Category Archives: supernova

Dive Into the Crab Nebula

One of the winter sky’s more intriguing objects (at least in the Northern Hemisphere sky) is the Crab Nebula. I’ve written about it a few times here over the years. That’s because it’s a fascinating thing. When we look at the Crab Nebula (through a telescope, it’s not a naked-eye thing), we are looking at the remains of a massive star. Yes, that’s it: a star that died. Due to the vagaries of light-travel time, people on Earth didn’t see the explosion until the year 1054, but the massive star that formed the Crab actually exploded some 6,500 years earlier. It took 6,523 years for the light to reach Earth.

The Crab Nebula in multiple wavelengths of liight.
The Crab Nebula as seen in multiple wavelengths of light. Courtesy NASA/ESA/STSCI/Chandra/Spitzer/NRAO.

At that point, a relatively quiet and dark part of the sky hosted a “new star” six times brighter than Venus. It stayed bright for months before fading out to obscurity. Today, we know it as the Crab Nebula, and astronomers study it as the best example of the death of a supermassive star. It’s the current subject of a news story coming from the American Astronomical Society meeting in Hawai’i this week, and for good reason. Scientists are peering into it with everything they’ve got!

Forming the Crab Nebula

When the progenitor star that formed the Crab exploded, it sent huge clouds of material out to space. The “leftovers” of the star became a neutron star, a ball of neutrons packed together in an area about the size of our planet. Essentially, it’s the crushed core of the star. It’s spinning 30 times a second and sends bursts of radiation that we can detect here on Earth. The rhythmic pulsing of that radiation is what astronomer Jocelyn Bell measured in her ground-breaking discovery of the first pulsar in 1967. Surrounding the neutron star are filamentary remains of the star. They’re made up of gas and dust ejected from the star. Some of the remains are threaded through with magnetic fields. Interactions between the gas, dust and magnetic fields cause the clouds to glow in infrared light.

Probing the Crab

The object also gives off radio waves, x-rays, and visible light. All that light is detectable from here on Earth. So, astronomers put together an observing program using Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-Ray Observatory. They all observed the Crab, and their data were combined to make a three-dimensional “image” of the Crab Nebula. There’s a nifty little movie that explains it all, which you can watch here.

Why We Study the Crab Nebula

Why combine all these wavelengths to look at the Crab? It’s one thing to look at this object through a telescope; a typical backyard-type instrument will show you a greyish-greenish “fog”. But, focus on it with high-resolution purpose-built telescopes tuned to different types of light, and you start to see great details.

We learn more about the actions of the material in the Crab and the distribution of gas and dust around the neutron star. And, we get a much more satisfying understanding of just what happens when a massive star dies and spreads itself out to space.

That’s why we spend time and money on astronomy research. It tells us more about our universe and the things in it. It may not show us how OUR star will die; it’s not a supermassive star, after all. But, supernovae provide the heavier elements needed to create planets like ours, and so studying them is like a look into our very distant, very ancient past.

A Supernova Named Crusher

What a Brilliant Flash in a Distant Galaxy Can Tell Us

Every once in a while I see a posting by an astronomer who has discovered something unique. The latest one is from Dr. Melissa Graham (@mlg3k) at the University of California-Berkeley. She was working with some colleagues on a project called the Hubble Frontier supernova survey and spotted the telltale “light-up” of a Type Ia supernova in a view of a galaxy cluster some 6 billion light-years away.

For her work, not only does Melissa get a great discovery, but she gets to name the supernova. The usual practice for the Hubble Frontier Field survey is to use Star Trek names. So, she called her object SN Crusher, which is pretty darned cool. (Its official title is HFF15Cru.) And, lots of folks are taking notice, including actor Wil Wheaton (@WilW), who played Wesley Crusher. He caught the news, and congratulated Dr. Graham on her finding! He also let his “Trek” mom (Gates McFadden, who played Dr. Beverly Crusher) know, as well. It all played out on Twitter, where I happened to catch Dr. Graham’s first announcement.

What’s a Type Ia Supernova?

A binary system about to create a Type Ia supernova.
Artist’s concept of a binary system, with material from one star accreting onto a white dwarf.

There are two types of supernovae: Type I and Type II. The second kind, a Type II supernova, is a catastrophic explosion of massive supergiant star.  They’re cool, but that’s not what Dr. Graham saw. She spotted a Type Ia supernova. These events have interesting implications, particularly for understanding distances in the universe.

Usually Type Ia supernovae occur where two stars orbit as a close binary pair. One of them is likely a white dwarf. As they dance around each other, material from one star escapes and “accretes” (gathers) onto the surface of the other. Eventually, the second star reaches a limit to how much material it can accept, and it explodes, providing an extremely bright flash of light and other radiation. The exact mechanism for this explosion is still being modeled by astronomers.

These explosions eject material out to space, and are so bright they can outshine their home galaxy for a short period of time. Type Ia supernovae emit a pretty standard brightness level and astronomers have used them as so-called “standard candles” to measure distances in space. In 1998, astronomers observing Type Ia supernovae found an interesting result as part of their observations: the expansion of the universe appears to be accelerating.

The Frontier Fields Survey

To see how the expansion of the universe is changing over time, astronomers want to look at Type Ia supernovae throughout the cosmos. To do this, they gather light from objects that exist across great distances in space. That’s where the Hubble Frontier Fields Survey comes in.

Frontier Fields observers study massive clusters of galaxies using the the Hubble Space Telescope. The gravity of these clusters warps and magnifies the faint light of the distant galaxies behind them. As a result, astronomers can see some of the most distant galaxies as they looked back in the infancy of the universe. And, they’re also finding — as Dr. Graham did — the signals from ancient Type Ia explosions. Those events give them even more information about the expansion rate of the universe at the time those events occurred.

Want to see more info about this supernova? Check out Dr. Graham’s discovery telegram here.