Exploding Stars in an Accelerating Universe

Exploding Stars in an Accelerating Universe
# 50 Exploding Stars in an Accelerating Universe Dr. J. Craig Wheeler October 19, 2007 Produced by and for Hot Science - Cool Talks by the Environmental Science Institute. We request that the use of these materials include an acknowledgement of the presenter and Hot Science - Cool Talks by the Environmental Science Institute at UT Austin. We hope you find these materials educational and enjoyable.

in an Accelerating Universe
Exploding Stars in an Accelerating Universe This evening’s lecture is all about exploding stars, supernovae, how they work, how they manifest and what they have taught us about the universe. Sponsored by the McDonald Observatory and Department of Astronomy Board of Visitors J. Craig Wheeler Department of Astronomy The University of Texas at Austin

Supernovae Catastrophic explosions that end the lives of stars
Provide the heavy elements on which planets and life as we know it depends Energize the interstellar gas to form new stars Produce exotic compact objects, neutron stars and black holes Provide yardsticks to measure the history and fate of the Universe Supernovae are catastrophic explosions that end the lives of stars in various ways. They provide the heavy elements: carbon, oxygen, iron, calcium, etc. that are essentially the building blocks of human life and are the basis for planetary life. When supernovae explode they release energy back into interstellar gas to form new stars via compression. Thus, supernovae create a life cycle for stars and an ecology of the universe. In some cases, when supernovae explode they produce exotic compact objects like neutron stars and black holes. Because supernovae are very bright, you can see them from great distances. Because of this characteristic they can be used as yard stick to measure the history and fate of the universe. This final characteristic is one of the biggest stories to tell in this lecture.

Crab Nebula Left-over jet Chandra X-Ray Image Chandra Optical Image
These images are of one of the most famous supernovae, the Crab Nebula with an irregular spider-like shape. The image on the left is the optical image seen through a regular telescope. The center, which is difficult to see in this image, is a point of light that represents a neutron star that formed in the explosion. On the right is an image taken by an x-ray observatory, so it is a full color image but basically on the same scale as the left image. The tilted bagel-shaped object has a jet that extends through the middle. This is a very typical shape of supernovae. We will explore how this might happen and what the significance of the shape might be as we move through the lecture. Left-over jet Chandra Optical Image

Supernova (SN 1987A) Elongated debris Bi-polar symmetry
This is another very famous supernovae, SN 1987A, who’s light was perceptible here on earth 20 years ago, in February of It is a very interesting object, and astronomers are still investigating how it worked. On the left is a picture taken from the Hubble Space Telescope. Directly down the center is the red dot indicating the supernovae. Surrounding it is a ring of material that has been blown off the star prior to explosion. There are also two fainter rings that are off-set, that give an hourglass shape, with the supernovae in the center. All of these structures formed first, then the supernovae blew up in the middle of it all. On the right is another Hubble Space Telescope image, showing the inner ring on a larger scale. Notice the supernovae is not a point of light, but rather a blur that has elongated vertically, as indicated by the black line. This will be a recurring theme as we look at other examples. Bi-polar symmetry

This is an updated image taken by the Hubble Space Telescope of the same supernovae, SN1987A. The outer ring is still visible, pink in this image. The material that was blasted from the supernovae traveled for 15 years or so, until it finally started to hit the ring. The impacts created a clumping of material that light up within the ring as a result of the collision of energy. This collision is still occurring as we speak. There is much ongoing research about this aspect of the supernovae. You can still see the ejected material in the center is quite extended. The entire image is tilted about 45o on the sky. SN 1987A SINS Kirshner et al. 2003

