Category: Astronomy

When stars like our Sun run out of fuel, they die a long slow death as a white dwarf, slowing cooling down over billions of years. But now an international team of astronomers has found an unusual form of white dwarf with a bare carbon core; one that might suggest a new sequence of stellar evolution. A fate for stars right on the edge of detonating as supernovae.

The majority of stars that die eventually become white dwarfs in the end. The most massive 2-3% of stars will actually detonate as supernovae when they end their lives. But these newly discovered objects might have been right at the borderline. If they were just a little more massive, they would have detonated as well, but instead, they didn’t quite make it.

The evidence was gathered by astronomers from the University of Arizona, Université de Montréal and Paris Observatory. They reviewed more than 10,000 new white dwarfs found in the most recent update to the Sloan Digital Sky Survey. This automated survey has turned up four times as many white dwarf stars as previously known.

As stars run out of hydrogen, they switch to helium, and as this burns off, they’re left with a core of carbon and oxygen surrounded by an atmosphere of hydrogen or helium. That’s what a normal white dwarf looks like.

But a small group of these white dwarfs have a very bizarre appearance. They’re just a bare core of carbon, without any surrounding atmosphere of hydrogen or helium.

From the press release announcing the discovery, researcher Patrick Dufour describes the discovery, “when I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones. But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn’t agree with the SDSS data. Out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked. I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed. No one had calculated a pure carbon atmosphere model before. No one believed that it existed. We were surprised and excited.”

The researchers believe you need to have a star with 9-11 solar masses to create a carbon star like this. They’re planning follow up observations to better pinpoint the masses of the objects they’ve discovered so far.

Original Source: UA News Release

As humanity becomes a spacefaring civilization, we’re going to come up with tricky situations that challenge current laws and concepts of nationality. For example, what’s your country if you’re born on the Moon? Or if two astronauts get into a fight while in orbit, whose laws are followed? If you break a piece of an international module, where do you send the cheque? During a recent conference in Europe, scholars and space scientists met to propose unusual circumstances that might happen in space exploration

Law in space is currently covered by the Outer Space Treaty of 1967. It’s been ratified by 98 states, and follows the tradition of maritime law – states have legal jurisdiction within their own spacecraft. But what happens when a spacecraft has been built by several nations, such as the Columbus laboratory module, due to fly to the International Space Station in December.

The recent conference, called Humans in Outer Space – Interdisciplinary Odysseys was held on October 11-12 in Vienna, Austria.

The partner nations working to build the International Space Station have already rejected a proposal that the entire station falls under US law.

“It was agreed that each state registers its own separate elements, which means that you now have a piece of the US annexed to a piece of Europe annexed to a piece of Japan in outer space, legally speaking”, said Dr Frans von der Dunk of the International Institute of Air and Space Law at the University of Leiden.

Since each module is operated by a different nation, that sort of works. But in the case of the Columbus module, it was built and operated by several European nations. Since it’s a collaboration, it can’t be registered to any single state since there isn’t an entity called “Europe”.

There are issues of criminal law; what if one astronaut from one country punches another while in an international module? There are also patent law problems; where should an invention be patented? And there are civil law concerns; what happens if an astronaut damages a part of the station?

The meeting looked far into the future too, when bases are established on the Moon and Mars. Since the 1967 treaty defines the Moon for the good of all humanity, it can never be considered a territory of any country back on Earth. So what nationality would a child have?

The 1979 UN Moon Agreement provides rules on how nations should explore the Moon, but doesn’t go beyond to issues of civil and criminal law.

For now, if you’re born on the Moon, you’re from nowhere on Earth.

Original Source: ESF News Release

There are a few ways that stars can go kaboom, and each variant is different enough that astronomers can figure out what kind of object detonated as a supernova. But when a bizarre explosion was detected last year, it left astronomers puzzled. Now, it looks like SN 2006gz might have been caused by two white dwarfs colliding together.

Millions of years ago, there were two regular stars in a binary system, orbiting one another. Over time, one, and then the other ran out of hydrogen fuel, bulged up as red giants, and then settled down to live out their futures as slowly cooling white dwarfs. But instead of billions of years of quiet cooling, the two stars had decaying orbits. They spiraled inward, and finally collided, detonating as supernova 2006gz.

When SN 2006gz was first discovered last year, astronomers thought they were dealing with a type Ia event. This is where a white dwarf is in a binary system with another star. Like a vampire, the white dwarf feeds on material from the companion star until it reaches 1.4 times the mass of the Sun. This magic point, called the Chandrasekhar limit, is the upper limit of mass a white dwarf can have. And when that limit is reached, the white dwarf detonates as a supernova, visible for billions of light years.

The problem is that SN 2006gz seemed to be too bright. In other words, it must have gotten more mass than the Chandrasekhar limit before detonating. It also had the strongest spectral signature of unburned carbon ever seen.

And that was the key. Mathematical models suggested that colliding white dwarfs would generate this specific signature of unburned carbon. They also suggested that an explosion should contain evidence of compressed layers of silicon, created during the explosion and then compressed during the shockwave that rebounded from the surrounding layers of carbon and oxygen – this too was seen.

Thanks to good observations, SN 2006gz was relatively easy to recognize as a collision between white dwarfs. But this event might be more common in the Universe, and astronomers will need to go back and carefully analyze supernovae on record to see if they’ve been wrongly categorized.

Original Source: CfA News Release