How standard are “standard candles”?

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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

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

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…]

Now it can be told: I will be writing a weekly post for The Daily Beast (making me The Weekly Beast?), on space, astronomy, and such things. My first column is about inflation, and why it’s a big deal:

If you compare any two points on the night sky, their temperature as measured in microwave light is identical to a few millionths of a degree. That light, known as the cosmic microwave background, comes to us from nearly the beginning of the Universe, so it has been traveling for 13.8 billion years. Even with the expansion of the cosmos, two points on opposite sides of the sky were never in the same place, yet they have the same temperature… assuming the current rate of the expansion of the Universe has been roughly the same since the beginning.

But maybe it hasn’t. The cosmic temperature coincidence (which would be a great band name), along with several other annoying aspects of the Universe, led a group of researchers to propose the theory of inflation. [read more…]

The Daily Beast’s latest astronomy columnist is…me!

The Cassiopeia A supernova remnant. [Credit: NASA/CXC/SAO]

The Cassiopeia A supernova remnant. [Credit: NASA/CXC/SAO]

Nearly every atom of your body was forged in a supernova explosion and dispersed into space. But how do massive stars explode? The details are complicated, pushing the limits of computer simulations and our ability to observe with telescopes. In the absence of very close-by events, the best data come from supernova remnants: the still-glowing gas ejected during the explosion. A new set of observations of X-ray emissions from radioactive titanium in the Cassiopeia A supernova remnant show that it was a lumpy space princess highly asymmetrical explosion. That agrees with theory, but the researchers also turned up an odd disconnect between the titanium and other materials.

Cassiopeia A (abbreviated Cas A) is a historical oddity. The supernova was relatively close to Earth—a mere 11,000 light-years distant—and should have been visible around CE 1671, yet no astronomers of any culture recorded it. That’s in stark contrast to famous earlier explosions: Tycho’s supernova, Kepler’s supernova, and of course the supernova that made the Crab Nebula. This mysterious absence has led some astronomers to speculate that some unknown mechanism diffused the energy from the explosion, making the supernova far less bright than expected. [Read more…]

Supernovas: mysterious and lumpy space explosions

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]

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!


Green Peas were all my joy, galaxies were my delight

Most galaxies are somewhat red or blue in appearance, depending on the populations of stars that comprise them. However, citizen scientists working with the GalaxyZoo project identified a previously unknown type of galaxy: Green Peas, so named because they are small and green. The color comes from ionized oxygen, a particular form of emission that only happens under unusual conditions. A new study shows that Green Peas could resemble a kind of early galaxy responsible for reionization: the breakdown of atoms due to aggressive star formation when the Universe was young.

A new paper by A. E. Jaskot and M. S. Oey argues that galaxies much like the Green Peas could be responsible for the reionizing radiation. They analyzed the light emissions from the galaxies, and determined that their gas is thinner than in typical star-forming galaxies, which could allow more ultraviolet light into intergalactic space. The researchers also found signs in a few Green Peas of extremely massive stars, the ones most responsible for ionizing radiation. [Read more…]

V838 Monocerotis

A mystery: an unknown star, too faint to notice, suddenly expanded to a huge size, increasing in brightness to become one of the most luminous stars known. This star doesn’t even have a real name, just a “license plate” catalog number: V838 Monocerotis, indicating that it’s a not very important star in the constellation the Unicorn (Monoceros). However, a new paper has proposed the powerful flare could be explained by a well-accepted theory of binary star behavior, in which one star strips enough matter off the other until it suddenly grows to a huge size. These common envelope events (as they are known) could  explain the V838 Monocerotis outburst, along with some other currently mysterious flares.

A new Science paper proposes that a class of violent astronomical events that we’ve observed may be due to common envelope stars, providing more direct evidence for their existence. These cataclysms are known as “red transient outbursts,” and in brightness terms, they’re somewhere between novas (flares of nuclear activity at the surfaces of white dwarfs) and supernovas, the violent deaths of stars. N. Ivanova, S. Justham, J. L. Avendado Nandez, and J. C. Lombardi Jr. identified a possible physical model for these outbursts, based on the recombination of electrons and ions in the plasma when the stars’ envelopes merge. [Read more…]

Cannibal binary star could explain mysterious nova-like outbursts