Cassiopeia A by Chandra
Jet This is an image of Cassiopeia A, which went off about 1680 around the time Isaac Newton was alive, however, there was no clear visual display at the time. It is the brightest radio source in the sky. The image is a false color x-ray image taken by the Chandra Observatory, taken as a publicity photo for the opening of the observatory. The photo turned out to be a remarkable scientific result. In this image are some particular aspects to concentrate on. For example there were many people searching for the compact object in the center that was thought to have been left behind in this massive star supernovae, approximately about 20 solar masses at birth. The thought was that it would produce a neutron star or black hole, but none had been found since The search was intensified within the past years of modern astronomy, but yielded no positive results. Yet, in this publicity photo, the Chandra observatory found it, indicated by the arrow. The compact object is about 10 miles across formed during the explosion of the supernovae. So, our theories are right in this regard, but we are still unsure if the object is actually a neutron star or a black hole. This is still an ongoing mystery. In the upper left is a striated gas flow, a jet of material moving out of the compact object. A counter jet extends to the lower right. The two directions of debris highlights the elongation of the supernovae as seen in SN 1987A and the Crab Nebula. Counter Jet Compact remnant mass of Sun, size of Austin

Recent Chandra Observatory X-ray Image of Cassiopeia A
These are updated shots of Cassiopeia A, again from Chandra Observatory in false color, x-ray images. The jet is fairly easy to see moving in two directions, as well as three pieces of bright light. The bright light on the left is the focus of my research, which I believe is the main point of explosion. On the right is an x-ray image taken in the radiation emitted by silicon that shows up in certain circumstances with x-ray radiation. The jet and counter jet are very clearly shown, the question is whether this is primary or secondary behavior. This also illustrates that the supernovae is not round, and putting out energy equally in all directions, but in very special directions.

a neutron star, or perhaps a black hole
One type of supernova is powered by the collapse of the core of a massive star to produce a neutron star, or perhaps a black hole The mechanism of the explosion is still a mystery. There are three types of supernovae to discuss, two main types and one that has just been discovered. The first type of supernovae is powered by the collapse of a massive star, more than ten times the mass of our sun, that evolves to make a dense core that collapses to make an exotic object. One of these objects could be a neutron star, which would have the mass of the sun but the size, roughly, of Austin. The black lines emanating from the image on the left represent magnetic lines of force, leading from a north magnetic pole threading to a south magnetic pole. The collapse of the massive star could also overshoot the neutron star to become a black hole. These black holes exist, trap light, objects fall in where they cannot escape, and are formed from stars and galaxies. Much time is being spent to determine how they manifest, evolve, and what role they play in the universe. So, we know we need massive star to accomplish this, but don’t understand how the collapse of a core gets turned around and is sent back out as an explosion. The actual circumstances that result in either a neutron star or a black hole are still a mystery. When electrons spiral in magnetic fields they emit very specific types of radiation, called, among the specific processes, synchroton and cyclotron radiation. These radiations were first seen and understood on the ground, but are now widely seen and diagnosed in astrophysical objects as evidence for magnetic fields. Detailed studies in all wavelengths, radio, optical, X-ray have established that neutron stars are frequently substantially magnetized. The radiation from pulsars is specifically known to come from a spinning neutron star that causes its magnetic north and south poles to sweep around the sky, generating radiation. It is also true that if a given neutron star, say a very old one, did not have, or had lost, its magnetic field, it would be very hard for astronomers to find it. There are undoubtedly lots of old, weakly magnetized neutron stars floating around in our Galaxy.

Observations begun at McDonald Observatory suggest that these explosions are powered by jets.
One pursuit that has begun with observations begun at McDonald Observatory and now using some of the largest telescopes on the planet in Chile, is the actual shape of stars. It turns out that the theme we are looking at is whether or not a jet like flow is what is causing the supernovae. This is the first new idea to help understand how these massive stars might work. This computer simulation is a model of what would happen if a jet blew up in a star. The image indicates a jet blowing out of the star, characteristic behavior as seen in the Crab Nebula, SN 1987A, and Cassiopeia A. The bulk of the star is blown out along the equator in a torus, or bagel, shape around the star. Resulting in a “jet-bagel” shape that is common in several supernovae. Understanding how the jets form and how they work is another mystery that is currently being researched. These supernovae may be related to gamma-ray bursts. This is the first new idea to understand these supernovae in thirty years. Khokhlov et al. 1999

Jet propagating through a Star
This computer simulation is researching cosmic gamma ray bursts, essentially what happens when a jet comes out of a star. The jet here is coming out of the star, and as it forms a sort of bow wave forms in front of it, and sends a blast wave sideways into the space around it. The explosion going out can be quite interesting, but what it does to the star, sending blast rays sideways as it propagates out through the star and into the space around it, is also important. Weiqun Zhang & Stan Woosley, UC Santa Cruz

