How standard are “standard candles”?


Not-so-standard candles

From Physics World:

The story is already legendary. In the late 1980s and early 1990s, two groups of rival researchers set out to measure the deceleration of the expanding universe. These groups often used the same observatory, sometimes even using the same telescope on consecutive nights. And they both found the same thing, publishing their results at roughly the same time in 1998–1999: the expansion of space–time isn’t slowing down at all. In fact, it’s getting faster. The leaders of those collaborations – Saul Perlmutter and Brian Schmidt – along with Adam Riess of the latter’s group, won the Nobel Prize for Physics in 2011 for this discovery. The implication of the result was that the universe consists not only of visible matter and dark matter, but also a gravitationally repulsive substance. Known as dark energy, the nature of this weird stuff remains as mysterious today as when it was first discovered.

Both groups used certain kinds of exploding stars called type Ia supernovae for their measurements. These supernovae brighten and fade in very similar ways and the current thinking is that this is because they have a common source: the explosion of either one or two white dwarfs, which are the stellar remnants of small-to-medium-mass stars such as the Sun. This consistent brightness allows astronomers to determine how far away the object was when the light left it and for that reason, type Ia supernovae are known as “standard candles” – reliable light- houses in the measurement of cosmic distances.

Or so we all thought.

The rest of this story is in the print edition of Physics World, which you can subscribe to through membership in the Institute of Physics, which costs £15, €20, or $25 per year. You can join by clicking here. You can also get a nice mobile- and tablet-formatted version of the story using the Physics World app, available in the Google Play and iTunes stores. However, if you just want to read the rest of this article, Physics World has kindly allowed me to offer it to you as a PDF download, which looks exactly like the printed version!

A white dwarf murder mystery


What killed the white dwarfs? (Aside from the giant explosion)

Merger or extra matter? Two papers come to opposite conclusions

For Ars Technica:

Type Ia supernovae are explosions that occur when white dwarfs strip matter off a companion star, exceed their maximum possible mass, and blow up.

No, wait: type Ia supernovae are the explosions caused when two white dwarfs collide.

While it’s reasonably certain that white dwarfs—the Earth-size remnant of stars similar to the Sun—are involved, the observational evidence for how these supernovae actually explode is messy. This week’s issue of Nature is a prime example: two back-to-back papers provide evidence for a white dwarf-companion star explosion and a two-white-dwarf collision scenario, respectively. Ultimately, these apparently contradictory results could mean there are two distinct types of white dwarf supernovae… or that we still don’t understand what’s going on.

The stakes are high. Unlike other supernovae, which involve the death of a star much more massive than the Sun, type Ia supernovae all explode in very similar ways. The pattern of light they emit during and after the explosion provides a reliable measurement of how far away they are. Since supernovae are bright enough to be visible from billions of light-years away, astronomers use them to measure the expansion and acceleration rate of the Universe, as recognized in the 2011 Nobel Prize in physics. Because they are so important to cosmology, researchers want to understand what objects are involved in the explosion and exactly how they blow up. [Read the rest at Ars Technica…]

Two images of the supernova detected early this morning in M82, the Cigar Galaxy. The bright circle near the image center is the supernova, which you can see more clearly in the negative-color version at the right. [Credit: Ernest Guido, Nick Howes, Martino Nicolini]


Pardon me, I’m a little excited. When I logged onto my computer this morning, I found that every astronomer and astronomy fan was talking about the same thing: a new observation of a probable white dwarf supernova in M82, also known as the Cigar Galaxy. This is exciting because M82 is practically a neighbor in cosmic terms, a mere 12 million light-years distant. That makes this supernova the closest of its kind in decades (though I’m still trying to sort out which was closer, and when it happened). Suffice to say, the galaxy is close enough that the supernova is sufficiently bright to be visible with relatively small telescopes, and will continue to get brighter over the next few weeks. It’s projected to reach a magnitude of +8, which is bright enough to be seen with binoculars!

Type Ia supernovae are triggered either by the explosion of white dwarfs that accrete too much matter and exceed their maximum stable mass, or by the collision of two white dwarfs. (That’s as opposed to core-collapse supernovae, which are the explosions of stars much more massive than the Sun.) Because they all explode in very similar ways, Type Ia supernovas are “standard candles”: objects that can be used to measure distances to very distant galaxies. The use of them to track the expansion of the Universe was recognized by the 2011 Nobel Prize. [read more…]

What’s cool is that various astronomers, including a number of amateur astronomers, spotted the supernova before it was identified as such. M82 is a popular observing target because it’s distinctive and (yes) not far away. My colleagues at Universe Today and CosmoQuest actually highlighted the galaxy during their Virtual Star Party on Sunday evening, meaning they saw the supernova before we knew what a big deal it was going to be!


White dwarfs are the remnants of the cores of stars like our Sun. They have the mass of a star packed into the volume of Earth, but when they die, their light can be detected across the observable Universe. Researchers using the Hubble Space Telescope identified the farthest white dwarf supernova yet seen, one which exploded more than 10 billion years ago.

Only 8 white dwarf supernovas have been identified farther than 9 billion light-years away. (Some core-collapse supernovas, which are the explosions of very massive stars, have been seen farther than Supernova Wilson.) Since all such explosions happen in a similar way, cosmologists use them to measure the expansion rate of the Universe. [Read more…]

I gotta say, though: this supernova was nicknamed “Woodrow Wilson”, which kind of bugs me. Wilson was a war president, which means we Americans tend to give him a pass on a lot of things, but both his foreign and domestic policies reeked of racism. He worked against racial equality at home and abroad, stamping on egalitarian movements in the League of Nations and segregating the Federal Government. (The previous Republican administrations, for all their faults, had been making efforts to give African-Americans a voice after the Civil War.) Anyway, that’s mostly beside the point. If you want to read about a supernova named for someone whose work I do admire (prickly though he was), see my post about Supernova Mingus.

Death of a white dwarf, 10 billion years later

White dwarf supernovas—more officially known as type Ia supernovas—are important to cosmologists because they all explode in very similar ways. That means they can be used to measure distances to faraway galaxies. However, a peculiar type of supernova, first identified in 2002, has a lot in common with type Ia explosions, but with a lot less energy. Some astronomers are now saying this could be a new class of white dwarf supernova that produces much less light and sends material into interstellar space at far lower speeds.

Beginning in 2002, astronomers started recognizing a peculiar type of explosion. Since then, they’ve identified 25 of them; they resemble white dwarf supernovas in many respects, but strongly differ in others. A new paper by Ryan J. Foley and colleagues offered an explanation: these were an entirely new type of white dwarf explosion, one involving less energy and more material from a companion star. So much less energy, in fact, that the authors suspect that the white dwarf may not be fully destroyed in these odd events. [Read more…]

Baby boom-ers could be a new type of white dwarf supernova