of the Earth. The tiny and very dense remnant is so hot at first that it glows with a bluish-white light -- hence the name white dwarf. Left to itself, a white dwarf will very slowly cool off, fade out, and turn into a cold, solid lump.
But, if the white dwarf happens to be a member of a binary star system, in a very tight orbit, then it may accrete material from its companion.
Image copyright
David A. Hardy/AstroArt.org
As the white dwarf accretes material, the weight of all the outer layers increases, squishing the center of the dwarf harder and harder. If enough material builds up on the surface, the center of the dwarf can reach a hot enough, dense enough state that thermonuclear reactions start to occur -- and then feed themselves into a runaway reaction. Nearly the entire white dwarf undergoes a thermonuclear explosion, blowing it into teeny little pieces which fly outward at several percent of the speed of light. KABOOM!
Either way, when a star explodes, it emits a HU-U-U-U-UGE amount of energy for a short time. For several weeks or months, it may grow as bright as an entire galaxy of billions of ordinary stars.
Image courtesy of NASA
The light curve of a supernova -- the graph of its brightness as a function of time -- has a shape which (in many cases) follows a very typical pattern. Astronomers can use the shape and brightness of this light curve to estimate the absolute luminosity of the supernova and, by comparing that to its apparent brightness, figure out its distance.
Light curve of SN 2011fe in M101 based on
measurements made at RIT and Michigan State University
The important points here are
A gravitational lens is just what it name implies: a situation in which light rays are bent, not by a piece of glass, but by the gravitational field of a massive object. In some cases, the background object may appear distorted and magnified in size and brightness; we may even see multiple images of the object!
If we do see multiple images of a single lensed object, then, in general, the light from each image has to travel a different distance to reach us. That means that if the object should happen to grow brighter (or fainter) for some reason, each image will appear to grow brighter at a different time.
The amount of this delay depends on the exact alignment of the observer and lens and background object, and on the mass of the lens, and on the distance to the lens and background object. If we can measure a time delay between the images, we might be able to compute the distance to the object.
Distance.
Astronomers have a difficult time measuring the distances to celestial objects. We can do a good job for stars in the solar neighborhood, and make decent measurements to stars within the Milky Way Galaxy; but as soon as we try to estimate distances to other galaxies, we run into all sorts of problems. Don't believe me? Do a little reading on "the cosmological distance ladder."
Astronomers are always looking for a new and better way to measure distances, especially to objects which are really, really, really far away. Suppose that we could find a supernova in just the right position behind a massive object that its light was strongly bent by the gravitational field. That would yield
This very rare circumstance would be an ideal opportunity for astronomers to determine the distance to a very distant object -- which in turn would provide a check on our models for the size and expansion rate of the universe.
Astronomers have spent years scouring the sky for big, massive clusters of hundreds or thousands of galaxies; clusters so massive that their gravitational field bends the light of objects which are behind them. We have created catalogs with many of these clusters, and studied some of them in different ways.
One of these clusters is called MACS J1149.6+2223. The name tells astronomers its location in the sky. Here's a picture of the cluster taken by the Hubble Space Telescope (click on the image for a full-sized version):
If you look closely, you'll see that there are many short, thin "arcs" surrounding the center of the picture. These "arcs" are background galaxies -- the light from them has been bent and distorted as it passes through the cluster.
The fuzzy yellowish objects are galaxies in the MACS cluster. The light from these galaxies has a redshift of z = 0.544
Q: If we observe light from this cluster
at a wavelength of 550 nm (yellow),
what was the actual wavelength of that
light when it was emitted by the galaxies?
[Hint: observed wavelength = emitted wavelength * (1 + z) ]
Q: How fast is this cluster moving away from us?
Hint:
Now, let's zoom in to look at the center of this cluster.
The very bright whitish object with colorful spikes is a star in our own Milky Way Galaxy that happens to lie in this direction. It isn't involved in the lensing at all. The big fuzzy yellow object is one of the biggest, most massive galaxies in the cluster.
Between the star and the big yellow galaxy lies a smaller yellow galaxy -- also a member of the cluster -- and blueish spiral galaxy. The blueish galaxy is actually not a member of the cluster. Instead, it is far, far behind the cluster, much farther away from us. The redshift of this galaxy is z = 1.491
Q: If we observe light from this spiral
galaxy at a wavelength of 400 nm (blue),
what was the actual wavelength of that
light when it was emitted by the galaxy?
[Hint: observed wavelength = emitted wavelength * (1 + z) ]
Q: How fast is this background galaxy moving
away from us?
Actually, the light from this spiral galaxy has been bent so much by the cluster of galaxies that we can see three different images of it near the center of the field:
Astronomers used HST to take pictures of this cluster of galaxies back in 2011. In November of 2014, they took another set of images. A comparison of the two sets of images revealed something NEW in the 2014 pictures. Can you find it?
Yes -- you are seeing three bright (and one faint) image of a (likely) supernova which has exploded in one of the spiral arms of the background galaxy.
As soon as the supernova was noticed, astronomers requested special time on the Hubble Space Telescope to come back to this part of the sky every few days to monitor the brightness of these images.
Under ordinary circumstances, the images taken by HST are marked as proprietary for a period of one year after they have been taken; only the scientists who requested the images can access them. That gives the scientists a chance to analyze the images carefully and write up papers before other scientists could scoop their results.
However, these "special time" images are not covered by that policy. Instead, anyone -- yes, anyone -- can download the images from the HST archives, as soon as they have been beamed down to the Earth and run through the regular pipeline processing.
I've grabbed the images taken in the past few weeks and turned them into a very low-key movie. Can you see any changes in the little dots of light representing the SN?
Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.