We are working on the theory of how to convert the rotation of the neutron star to coiled magnetic fields that launch the jet. One of the key questions is how do you make a jet down in the inside of the star? The most interesting thing about these compact objects, such as a neutron star, is that it generates its own magnetic field as it forms, a magnetic field that wraps up and provides energy to the explosion. This invisible force field acts similar to a rubber band in the way that it can be elastically wrapped up and get kinks in it, and it can propagate that kink energy up through the magnetic field. Then, down to the lower left and lower right are movies, done by super-computer simulations where you have a magnetic field tangling through a rotating ball of gas, a star of some kind. The magnetic field threads this rotating object. As the object rotates, it will send energy up the magnetic field, effectively making a jet. We see jets in lots of astronomical objects; stars when they are first born, quasars blow jets out of them. We think something to do with rotation of magnetic fields is also at work in the middle of supernovae. That’s a hypothesis, we haven’t proved it yet. Uchida et al , Ap & SS, 264, Nakamura et al. 2001, New Astronomy, 6, 61-78

Jet Erupting Through Star
Here is an animation of if a jet might form down on the inside of a star, we that the jet will come blasting out through the surface of the star and then sideways from that jet, it will blow a blast wave that moves out away from the jet down toward the equator. If it is a bipolar jet, one going out the north, one going out the south; there will also be a south-going jet that will blow a blast wave that will come up towards the equator. The blast waves then collide along the equator, and send out this donut-like structure along the equator. What we see in this NASA animation is the jets blowing out through the star, creating the blast waves. The blast waves collide along the equator and the blow out a donut perpendicular to the original jet. In this animation, what is left behind is something that looks like the crown nebula. NASA

Gamma-Ray Bursts 30-year Old Mystery
Seen across the Universe. Swift Satellite Cosmic explosions, flashes of gamma-rays lasting about 30 seconds, detected by satellites. This is so-called cosmic gamma-ray bursts. We have known about these cosmic gamma-ray bursts for something like 30 years. They were discovered by satellites that were launched back in the 1960’s. They consist of flashes of energy, concentrated in very high energy electromagnetic radiation, so-called gamma-rays. They only lasted about 30 seconds, detected by satellites. People would try to find them, but they would be over by the time people could really look for them. So, thirty years went by without knowing what they were, where they were, how much energy was involved in them. That mystery was finally dispelled when, ten years ago, when people managed to correlate satellite observations with ground-based observations and found out that the explosions were indeed coming from across the universe. We also found out that the energy expelled not equally in all directions, but in narrow jets. Then we found out that the energy involved is comparable to that in a supernova, but rather than moving matter around like a supernova, all of its energy is in gamma-rays. These gamma-rays blast of into space, creating the afterglow which can be seen for days or weeks. The hypothesis is that what we are witnessing here is the birth of a black hole. Energy is expelled in narrow jets. Energy comparable to that of supernovae, but all in gamma-rays, with later afterglow in X-ray, radio and optical radiation. Birth of a black hole?

Burst and Afterglow Einstein’s Special Relativity in Action
Deceleration takes months, but because shock chases its own light, we perceive a rapid speed-up of the process playing out in days through our telescopes. Billion light years Afterglow radiation Shock wave To understand gamma-ray bursts, it turns out, there is matter moving at very nearly the speed of light. One cannot do this analysis without Einstein’s theory of special relativity. The idea is that there is some kind of explosion. You eject material in a jet, though we are unsure of what kind of material is ejected. That material comes out and collides with the interstellar gas around the explosion. You create a shockwave moving out through the interstellar gas. The shockwave then creates radiation; that radiation shines out in front of the shockwave in various directions creating the afterglow. That radiation is coming out at something like 99.99% the speed of light, and then it decelerates in the interstellar medium causing the afterglow. This radiation continues to propagate out through space for billions of light-years, and the radiation that reaches telescopes on Earth is what we use to analyze that light. By the time it reaches our telescopes, it appears that it has only taken days, while in fact it has taken months. The difference in the perceived time is because the source of the radiation is chasing its own light. Jet emerges from exploding star at more than 99.99% the speed of light, decelerates by colliding with the interstellar medium. The resulting shock wave radiates the afterglow.

The raging issue: are gamma-ray bursts produced in some
form of core collapse supernova? Circumstantial evidence... THEN PROOF! GRB was nearby, only 3 BILLION light years away! relatively bright. SN2003dh was discovered a week later! Early spectra (day 2) GRB afterglow, but no supernova We know that gamma-ray bursts were comparable to supernova in terms of being distant and very energetic, so the question arose, are they related to supernovae, in particular core collapse supernovae. In 2003, the evidence came forth. This was GRB (gamma-ray burst on March 29, 2003). It was only 3 billion light years away, which meant it was relatively close and therefore relatively bright. That meant that people would actually be able to analyze the light with large telescopes. This graph shows a spectrum portrayed in a graphical way. It is the power radiated in a certain wavelength, a certain color of light, versus the wavelength of the light in a numerical scale across the bottom. In the top of the graph you can see the spectrum just a couple days after the GRB while the afterglow is going on. It was a very smooth graph with no evidence of a supernova. But a week later, the lower graph is shown with a “bump” of light as evidence of growing supernova light. This was proof that, associated with this gamma-ray burst was a supernova of a certain known type. So why is there a raging issue? Several reasons. One is that the rush of evidence as to their true nature has only come in the last few years after their being a total mystery for several decades. We now know that they are seen from huge distances and that dramatic revelation has made them extremely interesting as possible probes of the early Universe. Lastly they are very energetic, related to the deaths of stars, and probably the births of black holes, but we do not know the mechanisms. There is great interest in gaining a deeper understanding so that we can, in turn, understand the deaths of stars and the births of black holes. A week later, a “bump” due to growing supernova light

Black Holes - the quintessential legacy of Einstein
The path of light swallowed by a black hole This is the quintessential legacy of Einstein, that you can have a region of space/time so compact that it will swallow anything. This was a simulation done by an amateur illustrating what happens when you pass a beam of light closer and closer to a black hole. The blue beam is the behavior of a beam of light if there was no black hole, and the yellow represents what it actually does. As you pass the light closer and closer to the black hole, the black hole will grab it and bend it along the curve of the black hole. Once the light gets down inside, it can’t escape and disappears down the black hole.

They are typically associated with jets.
We have identified stellar-mass black holes in binary star systems and supermassive black holes in the centers of galaxies. The black holes here are in stars where we see them associated with a star that has collapsed and created a black hole. Here is an animation of how we see black holes in other non-supernova situations. This is a black hole orbiting this blue star. There is matter being pulled off the companion star by the gravity of the black hole. It comes down and swirls around the black hole. As it gets down near the black hole, it is arguably affected by magnetic fields and there is this jet of material coming off the top. We see that in black holes in binary systems where they are sucking matter of other stars, making a jet and we think that the same thing could be happening with neutron stars or black holes in the middle of supernovas. They are typically associated with jets.

Black Hole Forming in Star, producing jet and Gamma-Ray Burst
Here is another animation of how one thinks a gamma-ray burst might work. We must imagine that there is a massive star that blows matter off its surface. Inside the star we see the rotating material form, a rotating black hole form, a jet come out of it, then we will pan back and see this jet come bursting out of the surface of the star. It sends a blast off sideways. This is what we see, and we can deduce that this happens about twice a day somewhere in the universe. Every burst, twice a day somewhere in the Universe - the birth of a black hole? NASA

Robotic Transient Source Experiment (ROTSE)
We have joined the U. of Michigan Robotic Transient Source Experiment (ROTSE) collaboration. Four ROTSE telescopes around the world. Texas, Australia, Namibia and Turkey. 18 inch mirrors, 1.85 degree squared field of view. We in Austin have joined a team of people that center at the University of Michigan who have built a robotic telescope. There are four of these Robotic Transient Source Experiment (ROTSE) telescopes stationed around the world. Each of the telescopes (including one at the McDonald Observatory) are spread across the circumference of the Earth; two in the northern hemisphere and two in the southern hemisphere; so that at any time, there is one telescope with a view of a black sky. These are small telescopes with a wide field of view. The idea is that as a satellite spots a gamma-ray burst, it will send a message electronically to the ground, causing the telescope to start searching in hopes of capturing the gamma-ray burst light or the afterglow radiation.

ROTSE ROTSE can point and shoot within 6 secs of electronic satellite notification, take automatic snapshots every 1, 5, 20, 60 secs. The power of the camera on the ROTSE telescope is that it can point and shoot within about six seconds. It is the fastest thing on the planet to do this. ROTSE has discovered the optical transient during the 30 second gamma-ray burst. It has followed the light in unprecedented detail and relayed the discovery and coordinates to the HET for spectroscopic follow up. This whole program and our contribution to the discoveries is all very exciting. ROTSE has: Discovered the optical transient during the 30 second gamma-ray burst Followed the light in unprecedented detail Relayed the discovery and coordinates to the Hobby-Eberly Telescope for spectroscopic follow up

A New Type of Supernova Texas graduate student, now Postdoctoral Fellow, Robert Quimby used ROTSE to conduct the Texas Supernova Search, covering unprecedented large volumes of space. Quimby discovered the intrinsically brightest supernova ever seen! (at the time, Fall 2006) This is a completely different type of supernova, but it also deals with ROTSE. Robert Quimby conducted the Texas Supernova Search. Robert realized that this telescope could search for supernovas every night by taking pictures every night and looking for new lights. Last fall, he discovered intrinsically the brightest supernova ever seen. Recently is was hypothesized that these stars happened early in universe, but are just now being seen. I proposed that it was yet a different kind of explosion, proposed theoretically 40 years ago, hypothesized to occur among the first stars ever formed in the Universe, but never seen.

Quimby has since found an even brighter supernova!
A very massive star, more than 100 times that of the Sun, gets so hot that its radiation, gamma-rays, convert some energy to matter and anti-matter, specifically pairs of electrons and anti-electrons, otherwise known as positrons. This process makes the pressure decline, the oxygen core contracts, heats, undergoes thermonuclear explosion, totally disrupting the star: a pair-formation supernova. One hypothesis: The progenitor resembled Eta Carina Here is what we think may be going on: we are looking at an especially massive star, anywhere between ten and one hundred times the mass of the sun. The theory is that they get so hot on the inside, you have to have a high pressure to hold the star up against gravity. That requires a high temperature, and the radiation trapped down in the middle of the star is so intense that it creates gamma-rays. This radiation can convert energy inside the star into matter and anti-matter, specifically into pairs of electrons and anti-electrons (otherwise known as positrons). Basically, you are reducing the pressure when you make these matter and anti-matter pairs. This process makes the pressure decline, the oxygen core contracts, heats, undergoes a thermonuclear explosion, totally disrupting the star. The result is called a pair-formation supernova because it is generated by making these electron and positron pairs. Quimby has since found an even brighter supernova!

The other principal type of supernovae (Type Ia) is thought to come from a white dwarf that grows to an explosive condition in a binary system. Chandra X-ray Observatory image Of Tycho’s supernova of 1572 There are two main types of supernovae; the ones that produce black holes, and those called Type 1a. These supernovae are thought to come from a white dwarf star, stars of mass less than ten times the mass of our sun. Under the right circumstances, if there were a companion star, the white dwarf would pull matter from the companion star and grow to the most mass it can have, at which point it would ignite its thermonuclear fuels and blow up as a supernova explosion. On the right is the current view of Tycho’s supernova, witnessed in It follows they theory that states that this type of supernova should blow up completely and leave no compact object (such as a neutron star or black hole) behind. These explode completely, like a stick of dynamite, and leave no compact object (neutron star or black hole) behind.

This type of supernovae is generally the brightest and can be seen at cosmological distances.
They were used as cosmological probes... to discover the acceleration of the Universe... This type of supernovae turns out to be one of the brightest supernovae there is. And because they are very homogeneous in their properties, they are good tools for measuring distances. Also, it was thought that the universe was expanding at a decelerating rate, but through the observation of these supernovae, it has been discovered that the universe is actually accelerating. the Science Magazine scientific Breakthrough of the Year in 1998

The Accelerating Universe
“The resulting acceleration of universal expansion is a new development in physics, possibly as important as the landmark discoveries of quantum mechanics and general relativity near the beginning of the 20th century.” -- From a National Academy of Science Report From a National Academy of Science Report: “The resulting acceleration of universal expansion is a new development in physics, possibly as important as the landmark discoveries of quantum mechanics and general relativity near the beginning of the 20th century.”

Dark Energy One of the greatest challenges to astrophysics now is to understand the nature of the Dark Energy that drives the acceleration. The dark energy is probably a field (like a magnetic field, but different), but it is 120 orders of magnitude smaller than physicists would expect. No current theory of physics accounts for it. If it stays constant, the Universe will expand to a Dark Oblivion. If it reverses, the Universe could slam shut in a Big Crunch (in more than 10 billion years). To test the behavior of the dark energy in space and time, supernovae remain a key tool of choice for precision measurements. One of the greatest challenges to astrophysics now is to understand the nature of the Dark Energy that drives the acceleration. The dark energy is probably a field (like a magnetic field, but different), but it is 120 orders of magnitude smaller than physicists would expect. No current theory of physics accounts for it. If it stays constant, the Universe will expand to a Dark Oblivion. If it reverses, the Universe could slam shut in a Big Crunch, but don’t worry, it wouldn’t happen for at least 10 billion years. To test the behavior of the dark energy in space and time, supernovae remain a key tool of choice for precision measurements.

Dynamic Behavior of Dark Energy
This video illustrates the idea of Dark Energy. You can see the idea that the universe is expanding, and as the dark expands, there is more dark that fills in. So, you never run out of dark. It is always equally dark, no matter how the universe expands.

The Ragged Edge of Research
Conclusions The Ragged Edge of Research All core collapse explosions are asymmetric, maybe produced by magnetic jets. How can this be proved? Gamma-ray bursts are caused by jets of material moving at nearly the speed of light. Do they mark the birth of black holes? At least some gamma-ray bursts (and maybe all) arise in supernova explosions. How does this work? Have we discovered pair-formation supernovae? Type Ia helped to discover the Dark Energy, but they must be studied and understood in unprecedented detail to learn what the Dark Energy is - the biggest problem facing physics today. Here are some conclusions that I like to call The Ragged Edge of Research because our answers are hardly ever clear-cut, and many questions still arise. First, all core collapse explosions are asymmetric; they may be produced by magnetic jets. How can this be proved? Next, gamma-ray bursts are caused by jets of material moving at nearly the speed of light. Do they mark the birth of black holes? Also, at least some gamma-ray bursts (and maybe all) arise in supernova explosions. How does this work? Have we discovered pair-formation supernovae? Finally, Type Ia helped to discover the Dark Energy, but they must be studied and understood in unprecedented detail to learn what the Dark Energy is. This is the biggest problem facing physics today.

Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe
Written for course of same title Covers all the topics of the lecture, and more

Film - scientific premise related to tonights topics.
Available at:

Dr. J. Craig Wheeler J. Craig Wheeler is the Samuel T. and Fern Yanagisawa Regents Professor of Astronomy at the University of Texas at Austin, where he was chair of the department from 1986 to He was a Research Fellow at Caltech working in Nobel Laureate Willy Fowler's group from 1969 to From 1971 to 1974,he was an Assistant Professor of Astronomy at Harvard. In 1974, he moved to Texas as an Associate Professor of Astronomy. He specializes in the astrophysics of violent events: supernovae, neutron stars, black holes, gamma-ray bursts and the relation of these events to astrobiology. He was elected to the Academy of Distinguished Teachers in He is serving as President of the American Astronomical Society from 2006 to He has published about 200papers in refereed journals and conference proceedings, has edited books on supernovae and accretion disks. He published a novel, "The Krone Experiment," co-authored the screenplay, and played a role in the independent film made in Austin. He has also written a popular astronomy book, "Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts and Adventures in Hyperspace" the second edition of which was released in December 2006.

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